this post was submitted on 29 Sep 2023
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You want to put the batteries in parallel, so you'll have double the capacity. Installing them in a series would increase voltage and most likely damage something (mixing 12V systems with 24V battery pack doesn't really work). Increasing capacity with another battery can cause strain on charger components, so make sure they're beefy enough or at least have proper protection against overcurrent.
People are correct that you should use the same capacity batteries, preferably the same make/model and age. Mixing batteries can cause problems where one battery drains faster and other(s) start to charge the lower level one so you'll have less useful amp-hours and that degrades batteries faster.
Switching batteries with a relay or a switch is possible, but you need quite big and good quality relays/contactors for that as current can be pretty high which can cause arcing and even weld contacts together eventually. With proper parts it's a safe way of doing it, but personally I'd just get two batteries in parallel since there's fewer components to malfunction and adding complexity with arduino+contactor doesn't save that much money, specially if you place any value for your time (which of course isn't necessary, tinkering itself is often worth the time spent).
The issue is that I cant place two of the same batteries. The extra one has to go under the passenger chair so it will be a completely different battery. I'm not completely sure that we won't be running into issues then when linked in parallel.
I agree that the relay option it's added complexity and finding the right relay is a bit of a challenge haha. Also there probably will be a short drop in voltage or I have to switch them shortly after each other. So both relays are closed at the same time for a little bit. I don't know if that would fix the arcing? It will be linked in parallel for a little bit then I guess.
Get a switching relay or one normally closed and another normally open one. That way there's no paraller connection at all. Connecting two batteries together with different voltage levels causes a huge current spike, think jump starting a car and how thick those cables are. Arcing will happen on the relay contacts no matter how you switch it if there's load connected.
And since you're not talking about a trailer, is one of the batteries for the car itself? Since if you're planning to use the secondary battery as a car battery too you need very heavy wiring to give starter enough amps to run plus running that over a relay is a whole another beast to manage since starter motor can pull hundreds of amps momentarily.
Is an extra utility battery. So that shouldn't give any issues for starting. That's a separate battery. I'll try to find one of those relays. I know I can search myself but if you have any suggestions they are always welcome:)
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.