Vanlife Essentials: Installing a Smart Battery Isolator

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

140 Amp Dual Battery Smart Isolator by KeyLine Chargers - VSR - Voltage Sensitive Relay Specially Designed for ATV, UTV, Boats, RV's, Campers 5th Wheels Off Road Vehicles Rhino Polaris Artic Cat ETC

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.

WirthCo 20092 Battery Doctor 125 Amp/150 Amp Battery Isolator

Installing a Battery Isolator in Your Van

Step 1: Disconnect the Negative Battery Terminal

disconnecting 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

mounting isolator bracket

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

connect grounding wire

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

cables on 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

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

cable under vehicle

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!

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alexander fowl
alexander fowl

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.


Did y’all connect the negative ends of the starter and aux batteries?

Freya Heeks
Freya Heeks

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


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.


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?


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?


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

Stephen O
Stephen O

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 »

Joe J
Joe J

Did y’all use any fuses for this system?

Todd Bushman
Todd Bushman

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 »

Nick Cheply

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 »

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