APC - We don't charge batteries

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.