A battery pack including: one or more battery cells; and a connector portion including a first terminal portion, wherein the connector portion is configured to be detachably engaged with a connection portion of an electrical device or a charging device including a second terminal portion and a drain hole, and wherein the first terminal portion is configured to be connected to the second terminal portion. The connector portion is configured to be detachably engaged with the connection portion by rotation, thereby closing the drain hole by a part of the connector portion.