Conformal heat exchanger for battery cell stack

A heat exchanger structure (18) for use in a battery unit (10) that comprises a first battery stack (12) comprising a plurality of battery cells (16) and a second battery stack (12) comprising a plurality of battery cells (16). The heat exchanger structure is disposed between opposing surfaces (26,28) of the first battery stack and the second battery stack and defines one or more fluid flow passages (30), the heat exchanger structure being dimensionally compliant to compress under expansion of the first and second battery stacks and expand under subsequent contraction of the first and second battery stacks.