Disclosed herein is a battery assembly including at least two battery cells connected in series or in parallel. Each of the battery cells includes an electrode assembly including a cathode, an anode and a separator interposed between the cathode and the anode and a battery case in which the electrode assembly is mounted. An electrode terminal of a first battery cell and an electrode terminal of a second battery cell are formed as a single member at a series or parallel connection portion between the first and second battery cells.