I have a APC Back-UPS 280 I use for my server at home, and since power rarely fails, it is barely used. It only keeps the battery charged. The battery is 12 V, 7 Ah.
After about 18 months it started signaling battery to be replaced and I'm surprised, since it is a very short time. A APC Smart-UPS I also have has been going well for no less than 3 years, that is what I expected from the other one as well. Maybe the battery is of lower quality on the Back-UPS (neither of them is original).
Is it possible that the early failure depends on lack of "priming"? Maybe I could discharge the weak battery at 1 A (0.15 C) down to 11.8 V and recharge it to 14.0 V two or three times with an external charger.
After reading the datasheet and the datasheet of a similar cell I found that the battery can stay continuously charged without the need of discharge cycles to maintain its capacity. The lack of discharge cycles was not the issue. The temperature inside the UPS could have caused a shorter life, but that much.
I opened the cap and I checked inside. I could see the gel but in one cell the gell appeared drier. I tried to add some liquid to that and to other cells and I connected a 2.5 A load to discharge the battery before recharging it. The initially drier cell was the only one gassing.
After flooding at this point all the cells (there was nothing to lose) I charged it with the UPS, to equalise the cells. When complete (5 hours, it was already mostly charged) I connected an even higher load (5 A) and now no gassing in any cell (as I said, they are all flooded now). It held 5 minutes without issues and with the expected voltage decrease as from the datasheet. The UPS now recognises the battery as good. I guess one cell was weaker and gassed during charge, resulting in drier gel and poorer performances.
I also made some ventilation holes in the case of the UPS.