UPS batteries have a short life, 3-6 years on average. The usual indicator of a failed battery is a failed UPS self test. Till last evening, I had purchased a new battery immediately on a self-test fail, had it arrive next working day, installed the new one, and taken the old one to the recycling centre. No dramas, just a period of tension between old battery fail and new battery fitting, in case of power problems.
Last evening was special. Self test fail, new battery ordered, enter tension state as always. Then something new, horrendously nasty smell, and unusual irregular beeps from the UPS, not to mention the batter charge lights going off and coming on. So I turned off all the equipment, and found that the whole UPS casing around the battery was too hot to touch. I let it cool overnight, to discover the battery casing was severely mis-shapen. The previous evening the battery had got so hot it had started melting its casing. It is a good job the human olfactory sensors detected the problem, it seems the UPS internal sensors didn't have this as a use case.