A secondary battery module includes a plurality of secondary battery cells each having a measureable temperature and each spaced apart from an adjacent one of the cells to define a cooling channel therebetween. The plurality of cells includes a first one of the cells having a measureable first temperature, and a terminal one of the cells having a measureable terminal temperature and separated from the first one of the cells by at least one other of the cells. The module includes a fluid flowable within each of the cooling channels and in thermal energy exchange relationship with each of the cells. The module includes a housing having a surface defining an inlet channel disposed in fluid flow communication with each of the cooling channels and configured for directing the fluid flow to each of the cooling channels. The surface has a shape defined by a spline.