Abstract: A reserve power supply for providing backup power to a computer system includes a first battery and a second battery which are switchably interconnected in series or parallel by a switch means. When the batteries are connected in parallel they may be charged by a primary power supply which normally drives the computer system. When the batteries are connected in series they provide power to the computer when the primary power supply fails. This association with the primary power supply and the computer system is effected by a coupling circuit of the reserve power supply. The reserve power supply further includes an indicator circuit which visually and audibly signals when the batteries are providing power to the computer system. The reserve power supply further includes an electrical circuit which connects the voltage from one of the batteries to a second voltage output of the primary power supply and a second voltage input of the computer system.