A battery pack that includes a bare cell, a bottom case and a resin part disposed on a lower surface of the bare cell. The bottom case has a number of holes. The resin part is disposed on a surface of the bare cell affixing the bottom case to the bare cell. Further, a number of protrusions is provided and composed of the same material as the resin part in which each protrusion corresponds to a hole. The protrusions and the holes have a circular, tetragonal or polygonal cross sectional shape in which upon thermal compression of the protrusions the bottom case is solidly coupled to the resin part.