I was working in the data room, and something suddenly started beeping. My ears took me to one of my big UPSes. It soon stopped beeping, but there's a red light showing, indicating battery failure. They haven't actually failed, but my label on the UPS tells me that I last put in new batteries in 2011, so they're four years old, and these batteries last three to five years.
So I've ordered a replacement set of batteries, which should arrive soon.
I also thought a bit about which of my servers actually need to be on a UPS and which don't - this led me to powering down two servers, and putting two others onto non-UPS power.
And I've done a census; I have a non-working APC 3000 (last time I powered it up after putting in batteries, there was a flash and a bang), three non-working BPC UPSes and severs smaller units that don't work. One of more of the smaller units might be repairable, so I've ordered batteries for one of them.
A conclusion I draw from this, is that APC 3000s are more reliable than others I've used in the past. Of course, they all need new batteries every 3-5 years.