Battery pack

A battery pack (10) according to the present invention includes a battery (31), a control unit (32), a power supply circuit (33), and a supply source selection unit (32). The power supply circuit (33) is configured to receive electric power selectively from one of the battery (31) and a charger (20), and generate a voltage for driving the control unit (32). The supply source selection unit (32) is configured to select the charger (20) as a power supply source to the power supply circuit (33) when the battery (31) is in a state of being able to supply electric power to the power supply circuit (33), the battery pack (10) is attached to the charger (20) and it is possible to supply electric power to the power supply circuit (33) also from the charger (20).