We do sell battery replacement cartridges for the BR1500LCD
This may be a better link
It's about $80 and includes pre paid shipping of your old batteries back to us. However if the batteries fail within the 2 year warranty then they will be replaced (the whole unit) under said warranty.
Not that both the APC and Cyber units are step approximated square waves when on battery and I do not recommend using any type of surge protector to extend the number of outlets, only a PDU as mentioned above.
Normal battery life expectancy is 3-5 years (all usp systems) where 3 years if you have the unit placed in a room or closet with no ventilation, high temperatures and constantly switching back and forth to battery.
Disclaimer, I work for APC