UPS turned off one night and it seemed that the 3 year old batteries gave out.

Replaced with a new pair, but the UPS software claimed the battery voltage was only 18 volts and needed to charge. After 24 hours, the UPS claimed only 60% charge and 20 volts (although FWIW, my meter sees 27v).

...then I found various Cyberpower threads here with some guidance on what to check.

C1 (22uF) tested at 2, so I replaced that.

Put it all back together and didn't see a change on the reported battery voltage. Although, I did run the quick battery diagnostic...