Having solar power 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.
But sometimes solar power 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 limits the amount of sunlight getting to your panels. Even with our larger four-panel solar system, we’ve run into problems with battery drain after 4-5 days in poor sunlight conditions.
That’s why we highly recommend installing a battery isolator in your DIY van build. A battery isolator 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 with nothing but a good isolator and a deep cycle battery.
Adding our KeyLine Chargers Iso-Pro140 Smart Isolator to our rig was a game changer for us. Read on to learn why having a battery isolator is essential for vanlife, and how you can install your own.
Page Contents
What is a Battery Isolator and Why Do You Need One?
Every vehicle has an alternator, which is a device that converts the mechanical energy from your van’s engine into electricity and uses that electricity to charge your starting battery. A battery isolator is a device that allows you to charge a second (auxiliary) battery from your van’s alternator.
You can easily use your alternator to charge your auxiliary battery simply by connecting the positive terminals of both batteries. But connecting your batteries this way 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!
That’s where a battery isolator comes in. An isolator isolates your batteries so that your alternator will only charge your auxiliary battery when the engine is running, and your electrical loads won’t drain your starting battery.
Battery isolators are essential for vanlife. Life on the road means a fair amount of driving, and having the ability to charge your batteries from your engine is a great way to increase your power efficiency and make sure you have electricity when you need it.
On a budget? Start with a battery isolator.
A good battery isolator paired with a deep cycle battery is the easiest and cheapest way to add electricity to your van build. You can always expand your system and add solar later.
Why You Want a Voltage-Sensing “Smart” Isolator
There are three types of battery isolators out there: solenoid, solid state, and voltage-sensing “smart” isolators. In our opinion, a smart isolator is far and away the best choice. Here’s a rundown of each type:
Solenoid Isolators are essentially mechanical switches – electrical current causes the switch to close, sending charge to the auxiliary battery. These isolators are the cheapest (around $20 or so), but they’re also the most complex to install. To prevent draining your starting battery, you’ll need to wire it into a circuit that’s only active when the vehicle’s running. Since they have moving pieces, they also come with some risk of mechanical failure.
Solid State Isolators use electrical diodes to split the charge coming from the alternator between the starting and auxiliary batteries. But these types of alternators are not nearly as efficient, introducing about 0.7A of voltage drop. This means it will take longer to charge your batteries, and neither battery may reach full charge. Solid state isolators are generally in the $50 range.
Voltage-Sensing “Smart” Isolators automatically sense 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, the isolator “cuts out” to prevent your starting battery from draining. A smart isolator will run you about $60-$80+, but the efficiency, reliability, and ease of installation are definitely worth it.
Recommended Battery Isolators
KeyLine Chargers Iso-Pro140
We had the KeyLine Chargers Iso-Pro140 Smart Battery Isolator installed in our van initially, and we can’t recommend it highly enough. 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. IFor durability and reliability, this isolator is an excellent choice.
Wirthco Battery Doctor 125A/150A
This the battery isolator that we have now, and we highly recommend it. Although we think the Keyline isolator is excellent, we decided to switch to the Wirthco Battery Doctor to pick up one key feature: the ability to reverse the current and jump the starting battery from the aux battery. This is a handy feature to have if your starting battery dies in the middle of nowhere. The Wirthco Battery Doctor is also more affordable and comes with clearer instructions than the Keyline.
Installing a Battery Isolator in Your Van
Step 1: Disconnect the Negative Battery Terminal
This is an important safety step that isolates the starting battery so you won’t get shocked.
Step 2: Mount the Battery Isolator in the Engine Compartment
Find an easily-accessible spot near the starting battery to mount your isolator. The Iso-Pro140 comes with a rear mounting bracket. Use this as a template to mark where to drill, and drill the holes for the mounting screws. Screw down the mounting bracket using the two short screws.
Note: Depending on how your engine compartment is arranged, you may need to temporarily remove your starting battery for this step (we had to).
Step 3: Connect Black Ground Cable to Vehicle Body
Crimp a blue ring terminal onto the isolator’s black ground wire. Attach the ground wire to the metal vehicle body. The best option here is to add it to an existing ground screw.
Step 4: Cut and Crimp Battery Cable and Attach to Back of Isolator
Cut your battery cable to length – one short cable to connect the isolator to the positive terminal of the starting battery, and one longer cable to connect to the positive terminal of the auxiliary battery. Crimp battery terminals onto the ends of the cables. Then, attach the cables to the posts on the back of the isolator (which post goes to which battery should be marked).
Step 5: Attach Short Cable to 150A Inline Fuse, Then Run Wire to Positive Terminal on Starting Battery
Attach the other end of the short cable to a 150A inline fuse or circuit breaker. Mount fuse/breaker in the engine compartment if needed. Run a short piece of wire from the fuse to the positive terminal on the starting battery.
Important Note on Fuses
The in-the-box instructions for some battery isolators (including the one we have) does not call for any fuses. But adding two 150A 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!
Step 6: Run Long Cable Underneath Vehicle to Auxiliary Battery
String the long cable underneath the vehicle back to where you have your auxiliary battery, tying it out of the way with zip ties as you go. You may need to drill a hole up through the floor to get the cable up to your auxiliary battery (make sure to seal the hole with silicone caulk). Then, attach the end of the cable to a 150A inline fuse/breaker, and run a short cable from the fuse to the positive terminal on your auxiliary battery.
Step 7: Reconnect Starting Battery and Make Sure Everything Works
After reconnecting your starting battery, test your system to make sure it works. First, turn on your engine. Wait a few minutes for the system to reach cut-in voltage. Using a multimeter, voltmeter, or battery monitor, check the voltage on your auxiliary battery. It should be above 13.0 to indicate that it’s charging. Now you should be good to go!
Electricity on the Road in Any Conditions!
We think 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 battery isolator is 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, having a battery isolator ensures that you can keep your batteries charged in all conditions.
For more build guides and vanlife tips, be sure to follow us on Instagram @gnomad_home and on Facebook. Cheers!
Authors
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John -
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.… Read more »
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!
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… Read more »
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… Read more »
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.… Read more »
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
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… Read more »
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… Read more »
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… Read more »
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
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
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
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… Read more »
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… Read more »
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,… Read more »
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!
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… Read more »
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… Read more »
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… Read more »
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… Read more »
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… Read more »
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… Read more »
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
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… Read more »
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
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… Read more »
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… Read more »
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
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!
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.
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
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… Read more »
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… Read more »
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
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… Read more »
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 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… Read more »
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… Read more »
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
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… Read more »
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
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
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
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
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
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… Read more »
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… Read more »
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….… Read more »
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
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
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)?
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
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… Read more »
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
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… Read more »
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
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… Read more »
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 commenting@ 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
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… Read more »
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. Typically, your charge controller is wired directly to your battery (with an inline fuse on the positive cable). There is then a separate set of wiring connecting the charge controller to your 12V fuse box. The way the fuse box is wired in that system is functionally the same as wiring it directly to your battery, which should work fine, but not going through your charge controller means… Read more »
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… Read more »
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… Read more »
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… Read more »