Comment by freedomben

I agree, this is a pretty bad idea. I've used batteries in parallel many times, but it's always in the context of treating it as a single "bank" that is always charged at the same level, not as individual batteries which can be swapped separately. Swapping a full battery in when the other one is low is going to result in a rapid rebalance of power until they get to the same level!

You can think of it like having two big swimming pools side by side, slowly draining out into a little stream. If the two pools are at the same level and the wall between them disappeared, they're largely going to be fine and keep on going.

Now imagine one pool is nearly empty and the other is full. As soon as you disappear that wall in the middle, there's gonna be a very powerful wave as the water from the high pool rushes in to fill the empty pool.