A battery pack having a secondary battery; a measurement unit measuring a terminal voltage and a current of the secondary battery; a protection unit preventing the secondary battery from being overcharged when the terminal voltage measured by the measurement unit is equal to or higher than an overcharge detection voltage; and a control unit receiving the terminal voltage and current measured by the measurement unit, the control unit having a first mode and a second mode in which current is less consumed than the first mode. A voltage in a range of from a full charge voltage to less than the overcharge detection voltage is defined as a voltage threshold. The control unit changes from the first mode to the second mode when a state in which a transition requirement is satisfied lasts for a predetermined period of time.