A backup power supply device is provided with: first to fifth battery packs connected in parallel; a charger that supplies a charge current to each battery pack; first to fifth charge switches that individually connect and disconnect charge paths of the respective battery packs; and a controller that controls each charge switch. The controller divides the battery packs into a plurality of groups and performs pulse-width modulation (PWM) control of each charge switch at a duty ratio corresponding to the number of batteries in each group to charge the battery packs for each group.