APC Smart-UPS X 48V External Battery Pack Rack/Tower - Battery enclosure (rack-mountable) - 2 x battery - Lead Acid - 864 mAh - 2U - black - for P/N: AR3106, AR3106SP, SMX1000C, SMX1500RM2UC, SMX1500RM2UCNC, SMX750C, SMX750CNC