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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.

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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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