The present invention relates to a battery cell that can include an electrode assembly configured with two or more unit stacks connected to each other in series or in parallel, a battery case configured to receive the electrode assembly therein, and an insulative protection member configured to protect an electrode tab portion of the unit stacks. The overall length of the battery case can be three or more times the overall width of the battery case. A positive electrode lead and a negative electrode lead can extend outwards from the battery case protruding in opposite directions. A plurality of unit stacks can be used to implement a long battery cell, and a connection portion at which the plurality of unit stacks are connected is reinforced to improve safety of the battery cell.