A battery pack includes a plurality of unit batteries, each including a pair of first side surfaces and a pair of second side surfaces perpendicular to the pair of first side surfaces, the unit batteries being side-by-side in a first direction such that the pair of first side surfaces face each other. A protective circuit module including a substrate and protective devices mounted on the substrate is electrically connected to the unit batteries adjacent to top portions of the unit batteries. A first cap cover accommodates the protective circuit module and the top portions of the unit batteries. A second cap cover is spaced apart from the first cap cover with the unit batteries therebetween. The second cap cover accommodates bottom portions of the unit batteries. At least one of the first cap cover and the second cap cover includes a rib extending in a region between adjacent unit batteries.