Disclosed herein is a battery pack including a module assembly consisting of two or more unit modules having different outputs and capacities, a detection unit for detecting the current and/or the voltage of the battery pack and transmitting the detected current signal and/or the detected voltage signal to a control unit, the control unit for selecting an optimal unit module from the module assembly based on the current signal and/or the voltage signal received from the detection unit, and a switching unit for electrically connecting a specific unit module of the module assembly to an external input and output terminal under the control of the control unit. In the battery pack according to the present invention, an appropriate unit module is selected and operated depending upon the operating conditions of a device in which the battery pack is mounted, whereby the operating efficiency of the battery pack is improved.