Patent ID: 8237404

Claim:
An electric tool comprising: an electric motor; a switch that is connected to the electric motor in series; a battery pack, including a plurality of lithium battery cells, that serves as a driving source for the electric motor and includes a protection means for monitoring each voltage of each lithium battery cell and current flowing to each lithium battery cell, and for outputting a signal indicating one of an overcurrent situation and an overdischarge situation of the lithium battery cells when at least one of the lithium battery cells is in one of the overcurrent situation and the overdischarge situation; and an adaptor attached on the battery pack to be located between the electric motor and the battery pack, the adaptor including: a first terminal electrically connectable to the battery pack; a second terminal electrically connectable to the electric motor; a third terminal for receiving the signal from the protection means of the battery pack; an effective voltage converting unit for converting an effective voltage inputted from the first terminal and outputting the voltage from the second terminal; a cutting-off unit for cutting off a closed circuit between the electric motor and the battery pack; a switching control unit for controlling a switching operation of the switching unit by changing a switching frequency and a switching duty of the switching unit; a constant-voltage unit for supplying a constant voltage to the switching control unit; a power supply unit for supplying a power voltage to the constant-voltage unit in accordance with an operation of a switch provided in the electric tool; and a power supply maintaining unit, to which the constant voltage is supplied from the constant-voltage unit, for maintaining the supply of the power voltage from the power supply unit, wherein: the cutting-off unit includes a switching unit that is connected between the first terminal and the second terminal and is configured to be turned off for cutting off the closed circuit when the cutting-off unit receives the signal indicating said one of the overcurrent situation and the overdischarge situation of the lithium battery cells via the third terminal, the power supply maintaining unit monitors the supply of the power voltage from the power supply unit and cuts off the power supply unit depending on whether the electric tool is connected to the second terminal or not, and the power supply maintaining unit counts a power supply time of the power supply unit and cuts off the power supply unit after a predetermined time.