Having electricity in your van is just plain awesome. It means you can live completely off the grid with lights, a fridge, phones and computers – all while not worrying about electric bills or power outages.
Many vandwellers install solar power in their rigs. But sometimes solar just isn’t enough – especially if your budget doesn’t allow you to throw down for a huge multi-panel system.
Cloudy weather, wildfire smoke, and camping in shady forests can limit the amount of sunlight getting to your panels, leaving you scrambling for adequate sun to recharge your depleting batteries. Even with our larger 400W system we’ve run into problems with battery drain after about 4-5 days in poor sunlight conditions.
That’s why we highly recommend setting up your campervan electrical system to charge your batteries from your van’s alternator while you’re driving.
Charging from your alternator is a great way to supplement your solar panels and make sure your batteries stay topped off no matter the weather. And if you’re on a tight budget, you can even skip the solar and still have basic electricity in your DIY van build.
Life on the road means a fair amount of driving, and having the ability to charge your batteries while driving is essential for vanlife.
How To Charge Your Van’s Batteries While Driving
Every vehicle has an alternator. An alternator is a device that converts the mechanical energy from your van’s engine into electricity, and uses that electricity to power electronics in your van and charge your starting battery.
You can easily use your alternator to charge your second (auxiliary) battery simply by connecting the positive terminals of both batteries so that they’re in parallel. But paralleling your batteries means that when the engine’s off, your electrical loads will also drain the starting battery – not good if you want to start your van in the morning!
So you need a device that allows you to charge a second (auxiliary) battery from your van’s alternator without draining your starting battery when the engine isn’t running.
There are two kinds of devices for this: DC-DC chargers and Battery Isolators.
We’ll go over both in this post, but in general we recommend that most people get a DC-DC charger for their rigs.
What is a DC-DC Charger?
A DC-DC charger (also known as a battery-to-battery charger, or b2b charger) is a device that takes the input from your alternator/starting battery and uses it to charge your aux battery.
DC-DC chargers are able to charge just about any type of battery (including lithium), they work with modern variable voltage alternators, and they employ multi-stage charging to fully and properly charge your battery bank.
DC-DC chargers come in two varieties: single input and dual input.
Single Input DC-DC Chargers
Single input DC-DC chargers do one thing, and one thing only: charge your aux battery from your alternator. This is what you want to get if you already have your solar equipment, or if you want the flexibility to pick and choose the exact specs you need for each component.
Pros
- Variety of amperages available to suit your specific requirements
- Easy add on to Renogy solar kits or existing solar setups
Cons
- Another component taking up room
- May require tapping into ignition circuit
Smart DC-DC charger from Victron with Bluetooth connectivity. Comes in models ranging from 12A to 30A.
Dual Input DC-DC Chargers
Dual input DC-DC chargers, on the other hand, also function as solar charge controllers. So with just a single unit, you can charge your batteries from your solar panels and from your engine. This makes installation a whole lot easier, and it makes your electrical system look a bit cleaner.
However, combined units like this take away the flexibility to really customize your solar setup independent of your engine charging, since you’ll be locked in to the specs of your DC-DC charger (For example, Renogy’s DCC50S can only accept 25V solar input, which means you must wire your panels in parallel to stay under that voltage).
But, if you were planning on wiring your panels in parallel anyway, then the DCC50S allows you to have one less component in your system.
Pros
- One unit handles both solar and DC-DC charging
- Easy to install (usually no ignition tap)
- Also tops off your starter battery
Cons
- Less flexibility with charging parameters
- Need to assemble your own solar components vs buying a kit
40A dual input charger that allows you to charge from both solar and your alternator. Also available in a 50A version.
What is a Battery Isolator?
A battery isolator is a device that allows you to charge an auxiliary battery from your van’s alternator, while keeping your starting and aux batteries “isolated” from each other.
Battery isolators are inexpensive, and they’re generally pretty easy to install. However, they are not always the best choice for your van’s electrical needs, and in most cases a DC-DC charger is what you should go with.
Battery isolators may not work properly with modern variable voltage alternators, won’t work with lithium batteries (unless you pay through the nose for a lithium-specific isolator), and may not maintain the proper voltage to fully charge your aux batteries.
There are three types of battery isolators out there: solenoid battery isolators, solid state battery isolators, and voltage-sensing relays (or “smart” isolators). Voltage-sensing smart isolators are far and away the best choice, so we’ll focus our discussion on these.
Smart battery isolators work by automatically sensing the voltage of your starting battery. When the voltage reaches 13.3V (meaning the engine is on and the battery is fully charged), the isolator “cuts in” and sends 100% of the alternator’s current to your auxiliary battery. When the starting battery voltage drops to 12.8V (meaning the starting battery is no longer charging), the isolator “cuts out” to prevent your starting battery from draining.
Rugged and durable smart battery isolator for charging your aux battery while driving. Easy to install, and IP65 certified.
The KeyLine Chargers Iso-Pro140 Smart Battery Isolator has worked out great in our van. It’s small and compact, it’s very simple to install (the hardest part is running battery cable from the engine compartment to the rear of your vehicle). And it’s IP65 certified, which means you won’t have to worry about it failing after driving the dusty road to Burning Man.
Everything you need to install your smart battery isolator. Includes the Iso-Pro 140, battery cable, crimp terminals, and lugs.
The KeyLine Iso-Pro 140 is also available as a kit that includes wiring, rings, terminals, etc, which should help make installation a whole lot easier.
When to Use a Battery Isolator (and when not to)
Battery isolators will work in your rig if all of the following are true:
- You have an older van with a fixed voltage alternator. Battery isolators need a consistent voltage to work properly. If you have a newer vehicle (about 2015 or newer) with a variable voltage “smart” alternator, an isolator probably won’t work for you.
- Your auxiliary batteries are lead acid (AGM, gel, flooded lead acid). Most isolators won’t function properly with lithium batteries. (There are lithium-specific isolators out there, but they’re super expensive and thus kind of pointless.)
- You’re on a tight budget. Battery isolators are cheaper than DC-DC chargers, but that’s about the only advantage they have. If you’re not on a tight budget, you’ll be better off with a DC-DC charger.
If all three of the above apply to you, then awesome – get a battery isolator for your rig.
However, if any of the above do not apply to you, then you need a DC-DC charger.
DC-DC Chargers vs. Battery Isolators
On the surface, DC-DC chargers seem very similar to battery isolators. Both allow you to charge your auxiliary battery while driving, and both prevent your starting battery from draining when your engine is off. But the difference is in how they charge your aux battery.
Battery isolators simply parallel your starting and aux batteries together, which puts them at the same voltage. So if your alternator is sending 14.4V into your starting battery, the battery isolator connection will put your aux battery at 14.4V also (meaning it’s charging).
There are a few issues with this:
- With modern variable voltage alternators the voltage output can fluctuate, preventing the battery isolator from kicking in.
- If your alternator isn’t putting out enough voltage, your isolator may only partially charge your aux battery. This can lead to battery degradation over time.
- Voltage drop can be an issue if you have a long wire run connecting your isolator to your aux battery.
DC-DC chargers, on the other hand, take voltage input from your alternator/starting battery and boosts it to the proper voltage for charging your aux battery. They do this by putting a “load” on your alternator, so that you alternator treats it like it would, say, a light bulb, and sends power to it. No matter what voltage your alternator is putting out, a DC-DC charger will send the proper charging voltage to your aux battery.
There are a few advantages to ths:
- DC-DC chargers can handle the fluctuations of modern variable voltage alternators and still charge your aux battery properly
- DC-DC chargers can employ multi-stage charging, so you know your batteries are being properly and fully charged.
- DC-DC chargers can work with different charging profiles, meaning you can use them to charge different types of batteries (including lithium).
What are the downsides to DC-DC chargers? Mainly that they are slightly more expensive than battery isolators, and they may be a little more difficult to install (since some DC-DC chargers require you to tap into your ignition circuit).
But DC-DC chargers are way more flexible and capable than battery isolators, and we think they’re the best overall choice for vanlife.
What Size DC-DC Charger or Battery Isolator Do You Need?
DC-DC chargers and battery isolators comes in different sizes, indicated by amperage (i.e. a 60A DC-DC charger, or a 140A battery isolator). How do you pick the right size for your van?
Sizing a DC-DC Charger
When selecting a DC-DC charger, you want to size it based on the charge rate of your aux batteries. This is based on your battery chemistry – so an AGM battery has a different charge rate than a lithium battery.
Here’s the general rule of thumb for battery charge rates:
- Lithium batteries (LiFePO4, etc.) can be charged at 0.5C (or, 50% of capacity***). This means that a 100ah battery can be charged at 50A.
- Lead acid batteries (AGM, gel, FLA, etc.) can be charged at 0.2C (or, 20% of capacity***). This means that a 100ah battery can be charged at 20A.
***Note: These are general guidelines only. Check the specs of your specific batteries before selecting charging components.
DC-DC Charger Sizing Calculator
Keep in mind, this is the maximum charge rate. You could undersize your charger, but don’t oversize it (some DC-DC chargers, like the Renogy models we recommend, have the ability to set a lower charge rate if needed).
Again, double check your specific battery’s specs to make sure you’re getting the right size DC-DC charger.
Sizing a Battery Isolator
The general guideline is to size a battery isolator based on the maximum output of your alternator. You should be able to find this number either in your vehicle’s spec sheet, or stamped onto the alternator itself.
So, if your alternator max output is 175A, then in theory you would need at least a 175A battery isolator.
However, although your alternator may be capable of outputting 175A, not all of that is available for charging your aux battery. Some of that is being used to power the other systems and electronics in your van, so the amperage actually being sent through your battery isolator may be substantially less.
On top of that, most battery isolators out there come in sizes ranging from 125A to 150A. While there are larger isolators available, they get pretty expensive above 150A, and at that point you might as well get a DC-DC charger anyway.
Long story short, if you’re going the battery isolator route a standard 125A to 150A smart isolator should have plenty of capacity in most situations.
Installing a DC-DC Charger or Battery Isolator in Your Van
What You Need
Components
- DC-DC charger or battery isolator
- Deep cycle battery
- Battery cable (size cable based on specs for your specific unit)
- Battery terminal lugs (size for your cable) and crimping tool
- (2) Inline ANL fuses (one for each battery – see fusing specs for your specific unit)
Tools
- Cordless Drill
- Mechanic’s Toolset
- Multimeter
- Zip Ties
- Cable sheathing/flexible conduit (sized for your cable)
Instructions
- Disconnect the negative battery terminal from your starting battery. This is an important safety step that isolates the starting battery so you won’t get shocked.
- Mount the charging unit. Find an easily accessible spot. Battery isolators are typically mounted within the engine bay (you may need to temporarily remove your starting battery to make room). DC-DC chargers are typically mounted back by the auxiliary battery so they are out of the elements.
- Run battery cable from the engine bay to your van’s electrical hub. You may need to run this underneath your van. Cover the battery cable with sheathing or flexible conduit to prevent shorts. Use zip ties to keep it out of the way. Make sure there the cable is tight and that there is nothing loose hanging down. Drill a hole up through your van’s floor to route the wire inside. Seal this with silicone caulk.
- Ground the charging unit. Attach the DC-DC charger or battery isolator to a common ground point on your van’s chassis. It’s best to use an existing ground screw.
- If needed: Tap the charging unit into your vehicle’s ignition circuit. Some DC-DC chargers (and battery isolators) require that you tap into your van’s ignition circuit.
- Attach the charging unit to your starting battery. Cut and crimp battery cable to the size that you need. Run a cable from the DC-DC charger or isolator to an inline ANL fuse, then another cable from the fuse to your starting battery (for DC-DC chargers, this is the long cable you ran under your van. Battery isolators are mounted in the engine bay).
- Attach the charging unit to your aux battery. Cut and crimp battery cable to the size that you need. Run a cable from the DC-DC charger or isolator to an inline ANL fuse, then another cable from the fuse to your aux battery (for battery isolators, this is the long cable you ran under your van. DC-C chargers are mounted near the aux battery)
- Reconnect your starting battery and make sure everything works. Fire up your van, wait a few minutes, and check to make sure your aux battery is charging. DC-DC chargers should give you a readout. Battery isolators will have indicator lights, and you can also check the voltage at your aux battery terminals using a multimeter.
Step 1: Disconnect the negative battery terminal from your starting battery.
Find an easily accessible spot. Battery isolators are typically mounted within the engine bay (you may need to temporarily remove your starting battery to make room). DC-DC chargers are typically mounted back by the auxiliary battery so they are out of the elements.
Step 2: Mount the charging unit.
