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A particular method of providing power to an implantable medical device includes providing a first signal to a primary coil that is inductively coupled to a secondary coil of an implantable medical device. The method also include determining a first alignment difference between a voltage correspondi

A particular method of providing power to an implantable medical device includes providing a first signal to a primary coil that is inductively coupled to a secondary coil of an implantable medical device. The method also include determining a first alignment difference between a voltage corresponding to the first signal and at least one of a current corresponding to the first signal and a component voltage at a component of a primary coil circuit. The method further includes determining a frequency sweep range based on the first alignment difference. The method also includes performing a frequency sweep over the frequency sweep range.

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1. A method comprising: providing a first signal at a first frequency to a primary coil that is inductively coupled to a secondary coil of an implantable medical device;determining a first alignment difference between a first signal voltage and at least one of a first signal current and a component

1. A method comprising: providing a first signal at a first frequency to a primary coil that is inductively coupled to a secondary coil of an implantable medical device;determining a first alignment difference between a first signal voltage and at least one of a first signal current and a component voltage at a component of a primary coil circuit;determining a second signal at a second frequency based on the first alignment difference; andproviding the second signal at the second frequency to the primary coil that is inductively coupled to the secondary coil. 2. The method of claim 1, further comprising: determining a second alignment difference between the second signal voltage and at least one of a second signal current and a second component voltage at the component of the primary coil circuit;determining a third signal at a third frequency based on the second alignment difference; andproviding the third signal at the third frequency to the primary coil that is inductively coupled to the secondary coil. 3. The method of claim 1, wherein the second frequency is approximately a resonant frequency of a circuit that includes the primary coil. 4. The method of claim 1, wherein the implantable medical device comprises a nerve stimulation device, wherein the nerve stimulation device includes the secondary coil, a battery and a battery charging circuit within a conductive housing, wherein the secondary coil is responsive to the charging signal applied to the primary coil to provide energy to charge the battery, and wherein a temperature of the conductive housing increases by no more than two degrees Fahrenheit during charging of the battery. 5. The method of claim 1, further comprising receiving information from the implantable medical device, wherein the information indicates a value of a measured electrical property of the implantable medical device during the frequency sweep. 6. The method of claim 5, wherein the information indicates a voltage applied to a battery charging circuit of the implantable medical device, wherein the voltage is responsive to current in the secondary coil. 7. The method of claim 5, further comprising in response to the information received from the implantable medical device, modifying a duty cycle of the charging signal while maintaining the charging frequency of the charging signal. 8. The method of claim 1, wherein a current waveform and a voltage waveform are generated in response to the first signal being applied to the primary coil and wherein determining the first alignment difference comprises: determining a first time difference between a first portion of the current waveform and a corresponding first portion of the voltage waveform;determining a second time difference between a second portion of the current waveform and a corresponding second portion of the voltage waveform; anddetermining a first phase difference by comparing the first time difference and the second time difference. 9. The method of claim 1, wherein a current waveform and a voltage waveform are generated in response to the first signal being applied to the primary coil and wherein determining the first alignment difference comprises: determining a first time difference between a point at which the current waveform has a reference current value and a point at which the voltage waveform has a reference voltage value;determining a second time difference between a next point of the current waveform that has the reference current value and a next point of the voltage waveform that has the reference voltage value; anddetermining a first phase difference by comparing the first time difference and the second time difference. 10. The method of claim 1, wherein the first alignment difference comprises at least one of a phase difference and a time difference. 11. A device comprising a primary coil coupled to a circuit and operable to inductively couple to a secondary coil within an implantable medical device to transfer energy to the secondary coil within the implantable medical device responsive to a signal of the circuit;a sensing system coupled to the circuit, the sensing system operable to detect an indication of an alignment relationship between a signal voltage and at least one of a signal current and a component voltage at a component of the circuit; anda control system responsive to the sensing system, the control system operable todetermine a second signal at a second frequency based on the first alignment difference; andprovide the second signal at the second frequency to the primary coil that is inductively coupled to the secondary coil. 12. The device of claim 11, wherein the primary coil is at least partially contained within a portable housing, and wherein the control system adjusts the frequency of the signal and a duty cycle of the signal in response to a change in an electrical property of the circuit due at least in part to relative motion of the primary coil and the secondary coil. 13. The device of claim 11, further comprising a receiver operatively coupled to the control system, the receiver operable to receive information from the implantable medical device, wherein the information is indicative of an electrical property associated with the implantable medical device or a component of the implantable medical device, and wherein the control system modifies the signal responsive to the information. 14. The device of claim 11, wherein the circuit includes a capacitor and wherein the control system estimates a resonant frequency of the circuit during the transfer of the energy. 15. A non-transitory computer-readable storage medium storing instructions executable by a computer system to: cause a first signal to be provided to a primary coil that is inductively coupled to a secondary coil of an implantable medical device;determine a first alignment difference between a first signal voltage and at least one of a first signal current and a component voltage at a component of a primary coil circuit;determine a second signal at a second frequency based on the first alignment difference; andcause the second signal at the second frequency to be provided to the primary coil that is inductively coupled to the secondary coil. 16. The non-transitory computer readable storage medium of claim 15, further including instructions executable by a computer system to: determine a second alignment difference between the second signal voltage and at least one of a second signal current and a second component voltage at the component of the primary coil circuit;determine a third signal at a third frequency based on the second alignment difference; andprovide the third signal at the third frequency to the primary coil that is inductively coupled to the secondary coil. 17. The non-transitory computer readable storage medium of claim 15, wherein the second frequency is approximately a resonant frequency of a circuit that includes the primary coil and the secondary coil. 18. The non-transitory computer readable storage medium of claim 15, wherein a current waveform and a voltage waveform are generated in response to the first signal being applied to the primary coil and wherein determining the first alignment difference comprises: determining a first time difference between a first portion of the current waveform and a corresponding first portion of the voltage waveform;determining a second time difference between a second portion of the current waveform and a corresponding second portion of the voltage waveform; anddetermining a first phase difference by comparing the first time difference and the second time difference. 19. The non-transitory computer readable storage medium of claim 15, wherein a current waveform and a voltage waveform are generated in response to the first signal being applied to the primary coil and wherein determining the first alignment difference comprises: determining a first time difference between a point at which the current waveform has a reference current value and a point at which the voltage waveform has a reference voltage value;determining a second time difference between a next point of the current waveform that has the reference current value and a next point of the voltage waveform that has the reference voltage value; anddetermining a first phase difference by comparing the first time difference and the second time difference. 20. The non-transitory computer readable storage medium of claim 15, wherein the first alignment difference comprises at least one of a phase difference and a time difference.