Method for charging lithium ion secondary battery and battery charger

A battery charger comprising:

• a power conversion unit;
• a charge control switch and a discharge control switch each connected to the power conversion unit in series;
• a control circuit configured to control output of the power conversion unit;
• and
• a temperature detection unit detecting a temperature of an environment in which a secondary battery is used,
• wherein:
• the control circuit comprises:
• a current-voltage switching unit;
• and
• a temperature control switching unit,
• the temperature control switching unit is configured to output signals for supplying output of the current-voltage switching unit to the power conversion unit in accordance with the temperature,
• the signals comprise:
• a first signal for permitting constant current charge, constant voltage charge, and discharge, when the temperature is lower than T1;
• a second signal for prohibiting constant voltage charge and permitting constant current charge and discharge, when the temperature is T1 or higher and lower than T2, T2 being higher than T1;
• a third signal for prohibiting constant voltage charge and constant current charge and permitting discharge, when the temperature is T2 or higher and lower than T3, T3 being higher than T2;
• and
• a fourth signal for prohibiting constant voltage charge, constant current charge, and discharge, when the temperature is higher than T3, and
• the control circuit is configured to determine whether charging in accordance with the temperature, and to shift from constant current charge to constant voltage charge when a voltage of the secondary battery reaches a given value during charging.