Find an easily-accessible spot to mount your charging unit. DC-DC chargers typically mount back by your aux battery. Battery isolators are typically mounted in the engine bay (you may need to temporarily remove your starting battery for this step).
Step 3: Run battery cable from the engine bay to your van’s electrical hub.
You may need to run this underneath your van. Cover the battery cable with sheathing or flexible conduit to prevent shorts. Use zip ties to keep it out of the way. Make sure there the cable is tight and that there is nothing loose hanging down. Drill a hole up through your van’s floor to route the wire inside. Seal this with silicone caulk.
Step 4: Ground the charging unit to a metal point on your vehicles chassis.
Attach the DC-DC charger or battery isolator to a common ground point on your van’s chassis. It’s best to use an existing ground screw.
Step 5: If needed: Tap the charging unit into your vehicle’s ignition circuit.
Some DC-DC chargers (and battery isolators) require that you tap into your van’s ignition circuit.
Step 6: Attach the charging unit to an inline fuse then to your starting battery.
Cut and crimp battery cable to the size that you need. Run a cable from the DC-DC charger or isolator to an inline ANL fuse, then another cable from the fuse to your starting battery (for DC-DC chargers, this is the long cable you ran under your van. Battery isolators are mounted in the engine bay).
Important Note on Fuses
The in-the-box instructions for some battery isolators may not call for any fuses. But adding two inline fuses (one as close as possible to your starting battery and another one close to your auxiliary battery) is an important safety feature.
The purpose of a fuse is to break the circuit in case of an electrical short. When you install an isolator, you’re likely running electrical wire underneath your van. If that wire somehow shorted out and both of your batteries were not fused, you could have a serious problem on your hands.
So – it’s a good idea to fuse both batteries when you install a battery isolator. When in doubt, add a fuse!
How big of a fuse do you need? Unless it’s outlined in the instructions for your battery isolator/DC-DC charger, it’s a good idea to fuse based on the charge rate of your battery.
Step 7: Attach the charging unit to your aux battery.
Cut and crimp battery cable to the size that you need. Run a cable from the DC-DC charger or isolator to an inline ANL fuse, then another cable from the fuse to your aux battery (for battery isolators, this is the long cable you ran under your van. DC-C chargers are mounted near the aux battery).
Step 8: Reconnect your starting battery and make sure everything works.
Fire up your van, wait a few minutes, and check to make sure your aux battery is charging. DC-DC chargers should give you a readout. Battery isolators will have indicator lights, and you can also check the voltage at your aux battery terminals using a multimeter.
Electricity on the Road in Any Conditions!
We think a DC-DC charger (or a battery isolator) should be one of the first things you add to your van’s electrical system. Sometimes solar isn’t enough, or you may not have the budget for solar right away. In either case, a DC-DC charger is a great solution.
No matter if you’re traveling in overcast climes, in the deep forest, or other areas where you may not get enough sunlight, charging your batteries while driving ensures that you can have the power you need in all conditions.
Hi John, hoping you can help me among a few thousand others with similar situation.
I have a GZ Yeti 3k with a link module that can be connected to starter battery for fast charging from alternator while driving. This works well if the alternator is a static one, but does not work well on a smart alternator.
So here are my two questions:
1 – can I connect a dcdc charger directly to the GZ link module to charge the GZ?
2 – if answer to 1 is a negative. Then can I setup a small, 50ah lithium house battery and connect the GZ link module directly to the lithium?
The link module requires at least at 13.8v to start charging and will stop once the GZ is full.
Really appreciate your thoughts on this topic. Thanks.
Hello, unfortunately I don’t have direct experience solving this problem with the Yetis. However, based on what I’ve seen, a DC-DC charger that boosts the voltage above the cut off should work in theory. You would want to make sure the DC-DC charger you use outputs the proper amperage that the Yeti is able to accept. The battery solution I’m not sure would work, because then you’re introducing a third battery in the chain, and the first DC-DC charger will be sensing the voltage of this battery to determine its charging behavior. Just a quick note – per the Goal Zero website, they are planning a firmware update with the goal of fixing this issue with smart alternators. Hope that helps!
Hi John, very helpful article.
I currently have a completely isolated house battery system powered via solar charge controller and has it’s own battery monitor).
So far, the solar completely covers my needs, however, I was hoping to tie the starting battery and house batteries together to cover some contingencies. I’d like to self jump start the starter battery, use solar to charge the starter battery and charge the house batteries from the alternator. Is this possible with a DC-DC or battery isolator?
Also, if the house batteries need to be grounded to the frame, would that interfere with the battery monitor shunt that’s currently installed?
Hi James, the Renogy DCC50S can meet most of those needs. It’s a combination DC-DC charger and solar charge controller that charges the house batteries from the alternator and charges both your house and starting batteries via the solar. I don’t believe it has the ability to jump the starting battery, but it will keep it topped off. The other downside is that it’s limited on the amount of solar input voltage it will accept, so you will have to wire your panels in parallel (series or series/parallel will not function).
There are a few other companies, such as Victron and Redarc, that also make multi-input DC-DC/charge controller units that should meet most of these needs as well.
As far as battery isolators go, the Wirthco Battery Doctor does has jumpstarting capabilities, but it does not accept solar input and it’s not the most reliable unit.
Hope that helps!
~John
hi this is really helpful and well written, so thanks. But I am clueless on everything, so some things like “it’s a good idea to fuse based on the charge rate of your battery” are not clear to me. Do you mean the car battery or the leisure battery? I have the 30A renogy dc to dc with mppt, 90ah of LifeP04 and an old non-smart alternator car. What size fuses should i use please?
Hi Anna, that ultimately depends on your specific battery. However, in general, LiFePO4 batteries can be charged at 50% of capacity, so a 90Ah battery can be charged at 45A (we recommend double checking the specs of your specific battery). Therefore, you would want to fuse the battery at 45A or less. Hope that clarifies!
I already have a working solar system with a controller. I want to add the renergy 20 amp dc2dc. Can I have the 2 separate systems charging my house battery at the same time, or will it over charge my house battery?
Hi Charlie, you can have both systems, and they won’t interfere with each other. Both systems are sensing your battery voltage and adjusting their charging parameters, or whether they’re charging at all, based on that. Hope that clarifies!
What a cool site. I have a 2003 Chrysler town and country. I am not a van-lifer. I am a pro disc golfer. So I touring the usa each year in van for 8-9months. Then home for winter. I want to put a small fridge like the boug 23 or 30q. I would be charging my phone and iPad. Sometimes run a small fan. And maybe one of the 12v ovens or single burner stovetop. That’s about it. I’d like to start with the dc-dc from renogy. (Add solar later). My questions are thus.
If I get the dc-dc and a backup battery, what do I hook up to the back up battery to be able to run my fridge and charge stuff?
Also with fixed budget is it better to get a less expensive lithium/lifepo4 battery or a better agm,etc..?
Thanks
Mikey
Hi Mikey, thanks for reaching out! In order to power things from a battery, for DC devices (like your fridge, phone/ipad with a car charger, etc), you would need a DC fuse block wired to the battery, then connected to cigarette lighter or USB outlets. For AC devices (anything with a wall plug) you would need to add on an inverter wired to the battery. Most inverters have outlets built in. On top of that, you would need some kind of enclosure to contain the wiring. We have a post about DIY electrical systems that might be helpful.
As far as batteries go, LiFePO4 is definitely better than AGM if you can afford it. AGM batteries are much heavier, and you have to worry about draining them down too far (not really an issue with lithium).
However, from what you described, I would actually steer you towards a portable power station, like those on offer from Bluetti or Jackery. We wrote a review of the Jackery 1000, and were actually surprised by how well it did. With these you get a battery with built in charging, inverter, and outlets, and you don’t have to do any wiring. And power stations are also portable, so you can take it off in the woods somewhere, or use it around your house, etc. Both Jackery and Bluetti offer a range of sizes and price points. Bluetti is a bit better because they use LiFePO4 batteries and will last longer, whereas Jackery uses Litium NMC batteries.
As far as your usage, you can easily power a fridge, small fan, and charge devices with a small (100ah lithium or 200ah AGM or so) system, or one of the portable power stations. However, powering any sort of electric heating element like a stove or oven is difficult to do without a much larger system. I would advise looking at small camp stoves for cooking.
Hope that helps!
John
Did you tap into the ignition circuit for the renogy model?
Hi Skyler, the Renogy 20A/40A/60A DC-DC chargers do require tapping into an ignition circuit. The DCC50S charger does not, and also functions as a solar charge controller. Hope that helps!
This is very well written.
Good article. I want to install a DC-DC but I also want to be able to start my Van battery by using the House battery during emergency when my Van battery is drained. How would I do this?
Hi Edgar, the Wirthco Battery Doctor isolator includes a way to reverse the flow so that you can jump your starter battery. However, these units are not super reliable in practice. If you’re building out an energy system from scratch, another option is the Renogy DCC50s. This unit is a DC-DC charger and solar charge controller in one. Not only does it charge your house battery from your alternator, it also charges your starting battery from excess solar when you’re parked. That way you’ll always be topped off. Aside from that, we recommend carrying a portable jumpstarter with you, expecially if you’re going off into wild inaccssible areas. Hope that helps!
~John
Great suggestion. Thank you
John-I’m seeing a lot of videos describing people that put a physical switch between the DC2DC and the Inverter. Their explanation is they only turn on the DC2DC whenever the car is running. Then in a follow up, they say they forget to turn off the switch and it drains their battery.
1. is this switch necessary with the DCC50s?
2. How does the inverter know where to draw the power from? Solar, Shore power and/or car battery? Do I need something else to make this decision?
Hi Edgar, every DC2DC charger I’m aware of has built in mechanisms to prevent draining the starting battery, either via an ignition tap or via sensing voltage. I’m assuming these people have chargers that require tapping into the vehicle ignition and they just installed a manual switch instead.
No switch is necessary with the DCC50S, and would in fact disable one of its big benefits. The DCC50S takes dual input from solar and the alternator. It basically works off of the voltages of your aux battery, starting battery, and alternator. Keeping the aux battery charged is always the priority, and solar is always the priority input. If there is not enough solar coming in to hit the maximum charge rate, the DCC50S may supplement with alternator power if and only if the starting battery and alternator are above a certain voltage. Meaning, that it will automatically cut off drawing power from your starting battery/alternator if the vehicle is not running, because the voltage will drop below the threshold when the alternator is not actively charging the starting battery. This will also prevent it from taking too much power from your starting battery/alternator while the vehicle is running, because it will only do so when the voltage indicates that the battery is fully charged and there is excess alternator power available.
Now, one of the awesome benefits of dual input chargers like the DCC50S is that while you’re stationary it will also keep your starting battery topped off from your solar. Trust me, it is not fun having your battery die in the middle of BLM land with no cell service, so automatically keeping it topped off is huge in my eyes. Installing a switch to close this connection would eliminate this.
Regarding your inverter question, that all depends on your inverter, which is separate from the solar/alternator charging setup. If you get an inverter/charger that can accept shore power, it probably has an automatic bypass switch that would switch output from battery to shore power when you’re plugged in. DC devices would still run off the battery, but the inverter/charger and your solar, etc, would be charging it. Another route to go is to just install a regular inverter, and use a separate charging unit for shore power plug in. In this case, your inverter would always be drawing on your battery, but when plugged in your battery would be charging from shore power simultaneously.
I hope that answers your questions!
~John
Thank you for the very thoughtful answer. This all makes sense now.
I have a key line battery isolator and a Renogy 60amp dcdc charger. Can I use both or just use the 60amp Renogy dcdc charger? Also, I have (2) 200amo Renogy amg batteries
Hi Lyne, there wouldn’t be a reason or benefit to using both. I would go with the DC-DC charger, since that will do a better job of fully and properly charging your batteries. Hope that helps!
I found this information very interesting thank you
So glad this was helpful!
Hi John,
If we hook up a DC-DC charger or a battery isolator to my two 100 AH Renogy gel batteries do we have to disconnect the Renogy charge controller while the DC-DC or battery isolator is charging?
Hi Stephen, no need to disconnect the charge controller, they won’t interfere with each other.
Hi John! I saw your article about DC to DC chargers vs isolators. I have a 2015 Chevy Express with an isolator, and I seem to be running into the problem you talked about: the variable voltage of my alternator doesn’t really charge my aux battery very well. I want to switch to the Renogy DC to DC, but I’m not sure whether to get the 20 or 40 amp. A call to Renogy didn’t really clear up matters much. I have Renogy’s 200w solar kit with the Rover 20a charge controller. This is feeding a single VMAX 155ah battery. Also, should I fuse this? Thanks!
