A system, comprising an implantable electrical device adapted for mounting within the body of the patient, and an implantable power unit adapted for mounting within the body of the patient, the power unit having a receiving coil, a power circuit connected to the receiving coil so that power applied to the receiving coil can be transmitted to the implantable device. The system further comprising an external power supply having a transmitting coil adapted for transcutaneous inductive coupling with the receiving coil to define a system having a resonant frequency varying in relation to a distance between the transmitting and receiving coils, a drive circuit operable to apply an alternating current to the transmitting coil at a variable drive pulse width. A control circuit is operable to (i) monitor a parameter related to mutual inductance between the transmitting and receiving coils； and (ii) adjust the variable transmission frequency to a value having a predetermined difference from the resonant frequency, so as to maintain the transmission frequency with a predetermined difference from the resonant frequency. The power circuit further includes a battery and a routing circuit arranged to route power from the receiving coil to the implantable device to the battery or both when power is received through the receiving coil and from the battery to the implantable device when power is not received by the receiving coil, the control circuit being operable to vary the current in the transmitting coil so as to maintain the monitored parameter within the desired range during the charging operation and during operation when the battery is fully charged
