I wonder, they don't seem to put protection / monitoring circuitry in on the per-cell level (which would be prohibitively expensive). How do you know when a cel

I wonder, they don't seem to put protection / monitoring circuitry in on the per-cell level (which would be prohibitively expensive). How do you know when a cell or a block of cells go bad? Or does it not matter? I think it could be bad if the resistance of one increases and you have a bunch of them in series...