A system is disclosed which comprises an implantable electrical medical device (100) with an internal energy receiver (102). The system also comprises an external energy source (104) located externally for supplying wireless energy to the internal energy receiver (102). The external energy source (104) has a primary coil (11) for transmitting energy inductively to a first secondary coil (10) in the energy receiver (102). The system is arranged to measure parameters related to a first coupling factor (C1) between the primary and the first secondary coil (10), and the external energy source (104) is adapted to transmit energy to the energy receiver (102) for enabling the medical device (100) to detect information related to the first coupling factor (C1). The medical device (100) is adapted to wirelessly send feedback information related to said first coupling factor (C1) to the external energy source (104) with the external energy source (104) being arranged to receive the feedback information and to perform a predetermined action based on said feedback information.