One of my Back-UPS 550 units started emitting an annoying continuous sound in the middle of the night a couple of weeks ago. Unplugging it did not silence it and I had to disconnect the battery to keep it quiet.
In the morning, I checked my records and it turned out that the battery in this unit was replaced about a year ago, which was surprising. Nevertheless, I had a spare battery just for such occasion and I thought I won't hear this UPS again for a while, but a day later same same sound came to haunt me again.
Soft power switch
It was apparent that the problem was in the Back-UPS itself, so purchased a new one. Curious about the two relatively new batteries it reported as bad, I put the APC-branded battery from the old UPS unit into the new one and it started emitting the same continuous sound right away, with the green light half-lit.
Turns out that the power switch in these new Back-UPS 550 units is not a physical switch, but it rather flips some relay inside the unit, so I had to put the new battery in, press the power button to turn the UPS off, then replace the battery and only then plug it in, without powering it up, to keep it quiet and, hopefully, charge the battery.
We'll top it up, but that's it
I kept the UPS plugged in with the previous APC-branded battery for a few hours and it started emitting the same sound once I turned it on. Doing the same with the new battery I got on New Egg produced the same effect. Even keeping the UPS turned on with the nagging sound for a few hours didn't help. I measured voltage in each battery, and it was 5V and 7V - it seemed these batteries were gone.
Throwing away two batteries I got in the last 2-3 years seemed like a waste of money, but there wasn't much I could do, so I brought both to the pile of old batteries in the garage. While there, I recalled that I had a 12-volt trickle charger for my car battery and seeing how Back-UPS 550 also uses a 12-volt battery, I decided to give it a try.
I wasn't sure how this whole setup will behave, so I set it up in a safe place and plugged it all in. Nothing blew up and all the right lights lit up, so I left it like this for a few hours.
After this, I put the battery into the new UPS unit and, just as if by magic, it started working. I repeated the exercise with the second battery and it was brought back to life just like the first one.
I put the recovered APC-branded UPS battery into the new UPS unit and plugged in all of the usual stuff. It has been working now for a couple of weeks, with a few brown-outs in between, holding up as it should.
Out of curiosity, I tried putting a revived battery into the old UPS and it turned on just fine, but after a few hours it started emitting the same sound. It appears that its charging circuits were fried, so it discharged the battery for as long as it could and then reported this battery as a goner.
Conclusion
Electronics do go bad from time to time. This is expected and I cannot complain about the broken Back-UPS 550, as it served me well for a few years. What I do find strange, however, is that the new UPS unit wouldn't charge a functioning discharged battery, even when the external outlets are powered down. Nobody expects it to maintain the output voltage while the battery is badly discharged, but to indicate that it should be thrown away just shows poor design.
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