A battery pack includes a case, a first set of battery cells and a second set of battery cells received in the case, a circuit board, and a main power output interface. The first set of battery cells and the second set of battery cells can be connected in series or in parallel, and output two kinds of working voltage through the main power output interface. The battery pack further includes an extended power output interface and a switch provided between the first set of battery cells and the second set of battery cells. The battery pack can output a third voltage for an external device through the extended power output interface when the first set of battery cells and the second set of battery cells are connected in series through the switch. It's safe and reliable when charging the external device.