A power tool (13, 23, 33, 43, 100), comprising: a motor (138, 236, 338, 435); a first battery pack (11, 21, 31, 41) having a first battery; a second battery pack (12, 22, 32, 42) having a second battery connected in parallel with the first battery; a first battery receptacle for receipt of the first battery pack; a second battery receptacle for receipt of the second battery pack; a first switch device (131, 231, 333, 414) connected between the first battery and the motor so as to form a first electrical connection between the first battery and the motor or cut off the first electrical connection between the first battery and the motor; a second switch device (134, 223, 336, 424) connected between the second battery and the motor so as to form a second electrical connection between the second battery and the motor or cut off the second electrical connection between the second battery and the motor; a controller (139, 139a, 139b, 212, 222, 339a, 339b, 413, 423), characterized in that the controller is operable to: detect a first voltage drop across the first switch device from a respective high potential point to a respective low potential point, and control the first switch device to cut off the first electrical connection between the first battery and the motor when the first voltage drop is less than a first control voltage; detect a second voltage drop across the second switch device from a respective high potential point to a respective low potential point, and control the second switch device to cut off the electrical connection between the second battery and the motor when the second voltage drop is less than a second control voltage, detect a voltage of the first battery pack, and cut off the connection of the first battery pack and the motor when the voltage of the first battery pack is less than a first voltage threshold; and detect a voltage of the second battery pack, and cut off the connection of the second battery pack and the motor when the voltage of the second battery pack is less than a second voltage threshold. a motor (138, 236, 338, 435); a first battery pack (11, 21, 31, 41) having a first battery; a second battery pack (12, 22, 32, 42) having a second battery connected in parallel with the first battery; a first battery receptacle for receipt of the first battery pack; a second battery receptacle for receipt of the second battery pack; a first switch device (131, 231, 333, 414) connected between the first battery and the motor so as to form a first electrical connection between the first battery and the motor or cut off the first electrical connection between the first battery and the motor; a second switch device (134, 223, 336, 424) connected between the second battery and the motor so as to form a second electrical connection between the second battery and the motor or cut off the second electrical connection between the second battery and the motor; a controller (139, 139a, 139b, 212, 222, 339a, 339b, 413, 423), characterized in that the controller is operable to: detect a first voltage drop across the first switch device from a respective high potential point to a respective low potential point, and control the first switch device to cut off the first electrical connection between the first battery and the motor when the first voltage drop is less than a first control voltage; detect a second voltage drop across the second switch device from a respective high potential point to a respective low potential point, and control the second switch device to cut off the electrical connection between the second battery and the motor when the second voltage drop is less than a second control voltage, detect a voltage of the first battery pack, and cut off the connection of the first battery pack and the motor when the voltage of the first battery pack is less than a first voltage threshold; and detect a voltage of the second battery pack, and cut off the connection of the second battery pack and the motor when the voltage of the second battery pack is less than a second voltage threshold. detect a first voltage drop across the first switch device from a respective high potential point to a respective low potential point, and control the first switch device to cut off the first electrical connection between the first battery and the motor when the first voltage drop is less than a first control voltage; detect a second voltage drop across the second switch device from a respective high potential point to a respective low potential point, and control the second switch device to cut off the electrical connection between the second battery and the motor when the second voltage drop is less than a second control voltage, detect a voltage of the first battery pack, and cut off the connection of the first battery pack and the motor when the voltage of the first battery pack is less than a first voltage threshold; and detect a voltage of the second battery pack, and cut off the connection of the second battery pack and the motor when the voltage of the second battery pack is less than a second voltage threshold.