Hi Tim, you will want to base the size of your DC-DC charger off the charge rate of your battery, which is a function of your battery capacity and chemistry. For AGM batteries like the VMAX, the general rule of thumb is a charge rate of 20% of the capacity. So, the max charge rate of a 155Ah AGM battery is about 31A (155 * 0.2 = 31). The 20A should be your best bet here, although I **believe** the 40A has a switch to make it charge at 20A if you want more ability to expand in the future. Hope that helps!
~John
Thanks John, that helps a lot. Do you think it’s necessary to put an inline fuse on the power wire from the battery?
Hi Tim, it’s always a good idea to fuse the battery connection. Your battery manufacturer should have fusing recommendations. Hope that helps!
Hi John, I hope you can help. I ordered the Renogy 40A dc-dc charger yesterday but now I think it might be too big?? I have a Beaut 200Ah AGM battery and looked at the specs where it said “Recommended maximum charge current limit: 60A” this means I can charge it at max 60A right so the 40A dc-dc is still good for now. Next, I contacted the van’s garage to ask about the alternator and it is a 150A alternator so this should also be fine for the 40A dc-dc charger. Now… I totally forgot about my 325W solar panel on the roof… I also didn’t read it in this post so hope it’s ok but… My thought now is that the solar panel generates 27A (325/12v) combine this with the 40A dc-dc would be 67A.. which is more than the max charger limit of my AGM battery… Did I screw up, should I have bought the 20A dc-dc? I read this in your article “(some DC-DC chargers, like the Renogy models we recommend, have the ability to set a lower charge rate if needed).” How do you do this? Can I set it to 30A? which would make a combined 57A. Or can I put an on/off switch on the D+ line that goes to the ignition to turn the charger off when it’s sunny outside and on when it’s cloudy? How do I do this and where do I buy the on/off switch? Really hope you can help me.
Hi Thomas, thanks for reaching out. One thing to keep in mind is that not all of that available amperage is going into your batteries all at once – both the DC-DC charger and the charge controller are going to be charging as needed. They both have multi-phase charging, meaning that they’ll charge at different amperages based on the current state (voltage) of your battery. They should not conflict with each other. However, you could fuse your batteries at 60A – so, run the positive cables for your charging sources to a buss bar, then connect the buss bar to your battery via a 60A inline fuse. That way anything greater than 60A would trip the fuse. You could also install an on/off switch on the D+ line, which would give you further control over the alternator charging. Hope that helps!
~John
Hello John,
I want to build a dc-dc charger in my van, when it doesnt have one yet. BLast summer went quite good with the setup atm. 60ah lead acid batter charged by solar, and a ” portable” 40ah lithium( for use with microwave oven and coffee maker etc.) with 230v charger that i charge at home or work.
My question is that how SMALL can the DC/DC charger be? In my mind i wouldnt need any bigger than 5-10a charger, that would top up my small batter more than enough.
In the same way i save a lot of money buying a smaller dcdc charger, and the wiring could be smaller also saving money and weight.
I have also seen ” dc-dc chargers” that you can hook up to the cigarette 12v plug to charge a second battery? Is this a working syster?
When will the alternater break a too small dc-dc charger? Because the alternator is giving out a lot of amps…? ♂️♂️♂️
Hi Robin, I don’t think there’s a limit on how small you can go. There is an upper limit based on your battery capacity, but at the lower end it would just be based on how much current you need to meet your charging needs. 12V cigarette lighter chargers should be fine so long as they will work with lithium.
As far as the alternator goes – while it can put out a lot of amps, it’s not force-feeding amperage to anything. Your DC-DC charger should only take as much as it needs from the alternator. The danger is usually the other way around – high amperage chargers/components overtaxing the alternator. Hope that helps!
John
Hello! I didn’t build out my van myself (nor have much knowledge on this stuff) but my alternator used to charge my aux batteries so I assume there’s a DC-DC charger or isolator somewhere that I can’t see. But today my aux batteries are low, the charger controller won’t stop beeping, and turning the van on drains them more! When I turn the van off, the charge goes up a little. Any ideas??
Hi Emily, if the voltage increases when you turn your van off, then there might be a load being applied to the batteries when the van is on. If the alternator were charging the battery, then voltage should drop when you turn the van off. Hope that helps a bit!
~John
Hi John, – decking out a newly aquired van and auxillary battery questions arrise.
If I fit an isolater and it is joining the batteries together only when the ignition is on through a damper of sorts so as too much power goes either way should this be OK. – The van has an auxillary battery 120AH Lithium and solar panels, as the van is quite small weight and space are an issue. It has AC and a microwave and a electric dolly wheel not yet fitted. I am concerned about the 500w motor on the dolly wheel and if I should fit a small acid battery to operate it.
I am thinking of fitting a large auxillary battery and inverter to the rear of the towing vehicle (on order). Whilst camping a 240V lead could be run from the invertor to the van for operating the accessories. whilst driving the battery will be charging. Your thoughts?
Hi Maxwell, I’ve definitely seen people contain the bulk of their power components in a tow behind to save on living space in their van. That could work nicely for you if that is your preference. As far as the dolly wheel – is it AC or DC? If DC then you definitely could add a small battery to run it. When we bought our van it had a wheelchair lift installed, and it came with a small separate battery to run that and the fold down bench seat. Hope that helps!
~John
Hey this has helped me so much. Can you tell me what size wire you used to connect your inverter?
Hi Paula, so glad this has been helpful. In our van, we used 2 AWG wire to connect our inverter, and fused it at 100A. Your inverter instructions should tell you what size wire and fuse to use. When we did our first build, we didn’t feel like messing with wire thicker than 2 AWG (even though our inverter said to use 1/0 I believe with a 150A fuse), so we just used 2 and a 100A fuse. It’s vitally important to always size your fuses based on your wire gauge – this protects the wire from taking more current than it can handle, which can be a fire hazard. Hope that helps!
~John
Hi there, I recently contacted Renogy about a 20 amp DC to DC charger as I wondered if the charger disconnected the Starting battery during power draw. The answer was it disconnected from the starter battery at 8 volts !!! isn’t this way to low, most disconnect around 12.2. Can you please advise
Hi Russ, Renogy’s DC-DC chargers don’t use a voltage cutoff, because they are tapped into the ignition circuit and will not run if the engine is not running. I’m not sure why they would tell you that.
Thanks for the article! I can’t seem to find your more detailed writeup on the installation of the Keyline 140, including the addition of fuses. Is it still available?
Hi Scott, we had the images down to update this post, but we’ve since restored them. Hope that’s what you were looking for!
~John
Hello John,
What if you have 2 alternators, one for the engine and the other, a high-output 48V unit for the camper, wouldn’t the battery isolator and or the DC to DC converter be unnecessary?
Thanks,
Patrick
Hi Patrick, if you have a second alternator that’s already serving the role of charging your aux batteries then you would not need another charging device.
~John
Hey John. Great article. I am doing a bus conversion. I would like to use a DC to DC charger to charge my house batteries. The setup I would like to accomplish is like this: I would like to use 6 to 8 house batteries to run a small split unit AC, through an inverter, while I am driving. I am hoping that with 6 to 8 batteries and the DC to DC unit charging them through the alternator, that it will be enough power to keep the spit unit running while the engine is running. What is your opinion on this? I am trying to prevent from having to run a generator every time I need air conditioning while driving. Thanks.
Hi Eric, thanks for reaching out. The first question that pops to my mind is how many amps is your AC unit drawing. To determine this, look at the wattage for your AC unit and divide by the system voltage (12V). For example, a 900W split unit would pull about 75A per hour (probably more like 80A once you factor in inverter inefficiency). In that scenario, a 60A DC-DC charger would only replace 60A while driving – so you’d have a drain of 20A per hour. Even with that, you could easily drive all day long and power your AC, but you would need another way to charge up your batteries when you’re stationary.
Other things to think about:
–I believe you can wire multiple DC-DC chargers in parallel to increase the charging amperage. The battery bank you’re talking about could accommodate 400A or more, so you could easily wire in two DC-DC chargers and cover the draw from your AC.
–You may need to consider upgrading your alternator if you’re going to maintain that kind of load on it all the time.
–Adding in solar would also help a lot.
~John
This is good information. Thanks a lot. I have a 220 Amp alternator so I am thinking that will be big enough. Wiring multiple DC-DC chargers in parallel to increase the charging amperage is a good idea. I did see where Sterling makes a 120 Amp DC-DC charger which would probably be big enough. I will do some research into that. Thanks.
Thank you for the very informative and helpful article. I may be missing the obvious here. I understand how the isolator works when the vehicle is running and charging. Does it continue to function the same when the ignition is off. That is, when drawing power from the auxiliary battery, does the isolator disconnect the vehicle (starting) battery to prevent discharge? Thanks.
Hi Daniel, thanks for reaching out. Battery isolators detect when the engine isn’t running, and cut the connection to avoid draining your starting battery (they “isolate” the batteries, hence the name). Smart isolators (aka voltage sensing relays) sense the voltage of your starting battery. Once the voltage rises above a certain point (for example, 13.4V), the isolator knows that the battery is fully charged and the engine is on, so it opens the connection and charges your house battery. When the voltage drops below a certain point (say, 12.8V), then the isolator knows the engine is off and cuts off the connection.
However, this does not work with newer vehicles that include a variable voltage “smart” isolator. If you have a newer van, you will likely need a DC-DC charger instead (like this one from Renogy – use coupon code gnomadhome for 10% off). DC-DC chargers can accept variable voltages and boost them to the proper voltage to charge your house battery. They also cut off when the engine isn’t running.
Also, if you plan on using lithium batteries in your van build then you will need a DC-DC charger vs an isolator.
~John
Does anyone know if the
2003 Chevy Silverado 1500 SS 1500 6.0L V8 has the (HALL-EFFECT SENSOR) Current Load on the GROUND CABLE from – post of starting battery to engine block?
I’m wondering if I need a Smart Battery Isolator or not. That’s what is holding me back from moving forward installing my 1,200 watt AGM battery.
Hi John
I’m new to the forum and have 3500 express and was curious of could I run a regular inverter (8000 watts pure sine) and install a battery isolater for second system ? To run four 12 amp freezers and tv ?
Hi Omar, you can certainly use a battery isolator to help you charge up your batteries while driving. However, if you’re running four freezers and a TV through an 8000W inverter you may need a pretty large battery bank that could be difficult to charge up from driving alone (this also depends on how much you’re driving). You may want to think about having another charging source, such as solar or even a gas generator.
~John
We installed our solar system and isolated our self’s everything works great except there is a whiny noisy under hood..could that be from something we did wrong
Hi Casper, it’s tough to say without being there. If it’s coming from the isolator then you may have a defective unit.
Hi John,
I’m a total newb when it comes to 12 volt wiring. I definitely found your article very helpful and I would like to attempt to install an auxiliary battery to my 2000 Nissan Frontier 3.3L V6 truck camper. Weight is an issue and I am leaning towards a Lithium Iron Phosphate battery. My energy consumption is minimal as I would really only be powering some led track lights with a dimmable switch. I also would like to add a couple 120 volt sockets in order to power laptops. I am also thinking about adding a very small exhaust fan to use occasionally in order to remove smoke from cooking. The camper is tiny so I don’t believe I need a ton of energy and I don’t plan on boondocking for extended periods of time. Do you feel that this configuration will work without a solar panel? If I were to use a voltage sensing smart-isolator, would it be adequate? From what I have read, it doesn’t seem like my alternator would be able to fully charge the lithium battery to full capacity, would you agree? would a 100 ah lithium battery be enough energy? Does this set up seem realistic? Thank you for your time, it is greatly appreciated!
Cheers friend!
Hi Dave, so glad you’ve found our site helpful! The setup you’re describing should work fine with a 100ah lithium battery. As far as charging, make sure any sort of isolator/charger you use is set up for lithium batteries. For lithium systems, we usually recommend using a DC-DC charger like this one from Renogy (use the coupon code GNOMADHOME for 10% off). As far as whether you would need solar, I would say it depends on how frequently you’re driving and how long you’re out camping for. If you’re doing shorter trips and driving frequently enough, then that may be enough to keep you charged. However, if you’re spending days in one place, then that’s where solar really comes in handy. Even a 100W kit (panel and charge controller) would go a long way towards keeping you topped off. Another advantage of lithium batteries is that you won’t necessarily damage them by letting them get too low, unlike lead acid batteries. Hope that helps!
~John
Hello how can I use the auxiliary battery to power the vehicles 12v outputs like the cigarette lighter outlets?
Hi Mohammed, to do that you would attach a fuse box to the battery and wire 12V cigarette lighter outlets to that. We have a more detailed explanation of how to wire a campervan electrical system here: https://gnomadhome.com/van-build-solar-electrical-wiring/
Hope that helps!
