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

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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

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15 thoughts on “How-to Choose a Battery Isolator for a DIY Camper Van Electrical System”

  1. Hey, Nate,
    Here is your grandma question (aka alternator size for dummies):
    How do I know the size of my alternator?
    2014 Ram ProMaster Front Wheel Drive 20 feet long 6 feet wide and 8 1/2 feet tall…….. I know nothing about engines other than they start when I turn the key. The van moves forward when I shift to the D….

    1. 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.

      1. Hey, Nate, well, My alternator is on it’s last legs and is being replaced Monday! I I asked and it’s 220Amps.

  2. Hey Nate,
    I’ve purchased the si-arc and subsequently found out my alternator is 220 amp. Do I need to return and purchase the m series or can I just make sure I have the appropriate breaker installed so it won’t go over 100 amps. Thanks

    1. Darn! We actually did the same thing when we were learning about all of this. But, yes: I’d recommend swapping for the adequately sized ML ACR

  3. Assuming the people you asked about alternator size have Sprinters and Promasters, I’m amazed at the range of alternator sizes of your respondents. I was wondering if you have an idea of how many amps at 12v you get from the Li-BIM per hour while driving on the hwy. Reason I ask is BattleBorn also sells the Sterling battery to battery charger which tells you exactly amp output, however the Li-BIM cycles so it is hard to tell. The Sterling also has ability to be turned on and off, and some programing flexibility. Can the Li-BIM be turned on and off, and also will it work with a 24 system? Appreciate your help!

  4. Nate I have installed your Explorist.life system in my camper. Thank you so much for your DIY system. Every thing was plain, simple, and easy to follow.
    I only have a couple of things that are a bit puzzling.
    1- the operating of the current limit dial on the Digital multi controller.
    2- The Battery isolater LI-BIM 225. I hooked it up as specified in your drawing. Did not add a manual switch. I ran my batteries down to 83% then hooked it to the Truck to test for charging. I tested all wires with a 12v tester and all were hot after starting the truck. However no charge was coming in to the charge controller. I have researched this for a few hrs. know and recall at one point read on one of thees that they will not start charging until the battery’s are below 80%, Is this correct?
    Also I swapped the Ignition wire which was hot at the time to the switch position and the controller indicated that it was charging at 225.
    Do I need to ad a manual switch if I want to charge when the batteries are above 83%? and will it be possible to over charge with a manual switch. Also on your DIY diagram the manual switch is hooked to the +pos side. But on the precision circuit sight I think it indicates that the sig (manual switch) is hooked to a ground? I would like to figure this out as My wife are going on a 4 mo. off grid trip up the Great divide mountain bike route with a horse and wagon. Starting the 2nd of May.
    Thanks Bob

    1. 1- the operating of the current limit dial on the Digital multi controller.

      That is for limiting the current through the multiplus powerassist when connected to shore power. I’d recommend setting it to 30.

      2- The Battery isolater LI-BIM 225. I hooked it up as specified in your drawing. Did not add a manual switch. I ran my batteries down to 83% then hooked it to the Truck to test for charging. I tested all wires with a 12v tester and all were hot after starting the truck. However no charge was coming in to the charge controller. I have researched this for a few hrs. know and recall at one point read on one of thees that they will not start charging until the battery’s are below 80%, Is this correct?

      Here is a pdf of all of the voltage cutoffs for that particular unit that I think will answer most of those questions: https://drive.google.com/file/d/0BykETJ41Nu37b1B0OFE3NDBlbWI3OGIzQWo4VXdBTzEyZXBN/view?usp=sharing

      Also I swapped the Ignition wire which was hot at the time to the switch position and the controller indicated that it was charging at 225.

      Yep! That force combined the batteries.

      Do I need to ad a manual switch if I want to charge when the batteries are above 83%?

      Yes.

      and will it be possible to over charge with a manual switch.

      I’d contact Battle Born for clarification on that, but your Battle Born batteries have high voltage protection built into their BMS to negate that.

      Also on your DIY diagram the manual switch is hooked to the +pos side. But on the precision circuit sight I think it indicates that the sig (manual switch) is hooked to a ground?

      That, apparently, can go either way.

      I’m working on some more documentation on all of this because the documentation that’s available is… a bit underwhelming. Stay tuned.

  5. Hey Guys, keep up the great work, we really appreciate it!

    Quick question and I apologize if its a dumb question. I purchased the Li-BM isolator for charging my lithium battery bank, but do I need to ensure my coach battery (cabin) is also a lithium battery?

    Thank You in Advance!

  6. Hi Nate. What do you recommend to avoid the alternator from overheating while charging the lithium battery pack, specially if the engine is operating at low RPM. Thanks

      1. Hey Nate,

        What do you recommend for my Ford Transit?

        I have lithium house batteries and AGM starter batteries. The alternator puts out 14.7V.

        Thanks in advance!

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