Apparatus comprising: a battery pack for providing an output voltage at first and second output terminals, and an external device powered by the battery pack and being connected to the battery pack via positive, negative and preset resistance terminals, the battery pack comprising: a battery module including a plurality of battery cells and a plurality of switches for connecting the cells in series and/or parallel; a control unit arranged to control the switches so as to change the output voltage of the battery pack in dependence on an operating voltage of the external device; an authentication unit having a look-up table of resistances and corresponding operating voltages, wherein the external device has a preset resistance that corresponds to a resistance stored in the look-up table; and a third terminal TH at which the preset resistance of the external device is measured, the third terminal being connected to the authentication unit; wherein the authentication unit is arranged to measure the preset resistance of the external device connected to the third terminal TH, to determine the operating voltage of the external device from the look-up table, and to transmit a signal to the control unit to instruct the control unit of the operating voltage of the external device. a battery pack for providing an output voltage at first and second output terminals, and an external device powered by the battery pack and being connected to the battery pack via positive, negative and preset resistance terminals, the battery pack comprising: a battery module including a plurality of battery cells and a plurality of switches for connecting the cells in series and/or parallel; a control unit arranged to control the switches so as to change the output voltage of the battery pack in dependence on an operating voltage of the external device; an authentication unit having a look-up table of resistances and corresponding operating voltages, wherein the external device has a preset resistance that corresponds to a resistance stored in the look-up table; and a third terminal TH at which the preset resistance of the external device is measured, the third terminal being connected to the authentication unit; wherein the authentication unit is arranged to measure the preset resistance of the external device connected to the third terminal TH, to determine the operating voltage of the external device from the look-up table, and to transmit a signal to the control unit to instruct the control unit of the operating voltage of the external device. a battery module including a plurality of battery cells and a plurality of switches for connecting the cells in series and/or parallel; a control unit arranged to control the switches so as to change the output voltage of the battery pack in dependence on an operating voltage of the external device; an authentication unit having a look-up table of resistances and corresponding operating voltages, wherein the external device has a preset resistance that corresponds to a resistance stored in the look-up table; and a third terminal TH at which the preset resistance of the external device is measured, the third terminal being connected to the authentication unit; wherein the authentication unit is arranged to measure the preset resistance of the external device connected to the third terminal TH, to determine the operating voltage of the external device from the look-up table, and to transmit a signal to the control unit to instruct the control unit of the operating voltage of the external device.