Disclosed herein is a battery pack having a plurality of battery cells electrically connected to each other, the battery cells being chargeable and dischargeable, wherein the battery pack includes two or more kinds of battery groups having different capacities or sizes, wherein each of the battery groups includes two or more battery cells having the same capacity or size, the battery cells in each of the battery groups are connected in series to each other, and the battery cells between the battery groups are connected in parallel to each other.