A battery assembly includes a plurality of battery cells and a plurality of fins arranged in an array. Each of the cells has a side portion disposed against the fins. Each fin defines a serpentine fluid channel having an inlet and an outlet, and a plurality of parallel sections extending across the cells such that lengths of the sections increase from the inlet to the outlet. The length of at least one of the sections near the outlet is greater than the width of the cells and the length of at least one of the sections near the inlet is less than the width of the cells.