Hi John,
Thanks for this great guide and for supporting it so well afterwards. The comments section with your responses are equally as helpful.
I’m not a vanner, but wanting to rig up my Silverado for more camping. Wanting a sanity check on my plans before I buy everything if you don’t mind!
1) Truck has a 145A alternator so I think I’ll go Battery Doctor 150A, even though the Keyline would likely be perfect.
2) 150A Inline Fuse between the Isolator and the Aux Battery (which will be a short run, the truck came with a second battery tray)
3) I’ll have a long run from the Aux battery to a 12V Marine Panel I’m going to install in the back. Panel HERE.
Does this being a long run from my Aux Battery to this panel change any of my cable gauge requirements compared to it being a long run from the Isolator to the Aux battery? Gauge recs? I’d prefer to just KISS and use the same gauge for the whole build if possible.
Do I need a fuse between the Aux Battery and the Panel? How big?
I was previously going to have this done but I think with your guide I’ll take it on myself.
Thanks so much!
Hi Carl, so glad you found this guide to be helpful! Here are my thoughts on your setup:
To connect to the 12V marine panel, the wire size should be based on the max current going that the panel will use. Looking at the Amazon listing, it appears that the max current it is rated for is 20A. For a long run at that amperage, I would use 8 AWG wire. Anything thicker than 8 AWG will be difficult to connect to the marine panel, just because of the types of connectors you will need. You will want to run a wire from the positive terminal of your aux battery to the positive connector on the marine panel, and connect it using an 8 AWG female spade terminal (you can get these at the hardware store if you only need a few). For the negative side, I would run it through the chassis. So – run a wire from the negative pole on the aux battery to the vehicle frame in the engine bay (using an 8 AWG ring terminal attached to a self-tapping screw with lock washers on it). Do the same on the other end – attach a wire to the marine panel using a female spade terminal, then run that to a ring terminal/ground screw attached to the vehicle frame in the back. For fusing – use a 20A inline fuse on the positive wire.
I hope that helps! Let me know if I can clarify anything!
~John
I would like someone to help me, I am old and some how my ignition source came undone and I do not know nothing, is there anyone in Michigan or near me to hook it back up?
Hi Mary, sorry to hear you’re having trouble! I hope you were able to find someone to help!
If I use circuit breakers do I still need two , one for each battery?
Hi Daniel, we recommend having a fuse/circuit breaker near each battery to protect the batteries from shorts/surges. Hope that helps!
~John
What if I use an ordinary battery, not deep cell? I have an extra one.
Hi Nancy, I recommend going with a deep cycle battery (or a golf cart battery). An ordinary car battery will be way less than optimal for this usage. Ordinary car batteries are designed to deliver a large amount of amperage in a short period of time (to start a vehicle) and are not designed for deep discharge over a slower period of time. Deep cycle batteries are designed for deep discharge, and can deliver much more power over a longer period of time than a similarly-sized car battery. Hope that helps!
~John
~John
John, I’m connecting the Battery Isolator recommended in this article with the RV Tow Connector on my truck. I don’t have a trailer but am having a 7 Pin connector fabricated with only the Pos & Neg Pins used. Those 2 wires coming out of the connector will connect to the Isolator and then off the Isolator to my DX300 (DBOX12) Battery Box. I was thinking I won’t need fuses because the trucks 7 Pin connector should already be fused. You thoughts.
Hi Donald, I always tend to err on the side of more fuses. While the connector may be fused, most isolators recommend fusing as close to the batteries as possible. This would protect your batteries from shorts in the cable between your batteries and the trailer connector. Hope that helps!
~John
Aloha John, first off thanks for taking the time to post all this very helpful information, it’s awesome!!
My question is this, I’ll be using my van for mostly weekend trips around Tahoe for camping and skiing and occasional music festivals, Do I even need solar panels or could I just get a few Lithium batteries and charge them from my house (i do have a 30 AMP outlet in my garage) and also install a battery isolator to charge them when I’m driving? I’ll have some puck lights, USB outlets, a Webasto Heater and occasionally use a induction cook top. I’m not necessarily trying to save money I just want a simple system that’ll work for my needs. Thanks for your time and help!!
Hi Barry, if you’re mostly doing weekend trips, you could certainly hold off on getting solar panels – as long as you have enough battery capacity to last you the weekend. An isolator can certainly help keep you topped off, but if you’re going to be mostly stationary the whole weekend then its value might be limited. Isolators tend to make the most sense if you’re doing a lot of driving from place to place, and idling your vehicle to charge up your batteries is not a good long term strategy.
Side note: if you’re using Lithium batteries, you will want to get a DC-DC charger instead of a basic isolator. DC-DC chargers (like these from Renogy) are better suited for lithium batteries, and are also designed to regulate the voltage fluctuations of modern alternators.
So yes, if you have a suitably sized battery bank then you could definitely go without solar (and keep in mind that solar is something you can always add later if you think you need it) Again, just make sure to plan out your electrical usage ahead of time – an induction cooktop could be a power hog.
Cool post, John!
One question:
Why did you go with two 150 amp circuit breakers (CBs) or fuses? I am estimating my current draw to be 20.5 amps. Would I want to use 30 or 40 amp CBs? Or would I want a 150 amp CB near the starter battery and smaller one by the leisure battery?
-Corey
Hi Corey, we went with 150 amp fuses based on the recommendations in the instruction manual for our isolator. However, it’s highly unlikely that that amount of current will ever pass through those fuses, unless there’s a short of some kind. Also, most leisure batteries should not be charged at a current higher than 30A-60A (depending on the battery). We need to update this post – if we were to do this today, we would go with a DC-DC charger (like these from Renogy) instead of a basic isolator. DC-DC chargers are capable of dealing with the voltage fluctuations of newer alternators, and also are able to charge lithium batteries. Hope that helps!
~John
Thanks, John!
How is this unit connected having 3 secondary batteries?
Hi Bob, are you trying to connect an isolator to three secondary batteries? If you have three batteries, I’m assuming that they’re wired in series. All you need to do is connect the positive wire coming from the isolator to the positive battery terminal that you’re connecting everything else to (with an inline fuse close to battery). Hope that helps!
~John
Hi John, thanks for the great information! I do have one question though. Most deep cycle batteries (including the one you added a link to) have a max charge current of around 30-40 amps. I believe alternators can put out well over that and the isolator states 140 amps. Do you know if this is bad on the batteries to be charging with that much current, that fast?
Hi Camron, great question. My understanding of it is this: most people recommend sizing a battery isolator based on your alternator output. Let’s say your alternator puts out 140A – that does not mean that it puts out 140A at all times, or if it does that all 140A goes to your aux battery. A good chunk of that is going towards powering the electronics in your vehicle, then is going towards topping off your starting battery, then is going back to your aux battery. On top of that, alternators are designed to only supply the necessary current to achieve a target voltage in the electrical system. So in effect, the current that the alternator supplies is based on the loads that are put upon it – the amount of load from the van’s electrical system, and the amount of current demanded by the battery’s state of discharge. In effect, it means that the alternator should not supply more amperage than is necessary to keep the battery at the target charging voltage. Let’s say this is 13.8V. The alternator will only supply the voltage necessary to get the battery up to 13.8V, which means that it will only supply the amount of amperage actually demanded by the battery – it’s not forcing a bunch of unneeded electrical current onto it.
On top of that, the electronic voltage sensing isolators that we recommend sense the voltage at both ends and engage in multi-stage charging. However, if you have a newer vehicle with a computerized alternator, or if you have a lithium battery bank, we recommend going with a DC-DC charger instead of an isolator (Renogy makes some good ones). DC-DC chargers are better equipped to deal with the voltage fluctuations of newer vehicles, and are set up to properly charge lithium batteries.
Hope that helps!
~John
Hey! I have just installed the key line battery charger in my van exactly as per instructions, but now my vehicle engine will not start! The vehicle lights still come on so I’m not sure that the starter battery is dead. Is there anything I may have missed? There’s not much information online about troubleshooting the product. Thanks!
Hi Moll, sorry to hear that you’re having troubles! If everything is wired correctly, the first thing I would check is that all of your connections are tight. A loose connection to your starting battery could cause it to not work properly, and could also drain the battery (it can be drained below the level of being able to start your vehicle without being fully dead). If everything is tight, you may have drained it during the installation (which could happen if the terminal was making contact with metal somehow – i.e. the wire crept back and made light contact with the terminal). If the battery has been drained, you will need to jump start it and run the engine for awhile, but hopefully will work after that.
Another thing to check is that you hooked everything back up to your battery’s positive terminal. Our van has two separate wires in our engine bay that need to be attached to the battery, and if we forgot to attach one then something wouldn’t be getting power.
Hope that helps!
~John
John – I’m using the keyline coupled with solar. If the starter battery and isolator are both grounded – Can I skip grounding my aux battery? I always assumed I would, now that I am ready to install, not so sure. I see various opinions about it.
Hi Jerrod, in general we recommend grounding your aux battery for safety. The wiring diagram for the Keyline isolator also shows the aux battery negative terminal wired to the vehicle chassis. Hope that helps!
~John
John – If the keyline and house battery are grounded, will the inverter use the battery ground, or would we also ground the inverter to the chassis?
Essentially having all 3 items grounded to the chassis somewhere?
Sorry I’m new to the electrical side of Van Life and this question may be dumb: Can I use this process of alternator charging and a battery isolator with such power stations as the Goal Zero Yeti or Inergy Apex systems? or just “traditional style” deep cycle batteries? I’m planning my build as much as possible and I don’t think I will initially need solar but I do believe I’ll need more than just a 12 Volt car charger set up you know? Thank you for any information and advice.
Hi Amanda, thanks for reaching out! I don’t have direct experience connecting a Goal Zero or similar product to a battery isolator, so unfortunately I can’s offer much insight. However, Goal Zero specifically has an add-on product called Goal Zero Link that allows you to connect a Goal Zero to your vehicle’s battery system much like an isolator would do. Here’s a video with more details on how that setup works: https://www.youtube.com/watch?v=Zl-5qq9hPOg
Hope that helps!
Thanks,
John
Someone suggested i needed a solenoid and that is why my house battery is not charging then someone else said an isolator. Confused? Suggestions?
Hi KB, a “solenoid” in this context is a type of battery isolator. Solenoid isolators are basically mechanical switches that opens a connection to your aux battery when there is an electrical current present. Solenoid isolators are inexpensive, but they require tapping into a hot power wire in your vehicle to install them.
We recommend going with a “smart” battery isolator, AKA a voltage-sensing relay. This type of isolator is essentially a small computer that senses the voltage of your batteries and cuts in/out appropriately. They also have the ability to do multi-stage charging of your aux battery. Smart isolators are also extremely simple to install, since you don’t need to tap into any of your vehicle wiring – just hook it up to your batteries, ground it your chassis, and you’re done.
Hope that helps!
~John
Jon, I’m wondering if you have any tips about wiring when you do BOTH the keyline isolator AND the solar panels
Like I only have one battery and so, forgive me if this seems illogical, but I wondered if
a) it’s okay, for both to be in the same circuit loop (meaning the battery isolator’s wires go on on a fuse box, and so do the do the solar wires)
or
b) it makes a difference to wire the isolator directly on the battery as apposed to the fuse box)
Cheers,
Ben
p.s.
if you want a visual, I copied this guys system exactly (same parts)
https://www.mobile-solarpower.com/the-minimalist-great-for-small-vans-and-cars.html
John*
Hi Ben, thanks for commenting! I took a look at the link you provided. First off I must say that I’ve never seen an off grid electrical system wired like that before.
I’m a bit skeptical of this system because of what I outlined above, and also because blowing the inverter fuse would basically take the whole system down.
We wrote a detailed guide going over exactly how to install a campervan electrical system, which you can find here: https://gnomadhome.com/van-build-solar-electrical-wiring/
As to your question about battery isolators and solar, typically you would wire a battery isolator directly to your positive battery terminal (with an inline fuse). Again, in a typical setup, the charge controller is also wired directly to the battery terminal, so if you choose to wire it to a fuse box as in the setup you shared, I can’t imagine that it would function any differently.
However, I highly recommend wiring your isolator (and your inverter) directly to your battery, which will allow you to separate the circuits a bit more in terms of fusing. This would make it simpler to find and fix problems if/when you blow a fuse.
As far as charging, the isolator and the charge controller won’t have any impact on each other. The isolator will basically do a bulk charge on your batteries while you’re driving, but probably won’t be able to get them fully to 100% (due to voltage drop in the wire run from the engine bay, and because many alternators/isolators don’t put out a high enough voltage to take a deep cycle battery past the bulk stage). Your solar will then complete the charge. And since both battery isolators (at least the electronic ones) and charge controllers sense voltage and adjust accordingly, neither of them will overcharge your batteries.
