HS-BPHP550-2 - Ni-MH, 2.4 Volt, 700 mAh, Ultra Hi-Capacity Battery - Replacement Battery for SONY MDR-RF800/900 Cordless Phone Battery Replacement Battery for SONY MDR-RF800/900 - Ni-MH, 2.4 Volt, 700 mAh. 100% compatible with your OEM battery - 3 year warranty. Made with high quality Japanese cells. Synergy Digital is a Registered Trademark