This was originally posted on APC forums on 5/21/2017
If you have access to a DC volt meter, remove the batteries from the unit and check their voltage. As a pack, you should measure around 48 volts DC. Anything lower than that won't be enough for proper operation.
(It's possible for the battery voltage to measure normally under the very light load presented by a volt meter. I strongly suspect bad batteries are this unit's only problem, especially if they are more than five years old.)