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Two battery setup in camper
(lemmy.world)
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So you'll have total of 3 batteries at the car? One for engine, existing utility battery and now you're planning to install another utility battery, right? That should work. I don't know how much current you're pulling from the battery, so it's difficult to recommend anything. Common bosh switching relay is something you can find from pretty much every car part store for ~5€, but I think they're rated only up to 40A and I wouldn't push them to the limit. Check your inverter datasheet how much current it can draw and preferably get a relay which can do double the maximum rating for longevity and stability.
That is correct. 1 battery for the engine and 2 for utility. I guess it has to be quite a beefy one. The interter is rated for max 166 amps. So that's quite a bit. Which also makes it a bit hard to find a suitable relay.
12V 200A relays are pretty easy to find (I wouldn't get one from alibaba tho), but that much current requires quite beefy wiring as well. Personally I'd review options to place auxiliary batteries in parallel since that would simplify wiring and the whole system a bit, but as I don't know how your camper is built it can be tricky or you need to sacrifice storage space somewhere.
And as you're placing a battery at the same space where people stay be careful with hydrogen. Charging lead acid battery produces hydrogen and in the worst case scenario, specially if you have a gas stove be dangerous if not lethal.