The APC Smart-UPS 1500 (SMT1500) contains two 12V 9Ah sealed VRLA batteries connected in series to produce 24V, which feeds the internal inverter circuit. Both batteries are housed together and replaced as a single unit using APC Replacement Battery Cartridge RBC7 for tower models or RBC109 for rack models, depending on the specific configuration. The replacement process is straightforward and does not require a technician. The battery door is located at the front of the unit and secured with a single screw. It is important to replace both batteries simultaneously as a matched pair rather than replacing only one. Using genuine APC replacement battery cartridges ensures correct fit, proper charging, and clears the Replace Battery alarm reliably.