Hope that helps! Best of luck with your build!
~John
That does help a lot!
I think you explained why I’m not able to get my battery back up to the higher end of charge sometimes due to not high enough voltage to take deep cycle past bulk stage.
I’ll rewire your suggestions.
Cheers,
Ben
Hi John, thanks again. Follow up question now that I’m wiring: what is the difference between grounding the various items to the car frame vs. going to the battery. From searching the web all I find is that it’s more convenient but ultimately they are all connected. I wondered if there is any protective aspect to not grounding everything on the battery, but figured the grounds all connect to one another.
Hi Ben, as far as the battery isolator is concerned, grounding it to the vehicle chassis essentially saves you the hassle and expense of running another thick battery cable all the way back to the aux battery. The vehicle frame is basically one giant blob of wiring, and it’s very common in vehicle electronics to use the vehicle frame as the negative return path. In this case, you also need a common ground for the battery isolator, aux battery, and starting battery, and grounding to the chassis simplifies things quite a bit.
As far as your inverter goes, you could theoretically only have it hooked up to the pos/neg battery terminals with no chassis ground and it would function. However, grounding to the chassis using the additional ground terminal on the inverter is an important safety feature that reduces the chances of electric shock.
~John
Thanks John, very helpful. I rewired things a good bit. I have been watching my battery isolator (keyline) and notice that it takes my 12 volt 100 amp renogy battery from 12.3 volts when the car was off, to 13.2 volts when the car is on. This is an example from today when I took an instantaneous measurement by volt meter and turning on and off the car to see the difference.
I haven’t felt like it’s been charging my battery as quickly as when I first installed it. What do you experience or what’s the right way to quantify it’s charging? Would you think this is adequate or suggest any trouble shooting starters?
This may have been answered already (I couldn’t find it in the thread)… we are going to run two 100Ah LiFePO batteries. Can we use one battery isolator to charge both of the LiFePO auxiliary batteries?
Hey Mark, thanks for [email protected] Yes, you can use a battery isolator to charge two batteries since your batteries will be wired together. Just attach the wire from the isolator to the positive terminal of one of the batteries. Hope that helps!
~John
I had the Keyline isolator, but it somehow got fried (one of the terminals somehow is loose), and so I got the Wirthco isolator, and it’s having issues (see below if you want the details). When I called Wirthco they said it’s cause I am running a two battery system. Have you had any issues with a two battery system?
Details: my batteries were dead, which I how I first realized I had a problem. The Wirthco isolator kept blinking, and to get it to stop, I charged each battery individually. Then I hooked them back together, it seemed to be working fine. While parked for a day or so, the blue light went off, signaling the van battery was below 12.8v. When I went for a drive, the isolator was blinking again, which I believe signals that its not charging. Just curious if you’ve had anything like this happen with yours.
Hi Katie, so sorry to hear that you’re having issues! We have not had similar issues with ours. I personally don’t understand how having a two battery system would affect anything since they’re wired together. However, a handful of readers have reached out to us recently regarding other issues they’ve experienced with their Wirthco isolators, and we’ve chosen to remove this product from our website while we reassess our recommendation. Sorry again that you’ve been having issues!
~John
Thanks for the feedback and all you do for the vanlife community! I’ve benefited greatly from your efforts.
Are you still happy with the Battery Doctor? Based on Amazon reviews, it seems as though there are reliability issues with both but the Keyline has a longer warranty period (1 year versus 1 month).
Hi Richard, we haven’t had any issues with either the Battery Doctor or the Keyline. We started with the Keyline and had that for over a year. Then we switched to the Battery Doctor due to its ability to reverse the flow, and we’ve had that for over a year so far. I will say that the Keyline definitely seemed more robust/durable, and it seemed like it did a better job driving voltage to our house batteries. Another option to look at is the Blue Sea Systems Automatic Charging Relay.
We’ve since developed a short in the wiring under the van (causing the fuses to blow), so I have the Battery Doctor disconnected until I have time to check out the wiring and add a protective sheath, but this isn’t the isolator’s fault.
Hope that helps!
~John
What is the reason for installing the B2B charger in the main engine area or close to the van battery? Could the B2B Charger be installed in the rear of the van by the rest of the electrical components?
Hi Travis, the manufacturers’ instructions for these battery isolators say to install them in the engine bay. My guess is it’s because the isolator needs to be grounded near where the starting battery is grounded, but I’m not positive.
~John
Hello! I’m completely clueless to anything electric and was wondering where I should look for help with this. I was thinking car audio shop? Any ideas on what to even google to find some assistance? Also what kind of power are you going to get from this? I really am in need of some type of AC, was considering a swamp cooler so I would just need power to run a 12v fan. Halp!
BTW love you guys, thank you so much for all this valuable and FREE info! Y’all are awesome!
Hi Monica, thanks for reading and for the kind words! Electrical can definitely be intimidating when you’re first getting started. I’m honestly not sure where’s the best place to look for professional help, but you could certainly try a car audio shop. They should have at least some experience with 12V vehicle electronics, running wires from the engine bay to the rear, and wiring things to your battery. It certainly couldn’t hurt to ask.
As far as the amount of power you can get, it’s all going to come down to your battery capacity. More batteries = more ability to run things like a fan/swamp cooler, but keep in mind that if your only charging method is a battery isolator, then you may have to drive/run your engine at least daily to charge your batteries. AC units draw a lot of power, and are generally not feasible in a van unless you’re plugging in (at a campground, friend’s house, etc) or have a VERY large and expensive solar/battery setup. Fans/swamp coolers draw much less power, and combined with shade and reflective window coverings can help keep your van at a livable temperature.
I hope that helps, and best of luck with your build!
~John
I ordered the 75-100 battery isolater my battery is a 100 ah batter says max amp charge is 30 amps so would I need a smaller isolater
Hi Kyle, you shouldn’t need a smaller isolator. Both the Keyline and the Battery Doctor isolators that we mention are “smart” isolators that are designed to charge deep cycle batteries with multi-stage charging. Hope that helps!
~John
Happy I found this I wanted to add a battery to battery set up in my 18 transit I just ordered everything u guys used and am gonna copy u… I’m new to all this so I asked on forums for help and everyone was confusing the crap out of me saying battery’s have to have the same chemistry and this is. Isolater not a charger blah blah I’m only running a maxfan water pump andfew usb outlets so I ordered a 100ah battery to supply them … so I hope this works for me seems pretty straight forward I had everything connected to starter battery before
This is a great site! I am finding so much useful information. Thank you for sharing! I have a ProMaster 2500 159. It has a 180 Amp Alternator. Do you think this witch 150 would work for me?
Hi Chris, so glad the site has been helpful! Here is my understanding – even though your alternator is 180A, it’s not sending all of that to the battery (the other electrical components in your vehicle are also receiving some of the current). Given that, a 150A isolator should be perfectly fine for most vehicles and alternator sizes. Hope that helps!
~John
Thx! Great article, as usual for your site. What I don’t know is how you determine what amperage you need for the isolator and the fuses. Is it based on the alternator amps, or what? And how do you choose a fuse that is neither too high (as to properly protect) or too low (being tripped when you don’t want it to be)?
Thank you so much for all this useful information! We would be lost without you guys!
Of course, so glad we’ve been of help!
~John
I have my isolator installed in my van, and when the solar panels are engaged, the isolator makes a constant clicking noise. I believe it’s the isolator cutting off the attempt by the aux batteries to draw power off of the starter, but I haven’t seen the sound referenced anywhere, so want to make sure my hookup is actually working correctly and charging while I’m driving :\
Hi Alex, one thing you can check is your battery voltage when the isolator is engaged, which you can do with a multimeter connected to your battery terminals. If the isolator is working properly, the voltage should jump up (probably to 14V-14.5V). Is this a mechanical/solenoid isolator? I don’t have direct experience with those (we have an electronic smart isolator), but a constant clicking noise would definitely make me suspect something is wrong, especially if your not seeing the voltage increase. There shouldn’t be a whole lot of moving parts – it should just open the circuit when your engine is on and close it when your engine is off, so I can’t think why it would be constantly making a noise. Hope that helps!
~John
Nope, it’s the same electronic smart isolator that you guys have posted about! It definitely is getting juice, the only thing I can think of is that the clicking noise is somehow related to the isolator cutting ‘off’ an attempt at power draw from somewhere, but I don’t understand where that would be :\
Huh, I’m not sure then. I would try reaching out to the company – it may be a defective unit. Hope you get it figured out!
You didn’t ask but, I uploaded a video of what it’s doing. Figured the comments section might be a good resource for someone else having this problem, or for someone who has a solution! https://www.dropbox.com/s/4pus972n0fzrpnc/batteryisolator_click.MOV?dl=0
Hi Alex, that sound is coming from the isolator? My guess is that it’s a defective unit. I would contact the retailer where you got it or Keyline directly and see if you can exchange it for a working unit.
Decent article and blog but I wish it was as simple as the article makes it sound. One key is to adjust the solar charge voltage so it’s slightly higher than the alternator charge voltage (which is often too low anyway). This way they work harmoniously together. The alternator will do the bulk charge and the solar will do the absorption and float charge to get the house battery fully charged. If the solar and alternator are set to the same voltage they may interfere. One will click off and the other click on and visa versa. Click, click, click….
Alternator charging alone for deep cycle house batteries is notorious for undercharging the batteries, as this article mentions, unless a electronic smart isolator is used. But without solar they still may get undercharged. A adjustable solar energy system or adjustable alternator voltage regulator is even better for equalizing the flooded lead house batteries which can take up to 16.2 volts. If they are sealed house batteries they don’t take a equalizing charge but still may be getting undercharged without solar finishing. The sealed starting (sealed are not supposed to be equalized) battery will not be damaged because it is isolated by the electronic smart isolator from the higher voltage flooded lead acid batteries. Which requires equalization. Even sealed agm batteries can take some mini equalization charge or “conditioning” charge. Van and RV deep cycle batteries are notorious for failing as they are in use in a system that’s not designed for them. There’s no east way around the fact that they were not designed to charge deep cycles. Even with a electronic smart isolator. But it does help. And adding solar helps even more.
Perhaps a better way is to take out the charging system and start over. But thats a lot of time and money. Go about it another way, the other way around. Design the alternator and regulator to go straight to the house battery with a fully adjustable 4 stage regulator. Then use a adjustable DC to DC Charger to charge the starting battery from the house battery. But they are too cheap to put nice equipment like this in our vans and RVs so we have to do it or live with the compromise our vehicles are.
If you have a battery amp hour meter on your house battery it’s important where you bolt the cables from the electronic smart isolator or your meter will be very inaccurate. The positive cable from the ESI goes to the battery but the negative cable comes from the chassis to the load side of the shunt. The same side as your al, your inverter and loads load is connected or should be. This way the shunt can measure the current from the alternator and tell the meter how much is going into the house battery.
Hi Jeremy, thanks so much for all the detailed information! How all of these systems may work together in the best way is a very interesting subject, and this gives us much to think about as we work on updating/expanding this article for 2019. Cheers!
~John
John, I got a big 100 amp hour battery by Renogy.
Any suggestions on how to put some safety measures in place in case of a crash?
My goal is to build or buy something if it already exists, that would keep the battery from flying (and hitting me) in the event of a big crash.
Cheers,
Ben
Hi Ben, thanks for reading! Securing your batteries and other heavy objects in your van is definitely an important safety measure. The most secure way to do it would be some sort of metal bracket over the top of the batteries that’s bolted down to the vehicle frame. I’m not aware of specific ready-made products off the top of my head, but there may be something out there. I did find some “battery lock” products that might work, but they seem pretty pricey.
In our van, we have our batteries strapped down inside a homemade plywood battery box, which is then attached to the floor, and adjacent bedframe and bench using 1-1/2″ L brackets and screws. It’s very secure for day-to-day use, but I can’t vouch for how it would be in a crash. Mounting to the frame will always be the most secure.
Hope that helps!
~John
Hi there, awesome tutorial!
You do t happen to have a full circuit diagram for this setup, so it can be visualised as a whole product!
Thanks a lot
Hi there Max, we’re working on an update to this post that will include a diagram. Stay tuned!
~John
You’re directions are a little confusing. You’ve got the in-line fuse connected to the positive terminal and you’ve got the isolator connecting to the positive terminal
Hi there, thanks for pointing this out! It should be Isolator–>Inline Fuse–>Positive Battery Terminal. I clarified the language in the post, hopefully it’s easier to follow. Cheers!
~John
Incredible post. Just finished installing the isolator (used your amazon link 🙂 for my Honda Element. Thanks for answering the questions below cause that helped me out when I installed mine.
