An uninterruptible power supply comprises a power supply unit for generating DC power at a predetermined voltage from AC power supplied from the outside to supply the DC power to an electronic device, and a rechargeable battery unit including rechargeable battery cells for storing the power supplied thereto from the power supply unit for supplying the electronic device with the power stored in the rechargeable battery upon service interruption of the AC power. Particularly, the rechargeable battery unit comprises a battery state monitoring unit for monitoring a state of the rechargeable battery cells, and communicating means for notifying the electronic device of information indicative of the state of the rechargeable battery detected by the battery state monitoring unit, thereby securely guaranteeing the operation of the electronic device upon power failure, and sufficiently improve the operational reliability of the entire system.