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.
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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.
- Variety of amperages available to suit your specific requirements
- Easy add on to Renogy solar kits or existing solar setups
- Another component taking up room
- May require tapping into ignition circuit
3-stage charger for optimal battery charging. Works with all battery types (lithium, AGM, etc) and alternator outputs. Pair with the BT-2 Bluetooth module for monitoring from your phone. Available in 20A and 40A. Renogy also makes a 60A DC-DC charger.
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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).
- One unit handles both solar and DC-DC charging
- Easy to install (usually no ignition tap)
- Less flexibility with charging parameters
- Need to assemble your own solar components vs buying a kit
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.
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
- 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)
- Cordless Drill
- Mechanic’s Toolset
- Zip Ties
- Cable sheathing/flexible conduit (sized for your cable)
- 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.
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.