How old was the UPS ?

When did you last check it's battery runtime ?

When did you last replace the battery ?

What was the total load connected to the UPS ?

APC UPSs are no worse and somewhat better than the rest of the lot.

Failure is one I have seen before. Charging circut overheats because
of a failed shorted battery cell.

As others have mentioned don't buy the low capacity bottom feeders.

Hugh