A system comprising an electrically powered medical device (X100;) adapted to be implanted in a patient, in which system the medical device (X100;) comprises an internal energy receiver arranged to power the medical device (X100;), the system also comprising an external energy source adapted to be located externally to the patient for wireless supply of energy to said internal energy receiver, the external energy source being equipped with a primary coil from which the external energy source is arranged to transmit said energy inductively to a first secondary coil in the energy receiver, the system being arranged to determine by measurements on or more parameters related to a first coupling factor between the primary and the first secondary coil, the system being arranged to directly or indirectly measure over a predetermined period of time the difference between at least one electrical parameter related to the energy transmitted by the external energy source and at least one electrical parameter related to the amount of energy received by the internal energy receiver, and to determine the balance between said electrical parameters in order to determine said first coupling factor, in which system the external energy source is adapted to transmit a first amount of energy to the energy receiver for enabling the medical device (X100;) to detect information related to the first coupling factor, wherein the system further adapted to increase said first amount of energy until said first coupling factor is detected by the system, wherein the medical device (X100;) is adapted to wirelessly send feedback information related to said first coupling factor to the external energy source, with the external energy source being arranged to receive said feedback information from the medical device and in that the external energy source is adapted to perform a predetermined action based on said feedback information, in which said predetermined action comprises X optimizing the placement of the primary coil in relation to the first secondary coil in order to optimize said first coupling factor, and use said first coupling factor to X determine a second amount of energy to be transmitted by said external energy source to the internal energy receiver, said second amount of energy being optimized to power or charge the medical device, wherein the energy receiver of the system comprises at least one of: - a first switch (X102a) adapted to switch a connection between the first secondary coil and the medical implant on and off, in order to enable the system to measure the first coupling factor when the connection is off, - an electronic component connected to the first secondary coil for preventing flow of electrical current between the first secondary coil and the medical implant during measurement of parameters related to the first coupling factor, and - a half wave rectifying component for rectifying half of the pulse cycle of a received alternating current energy signal in the energy receiver, the system being adapted to measure parameters related to the first coupling factor during at least part of the non-rectified half of one or more pulse cycles, thus in which the system being adapted to let the first secondary coil be substantially without electrical load when measuring the first coupling factor, characterized in that the energy receiver further comprises a second secondary coil, so that the primary coil has the first coupling factor in relation to the first secondary coil and a second coupling factor in relation to the second secondary coil with the first secondary coil being arranged to receive energy for powering said medical device and the second secondary coil being adapted to supply information related to the second coupling factor, the second coupling factor having a predetermined relation to the first coupling factor, in which the one or more parameters related to the first coupling factor which are determined by measurements of the system being parameters related to the second coupling factor, thus the system being arranged to determine by measurements said second coupling factor, wherein the feedback information related to the first coupling factor which is transmitted to the external energy source is information which is also related to the second coupling factor. - a first switch (X102a) adapted to switch a connection between the first secondary coil and the medical implant on and off, in order to enable the system to measure the first coupling factor when the connection is off, - an electronic component connected to the first secondary coil for preventing flow of electrical current between the first secondary coil and the medical implant during measurement of parameters related to the first coupling factor, and - a half wave rectifying component for rectifying half of the pulse cycle of a received alternating current energy signal in the energy receiver, the system being adapted to measure parameters related to the first coupling factor during at least part of the non-rectified half of one or more pulse cycles,