A system for charging a small-capacity battery from a large-capacity battery. The large-capacity battery subsystem and the small-capacity battery subsystem cooperate with each other in different stages of a charging cycle of the small-capacity battery. The large-capacity battery subsystem includes a large-capacity battery, a large-capacity battery charging circuit and a constant-voltage and constant-current circuit which generates either a constant voltage or a constant current. The small-capacity battery subsystem includes a small-capacity battery and a linear charging circuit which uses the constant voltage or the constant current to charge the small-capacity battery. The charging cycle of the small-capacity battery includes a CC stage and a CV stage. In the CC stage, the linear charging circuit meets the ideal diode characteristics and outputs a constant current. In the CV stage, the linear charging circuit meets the linear charging characteristics and outputs a constant voltage.