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I have a 48 volts APC UPS (2200va/1980watts) attached to 4 65AH AGM batteries. A week ago after cleaning batteries and terminals, I accidentally put one battery in wrong direction. I turned the UPS on but it gave errors and showed red light and made a long beep noise. So I turned it off and noticed that one of the batteries was connected in wrong direction. I didnt have the tools near me, so 20 mins later I disconnected the 4th battery, put it in correct direction and turned it on. UPS turned on fine this time and has been running fine since.

But, Now the 4th battery voltage drops very fast down to 5-6 volts while other 3 are at 12.5 volts.

I would like to know, the batteries remained connected the wrong way for about 20 mins, what damage would this have caused to the 4th battery or may be all batteries.

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Sorry to be the bearer of bad news.

As you suspect, the one battery that was connected backwards has been damaged and needs to be replaced.

For the other batteries, the situation isn't as obvious. If the charge voltage was applied over three batteries instead of four, it could have overcharged them all. If your batteries were standard lead-acid or SLA (sealed lead-acid), then you should open the caps and check to make sure the fluid levels are correct. Since you are using AGM, however, they are supposed to stay sealed.

Assuming that these three are fine, there still may be a problem trying to match a new, fourth battery. If the new one has slightly less capacity, it will become depleted first, and may become damaged if the system gets severely discharged. If it has slightly more, it can cause a similar problem with the other three, although it will be more forgiving.

Here's what I would do:

  1. Get a new battery from the same manufacturer.

  2. If they were SLA's or standard "flooded" lead-acid types, check the fluid levels on the three "good" batteries. (don't do this for your AGM's!)

  3. Using some other charger, charge each battery individually .

  4. Allow them to sit for 12 hours or so to stabilize.

  5. Measure their voltages, and make sure they're all close. Remember, the charger had brought them all to the same level.

  6. If all looks good, go for it!

After allowing the system to discharge a bit, check the individual voltages again an make sure nothing is getting severely unbalanced. Worst case, you may have to buy four matching batteries.

Good luck :)

P.S. I had originally stated that you could top off the levels in an AGM battery, just like a standard flooded battery. I was incorrect: AGMs shouldn't have loose liquid sloshing around. At an earlier company, we had, in fact, done this to a compromised AGM. It worked, so we though it was the correct thing to do. It turns out that we basically converted it into a poorly-designed SLA :)

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  • \$\begingroup\$ in an AGM battery, the electrolyte is absorbed in a glass mat - there should be no visible fluid in the battery. Only conventional "flooded" batteries have visible fluid in the cells, and the cell caps should be easily removable for routine checks/topping up. \$\endgroup\$ – Peter Bennett Apr 30 '14 at 15:59
  • \$\begingroup\$ Thanks, @PeterBennett. We had topped off a bad AGM in the past, and it recovered, so I though it was OK. I did some research after your comment, and you are correct. I've modified the answer... \$\endgroup\$ – bitsmack Apr 30 '14 at 16:53
  • \$\begingroup\$ Thankyou for the prompt reply. Now I need to know if I can add a flooded battery of the same capacity. I was using 65AH AGM batteries, now one of them has gone bad so can I use a 70AH flooded battery. The same AGM is not available in the market, even if it were, it would cost me twice as compared to the flooded. I need to know, is it safe to have 3 AGMs and the fourth battery flooded in a 48v UPS setup. Thanks again, much appreciated! \$\endgroup\$ – Atif May 19 '14 at 18:14
  • \$\begingroup\$ @Lafanter I believe that they need to be well matched, in both capacity and charging characteristics. If you add a battery that has greater capacity, it won't get topped off as early as the rest. So, the rest will become overcharged before the system reaches the nominal float voltage. Sadly, I think you're going to have to get four new batteries at this point. You may want to post another question here on the board and see if anybody agrees or disagrees with what I'm saying, just in case... Good luck! \$\endgroup\$ – bitsmack May 19 '14 at 21:56
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I believe the only damaged battery is the fourth: if you think of a battery as an ideal voltage supply the UPS went mad because with the first configuration you only had 24V=(12+12+12-12)V at its inputs that probably is below the minimum acceptable voltage.

Current has flown in the wrong way in the reversed battery, that is not actually a problem, when you charge a battery current is flowing in the opposite direction. But it may be a problem if the current is too high, and that might have happened since you left it sitting for 20 minutes (!). The missing piece of the puzzle is what voltage did drop between the UPS's inputs, but I really can't guess that.

You should be fine just changing the damaged one, and please be really careful and have your tools with you since these batteries can even explode if not connected properly.

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