<p>A power supply (11) is energized by an external power source (10) and a battery (18), and supplies electric power to a load (17). The power supply includes a battery voltage detector (20) and a current detector (19) for detecting a discharge current (I19) from the battery (18). The remaining capacity (Tb in Fig. 3) of the battery (18) is detected in accordance with a change ( DELTA E in Fig. 3) in the battery voltage (E20) when the external power source (10) is free of power failure. The battery voltage change ( DELTA E) is defined as the difference between an initial voltage (V1 at t1) of the battery (18) and an active voltage (V1- DELTA E at t4) of the battery (18) obtained when the battery (18) is discharged by the detected discharge current (I19) for a predetermined period of time (Ta).</p>
