A secondary power source system, includes a first unit receiving a primary power input and restricting a current used for charging to a predetermined amount, a second unit including a device providing capacitance, receiving a first output from the first unit with restricted current, a third unit generating a second output of a certain voltage, and a fourth unit performing a logical OR operation with the primary power input, first output from the first unit and second output from the second unit, to generate a single third output of a certain voltage.