Cartridge #2 (RBC2). Battery Type: Sealed Lead-Acid; Battery Size: RBC2; For Use With: APC® BE500U, BK200, BK250B, BK250I, BK280B, BK300C, BK300X116, BK350, BK400, BK400B, BK500, BK500MC, BP280, BP280C, BP280IPNP, BP280PNP, BP280S, BP280SX116, BP420, BP42