I was given a few old APC UPSs the other day. mostly small ones 2 like 25-270 watt and a about 400 watter. all use 12V SLA cells small use 7ah large 11ah. tried to pull batteries and all were stuck in the device because of expansion. so to get them out had to tear apart hole unit. while in there I noticed C APC 1993. a few actually had cracks in the battery. one had lots of corrosion, and hard scale on case. put on protective gloves, glasses and soaked them in some water scale came off.
wondering what causes batteries to do that. age? charging too little, too much, or over discharge, freezing, salfatization on plates? like to know if it's electronic related so that I can try to change trimpots to prevent this. considering replacing battery if cheap enough, but don't want to kill another battery. tested inverter circuit and they seem to work. considering connecting a pair or all to an old car battery, then wiring output in parallel for more output.

 

Why do batteries age? Why does time only run in one direction?
Why does acid corrode stuff? Why does water run downhill?
Whatever the answers to these age old mysteries, do not connect the outputs of uninterruptable power supplies in parallel.