A battery cooling system (100) is provided to maintain more uniform temperature distribution in vehicle batteries (142). A battery (142) is provided with at least one exposed cell surface (143). A cooling shell (146) is provided with an interior surface spaced apart from the exposed cell surface (143). The interior surface defines a flow channel (147) for cooling fluid (150) such as air (114) provided by a system fan (112), and the air (150) flows in direct contact with the cell surface (143) from an inlet to an outlet of the channel (147). The channel (147) has a first hydraulic diameter at the inlet that is greater than a second hydraulic diameter at the outlet to the channel (147). The hydraulic diameter is varied or decreased along the length of the channel (147) such that the system provides a first surface heat transfer coefficient proximate to the channel inlet that is less than a second surface heat transfer coefficient proximate to the channel outlet.
 The United States Government has rights in this invention under Contract No. DE-AC36-086028308 between the United States Department of Energy and the Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory.