IN-VEHICLE BACKUP POWER SUPPLY DEVICE

An in-vehicle backup power supply device includes a battery unit having a plurality of unit batteries connected in series and a discharge circuit configured to perform a first discharge operation for supplying power to a third conductive path based on a charge accumulated in the battery unit, a balance circuit performs a cell balancing operation on the battery unit, and a control unit controls the balance circuit, and the balance circuit is performs a second discharge operation for supplying power to the third conductive path based on a charge accumulated in a plurality of power storage elements, and the control unit performs a first control for causing the balance circuit to perform the cell balancing operation and a second control for causing the balance circuit to perform the second discharge operation, and if a failure in which the first discharge operation cannot operate normally occurs, performs the second control.