A battery module having a cooling plate assembly made from two non-contiguous cooling plates is provided. Each cooling plate has a side panel contiguous with a base panel, and the cooling plates are disposed opposite one another such that the base panels are coplanar and separated by a gap. An inlet duct runs along an upper edge of each side panel, and a manifold having coolant channels in fluid communication with the inlet duct is located in each side panel. The base panels have an outlet duct in fluid communication with the manifold. The cooling plate is designed to decrease friction loss as coolant flows through the inlet duct, manifold and outlet cut, and coolant channels in the manifold are designed to maintain a uniform flow rate across the manifold. Individual cells are disposed in the assembly such that the cell sides and bottoms are in heat-transfer contact with the plate side and base panels, respectively. The design of the battery module reduces operational temperature variation in the cells across the battery module and within each battery cell. Methods for reducing coolant flow friction loss, for establishing a more uniform coolant flow rate, and for reducing temperature variation are also provided.