are straight sections 115 placed between every two battery cells 105 or every three battery cells not shown other arrangements are possible as would be understood by those of skill in the art for example expansion units 125 may be disposed between adjacent battery cells 105 as shown in fig2 and 6 for example every two battery cells see fig4 and 5 for example or between every three battery cells not shown referring next to fig7 a through 7f steps associated with one method of making the battery module 100 are illustrated first the metal sheet which in one non limiting example may comprises a flat sheet 109 of aluminum or aluminum alloy of fig7 a is corrugated into the wave fin 110 of fig7 b to define straight sections 115 with crest sections 120 joining two adjacent straight sections 115 because the coolant flow is only at the sides of the module 100 instead of between every battery cell or every other battery cell there are fewer seals with the wave fin 110 of the present invention than with other designs this reduction in sealing equipment means the design provides lower material and manufacturing costs likewise because seals which are very sensitive to surface cleanliness are not needed only normal cleanliness standards are required for the metal sheet as shown in general in fig7 c and in more detail in fig7 d expansion units 125 may be placed on one side of the straight wave fin sections 115 while locating grooves 145 can be incorporated into the wave fin 110 to provide.