Originally Posted by Mamassecret
We are finally ready to get the system up today and needed to know if there is any reason why these two batteries can't be paralleled?
I can't say I'd recommend it for a few reasons.
First, native LiFePO4 voltage is 3.2V nominal. "12V" is achieved by having 4 cells internally in series (3.2V + 3.2V + 3.2V + 3.2V = 12.8V nominal), and it is generally considered bad practice to parallel batteries already in series. It is best if you can parallel at the cell level.
Second, and compounded by the above issue a bit, is the fact that one of these batteries is twice the capacity of the other. I imagine this will mean that one battery will end up working much harder than the other while in regular service. Energy is also going to move from the higher voltage pack (likely from the higher capacity one) to the lower voltage pack as they approach a low or high SoC.
If you were to series these two batteries the second problem would be even worse- the lower capacity pack will start to bottom out (or top off in the case of charging) well before the larger one and I think it would be likely you would see the internal BMS disconnect it during modest, normal use, even though the larger pack was far from empty/full.
You could reach out to each manufacturer and get their take on the issue if in doubt... but I don't think it's likely they'd sign off on it. Its questionable to do this with identical batteries, let alone ones with vastly different capacities.