TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred System: In the thermal management system, a heat transfer surface area on the exposed surface is uniform within the channel from the inlet to the outlet. The fluid comprises air entering the channel at the inlet at a first temperature and exiting the channel at the outlet at a second temperature differing from the first temperature and where the system has a first heat transfer coefficient proximate to the inlet of the channel and a second heat transfer coefficient greater than the first heat transfer coefficient proximate to the outlet of the channel. The cell surface has a first temperature proximate to the inlet of the channel and a second surface temperature proximate to outlet of the channel and where the second temperature differs about 4 degrees C from the first temperature. The channel has hydraulic diameters each decreasing in magnitude from the first hydraulic diameter at the inlet to the second hydraulic diameter at the outlet of the channel. The hydraulic diameters decrease in magnitude linearly from the inlet to the outlet of the channel. The channel has a cross-sectional shape with a first area at the inlet and a cross-sectional shape with a second area less than the first area at the outlet of the channel. The cross-sectional shapes of the channel at the inlet and the outlet are each defined by a height measured from the cell surface to the interior surface of the cooling shell and where the height at the inlet is greater than the height at the outlet of the channel. The first and second hydraulic diameters have values selected so that a surface heat transfer coefficient proximate to the inlet of the channel is less than 50% of a surface heat transfer coefficient proximate to the outlet of the channel. In the battery cooling system, the interior surface is configured to provide the flow channel with hydraulic diameters and where the hydraulic diameters decrease in magnitude with increasing distance from the inlet end of the flow channel. The flow channel has a rectangular cross-sectional shape and where the interior surface is substantially planar. The second hydraulic diameter has a value of less than 50% of a value of the first hydraulic diameter. The first and second hydraulic diameters have values selected so that a surface heat transfer coefficient proximate to the inlet end is less than 50% of a surface heat transfer coefficient proximate to the outlet end of the flow channel. Preferred Vehicle: The cross-sectional shape dimension defines a hydraulic diameter for the channel and where the channel upper wall is configured so that the hydraulic diameter decreases along the cooling channel from the inlet to the outlet. For a particular volume and inlet temperature of the flowing air, the channel provides a surface heat transfer coefficient for the cell surface that increases along the channel from the inlet to the outlet so that the surface heat transfer coefficient is at least twice as large at the outlet than at the inlet of the channel. The cell surface has a temperature that is less than 4 degrees C different proximate to the outlet than a temperature proximate to the inlet during use of the battery and operation of the fan assembly to provide the flowing air in the channel. The cross-sectional shape is rectangular including the channel upper wall defines one of the rectangular cross-sectional shape. The battery pack comprises cylindrical or prismatic lithium ion batteries.