A system for controlling power delivery from an external power transfer system EPTS to at least one implantable neurostimulation system INS, said system comprising the EPTS configured to be disposed outside a body; and the at least one INS configured to be disposed beneath a dermis layer of the body; the EPTS including: one or more transmit coils disposed in series, each corresponding to a respective INS; a transmit coil driver circuit configured to drive the one or more transmit coils with a resonant current having a peak magnitude; and a back telemetry circuit configured to receive a message communicated by the at least one INS; each of said at least one INS respectively including: a receive coil (402; 521) tuned to the resonant frequency of the corresponding transmit coil and configured to receive power transferred therefrom when in proximity thereto; a plurality of electrodes (455, 533); an electrode driver circuit; a current regulator circuit (453, 458, 459) having an input to which the received power is coupled, and configured to output a regulated electrode current (VSTIM) to the electrode driver circuit for the plurality of electrodes (455, 533); a monitoring circuit configured to determine whether the received power is sufficient to achieve current regulation of the current regulator circuit (453, 458, 459); and a back telemetry circuit configured to communicate a message to the EPTS; wherein: each respective INS is configured to communicate a respective message requesting a change in power transfer from the EPTS based upon the respective current regulator circuit (453, 458, 459) determination; and the EPTS is configured to adjust the transmit coil driver circuit to change the peak magnitude of the resonant current, based on respective messages received from one or more respective INS. one or more transmit coils disposed in series, each corresponding to a respective INS; a transmit coil driver circuit configured to drive the one or more transmit coils with a resonant current having a peak magnitude; and a back telemetry circuit configured to receive a message communicated by the at least one INS; a receive coil (402; 521) tuned to the resonant frequency of the corresponding transmit coil and configured to receive power transferred therefrom when in proximity thereto; a plurality of electrodes (455, 533); an electrode driver circuit; a current regulator circuit (453, 458, 459) having an input to which the received power is coupled, and configured to output a regulated electrode current (VSTIM) to the electrode driver circuit for the plurality of electrodes (455, 533); a monitoring circuit configured to determine whether the received power is sufficient to achieve current regulation of the current regulator circuit (453, 458, 459); and a back telemetry circuit configured to communicate a message to the EPTS; each respective INS is configured to communicate a respective message requesting a change in power transfer from the EPTS based upon the respective current regulator circuit (453, 458, 459) determination; and the EPTS is configured to adjust the transmit coil driver circuit to change the peak magnitude of the resonant current, based on respective messages received from one or more respective INS.