An uninterruptible power supply is disclosed in which the backup energy source is connected to the power supply of the protected computer by means of a bi-directional power converter. This converter operates without sensing circuitry, relays, or switches, operating continuously when a load is present in either charging or discharging the backup energy source. This uninterruptible power supply can be placed within the available space of a functioning standard computer power supply enclosure, and may be configured so that it may be replaced during normal computer operation (i.e. it is "hot swappable"). Methods are described to interface the uninterruptible power supply to the computer, providing mutual control and communication. The device may be also used to power devices requiring external DC power, which are commonly found in association with computer operations.