Mine's a very old APC also
Mine's an APC, about 18 years old, 800kVA, enough to still keep a modern PC and its monitor alive (thank god for LCDs) for several minutes and an comfortable orderly shutdown.
I've had to change batteries twice, at 8 and 16 years, 50 EUR a pop, but why change a good thing?
Takes bog standard house alarm batteries, can find them anywhere.
Only grudge is that APC dropped support for the serial interface very quickly (in the mid-late 2000s, I think it was with Win2K) so no supervision from the PC.