Patent ID: 7977916

Claim:
A battery pack comprising: a secondary cell; a selection unit for selecting either a first operation state having a first power consumption or a second operation state having a second power consumption larger than the first power consumption; a judging unit for judging whether the secondary cell is in a used state or an unused state; an elapsed time period comparing unit for comparing an elapsed time period with a predetermined time period, the elapsed time period being a time period over which a voltage of the secondary cell is not less than a first voltage threshold value and the secondary cell is in the unused state; and a control unit for controlling the selection unit based on a comparison output from the elapsed time period comparing unit, wherein the control unit controls the selection unit to switch from the first operation state to the second operation state when the elapsed time period is longer than the predetermined time period, and the control unit controls the selection unit to switch from the second operation state to the first operation state either (a) when judged as being in the used state by the judging unit while the second operation is selected, or (b) when the voltage of the secondary cell becomes equal to or less than a second voltage threshold value which is smaller than the first voltage threshold value while the second operation state is selected.