An external processor device is described for an implantable prosthetic system. An external processor housing has a generally planar skin contacting surface and a central axis perpendicular to the skin contacting surface. A signal processor is located within the processor housing for developing an implant data signal. The processor housing also contains a transmitter coil for coupling the implant data signal across the skin to the implantable prosthetic system. A battery compartment is also located within the processor housing in an annular region around the central axis for containing a battery arrangement to provide electrical power to the signal processor and the transmitter coil.