The APC Smart-UPS 900XL is a UPS system that requires a 12V 26Ah AGM replacement battery. The compatible Universal Power Group replacement is the UPG UB12260 12V 26AH T3 AGM Battery (UB12260), a 12V 26Ah AGM sealed lead-acid battery. It uses T3 terminals. This is a direct drop-in replacement for the APC Smart-UPS 900XL requiring no modification to the device.
