A battery pack comprising: a case, a first set of battery cells and a second set of battery cells received in the case, a circuit board (3; 3'; 3"), a main power output interface connecting to the first set of battery cells and the second set of battery cells and an extended power output interface (4; 4'; 4") connecting to the first set of battery cells and the second set of battery cells; wherein the battery pack is configured to output two kinds of working voltages through the main power output interface by connecting the first and second sets of battery cells (21, 22) in series or in parallel, wherein the second circuit comprises a switch provided between the first set of battery cells and the second set of battery cells, and the battery pack is configured to output a third voltage for an external device through the extended power output interface (4; 4'; 4") when the first set of battery cells and the second set of battery cells are connected in series through the switch, wherein the battery pack has a conversion circuit, and wherein the conversion circuit is configured to convert the third voltage from the series connected first and second sets of battery cells (21, 22), wherein the third voltage output through the extended power output interface (4; 4'; 4") is different from the two kinds of working voltages output through the main power output interface, characterized in that the switch comprises a cover plate (51; 51'; 51") configured to shield the extended power output interface (4; 4'; 4"), a first terminal (52; 52') connected to one electrode of the first set of battery cells, and a second terminal (53; 53') connected to one electrode of the second set of battery cells, and wherein the electrode connected to the first terminal (52; 52') has a polarity different from the electrode connected to the second terminal (53; 53'), and wherein the cover plate (51; 51'; 51") is configured to slide along a front to back direction of the battery pack to connect or disconnect the first and second terminals (52; 52', 53; 53') from each other, and in that the first terminal is fixed on the cover plate, and the second terminal is fixed on the circuit board, wherein the second terminal includes a body portion and a clamping portion extending from the body portion, the first terminal is an insert piece disposed under the cover plate, wherein an upper part of the first terminal is insertable into the clamping portion of the second terminal to establish contact between the first terminal and the second terminal. a case, a first set of battery cells and a second set of battery cells received in the case, a circuit board (3; 3'; 3"), a main power output interface connecting to the first set of battery cells and the second set of battery cells and an extended power output interface (4; 4'; 4") connecting to the first set of battery cells and the second set of battery cells; wherein the battery pack is configured to output two kinds of working voltages through the main power output interface by connecting the first and second sets of battery cells (21, 22) in series or in parallel, wherein the second circuit comprises a switch provided between the first set of battery cells and the second set of battery cells, and the battery pack is configured to output a third voltage for an external device through the extended power output interface (4; 4'; 4") when the first set of battery cells and the second set of battery cells are connected in series through the switch, wherein the battery pack has a conversion circuit, and wherein the conversion circuit is configured to convert the third voltage from the series connected first and second sets of battery cells (21, 22), wherein the third voltage output through the extended power output interface (4; 4'; 4") is different from the two kinds of working voltages output through the main power output interface, characterized in that the switch comprises a cover plate (51; 51'; 51") configured to shield the extended power output interface (4; 4'; 4"), a first terminal (52; 52') connected to one electrode of the first set of battery cells, and a second terminal (53; 53') connected to one electrode of the second set of battery cells, and wherein the electrode connected to the first terminal (52; 52') has a polarity different from the electrode connected to the second terminal (53; 53'), and wherein the cover plate (51; 51'; 51") is configured to slide along a front to back direction of the battery pack to connect or disconnect the first and second terminals (52; 52', 53; 53') from each other, and in that the first terminal is fixed on the cover plate, and the second terminal is fixed on the circuit board, wherein the second terminal includes a body portion and a clamping portion extending from the body portion, the first terminal is an insert piece disposed under the cover plate, wherein an upper part of the first terminal is insertable into the clamping portion of the second terminal to establish contact between the first terminal and the second terminal.