In one embodiment, an external charging device for recharging an implanted medical device, comprises: a battery for powering the external charging device; a coil for radiating RF power; drive circuitry for driving the coil according to a duty cycle; circuitry for generating a signal that is indicative of an amount of current flowing through the coil; and control circuitry for controlling the drive circuitry, wherein the control circuitry is operable to process the signal from the circuitry for generating to detect when a coil of the implantable medical device temporarily ceases absorbing RF power, the control circuitry modifying the duty cycle in response to detection of the coil of the implantable medical device temporarily ceasing absorbing RF power.