In certain circumstances some APC UPS systems will continue to display a low battery warning even after the battery has been replaced. Often APC's Powerchute software (if in use) will automatically perform a Runtime Calibration to resolve this but in some cases, particularly when the replaced battery was in extremely poor condition, this warning will only disappear after a Manual Runtime Calibration has been carried out.

The procedure recommended by APC to perform a Manual Runtime Recalibration is as follows:

This is a manual procedure and should not be confused with the runtime calibration performed through PowerChute software. The batteries inside of the Smart-UPS are controlled by a microprocessor within the UPS. Sometimes it is necessary to reset this microprocessor, especially after the installation of new batteries.

Before beginning the calibration make sure the UPS has been online and charging for at leat 24 hours for just the UPS or 48 hours for a UPS with external batteries.

1. Stop the PowerChute software from running and disconnect the serial cable.
2. There must be at least a 30% load attached to the UPS during this procedure, and this load cannot fluctuate more than +/- 5%. This process will cause the UPS to shut off and cut power to its outlets, therefore, attach a non-critical load to the UPS and then force the UPS on battery by disconnecting it from mains power.
3.  Allow the unit to run on battery until it turns off completely. Make sure a 30% load or greater is present!
4.  Plug the UPS back into the wall outlet and allow it to recharge (it will recharge more quickly turned off and with no load present). Once the unit has recharged, the "runtime remaining" calculation should be more accurate. Remember that if the unit is an older model, then the runtime will not improve significantly.

APC offer additional guidance on this problem on their website here: http://nam-en.apc.com/cgi-bin/nam_en.cfg/php/enduser/std_adp.php?p_faqid=1497