A circulatory assist device comprising: (a) an implantable pump for mounting within the body of a patient, the pump having an electric motor, the pump having a power demand which varies with the cardiac cycle of a patient in which the pump is implanted, or a time-varying power demand; (b) an implantable power unit adapted for mounting within the body of the patient, the power unit having a secondary coil, a power circuit connected to the secondary coil for supplying power to the pump so that power applied to the secondary coil can be transmitted to the pump, and a monitoring circuit operable to monitor a condition of the power circuit and send a transcutaneous telemetry signal representing the monitored condition, (c) an external power supply having a primary coil adapted for transcutaneous inductive coupling with the secondary coil, a drive circuit operable to apply an alternating current to the primary coil, and a control circuit operable to receive the telemetry signal and adjust the alternating current in the primary coil at least in part in response to the telemetry signal, the monitoring circuit and control circuit being operable to monitor the condition of the power circuit and adjust the alternating current in the primary coil in accordance therewith so as to vary the alternating current in the primary coil substantially in accordance with the cardiac cycle, or to adjust the alternating current in the primary coil in response to the telemetry signal so as to vary the alternating current in the primary coil substantially in accordance with the monitored condition and maintain the monitored condition in the power circuit within a desired range; characterised in that the power circuit further includes a battery and a routing circuit arranged to route power from the secondary coil to the motor, to the battery or both when power is received through the secondary coil and from the battery to the motor when power is not received by the secondary coil, and wherein the monitoring circuit and control circuit are operable to vary the current in the primary coil so as to maintain the condition within the desired range during the charging operation and during operation when the battery is fully charged. (a) an implantable pump for mounting within the body of a patient, the pump having an electric motor, the pump having a power demand which varies with the cardiac cycle of a patient in which the pump is implanted, or a time-varying power demand; (b) an implantable power unit adapted for mounting within the body of the patient, the power unit having a secondary coil, a power circuit connected to the secondary coil for supplying power to the pump so that power applied to the secondary coil can be transmitted to the pump, and a monitoring circuit operable to monitor a condition of the power circuit and send a transcutaneous telemetry signal representing the monitored condition, (c) an external power supply having a primary coil adapted for transcutaneous inductive coupling with the secondary coil, a drive circuit operable to apply an alternating current to the primary coil, and a control circuit operable to receive the telemetry signal and adjust the alternating current in the primary coil at least in part in response to the telemetry signal,