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.


1 Answer 1


Even if you're not charging/discharging them these lead-acid batteries age and need to be replaced after some time.

You can discharge it and see if it still has some capacity left. If not or not enough, better replace it because when the power fails and you need it, it will not be able to provide the power needed.

  • \$\begingroup\$ I just saw this document sciencedirect.com/science/article/pii/S0378775303009340 explaining how concentration of the electrolyte and float voltage is related and how a imbalance can affect the rate of corrosion. I guess that the battery somehow either had a concentration too different from the expected value, or it was not meant to be matched to the UPS float voltage, resulting in much shorter life. I guess it was both (poor QA and not matched target value). \$\endgroup\$
    – FarO
    Nov 25, 2015 at 13:10
  • \$\begingroup\$ check my update on the matter \$\endgroup\$
    – FarO
    Nov 29, 2015 at 13:24

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