Batteries in the UPS typically last 3 to 5 years and it depends upon several factors. Including the number of times the unit must go on battery power and environmental conditions. There are usually several batteries in the UPS and while the battery voltage may show 13 volts, this may only be a float charge/voltage and a true indication of the battery voltage and condition requires the batteries to be tested under a load. If one battery is faulty in a set of batteries it will causes the whole battery system to fail and indicate a battery fault.
A faulty charging circuit will not charge the batteries and will also cause a fault condition.