Amazon.com: Synergy Digital Vacuum Cleaner Battery, Works with iRobot Roomba 531 Vacuum Cleaner, (Ni-MH, 14.4, 2800mAh) Ultra High Capacity Battery : Home & Kitchen
MaximalPower 2PK Ni-Mh 14.4v 3500mAh Power Tool Replacement Battery for iRobot Roomba 11702 80501 - Compatible with Roomba R3 500 600 700 800 900 Series 500 510 531 535 540 550 552 etc - Walmart.com
Roomba 531 Battery
Cameron Sino 4000mah Battery For Irobot Roomba 614,615,640,652,665,670,671,675,677,680,681,685,690,691,695,696,801,805,850,860 - Digital Batteries - AliExpress