I'm also pretty sure it's batteries. What's likely happened is that you rarely use the UPS function of your UPS, and while it stays on trickle charge - sensors have something to report.
When you updated the firmware on your unit, charging was off and your already long-dead battery simply discharged to nil and could not charge any more, while sensor did exactly what it should - alert about battery replacement.
BTW, Powerchute battery capacity/health status is frequently glitchy if you rely on it for monitoring. I only use it on my APC to monitor my server's power usage, and that's about it.
Um, no. You replace batteries when they stop working or you don't find the capacity satisfactory anymore. You don't replace batteries because the manual tells you to after xyz years. That would be extremely dumb.
If you like toasty-hot batteries, or electrolyte leaking all over your stuff - then definitely
UPS batteries aren't car batteries, they don't just gradually lose capacity, they also have consequences like rapid drying-out, heating up even on trickle charge or during normal operation, and past certain point can die in a snap (e.g. go from 30-40% capacity to 0, or "mildly" blow up from overheating inside the UPS, or expand while still "technically" functioning). It's the same for both lead-acid and gel batteries, not because of the form factor or special design, but because they are always used in the least optimal or healthy way (e.g. fully charged all the time, until every once in a while it gets drained to the bone).
As a rule of thumb, I change batteries at work, in my personal office, and in my attic server rack as soon as warranty expires (2 years for my favorite brands). The one on my desk is switched as soon as it gets noticeable warmer to the touch (around 3 year mark, like a clock).
I'm more accustomed to maintaining my UPSes in good health, cause in my area there are frequent repairs on transformer stations or power lines, and occasional 1-2 hour power loss is nothing special. In civilized places you simply don't notice any issues until it's too big or too late. Still, I wouldn't push it past 4 years even for the fanciest most expensive batteries on the market.
Too bad I have no pics. If you haven't seen the consequences of overdue battery replacement, I had a really bad case at local webdev office. They forgot about batteries in their server cabinet, and some genius put it tight between toasty NAS/app server and Cisco switch. It worked like that for 5 years. At some point the poor thing overheated and expanded so hard, that I had to power off the entire rack and bend it in order to pull anything from it. Rackmount UPS chassis looked like it had a kilo of TNT explode in it, but after disassembly I pulled a pair of APC-branded batteries mis-formed into a volleyball, and everything around it (including poor switch) was drowned in electrolyte. Server chassis was bent to shit(motherboard still alive). Switch was barely working on its last breath. They've only noticed when some RJ45 outlets stopped working.
I tried that once, the batteries were about to start on fire and the UPS emergency shut down after a year or two.
Probably some crappy batteries. I always use offbrand, cause it's much cheaper and gives me leeway to increase capacity.
Few of our local companies (LogicPower and Matrix) sell decent re-branded chinese lead-acid 9Ah units at half the cost of 7Ah APC or third the cost of, let's say, Panasonic. Claimed capacity is real and I personally tested it on many occasions, and warranty is exactly like APC (2 years). Never had issues with those.
Saved my ass last year, when I lost power for 4+ hours and our guys were uploading few hundred gigs of measurement logs that day.
If you have access to other batteries, as long as they are 12V, you can use them to test your UPS. The Ah rating will not matter for testing purposes.