Hi Ben, that’s awesome! So glad we were able to help 🙂 We almost bought an Element before we decided to go with a van, they’re kickass vehicles. Best of luck!
~John
The fuse link is for a 100a fuse and you refer to using a 150a fuse…is the 100a fuse sufficient?
Hi Rob, thanks for catching that! I’ll make corrections to the post. We actually used 100A fuses in our build, although Wirthco recommends using a 150A fuse. It works perfectly for us, but I’m not sure how it would do if we tried to jumpstart the starter battery from the aux battery.
~John
I’ve reading your blog with great interest. Really a great resource.
My home-brew camper van has a totally separate solar system feeding 3 deep cycle AGM batteries. I want to install a smart battery isolator. My van motor is a GM 6.6 turbo-diesel that has 2 full-sized starting batteries. The second battery is mounted under the van about mis-ship and almost directly underneath where my ‘house batteries’ . So my question is: can I mount the isolator to (and next to) the second starting battery rather than the battery under the hood? If so, I could do the whole install with very minimal wire lengths. Is this correct?
Thanks, – m
Hi Mike, thanks for reading! That’s a new one for me, but I would imagine that as long as your second starting battery is fed from your alternator, you should be able to hook up the isolator to it. Not 100% sure, but I can’t think of any reason that it wouldn’t work. That would be a pretty awesome setup!
~John
Hello, Great site. I’m wondering can you charge your auxiliary battery with your van running, charging from your alternator and charge the auxiliary battery with the solar at the same time with your wiring setup? Say it’s a nice sunny day and your driving down the road.
Hi Bruce, thanks for reading! The charging is all handled automatically with our setup. The alternator/isolator detect battery voltage and will only send charge to our aux batteries if they dip below a certain point. With our solar, the charge controller also monitors the voltage and charges accordingly. So it’s all completely hands off on our part, and each system works in tandem to charge only when needed. Hope that helps!
~John
Hi I bought a smart battery isolator through your site but now am doubtful of using it. One this video https://www.youtube.com/watch?v=9jjh3Y4xIL8 mentions not to use an isolator with vehicle with a computer controlled alternator. I’ve also read on the sprinter forum that alternators want the same type of battery they are tuned for(your starter battery). Thanks for any thoughts!
Hi there, the isolator they’re referencing in the video is a solenoid isolator, which works differently than the computer-based isolators that we recommend (Keyline IsoPro and Wirthco Battery Doctor). The manual for the Battery Doctor specifically states that it works with modern computer-controlled alternators and regulates the voltage appropriately. I haven’t been able to find a similar note about the Keyline, but if that’s the isolator you purchased, you could try contacting their tech support to make sure. Both isolators contain computers that regulate the voltage and are designed to charge deep-cycle batteries with multi-stage charging. Simpler mechanical or solenoid isolators do not have this capability, so that could be what those forums are referring to. I hope everything works out, and please check back in with any updates!
~John
HEy guys
Is it ok to use 8 awg cable instead of 4 awg to run the connection ? I have a lot of 8 awg cable left over
Hi Joe, undersizing wiring is a really bad idea. If you’re doing a very short run then 8 AWG might be okay, but you really need to base that on the length of the wire run and the max amperage going through it. You can use this calculator (https://www.wirebarn.com/Wire-Calculator-_ep_41.html) to determine the size wire that you need. If you use wiring that’s too small, you run the risk of overloading the wiring and causing an electrical fire. Whatever size wring you decide to use, make sure you use properly sized fuses, which will cut the circuit before the wiring gets overloaded. Hope that helps!
~John
Hey! What size wire did you use to wire the Isolator your house batteries?
Hi Jazmyn, we used 4 AWG wiring. This is what the Battery Doctor instructions suggest for an aux battery in the rear of the vehicle. The best route to go, though, is to use a wire size calculator (like this one https://www.wirebarn.com/Wire-Calculator-_ep_41.html) to make sure you’re using the proper wire size for the length of the run and the amperage. Hope that helps!
~John
Hi John, also trying to figure this out… But that calculator i put in: Amperage: 150 + 14 feet (4 meter) + 2% drop and it gives back 53.4mm2 (0 Gauge). Is that correct? Or what am i doing wrong?
Hi there, using this calculator from Blue Sea Systems I get back 2 AWG wire for those parameters. But yes, the longer the wire run the thicker the wire you will need. Another option is to use a thinner wire gauge with a lower fuse. We had 100A fuses on ours for a year, and others fuse even lower. It’s highly unlikely that you’d ever have 150A of current coming from the isolator. Hope that helps!
~John
HEy there! Love the site, it’s helping me tons! Was wondering though if I’m running two aux batteries how I would run the isolator with them? I am horrible with electricity!
Hi Josh, thanks for reading! To run an isolator with two batteries, the first step is to wire your batteries together. Then you would just run the isolator to the positive terminal on only one of the batteries. If you have 6V batteries, you will want to wire them together in series (which will make your battery bank 12V). If you have two 12V batteries, you will want to wire them together in parallel (which will keep them at 12V). Check out our epic electrical post for instructions on wiring your batteries and installing a campervan electrical system. Hope that helps!
~John
HI there! Awesome site.. so beautifully done. This isolater help was very much needed. With that said.. I ran into an issue. I have a ’18 Promaster. After everything was hooked up I turned on the van and bam. No start. I think the van is in an “anti- theft” mode and locked the key in the ignition. The key will not come out. Has anyone run into this issue??? Help! I’ve heard that it’s the Promasters way of saying there was short and that I’ve blown a fuse. I have no idea where to start looking or if that’s really the cause. If anyone has had this happen please do send some help. Thank you. And thank you John and Jayme again for all the hard and detailed work you’ve put into this site. I’ve ordered literally everything off y’alls links.
Hi Jayne, thanks for the kind words about our site, we love hearing that it’s been helpful!
That’s a bummer about your van! Unfortunately we don’t know a whole lot about Promasters specifically, but we know diagnosing your vehicle’s electrical system can be a pain. I would start by checking all of your fuses (our van has a fusebox in the engine bay, and another one down by the driver’s feet. Your van might be different). We one blew an engine fuse driving up a bumpy road – the engine just cut out on us, and we had no idea what it was. Turned out to be a simple fuse that had blown. We’re still not sure exactly what shorted it, but look for any loose connections or areas where bare metal may be contacting the wires.
You could also try posting over at Promaster Forum (https://www.promasterforum.com/), I’m sure someone there will have more knowledge of Promasters than we do.
Hope that helps a bit, and good luck with your van!
~John
Jumping in here. This site is heaven-sent thank you so much. I have a promaster and was going to install this weekend (crazy I just caught this comment). I was going to use 4 awg wire with a 125A fuse to protect the wire. I don’t know much about what could’ve happened there but would love an update! I will see if I can find anything to help.
Hi Eugene, glad the site has been helpful! Sounds like you’re on the right track with the isolator. We would love to hear an update from Jayne as well – hopefully things are working now.
~John
Thanks for this info John! Still getting a little lost. Could you post a schematic drawing of what you’re recommending? Thanks a bunch!
Hi Chris, we’ll be updating this article and adding a diagram in the near future. Are you confused about the wiring? I’ll try to elaborate a bit. It basically goes like this:
Starting battery –> Fuse –> Isolator –> Wire run to back of vehicle –> Fuse –> Aux battery
The isolator is also grounded to the chassis, as are both the batteries. Both fuses should be placed as close to the batteries as possible.
I hope that helps!
~John
Hi Chris, we’ll be updating this article and adding a diagram in the near future. Are you confused about the wiring? I’ll try to elaborate a little bit. It basically goes like this:
Starting battery –> Fuse –> Isolator –> Wire run to back of vehicle –> Fuse –> Aux battery
The isolator is also grounded to the chassis, as are both the batteries. Both fuses should be as close to the batteries as possible.
I hope that helps!
~John
i haven’t researched too widely but from what i can gather, the info you provide here is priceless, thanks s much for this. I have one question (for now), can this set up of battery and isolator be used with the Renogy 1000Watt 12V DC to 120V AC Pure Sine Wave Inverter Charger ?
Hi Michael, Glad you find the article helpful! You can definitely use a batter isolator with an inverter/charger – just connect both directly to your batteries. Both the isolator/alternator and the inverter/charger detect your battery voltage and won’t send any current if it’s not needed. Hope that helps!
~John
John, thank you, i’d read your article referencing the Xantrex 806-1206 PROwatt 600 SW Inverter for use with the 140 Amp Dual Battery Smart Isolator by KeyLine , was unsure if i could use the Renogy 1000Watt 12V DC to 120V AC Pure Sine Wave Inverter Charger with the same setup, as this takes a charge from shore. Ouf, electrical is totally beyond me.
The use of solar power for making power, energy is the very wisest idea because solar energy is a renewable source of energy but if you don’t have enough budget then solar energy is not enough. Also, the given information on installing battery isolator is properly explained here.
Agreed, glad you like the post!
Thanks for the great article. I couldn’t agree more about multiple forms of incoming charge. I’m currently working on building a bank. Probably only 200 or 400 amp hours to start out. I’m in a MB Sprinter v6 diesel. I have looked up the alternator and it appears to have a 180 amp output. Any suggestions on how to properly size the isolator?
Hi Dan, my understanding is that you want a battery isolator that can handle at least as much current as your alternator will send to your batteries during peak charging – which should be substantially lower than 180A. Your alternator will be powering the electric systems in your vehicle before it sends anything to the batteries, which will take up quite a bit of amperage. Even then, the amount of amperage the alternator is sending to your batteries is ultimately dictated by the batteries. If your batteries are fully charged, they won’t be “requesting” current. If they’re 30% discharged, they’ll “request” more current than they would if they were 20% discharged. A larger battery bank will “request” more than a smaller battery bank.
Unless you have a very, very large battery bank, a standard 140A or 150A battery isolator should be able to more than handle what your alternator will throw at it. Here’s a helpful article that digs into alternator output ratings: https://www.lifewire.com/understanding-alternator-output-ratings-534785
I hope that helps!
~John
Awesome! Thanks a bunch John. I’ll get right into it. I’ve had this Sprinter for exactly 3 weeks now and I feel a little overwhelmed. I know this conversion is going to be a daunting process and every bit of advice and help means a lot. I appreciate it and I might pick your brain again from time to time..
Thanks again,
Dan
I recently upgraded my alternator to a Bosch 150A. I noticed the isolator you recommend is rated for 140A. Will I run into problems if I follow this guide? Can you update your post to include your alternator specs, for the curious? Love your blog!
Hi there, based on my understanding you should not run into issues with either of the isolators we recommend (the 140A KeyLine or the 150A Battery Doctor). Your alternator is rated for 150A, but it will only output what is required, which will be substantially lower than 150A. Your alternator will also be providing power to your vehicle electrical systems before charging your batteries, so there is less amperage available to send to the batteries. Also, your alternator will only send the amount of amperage that your batteries are “requesting,” which again will be much lower that 150A unless you have a HUGE battery bank.
I actually don’t know what size our alternator is. But we are planning an update to this post soon, and we’ll definitely add a section about sizing a battery isolator. Hope that helps!
~John
Great info guys, just purchased my first van. As an avid fly fisher I’m tired of hotel prices.
Thanks again.
Hi Randy, thanks for the comment and congrats on the van! Best of luck with your build!
~John
Absolutely the most useful site out here for van conversions!! THANK YOU SO MUCH for the enormous amount of effort you’ve put into this site. My question is this: I’ve got a cruiser van with tons of DC outlets and interior lights already wired in. There is no auxiliary battery, however. How can I most easily hook up the deep cycle and isolator without reconnecting every individual thing to it? Sorry if this is a dumb question, but I know absolutely nothing about this topic.
Hi Chad, thanks for the kind words about our site! We put a ton of work into, and we love hearing that it’s helping others! 🙂
As to your question – I think the only way to do what you’re talking about would be to rewire everything to an auxiliary battery system. Right now all of your outlets, lights, etc., are wired in to your vehicle’s main electrical system. There’s likely a fusebox somewhere that these are connected to (our van has the fuses for all the rear stuff down by the driver’s feet), so if you can identify which circuits correspond to the things you want to wire to the auxiliary battery, then you can just rewire those circuits to another fusebox that’s tied into your auxiliary system. I’m not sure if you’re planning on taking your van apart at all, but another way would be to cut the wiring to each of these components, splice on new wire, and run that to your auxiliary battery system. In either case, you’ll need to identify/disconnect the wiring to your lights/outlets, and run the wiring to your auxiliary system instead.
I hope that helps, let me know if I can help any further!
