Charger alignment in an implantable medical device system employing reflected impedance modulation

The disclosed means of determining alignment between an external charger and an implantable medical device (IMD) involves the use of reflected impedance modulation, i.e., by measuring at the external charger reflections arising from modulating the impedance of the charging coil in the IMD. During charging, the charging coil in the IMD is pulsed to modulate its impedance. The difference in the coil voltage ( V) produced at the external charger as a result of these pulses is assessed and is used by the external charger to indicate coupling. If the magnitude of Y is above a threshold, the external charger considers the coupling to the IMD to be adequate, and an alignment indicator in the external charger is controlled accordingly. The magnitude of Vcoil can be assessed in addition to V to determine alignment with the IMD with improved precision, and/or to further define a high quality alignment condition.