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.
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 Smart Isolator: KeyLine Chargers Iso-Pro140
We have the KeyLine Chargers Iso-Pro140 Smart Battery Isolator installed in our van and we can’t recommend it highly enough. It’s small and compact, and 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.
If for some reason the Keyline isolator is unavailable, the Winco Battery Doctor 125A/150A isolator is a solid alternative.
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!
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 »
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 »
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.
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
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 »