A heat exchanger and battery unit structure is provided for cooling battery units (or cells) where the thermally conductive nature of the battery forms a cooling path. The heat exchanger is in the form of a cooling element provided with an engaging device formed on or attached to an outer surface of the cooling plate for receiving and/or engaging with a corresponding engaging portion on a battery unit (or cell). The interconnection between the battery unit (or cell) and heat exchanger creates a mechanical interlock between the two components that results in improved heat transfer properties between the two components.