Method for determining coupling coefficient for wireless power transfer

1. An external power source configured to supply power through inductive coupling of power from an external coil to an internal coil associated with an implanted medical device, the external power source comprising:

a memory;

a processor in communication with the memory, the processor configured to:

receive an internal coil output power;

determine an efficiency as a ratio of the internal coil output power to an amount of power applied to the external coil;

track a resonant frequency of the external coil based on adjustments to a driving frequency applied to the external coil;

determine a coupling coefficient based on the efficiency and the resonant frequency; and

generate an indication of an orientation of the external coil relative to the internal coil, the indication based at least in part on the coupling coefficient.

2. The external power source of claim 1 , wherein the indication of orientation is an audible tone that changes in frequency as the orientation changes.

3. The external power source of claim 1 , wherein the processor is further configured to determine the coupling coefficient based on a look up table stored in the memory, the look up table providing a different coupling coefficient for each of a plurality of efficiency values and received power values.

4. The external power source of claim 1 , wherein the processor is further configured to determine the coupling coefficient is according to a formula that relates the efficiency to the coupling coefficient.

5. The external power source of claim 1 , wherein the indication includes a variable graphical indicator that varies according to an alignment of the external coil relative to the internal coil.

6. The external power source of claim 1 , wherein the indication varies as an angle between an axis of the external coil and an axis of the internal coil varies.

7. The external power source of claim 1 , wherein the indication varies as a distance between the external coil and the internal coil varies.

8. The external power source of claim 1 , wherein the external power source is configured to adjust a driving voltage via a tuning capacitor to adjust the driving frequency.

9. A method for indicating an extent of inductive coupling of power from an external coil to an internal coil associated with an implanted medical device, the method comprising:

determining an efficiency as a ratio of an internal coil output power to an amount of power applied to the external coil;

tracking a resonant frequency of the external coil when inductively coupled to the internal coil based on adjustments to a driving frequency applied to the external coil;

determining a coupling coefficient that indicates an extent to which the external coil is aligned with and in proximity to the internal coil, the coupling coefficient being based at least in part on the efficiency and the resonant frequency of the external coil; and

generating an indication of the extent to which the external coil is aligned with and in proximity to the internal coil based on the coupling coefficient.

10. The method of claim 9 , wherein the indication is an audible tone that varies in frequency as an orientation of the external coil varies with respect to an orientation of the internal coil.

11. The method of claim 9 , wherein at least one of a cell phone or a wrist worn device in wireless communication with at least one of an external power supply having the external coil and the implanted medical device communicates the indication.

12. The method of claim 9 , further comprising adjusting a driving voltage via a tuning capacitor to adjust the driving frequency.