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How-to Choose a Battery Isolator for a DIY Camper Van Electrical System

How-to Choose a Battery Isolator for a DIY Camper Van Electrical System

A Battery Isolator combines the ‘house’ battery bank that runs your lights, fans, refrigerator, computers and such to your starting battery so it can be charged by your vehicles alternator.

Quick note before we get started.  This is just one part of an overarching “How to Install a DIY Camper Van Electrical System” series.  If you’ve just stumbled on this article directly without seeing that, there are likely some things we’ve already covered.  If you want to check out that step by step guide, you can do that here: https://www.explorist.life/diy-campervan-solar

Also, we have interactive solar wiring diagrams that are a complete, A to Z solution for teaching you exactly what parts go where, what size wires to use, fuse size recommendations, wire lug sizes, and all kind of other stuff to help save you time and frustration.  You can check that out here: https://www.explorist.life/solarwiringdiagrams/

How to Choose a Battery Isolator for a Camper Van:

Most Battery isolators are rated in terms of amps. This number is simply the amount of potential amps that could be flowing through the device from your alternator to your house battery bank.

How to determine Battery Isolator Size:

Your battery isolator will depend on the size of your alternator. Although you’ll likely never see your full alternator output amperage going through your isolator, it’s really the only base line you’ve got.

Size your battery isolator to handle the max amp output from your alternator.

The max amps your alternator will output can sometimes be found in the spec sheet of the vehicle. If you don’t have this or can’t find it. It will typically be stamped onto a metal plate on the alternator itself.

Lithium Battery Isolator

Due to the resting voltage rates of Lithium Batteries, you’ll likely want a lithium battery specific Isolator.

Although a normal battery isolator shouldn’t ‘damage’ your lithium batteries, it simply may not disconnect properly when your engine is off as it may see the higher voltage of the lithium batteries as ‘charging’, which will keep the batteries combined.

Wire size for Battery Isolator

Blanket Statement: Use 2/0 wire. 2/0 wire will be enough for 95% of applications. 2/0 wire with 105°C insulation under 50 feet in length is capable of handling currents of up to 330 amps.

Most applications will never see amps that high nor are most alternators capable of putting out that many amps.

With 2/0 wire, as long as the house battery bank is kept within 12 feet of the starting battery, even at charging rates up to 149 amps, a 3% voltage drop will still be achieved.

Questionably Reliable Crowd-Sourced Data

I wanted to gather some real-world data from those who were charging their house batteries with their alternator. More specifically, I wanted to find out what the max amps people have ever seen going from their alternator to their batteries.

So, what did I do? I reached out to various camper van facebook groups, THE home of Questionablly Reliable Crowd-Sourced Data.

After learning that:

  • 50% of respondents didn’t know how to find out how many amps they were charging at
  • 15% didn’t know what a shunt was
  • 18% didn’t know the difference between amps and volts
  • 21% would rather give me irrelevant advice rather than answer my question
  • 87% of the answers were estimated to be 92% inaccurate

But… I gathered the following data:

Wire size to reported amps:

This list is of what people reported their wire size was, alternator size was, and the most amps they have seen going to their battery:

Alternator SizeWire SizeReported Amps
2203/0100
1803/0100
1801/060
220450
160250

There were also some respondants who didn’t know how many amps they were charging their house batteries with, BUT they did know the fuse size that was on that wire from the alternator to the house battery, which we can then assume that their amps never go above the size of that fuse.

Alternator SizeWire SizeFuse Size
2202100
1904125
135430
130440
702100

What conclusions can we draw from this info? Not much, honestly… The biggest takeaway that I can gather from this information is that house battery banks charged by the alternator rarely charge at rates greater than 100 amps.

This sample size is small enough, though, and stats are questionable enough to need to disclaimer that this info should only be used as an “FYI” rather than actual good information.

This does back-up even further, though, my claim that 2/0 is a great choice of wire size for installing a battery isolator.

If you are bound and determined to use the smallest wire possible for this application, this chart tells you the max amps a given wire with 105°C insulation can safely handle:

Wire SizeMax Amps
2210
1245
1/0285
2/0330
3/0385
4/0445

If you go this route, keep in mind your voltage drop. Or just use 2/0 and call it good.

