What are the signs a UPS battery is failing?

Runtime shorter than expected on test or on a real event; individual monobloc voltages diverging from the string average; rising internal impedance against the baseline taken at installation; case swelling, terminal corrosion, cracking or any sign of electrolyte leakage; and a battery noticeably warmer than its neighbours. The last of these deserves urgency rather than a note in the log, a hot battery on float is a thermal runaway precursor, and AusNet lists thermal runaway from excessive charging among the failure modes observed across its fleet.