A battery pack comprises a plurality of electrochemical cells and a housing. The cells have similar shape and size. The shape is a rectangular prism with opposing major faces. The cells are aligned in a stack along an axis of the pack so that one of the major faces of each cell contacts the major face of the adjacent cell. The housing comprises a top portion and a bottom portion. The top portion comprises a top plate and four side plates joined to the top plate so as to form a cavity with an opening. The cross-sectional area of the opening is at least slightly larger than the cross-sectional area of the stack, and the cross-sectional area of the cavity in a plane closer to the top plate is sized so that the stack fits snugly therein. The battery stack is disposed in the housing. The bottom portion closes the opening.