Upon hauling home a failed APC Back-UPS RS 1500VA (BR1500I) and cracking it open in the shop, I was greeted by a new sort of battery failure:
Yes, the plastic casing on one of the batteries has cracked – and it isn’t even just a minor crack; about 80% of the end wall of the cell can be bent up with a screwdriver.
Amazingly, the cell is still working. The battery was charged at a decent 12,6V when I measured it. The ESR of the cell, however, is not as healthy. It would barely power the UPS when idle.
I very much doubt that this has anything to do with the UPS itself, it seems far more likely to be a manufacturing defect on the battery. The UPS charges the batteries normally, and the other battery in the pack shows no signs of abnormal wear, not even swelling.
I think this is a quite serious issue indeed. If a consumer was to remove the battery pack, oblivious to the potentially corrosive contents of the battery cells, someone could get seriously hurt.