Amazon.com: Replacement for APC Back-UPS CS 500 BK500 Battery - Replacement UB1290 Universal Sealed Lead Acid Battery (12V, 9Ah, 9000mAh, F1 Terminal, AGM, SLA) - Includes Two F1 to F2 Terminal Adapters :
Amazon.com: Mighty Max Battery 12V 8Ah Compatible Battery for APC Back-UPS CS 500, BK500, BK500BLK : Health & Household
Apc Back-ups-cs Series Battery Backup Cs-500 300watts (CS500) for sale online | eBay