A traction battery assembly may include an array of battery cells having opposing end faces, opposing side faces, and a bottom face. The assembly may also include a pair of end plates and a pair of side plates arranged to form a four-sided enclosure around the end and side faces and configured to compress and retain the cells without being mechanically attached thereto or covering the bottom face. The side plates may partially cover an upper portion of the array. The side plates may have a lower horizontal edge, an upper horizontal edge, and at least one diagonal reinforcement rib configured to extend from a location where the vertical edge and lower horizontal edge meet upward to the upper horizontal edge.