A battery pack includes a pack case that includes external terminals formed on one side and contains a plurality of battery modules inside; a protective frame that extends across the pack case and defines the module area; a busbar that electrically connects the battery modules and the external terminals of the pack case; and a battery management unit located on the other side opposite to the pack case from where the external terminals are formed. The protective frame includes slots extending along the length direction of the protective frame, the module area is compartmentalized by the protective frame and includes a first battery part formed on one side of the protective frame and a second battery part formed on the opposite side of the protective frame, the busbar is inserted and secured within the slots of the protective frame, and the protective frame surrounds its sides.