Battery Isolator Fuse Size

Since we’ve seen that alternator charging rarely goes over 100 amps, fuse size is less of a concern as it’s mainly just in place for catastrophic failures (if insulation breaks down and shorts to ground)

For fuse size, I recommend taking your rated alternator output and multiplying by 1.5 to get your fuse size. Verify that number doesn’t exceed the max amps of the wire listed here:

Wire SizeMax Amps
2210
1245
1/0285
2/0330
3/0385
4/0445

Recommended Battery Isolators:

Here are the 3 Battery Isolators I recommend:

For Lithium Battery Banks, I recommend the Li-BIM:

For alternators under 120 amps, I recommend the Blue Sea SI-ACR:

For alternators over 120 amps, I recommend the Blue Sea ML-ACR:

For the ACTUAL detailed installation instructions, you will need to reference the installation documentation in whatever isolator you decide to install.

Now that you know all the little details about picking a battery isolator, lets learn how to monitor the status of your batteries.  This next lesson will teach you how to wire the battery monitor and shunt so you can see how much power is actively entering or leaving your batteries.  It’l like a fuel gauge for your battery bank. Check that out here: https://www.explorist.life/how-to-install-a-diy-camper-van-solar-battery-monitor/

Everything that you are learning here is put to use in our FREE Interactive Solar Wiring Diagrams.  If you haven’t yet, check them out as they are a complete solution for a camper van electrical system.  Check them out here: https://www.explorist.life/solarwiringdiagrams/

Remember, this is just one part of a full camper van electrical educational series.  To see all of the individual guides, click here: https://www.explorist.life/diy-campervan-solar

Finally, If you found this guide helpful, It’d truly mean the world to us if you’d share it with somebody who can use it, pin it to pinterest for later reference, or share it to a facebook group when somebody has a question about this subject.  Click the bubble in the lower right corner to subscribe to be notified of future updates and as always, leave any questions you’ve got in the comments below.


Malena Bell

Wednesday 5th of May 2021

I just purchased a 98 four winds 5000. We went on a weekend outing and when we returned I accidentally left cell phone charger plugged in the cigarette lighter and when I went back to drive 3bweeks later I had a dead battery. Not thinking anything of it I unhooked the battery took to the local auto parts to see if it was bad or needed charge. Battery was still good but when I hooked it back up there was a noise I hadn't heard before a loud click constantly, and soon to find out that there was absolutely no charging going on to house batteries. After researching I found it's a possibility of isolate and if so my question is does the click automatically mean to replace or is there a way to reset? Please help

Steven

Monday 20th of April 2020

Hey Nate, Awsome articles on installation. Quick question on a full electrical set up. Can the charge controller handle the amps coming from the alternator in a backflow direction? By the looks of your wire diagrams it can. Hopefully, I can explain it better below. More explanation. You have solar panels to the charge controller to the second battery. From the alternator to the starter battery to the isolator to the second battery. The alternator will produce about 100 amps to the second battery but the charge controller is rated at 20 amps. I know in theory all power should run to the battery and not back towards the solar panels but electricity always follows the path of least resistance. Can the charge controller prevent the backflow of 100 amps coming from the alternator? Thanks, Steven

Nate Yarbrough

Tuesday 21st of April 2020

Hey Steven!

There is no issue with multiple charging points in your system backfeeding through the solar controller. Alternator, generator, shore power, secondary MPPT, etc.

Suzanne

Saturday 18th of April 2020

Hi Nate, What do you think about the Victron 12/12-30A DC to DC charger? How does it compare with the Precision Battery Isolator? I have 4 Renogy Lithium batteries in my Sprinter, and I am trying to decide of the two (unless another was recommended) what the best option is to charge from the alternator. Also, I'm hoping one day soon you'll make a Youtube tutorial on hooking batteries to an isolator and/or BtoB Charger. I've pretty much read or watched all of your van blogs/tutorials. Thank you for all the great info you are posting!

Nate Yarbrough

Tuesday 21st of April 2020

I very much like the idea of the Victron 12/12-30 DC charger. I actually used that in my newest wiring diagram here: https://www.explorist.life/3000w-inverter-400-600ah-400-to-1200w-solar-camper-solar-kit/

I currently think this is the best method of charging from the alternator for most people.

I'll definitely be doing more 'in-van' tutorials when we get moved into our new shop next winter. I know that doesn't help you now, but it's coming.

jack

Saturday 4th of April 2020

hi, How do I pick the house battery, how many "A" I need if I wanna connect also to an inverter that gives me 1000w. I need to charge a laptop, 2cellphonnes, and wanna plug a coffe maker and have an extra plug just in case I need it. thanks

andrew

Monday 27th of January 2020

in a travel trailer configuration where it is not desired to have a heavy gauge wire connecting the alternator to the trailer battery through a B2B or isolator, what does one do? i understand the amps over the 7pin connectors aux +12 volt leg are low, but is there any benefit/concern with connecting it to the trailer battery bank. Or do you just connect the ground between trailer and truck, and cut the +12 Aux?