Has there been a water leak recently?
If any water has entered the battery cabinets it can cause some of the battery fuses to trip or cause batteries to overheat, and fail, giving off rotten egg smell gas.
The UPS may continue working although it is working much harder trying to compensate for the failed batteries.
This will cause the UPS to overheat and again is a FIRE risk and Health & Safety risk.
ACT QUICKLY – Call your local Power Continuity UPS engineer or Get a new one
https://batterytools.net/
If your Power Continuity UPS engineer has already advised replacement of batteries – DO NOT DELAY – Action the advise