NOVELTY - A battery pack (201) has a module assembly including two or more unit modules having different outputs and capacities. A detection unit (240, 242) detects the current and/or voltage of the battery pack and transmits the detected current and/or voltage signals to a control unit (250). The control unit selects an optimal unit module from the module assembly based on the received current signal and/or voltage signal. A switching unit (260) electrically connects a specific unit module of the module assembly to an external input and output terminal (270, 272) under the control of the control unit.    USE - As a power source for electric vehicles, hybrid electric vehicles, robots, electric bicycles, or electric scooters (claimed).    ADVANTAGE - An appropriate unit module is selected and operated depending upon the operating conditions of a device in which the battery pack is mounted, thus the operating efficiency of the batter pack is improved. The service life of the respective unit modules, and thus the service life of the battery pack, are increased. Worn unit module may be replaced with a new one without disposal of the entirety of the battery pack.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for an operating method of the battery pack. Each of the unit modules includes nickel-metal hydride secondary batteries, nickel-cadmium (Ni-Cd) secondary batteries, capacitors and/or lithium secondary batteries as unit cells. The module assembly includes a relatively high-output and small-capacity unit module and a relatively low-output and large-capacity unit module. The relatively high-output and small-capacity unit module includes super capacitors and/or secondary batteries as unit cells. The super capacitors are based upon electric double-layer capacitors and/or pseudo capacitors. The secondary batteries are lithium secondary batteries. The switching unit is a mechanical relay switch or an electrical switch, such as a solid state relay or a semiconductor relay. The control unit is included in battery management system (BMS).    DESCRIPTION OF DRAWING(S) - The figure shows a battery pack.    Battery pack (201)    Current detection unit (240)    Voltage detection unit (242)    Control unit (250)    Switching unit (260)    Input and output terminals (270, 272)