A charger having a controller with an input and an output, a first terminal for connecting a battery pack to the controller, a connecting line disposed between the first terminal and the input of the controller, a current source connected to the controller for providing power to the battery pack. The current source provides power to the battery pack via the connecting line. The controller sends a pulse signal unto the connecting line via the output, so that the controller can determine whether the battery pack is connected to the charger by the presence of the pulse signal in the input of the controller.