A heat exchanger for cooling multiple rows of battery cells has a plurality of longitudinal flow sections defining at least first and second U-shaped flow areas, each underlying a row of battery cells. The flow sections includes inlet and outlet flow sections, and at least two intermediate flow sections. Inlet and outlet ports are in flow communication with the respective inlet and outlet flow sections, and a first bypass channel extends between the inlet port and at least one of the intermediate flow sections. The first bypass channel supplies relatively cold heat transfer fluid from the inlet to mix with warmer fluid in a second or subsequent U-shaped flow area, to improve temperature uniformity between the rows of battery cells. A second bypass channel may extend around the outer periphery of the heat exchanger, from the inlet flow section to a second or subsequent U-shaped flow area.