~John
Hey guys! I’m going to be starting my can build in a few weeks and have spent the past few months researching online. Love your blog and your super detailed explanations of things; I know I’ll be using this a lot during my build!!
I’m trying to sort out our electrical situation and I am thinking of relying primarily on the alternator to charge our cabin battery via a smart isolator but am also going to get a 100 watt solar panel to have to supliment it when we are camped in one place for long periods.
I was just wondering if you guys have your solar panel and alternator hooked up to your cabin batteries at the same time, or do you switch back and forth? I’ve been trying to find info online if the battery can be charged by both solar and alternator at the same time (with one suplimenting the other) or if it needs to be a one or the other type situation.
Any input would be greatly appreciated!! Thanks and keep up the great work!!
Nick
Hi Nick, glad you enjoy the blog! We have our isolator and solar connected to our batteries at the same time, and it works well. Both systems have mechanisms in place for detecting the voltage level of the battery, which prevents overcharging. On the isolator side of the equation, the vehicle’s alternator actually detects battery voltage and cuts out once it reaches a certain point (which is why it doesn’t overcharge your starting battery). Depending on the output of your alternator and the length/gauge of your wiring, there will also be some voltage drop between the isolator and your aux battery that will limit the current actually making it back to the aux battery.
On the solar side, you have your charge controller, which also detects battery voltage and regulates the solar current to charge your battery appropriately. Having both hooked up at the same time should not be an issue, but as always, make sure your system is properly fused for safety. You should fuse both starting and aux batteries when installing your isolator, and also fuse in between your solar panel and your charge controller, as well as in between your charge controller and your battery. Hope this helps!
~John
Thanks a ton for the detailed response! I sent that original message while we were in van-planning stage. We now have everything pretty much done (yay) but I am about to wire in my VSR Isolator. We also have a BMV-712 and I’m having a hard time figuring out how to wire the isolator into the BMV shunt so that it recognizes the charge coming in to keep the SoC reading accurate. The isolator just has the positive post to wire it to the starting battery and the positive post for the house battery and then a small black ground wire. You mentioned that you just attached the ground to the van’s chassis, but do you have it wiring through your BMV’s shunt at all? I’ve been looking online and can’t find any information (or anything that I can understand haha) your help would be greatly appreciated!
I have our house battery grounded simply by mounting them to the vehicle chassis in our electrical area.
Hey Nick, as far as I know the shunt is only wired to the battery. It basically intercepts the battery ground connection (wiring goes from negative battery post to the shunt, then from the shunt to chassis ground). There’s also a wire that connects to the positive battery terminal. So I think it takes all its readings mostly from the current passing through the ground wire, and doesn’t need to be directly inserted between charge/discharge sources like your battery isolator or charge controller. Not positive, but that is my understanding. Hope that helps!
~John
Hey John, I’m facing same challenge, did you find any way to connect Isolator to BMV-712
Hi Lameri, my understanding of how the BMV-712 and other battery monitors function is that they are connected to the battery ground wire only and read the current going through that. The BMV is not directly connected to any charge source (isolator, solar, etc), and shouldn’t need a direct connection to read the current on the ground wire. However, we no longer run a BMV-712, so I cannot test things at the moment. Hope that helps!
~John
Hey, I’m facing same challenge, did you find any way to connect Isolator to BMV-712
So I want to set up a smart isolator (the second one you recommend since the keyline is sold out) to charge an auxiliary battery off the alternator of my van. I am using a goal zero yeti 400 chained to an additional battery of the same size (33ah 12v deep cycle) to run devices off of in our van. Goal zero told me I can connect the isolator into the anderson port of the yeti directly. I’m not convinced since in your description you only run the positive battery cable from the isolator back to the aux battery (so how would that interface with an anderson port even if I could jury rig a cable? Can you give any guidance on that?
OR
Is it better or possible to run the isolator to the aux battery like you did when it IS chained to a goal zero yeti?
Thanks a million for any help you can provide! This blog is incredible.
Hey Todd,
Thanks for the kind words about our blog! Unfortunately I don’t know a whole lot about Goal Zero units or how they work in this scenario, so I don’t know if I can offer too much guidance. But if the anderson port needs both positive and negative feeds I’m not sure how that would work. Battery isolators typically connect to the positive terminals of the starter and aux batteries, and also have a wire for chassis ground to complete the circuit. I don’t see why you couldn’t connect directly to your aux battery (make sure you ground the aux battery to the chassis as well), but then I’m not sure if doing that would cause issues with the Goal Zero. I don’t see how it could, but I would lean on Goal Zero support, and/or post in the vandwellers subreddit to see if anyone there has done this. Sorry I couldn’t be of more help here!
~John
Thanks for the help John. Goal zero also helped me figure out that I can run an isolator direct to the aux battery (chained to the yeti) without any issue. But then I thought will the alternator charge it that much faster than the cigarette lighter 12v adapter that I can plug into the yeti for $6? And with the added hassle and cost of doing the isolator, I might hold off at first and see how it goes.
That’s a good point Todd, I’m not sure if there is any benefit. An isolator is definitely great if you’re working with a DIY system, but if you can plug the Goal Zero into vehicle power already it might make more sense to just do that. Let me know how it works out, I’m curious!
Hello I am wondering if this isolater set up is what I’m looking for. So connected the you have it, let’s say I arrive at my spot ready to start the weekend. I turn off my van and use say for instance , the interior lights, the stereo, power pots. Are these accessories using the aux battery or the starting battery. Keeping in mind I have not changed anything only add the isolater you speak of. My van is a 1992 Chevy Palm springs edition travel van mildly converted. Thx for you info.
Hi Mike,
Basically what the isolator does is allow you to charge a second auxiliary battery using the vehicle’s alternator while you’re driving without draining the starting battery. It’s basically just for charging, and doesn’t affect which battery your electronics use – they’ll use whatever battery they’re connected to.
If you have lights, a stereo, fridge, fan, etc, connected to your aux battery, then they will be using the aux battery (this would be the case even if you don’t have a battery isolator). But, if you’re using any of your vehicle electronics – like the dashboard stereo, the actual vehicle interior lights, etc. – the vehicle would need to be on and they will be using your starting battery.
I would advise adding some 12V outlets and lights connected to your aux battery (you could even wire in some LED Christmas lights fairly inexpensively), and you may be able to rewire the power cord for your stereo to connect to the aux battery (not sure how to do this exactly, but it may be a possibility).
I hope this helps!
~John
Did y’all use any fuses for this system?
HI Joe, the installation instructions for the isolator did not call for any fuses, so we did not use any.
~John
Thanks for the input. I purchased the battery doctor and they recommend a 150 fuse between the isolator and the auxiliary battery. Opted to put it in just in case, was only a $6 part.
Hey I have just received the battery doctor and wondering what size of wire did you use?
Hi Jazmyn, we use 4 AWG wire, but we always recommend double checking your wire sizing using a calculator like this one: https://www.wirebarn.com/Wire-Calculator-_ep_41.html
Hope that helps!
~John
Well that’s just plain silly. The current coming from the alternator certainly needs fused inline on its way to the auxiliary battery to protect against a short. Neither isolator you suggested protects against a short circuit. I’d suggest updating this page to reflect this crucial safety information. You’re disseminating very unsafe electrical data with this isolator installation guide.
Hi Fred, thanks for the input. I’m not sure why the instructions for the Keyline Isolator do not call for fuses (or include fuses, since it’s marketed as a complete kit). But it does seem silly, and you’re right, fusing both batteries is an important safety feature. I’ve updated this post to reflect adding inline fuses where necessary. Thanks for calling out this oversight!
~John
For anyone reading, the battery doctor calls for an inline fuse on the aux battery side. It also claims no fuse is necessary from the starter battery to the isolator because the battery doctor unit has internal protection.
Hi Trevor, thanks for commenting! Even though Battery Doctor does not include a fuse on the starter battery side in their installation instructions, we still recommend fusing at both ends anyway.
~John
I added inline fuses to both sides as well to be safe, but wanted to clarify Fred’s comment “Neither isolator you suggested protects against a short circuit”. At least from the Wirthco documentation, it seems like they implemented.
I just want to triple check that I understand: even though there is a 150 Amp fuse built into the Battery Doctor isolator that I have, I should still wire a 150 Amp fuse inline between my starter battery and the isolator, and another between my isolator and my auxiliary?
Hi Julian, the installation instructions for the Battery Doctor specifically say to install a fuse in between the isolator and your aux battery (as close to the aux battery as possible). This is to protect your aux battery in case there is a short, which the Battery Doctor’s internal fuse would not do. The Battery Doctor’s internal fuse should protect the device itself and your starting battery, so an additional fuse next to the starting battery is not strictly necessary and is not called for in the BD’s installation instructions (although it would add an additional layer of protection). However, not all isolators have internal fusing, so if you are not using a Battery Doctor you should have a fuse at both ends. Hope that clarifies!
~John
Thanks for clearing that up. 10/10 blog, best I’ve come across in many months of research. Wish I’d found it sooner! Keep up the good work and sincerely hope you are earning what you deserve from this great site!
I’m wondering about the need for a DC-DC charger / charge controller. Much like a solar charge controller manages the various stage of solar charging (Bulk, Absorption, Float, etc), i.e. you don’t just connect a solar panel to your battery, shouldn’t there also be something like this installed for the voltage coming from the alternator to ensure your batteries are being charged efficiently/effectively/correctly? I have no knowledge / experience of RV electronics.
I’ve seen other posts about people using such things and am wondering what your thoughts are, because I have to say, your post is so well documented it can easily be taken as an excellent source of information so I’d like to ensure that it is all sound advice and it’s not a case of the one-eyed leading the blind. No offense meant in the least!! 🙂
Hi Stephen, Those are very good questions. I’m not an expert on DC-DC chargers, but my understanding of them is that the big benefit of installing one is boosting the charge coming from your alternator if you’re seeing too much voltage drop of if you have a low voltage alternator. For example, if your aux battery bulk charges as 14.4V but your alternator is only outputting 13.7V, you won’t be fully/effectively charging your aux battery. Adding a DC-DC charger would boost up the 13.7V coming from the alternator to a higher voltage to better charge your aux battery.
Solar charge controllers, in turn, regulate the voltage down. Solar panels produce electricity at 17-19V, much higher than the voltage needed to charge a battery. If you connected a solar panel directly to your batteries without a charge controller, that could be very dangerous – but there’s no danger wiring your aux battery directly to your starting battery/alternator without a DC-DC Charger/Controller.
So my thoughts are this – a battery isolator may not go through the stages of charging like a controller does (although it does cut in/out at certain voltages), but there’s little danger of it sending too much voltage to the aux battery (in fact, it’s probably sending less than needed to fully charge it – you can test the voltage coming from your alternator to find out exactly what it’s outputting). The alternator does cut out once the system reaches a certain voltage, so that prevents overcharging. And we still have solar hooked up, so our solar charge controller can take care of boosting voltage up as needed.
So you can definitely consider a DC-DC charger, but based on my understanding I think it’s much more necessary if you’re getting low voltage from your alternator and you do not have a another charging system (like solar) hooked up.
I hope that helps!
~John
The use of isolators and relays works fine on older cars/alternators, but a a lot of modern cars are / have smart alternators, Euro 6, start/stop, regenerative braking, etc. and you can lose a lot of charging opportunity/efficiency while driving. A lot of these newer cars pound the amps in fast (with larger alternators) over a shorter period of time to increase the overall efficiency. But the net result is, you may not get your expected 2 hours of charge after a 2 hour drive and you find your batteries aren’t 100%. Indeed a DC-DC charger or a buck/boost charger will ensure your batteries are getting 14.8 v when they want regardless of what your smart alternator is doing in terms of fuel savings.
Example… VW Golf Bluemotion with start/stop and regenerative braking. Has an oversized alternator and battery. Alternator often turns off (to increase fuel efficiency) and so DC system runs at an under-load voltage of say 12.2 volt (no battery will be charging). During this time, the car would draw certainly from your starter battery (and possibly from your secondary battery, but the isolator/relay “should” stop this). Then you coast or hit the brakes and your 150Ah alternator kicks in at 16volts (yes, they can charge well above 14.8v) and charges your battery up fast. This cyclic approach is better for fuel economy, but can be hard on alternators, batteries and wires, which is why the VW Golf, for example, has a bigger and more expensive battery than a non-Bluemotion Golf. Now question is, is your secondary battery and wires going to it up to this cycling?
