Mark Fortune wrote:

If I remember correctly, APC units do tend to do this during a
calibration test, and it takes a while too, although I cant remember
exactly how long. I noticed this problem too when I installed an apc
unit and spent a few hours trying to get hold of apc to find out that
this was indeed normal calibration. It should recharge again after it
hits 0%. I wouldnt count on it for backup power purposes until it
finishes its calibration though.

Oh and by the way, if you disconnect the power to the UPS during the
calibration, I seem to recall it will charge the battery to full and
restart the calibration.