UPS Battery

I used to live in a neighborhood with elderly power and phone infrastructure. The 120V, 60 Hz main power fluctuated during spring and summer. I bought an APC Back UPS 1500 uninterruptible power supply in early June, 2020 to get whatever server I was using back then through the inevitable series of short power losses.

promotional image of the uninterruptible power supply case

The Back UPS 1500 has a weird USB-to-RJ45 cable that can connect a computer to the UPS. If you install apcupsd, your Linux computer can shut down itself and its software in some orderly fashion to prevent problems with files and on-disk file system.

The Back UPS 1500 has mostly been invisible. Occasionally, I look at its front panel to see what the input voltage to the house is (varies a few volts, from 120 to 123), and how many power fluctuation “events” it has detected. There have been more than I thought. Apparently the mains power is less reliable than the odd server reboot indicates.

March, 2026, I noticed that the Back UPS 1500 was beeping. There’s not a lot on the front panel of the 1500, but I did manage to shut off the semi-annoying beeping.

I had a vague recollection that a daemon process monitored the USB input. I discovered that the daemon process is named apcupsd, and that it made a journal entry related to the beeping:

$ journalctl -u apcupsd --since '1 day ago'
...
Mar 19 14:07:24 monarch apcupsd[1123]: UPS Self Test switch to battery.
Mar 19 14:07:32 monarch apcupsd[1123]: UPS battery must be replaced.
Mar 19 14:07:32 monarch apcupsd[1123]: UPS Self Test completed: Warning
...

That was it for alarming journal entries from apcupsd for a while. Normal journal entries like this appeared:

Apr 30 12:38:05 monarch apcupsd[1122]: UPS Self Test switch to battery.
Apr 30 12:38:14 monarch apcupsd[1122]: UPS Self Test completed: Battery OK

On June 11, the beeping started again. Triplets of lines like this appeared in the journal for unit apcupsd:

Jun 11 10:27:04 monarch apcupsd[964]: UPS Self Test switch to battery.
Jun 11 10:27:11 monarch apcupsd[964]: UPS battery must be replaced.
Jun 11 10:27:12 monarch apcupsd[964]: UPS Self Test completed: Warning

I tried to quell the beeping to no avail. I could press the leftmost button on the Back UPS 1500’s front panel, but the beeping would stop for only a few hours. I ultimately decided to replace the battery at $99.99 instead of a new UPS - the Back UPS 1500 is $190 right now, other models are more expensive.

Changing the batter is really easy. You don’t even have to unscrew an access cover, you push on two clips and slide off the cover, then slide in the battery.

The config file for apcupsd is /etc/apcupsd/apcupsd.conf. One interesting value in it is SELFTEST 336, two weeks between self tests. Running journalctl -u apcupsd does show self tests every two weeks.

Running apcaccess shows some interesting information about the UPS itself, and its battery.