Mountain View, CA
reply to PeeWee
Regardless of what any "status LEDs" show, try taking the batteries out of the system and examining them for any kind of bulging (sides, top, bottom, etc.).
I myself just replaced two TrippLite UPSes of my own (not SMARTxxxSLT models, lower-end stuff) due to them mysteriously kicking over to battery power for no reason. They worked great for about 4 years. I ended up debunking that situation by purchasing a new UPS (see my profile for what brand/model) and hooking up the TrippLite "behind" that, with no devices hooked up to it, and waited. After a few days the TrippLite units (at separate times) would kick on battery power for a few seconds then kick back onto AC power -- while the UPS providing AC power showed no AC power loss whatsoever. The things just went bad from what I can tell.
At my previous workplace we had a high-end (datacenter-grade) APC UPS with 11 external batteries linked/chained together for a multi-workstation NOC. So that's 12 batteries total.
After many years our UPS began to kick its fans on (very loud) on occasion. After 3-4 months of this (because nobody ever cares to do maintenance on things these days, sigh), the situation worsened when the area where the UPS + batteries were began to emit a high-pitch squealing noise that lasted 3-4 seconds, followed immediately by a very strong, nauseous smell which made a lot of people sick (of which I was one). I know it sounds funny, but it sounded like a fart. Once things got to that point I took matters into my own hands.
I examined the UPS + batteries found that of the 12 batteries we had, 5 were blazing hot to the touch (including the one which was inside the UPS). All 5 required using use of a prybar to bend the steel housing just to get the batteries out (due massive bulging), and one battery had what appeared to be a hole in it of some sort (still not sure from what; there was no white acid leaking from it).
We ended up replacing the 5 batteries which were in this condition and the UPS no longer kicked on its fan. The fan issue was caused by what VegasMan stated -- heat. The heat was caused by the batteries -- they were hot even when they weren't being charged, or possibly they thought they needed to be charged 24x7x365, I don't know. I just know that they were preposterously hot even when the last power outage was months prior.
However, the high-pitch noise continued, as did the toxic smell shortly afterwards.
It wasn't until we replaced the UPS entirely (giving me a chance to open it up and look at it) where we found many of its internal capacitors were bulging. My theory was that the capacitors would hold a charge most of the time, but occasionally would (quite literally) leak electrolyte, which is highly toxic. Leaking caps often tend to emit a high-pitch noise.
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.