Abstract: A system for supplying uninterruptible d-c power to a load. When the system operates in its normal mode, d-c load power is provided by the combination of a rectifier and a power switch. The d-c output of the rectifier may be used by a d-c to d-c down converter to recharge and maintain a battery if a separately powered source is not available for that purpose. A d-c to d-c up converter provides a relatively high amplitude d-c voltage from the battery voltage. The amplitude of this voltage is lower than the amplitude of the d-c output of the rectifier in the normal mode. When the amplitude of the rectifier output falls below the d-c to d-c up converter output voltage, the system changes to its failure mode and the battery provides d-c power to the load. The operation of the down converter if it is included in the system is inhibited.