A week ago I got the following error on my APC Smart-UPS 1000 which I muted.
Warning State:
Connect battery
Load: 55%
Batt: 100%
Today, I could smell a sort of sulfur/sulphur/rotten egg smell when I came into the office and the UPS is alarming again. There isn't a burning smell.
I have vented the office & server room and shutdown the UPS.
Got any other advice?
To answer the question: This is almost always a lead-acid battery failure causing the battery to vent hydrogen sulfide (H2S). The battery needs to be replaced as soon as possible.
As an additional note, H2S can be extremely dangerous at higher concentrations. If you experience eye irritation or difficulty breathing or your ability to smell the odor deteriorates noticeably, the concentration of the gas is dangerously high and you should see a doctor. At that point, you may need to hire a hazmat cleanup service to remove the battery and clean up the area.
Wikipedia says this on H2S toxicity:
It sounds like one of your lead-acid batteries has gone bad and is now leaking hydrogen sulfide (H2S) into the air.
You will obviously want to replace the battery, but I would think some cleaning may also be in order as both hydrogen sulfide and the sulfuric acid normally present in the battery are corrosive. (Do consider that hydrogen sulfide is also toxic.)