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My setup/problem:

I have two 12 V, 200 Ah batteries (deep cycle lithium) that I use as auxiliary batteries for my van conversion (total 400 Ah). I connected the batteries in parallel, but made the mistake of connecting them incorrectly to my inverter/charger - the positive and negative leads that are going out to the inverter/charger are coming from the same battery, instead of the positive from battery1 and negative to battery2.

My questions:

Can this cause damage? I've been running things like this for 6 days, and only realized my mistake because I ran out of power in half the expected time. Also, what exactly is going on with this false setup? Am I only using power from one of the batteries? I just charged the batteries via shore power (still with the same incorrect wiring), and the system read out went from 10.2 V (dead) to 13.2 V (..overcharged? problem?) in only 45 minutes... which seems way too fast even for one 200 Ah battery (though I do have a great charger that was pulling about 80 A). Clearly I'm a bit of a noobie, so any comments or words of caution are warmly welcomed!!

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    \$\begingroup\$ The question would be better with a visual representation of your connection \$\endgroup\$ Nov 12, 2021 at 2:00

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If the two batteries are connected in parallel (both positive terminals connected together, and both negative terminals connected together), it doesn't matter where on the positive side, and where on the negative side, you connect the wires to your loads.

There is a claim that connecting the load "diagonally" (positive on one battery, negative on the other) will balance loading on the two batteries better than connecting the load or charger to one battery. However, if decent size wires are used to connect the batteries the difference in charge or discharge currents in the two batteries will be insignifigant regardless of how you connect things.

A "12 Volt" lead-acid battery will typically be about 14.4 Volts when approaching full charge - don't know the charge characteristics of your Lithium batteries.

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Presuming they are LiFePO4 batteries, their nominal voltage would be 12.8 V and when fully charged 13.8 V.

The charging voltage should be between 14 V and 14.2 V in both CC and CV modes.

The charging current should be 0.4C.

A good 12 V LiFePO4 charger should be used.

It should be ensured that their terminal voltages are within 0.1 V, before connecting them in parallel.

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Your parallel connection is quite okay.

The problem could be the result of the batteries not having been fully charged.

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Well it didn't blow up. You connected them correctly. If you connected negative of battery 1 to positive of battery 2, you would get a series connection instead of parallel. You would get 24 V and something would blow up on the other end. Using wires as balancing resistors wouldn't work either since the resistance of a 12 V lithium ion battery is probably around 100-200 milliohms so no point making one wire longer than the other.

It is my guess here, two batteries did not have the same voltage when you connected them in parallel and you blew some sort of protection, which is probably complex enough to prevent the stronger battery from blowing up the weaker one. That would explain why only one of them seems to be active.

I would suggest to give up this setup and buy a more powerful battery if you need more power. There are too many variables that can cause something to blow up at such high currents. In the meanwhile I would disconnect the batteries and test each one to see how it performs and just use the better performing one for now.

A bit more detail below.

Connecting used lithium ion batteries in parallel requires an industrial setup that would include millivoltmeter and custom resistors and connectors. It is cheaper to throw them away and buy new ones. You would have to fuse the batteries together. There are no circuits on the market to balance parallel lithium ion cells. The weaker battery probably wouldn't blow up, although it might. It would experience rapid heat rise inside and the chemical that separates positive and negative layers within the battery would be permanently damaged leaving you with a small fraction of the battery's capacity.

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