[zzz61831]

Deep discharge is a factor, but not a big one, heat has very little effect, it only affects voltages at which battery will be considered overcharged or deeply discharged as specs are usually all for room temperature.

I've used UPSes that discharge batteries no lower than 10.5 (no deep discharge) and only once in a couple of years and that charge them to and keep them at 13.6 volts under stable room temperature all year. All batteries lost most of the capacity after 3 years, two almost all of it, one had like 40% left. In five years only that one was still surviving short minutes outages, while initially was able to do a few hours. For comparison, I had one battery just laying around for 6 years in the same room without touching it and it lost only like 30%.

[Decade]

It turns out that APC intentionally overcharges the batteries to maximize runtime during the 2-year target battery lifetime, at the cost of killing the batteries relatively young. And the overcharged voltage drifts upwards over time.

I found this forum posting while searching for how to maximize the runtime on my APC UPS: https://forums.overclockers.com.au/threads/hacking-the-newer...