Last night I came home to the scent of warm electronics. It wasn't bad enough to make me worry, so I didn't investigate for a bit. Later, the smell got worse. It was a buring chemical smell. I followed it to the room with my Dell workstation and found the scent to be quite near it. That's when I started to notice a hissing/fizz/buzz coming from the APC SmartUPS 1000 on a shelf underneath it. I had set an old junker laptop on top of it so I picked it up and noticed it was quite hot underneath. There were no vents on top to block, so I knew the laptop wasn't the issue. I checked the UPS and it was VERY hot in the front where the batteries are. I shut down the Dell, unplugged all the stuff the UPS ran (DSL modem, WiFi/enet router, Dell, LCD) and removed the UPS. It was very hot to handle in the front. I opened the front panel and unscrewed the inner plate to reveal two Pb-Acid batteries that were blowing gas and deforming their cases. They were so hot I couldn't do much other than take the UPS outside in the cold air to be left overnight. I'm not sure why the thermal regulator didn't blow. I think that's one right above the batteries in the photo. You can also see they tops are warped up, and if you look at the APC sticker on the battery you can tell it's warped outward too.
I'm just glad we came home and I caught it before I went to bed. I think it might have continued getting hotter and maybe even have melted something and leaked acid or started a fire.
When was the last time the UPS was used (as in, actively powering the kit attached to it, like in a power failure)? The circuit that cuts the power to the batteries when they're fully charged may have malfunctioned and the batteries were on constant charge. The smell was probably due somehow to the evaporating of the electrolyte from the batteries, but I could be wrong.
You can probably replace the batteries. By the looks of them, they look to be standard 6 or 12v Pb-Acid batteries and should be available at most electronic supply stores. I know I have several of them at home that I use when testing vehicular electrical kit in the basement. Although, you may not want to replace the batteries unless you can find out what went wrong and caused this situation in the first place.
Cheers,
The Czar
This "puffy-battery" thing happened to one of the APC 1400 2U rackmount systems at the office a few months ago. It was indicating a "bad battery", which isn't that unusual as they get older, so I tried yanking the hot-swappable battery drawer out so I could replace it with one from a UPS that had less "mission critical" equipment attached to it. Upon doing so I discovered that the batteries had swollen up so much I couldn't remove the drawer, as there was no longer sufficient clearance on the front benzel.
It was annoying to have to shut down the attached servers and swap the whole unit, but there was some amusement to be had in dissecting it later.