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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
How to Choose a Battery Isolator for a Camper Van:
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
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.
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
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 Size||Wire Size||Reported Amps|
There were also some
|Alternator Size||Wire Size||Fuse Size|
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 Size||Max Amps|
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 Size||Max Amps|
Recommended Battery Isolators:
Here are the 3 Battery Isolators I recommend:
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,
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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