Anyway, just remember that new cars can run at voltages varying from 12 to 17 volts (brand dependant… someone suggested 1 model that peaks higher) and you need to consider this. There’s a reason people are spending $300 on DC-DC chargers rather than $30 relays/isolators. Effectively, DC-DC chargers and boost converters will buffer the cycling and ensure that your secondary battery is getting its full potential charge even ALL the time the car is system is running, regardless of whether the system (smart alternator, etc.) is maintaining the car’s DC system at 12.2 (fuel saving mode) or 16 volts (charging / regenerative mode). We seem to be on the cusp where every new car is putting in some fuel-saving charging technology…
Hi David, thanks for the tips! We’re working on updating this post and will add in information relevant to newer vehicles. We typically deal with vans that are 20+ years old, so that’s where the bulk of our experience lies at this point.
~John
Of all the information I’ve seen about batteries and solar this has been informative. Thsnks so much for this info on isolators. Greatly appreciated
Glad it was helpful Mary! Good luck to you!
~John
Did you replace your starter battery with a deep cycle battery as well? I know it generally kills batteries when you have two different types of batteries wired together. Does the isolator prevent this from happening?
Hi Brett, the purpose of a battery isolator is to allow you to charge your auxiliary battery from your vehicle’s alternator, while also preventing your starting battery from draining. So, there’s no need to replace the starting battery. Battery isolators like the one we have constantly sense the voltage across both batteries. It only sends current to the aux battery when the starting battery is fully charged, and if it senses the starting battery voltage dip below a certain point it will cut the feed to the aux battery and top off the starting battery. Hope this helps!
~John
This is the best info yet for isolator I have found; I’m sold! What gauge wire did you use to run under your van? This might be a dumb question…I’m not sure. Looks like 2 maybe?
Hi Keefe, at the time we bought the isolator, Keyline chargers had a complete kit available that included all the wiring needed. I think the wiring might actually be 4 gauge, it’s definitely thinner than the 2 gauge wire we used with our inverter. Thicker wire would give you a little less voltage drop, but would be more challenging to work with as well.
The kit is not always available for sale (when we wrote this post it wasn’t), but I just checked Amazon and it looks like they have it right now. You can click through the links in the post and make sure to select the kit, or get to it directly here.
Hope that helps! Let me know if I can answer any other questions!
Thanks,
John
Thanks for the responses. As I clicked around, I realized the kit option. I’ll likely go with that option as it looks most straightforward.
The other question I have is monitoring the charge of the battery. I understand the isolator will not overcharge the aux battery. I am planning on solar panels, but might at first not get the panels and just have the isolator as I get my van all set up for the road. So, if I set up a solar charge controller without any panels, will it still read the charge of the battery? Is there another way to get the charge info? Maybe just using a multimeter? If so, is it volts I’m looking at…to not be under what amount? I’ll be starting off with one 12v 100ah battery, and I understand the importance of not letting it get lower than 50% charge.
The charge controller should still read the charge of the batteries even if you don’t have solar panels hooked in, but depending on what type of controller you get you may also need a remote display to see battery readings. You could also use a simple voltmeter/multimeter to read the battery voltage. Battery voltage is the most useful number to determine where you’re batteries are – oftentimes the “state of charge” percentage you see on charge controllers and battery monitors is just not very accurate. As a general rule of thumb, 12.2V is approximately 50% charge (although this can vary a bit between battery types, and even different brands). We have AGM batteries, which you can discharge a little further than sealed lead-acid batteries, so we try to stay above 12.0 (although we only get down that far if we’re both not driving and not getting much sun for several days). Many charge controllers have an automatic low voltage cutoff, which cuts off your batteries if they get below a certain voltage to prevent. Our charge controller cuts out at 11.8 for AGM batteries. Hope that helps!
Hi, I am very interested in what controller you would recommend (20, 30, or 40amps)?Also, if you have an Amazon link that would be super helpful! Thanks!
Hi Vinny, we generally recommend the Renogy Rover charge controllers because they work well and are affordable. Sizing depends on how big your system is, but we recommend getting components that can grow with your needs. The price difference between the 20A controller and the 40A controller is not that large, and getting the 40A would allow you to expand your system later on without purchasing a new controller. Renogy also sells solar kits that come with the controller, panels, wiring, and mounting brackets. Hope that helps!
~John
what’s the approximate time scale that it would take to charge up a 135ah deep cycle with this? Something on the order or 10min, an hour, 5 hours? Thanks.
Hi Ian, it’s tough to give an exact number because it will depend on your alternator, the state of discharge of your battery, and the length and size of the wire you have running back to your battery. But I would imagine you could bring it from a 50% charge up to full in under a couple of hours of driving. Hope that helps!
Thanks,
John
Hi! Just wondering how you went about having the batteries charging from using both the isolator and the solar panels and how it’s all connected. Thanks
Hi Freya, the isolator connects directly to the positive terminal of the auxiliary battery and charges it that way (the alternator detects voltage and prevents overcharging). The solar first runs through a charge controller, which detects the voltage (i.e. charge level) of the batteries, and charges them at the optimal rate. So if it detects the batteries are fully charged (which they are after driving for a bit), then it won’t overcharge.
We have a detailed breakdown of how we wired up our solar and electrical system that you can find here. Adding the isolator means just wiring it to the positive battery terminal, and connecting it to the starting battery as we describe in this post.
I hope that answered your question, let us know if we can help further!
~John
Hi , I was wondering , you stop the van the batt isolator is still in its connect mode as the start battery so is still over 12.6 so that means both are connected to the solar controller , so in theory they will stay connected because the controller by day is supplying enough voltage so the isolator never senses the start batt low until night or the next day when’s you have used the lights fridge ect without ext supply , which means there is a chance the isolator never kicks in or only sometimes , I can see that it’s not really a problem but would prefer the start batt not be always connected , Is a physical switch the only way to prevent this or do I have the function wrong. Thanks
Hi Anthony, smart battery isolators only open the connection between the starting and aux batteries when the vehicle is running. These units sense the voltage of the starting battery. Once the starting battery reaches a certain voltage (I believe it’s about 13.4 volts, which is the voltage the battery would be at when it’s receiving charge from the alternator), it opens the connection and begins charging the aux battery. When the vehicle is turned off and the alternator is no longer charging the starting battery, the voltage on the battery will drop automatically since it’s no longer receiving a charge. Once the battery dips below 12.8V (about the voltage of a fully charged 12V battery that’s not actively being charged), the isolator closes the connection. In practice, this means that your isolator is not sending power to your aux battery for more than a few minutes after you shut off your engine. Hope that helps!
~John
Hi John,
I came to open a new comment thread, but this one actually is close to what’s going on. Following your recommendation, I got the WirthCo Battery Doctor for my build and it has been working great. In the winter. Now that spring is here in the PNW, the sun is coming back out and I am noticing that my isolator is staying on after I’ve turned off the van, for hours. I believe this is because:
1. Turn vehicle on, alternator charges, isolator turns on connection between starting/aux batteries
2. Driving + solar gets everything fully charged, batteries are all full
3. Park in the sun, solar still picking up lots of voltage. Turn van off. Both batteries still full, over 12.8v
4. Solar panels have been charging both starting and aux batteries b/c they are connected by turned on isolator
5. After turning the van off, the solar panels are keeping the voltage of the aux and starting batteries pumped full, above 12.8v
6. Isolator never shuts off b/c vehicle is left in the sun and solar panels continue keeping everything above 12.8v
Now this would all be well and good, however when I go to turn the van back on and the aux batteries (deep cycle AGM, exactly like the ones you have) are still connected to the starting battery, we run into the situation of exposing the AGM batteries to the sudden discharge of vehicle ignition, which is definitely not good for them! One thought is that the engine battery is obviously much closer to the spark plugs, and the aux battery is way down the line (about 6ft of 2awg, plus the inline fuse) – I wonder if this really is hazardous to the aux batteries?
At this point, when I see this happen, I have been reaching underneath my storage area with the ol’ ratchet and disconnecting/unscrewing the nut holding the small black earth wire coming off the battery isolator to the ground bolt I’ve rigged up on the vehicle chassis (per instructions from the manual). This is unnerving because the entire aux battery system is also grounded on this bolt and I’m a little worried I’ll disconnect that each time I pull the isolator’s ground wire off the bolt in order to shut it off.
I’m a little rusty since it’s been months since I did all the wiring, but what risk am I running by accidentally ungrounding my aux batteries momentarily while disconnecting the isolator ground?
Long term, I’m thinking I’ll do some more research and find an isolator/charger that doesn’t have this problem. I’m in a money pinch right now (paying off the van/build expenses) so am looking for a cheap fix. I think in the short term I’m going to get an inline switch to install between the isolator ground wire and the ground bolt so if i see that the isolator is on when I want to start the van, I can just flip the switch to off which will shut the isolator down and close the connection. This will then allow the voltage levels to return to normal and then I’ll turn the isolator back on, which should not be in charge mode anymore, and start the van.
Let me know what you think? Thank you!
Hi Carl, thanks for reaching out! You could very well be correct with how the isolator is behaving. A few other readers have recently reached out to us with issues they’ve experienced with their Wirthco isolators, so we’ve chosen to remove it from our website for the time being until we can reevaluate it.
As far as how you’re disconnecting it, momentarily ungrounding other components won’t necessarily cause issues. You can run a battery and inverter setup without any type of ground and it will work just fine. The grounding is a safety measure to help prevent electric shock, however, and manually disconnecting is probably not the best thing to do regularly. You could consider installing a switch on the positive cable to the isolator to make disconnecting it easier. Hope that helps!
~John
Hi John, thanks for this incredible website and all it’s information. We referred to it a lot during our van build. I also got the Wirthco Battery Doctor based on your recommendation, and it works great. However, I am thinking of adding a portable solar panel that would only be connected when the van is not running. I was planning on connecting it with alligator clips to the starting battery, assuming that the Battery Doctor would behave just as if the alternator is running. Do you know if that’s OK to do? My house battery is buried in the back of the van and I really don’t want to have to add a set of special ‘solar terminals’ to it. (Does that make sense?) I appreciate your answer in advance.
Hi Dan, I wouldn’t connect a solar panel directly to a battery. The voltage delivered by the solar panel will be around 17-19V, which is higher than the charging voltage of a vehicle battery (If you connect the panel to your starting battery, it will apply current to that battery as well). Running it through a charge controller will avoid any issues and make sure that your batteries are being charged properly. Hope that helps!
~John
Hi John, and thanks! I was not going to connect the panel directly. What I meant is, that I am getting a portable panel that comes with a charge controller, and I wanted to know if the controller could be hooked to the van starting battery, and let the Battery Doctor isolator do it’s thing. My main question is that the van starting battery is a sealed lead-acid type and the house battery is a sealed lead-acid, AGM VRLA battery. Would it be OK to hook the alligator clips just to the starting battery in that situation? I would assume it will just act like the alternator is charging. -Thanks
Did y’all connect the negative ends of the starter and aux batteries?
Hi Topher, there’s no need to directly connect the negative terminals. Both batteries are grounded to the metal vehicle frame, which serves as a common ground and completes the circuit. Hope that helps!
~John
I second Topher’s question. But, does it matter if it’s a “common” ground — or just that they are both grounded? I suppose I’m concerned because the aux battery is so far away from the starter battery. And are you referring to the starter battery’s factory grounding? Thanks!
Hi Keefe, as long as they’re both grounded to the metal of the vehicle chassis you’ll be good to go. The van body basically acts as a big mass of wiring that connects the grounds and completes the circuit. Other components in your vehicle (like your tail lights, etc) are grounded in the same way – with simple ground screws into the metal frame. Hope that helps!
With the battery isolator does it shut off when the secondary batteries are 100% charged? Just wondering because I know some batteries you don’t want to over charge them or it can ruin them.
Hi Alexander, neither batteries (starting or auxiliary) will be overcharged with this setup. I was intitially concerned with overcharging too, so I talked to several people familiar with automotive electrical and how isolators work. My understanding is that the alternator contains a voltage regulator that prevents both batteries from overcharging – if the alternator detects that the battery voltage is above a certain level, it cuts out and stops sending current to the batteries. The Keyline isolator also detects voltage, and if the voltage on the starting battery hits 13.3 it “cuts in” and sends current to the auxiliary battery. But when the voltage of the whole system reaches a certain point (when both batteries are charged), the alternator cuts out and stops charging altogether. Hope this helps!
~John
Hmm this explanation is liable to cause confusion. Modern cars with smart alternators vary the output voltage and you should use DC to DC charging in this case, not a smart isolator. Older vehicles have alternators that will supply a constant 14V or so when the car is running regardless of the state of charge of the battery (a booster diode can help charge AGM house batteries). Overcharging is prevented as the current output from the alternator will drop as the battery approaches full charge.
Hi Carl, thanks for commenting! We are working on updating this post to include information about DC-DC chargers, which is what we recommend to anyone getting started now.
~John