A battery pack for an electric bicycle is disclosed. In one embodiment, the battery pack includes i) a lower case having a top, a bottom and an interior space formed between the top and bottom and ii) a battery cell placed in the interior space of the lower case, wherein the battery cell has first and second surfaces opposing each other, and wherein the first surface of the battery cell is closer to the bottom of the lower case than the second surface of the battery cell. The battery pack may further include i) a protection circuit board mounted on the second surface of the battery cell and placed in the interior space of the lower case, ii) an upper case formed over the top of the lower case and iii) a separator case formed between the protection circuit board and the upper case.