just picked up two nice rack-mount APC UPS units, free.
One seems to work fine, the other one has the ‘battery fail’ light on red. Tried swapping battery pack to other UPS, it too lit the battery-fail light.
The battery pack tray consists of four 12V gel cell batteries, two in series, two in parallel (24V output).
The good pack measured around 23V output, but the bad pack measured around 21V.
Each of the individual batteries in the bad pack measured in the 11V range. Neither pack has been charged.
I would assume that the UPS is looking at more than just voltage?
Internal resistance of the battery?
Or does it just try to put a load on the battery, and if the load drops, it lights the red light?
