[dattaway]

Lead based batteries often have a usable life of 3-5 years. Chances are, the others of that vintage are already dry and have already failed. Then they will rupture, often with smoke as their series connected brothers try to push electrons.

[micro-ram]

I have seen plenty of UPS batteries swell up so big they can't be removed without disassembly. The only indication was the failed self test. OP did say it was the UPS in the rack next to his production DB.

Don't forget Capacitor Plague. I still see it regularly.

https://en.wikipedia.org/wiki/Capacitor_plague

Have a plan, be safe.