How to tell if your UPS is shot
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.