Is reported battery voltage supposed to sometimes drop during charging?

[Anonymous user]

Updated summary: Something is definitely wrong with UPS voltage measuring, but charging seems fine. Actual voltage at the battery was 13.60V with the UPS off, and after turning it on and the self test it stays around 13.56V. That's a reasonable float voltage.

The voltage reported by the UPS via USB is much lower, around 12.8V. HWMonitor in Windows updates it a lot more often than apcupsd and shows a lot more fluctuation. It has gone from 11.05 to 13.10 while actual battery voltage was 13.55 to 13.56. Also, even though it had reached 100% and finished charging, the level is now dropping and at 77%. APC PowerChute Personal Edition 3.0.2 also reports 77%. Even if I don't believe the 3rd party software and the voltage it reports, I have enough evidence that something is wrong based on the percentage behaviour.

The problem was due to the power switch developing high resistance. I removed it from the board, disassembled it, cleaned it and put it back, and now everything seems fine. I wrote about it on my blog. Does anyone know a part number for a replacement in case I end up needing that?

Original post: I have a Back-UPS  ES 500 BE500U-CN and am monitoring battery voltage via apcupsd  3.14.12 in Linux. During charging, the reported voltage sometimes drops by one volt or more. It just dropped from 13.1 to 12.1 and then went back up to 13.0. At the same time the battery percentage dropped from 72% to 71%. I've seen the voltage drop much lower during charging, one time to 11.1.

Reported voltage is not fluctuating constantly. It stays constant for many minutes or occasionally rises by 0.1V, then suddenly drops and rises to its former level within a few minutes.

Such large drops seem weird. I can't even imagine how voltage could have truly dropped to 11.1 because that would require a large load. The inverter wasn't running, and voltage wouldn't even suddenly drop that low when powering my PC.

Is this normal or is the UPS malfunctioning? Is this some kind of load testing the UPS performs on the battery or inaccuracy in voltage reporting?

I was seeing similar fluctuations with the old battery, and it even sometimes reported 0% battery level even though there had been no power outage. The old battery was many years old and certainly worn out, so I thought that may be the cause. Now I'm seeing these fluctuations with the new battery, and wonder if the UPS is defective.

HWMonitor in Windows shows the same voltage drops as apcupsd. I don't recommend HWMonitor  though, because it causes blue screen Windows crashes. PowerChute Personal Edition only shows percentage and does not show battery voltage.

I just want to make sure the UPS is charging the battery properly, so it doesn't damage the new battery.