Patent ID: 8098053

Claim:
A charger for a lithium ion secondary battery comprising: a series circuit of a transistor reverse-current preventive switch, a transistor charging switch and a current-detecting resistor, the reverse-current prevention switch being continuously directly connected to a negative electrode of a lithium ion secondary battery to be charged, and the current detecting resistor being connected between the charging switch and a ground; a rectifying circuit connected on a first side to a transformer and directly connected on a second side to a positive terminal of the lithium ion secondary battery; a charging control circuit configured to control said reverse-current preventive switch and said charging switch, to make said lithium ion secondary battery repeat charging and opening at regular intervals, to detect a voltage difference between a specified voltage which is an upper limit voltage of the lithium ion secondary battery and an open circuit voltage of said lithium ion secondary battery during said opening at a position directly between the reverse-current preventive switch and the charging switch, and to generate a change-over signal when the detected voltage difference becomes smaller than a predetermined change-over voltage difference; and a constant-current/constant-voltage control circuit configured to cause constant-current charging at a first set voltage which is set in a range not exceeding the upper limit voltage of said lithium ion secondary battery, when said voltage difference does not exceed the predetermined change-over voltage difference, and configured to cause constant-voltage charging at a second set voltage, which is lowered from said first set voltage based on the change-over signal received from the charging control circuit.