Method of charging storage battery in power supply system having another battery of larger capacity

A power supply system for applying an output current to a separate load, the load current to the load being under the control of a first chopper circuit, the system comprising:

• a first battery;
• a second battery connected in parallel with the first battery, the second battery being higher in open-circuit voltage and larger in capacity but lower in maximum output than the first battery;
• a charging circuit for applying a charging current from the second battery to the first battery;
• a second chopper circuit arranged in the charging circuit;
• and
• means for controlling the operation of the second chopper circuit such that the second chopper circuit permits the charging current to flow therethrough continuously and without interruption when residual capacity of the first battery decreases to a first predetermined value, but periodically interrupts the charging current when the residual capacity increases to a second predetermined value, the interruption of the charging current being in such a manner that the amount of energy charged to the first battery in each cycle of interruption actions is close to but less than the amount of energy discharged from the first battery in each cycle of interruption actions of the first chopper circuits, thereby to allow the residual capacity to decrease gradually to the first predetermined value.