A transcutaneous energy transfer system for an implantable medical device. The system comprises an internal coil assembly implantable in a recipient; and an external data coil assembly positioned in a housing worn by the recipient. The data coil assembly is configured to be inductively coupled to the internal coil assembly in order to transcutaneously transfer data to the internal coil assembly. One or more elements of the data coil assembly are physically adjustable with respect to the housing to alter the inductive coupling.