A charging system for an implantable medical device having a secondary coil. The charging system includes an external power source having at least one primary coil, a modulation, circuit operatively coupled to the primary coil and capable of driving it in a manner characterized by a charging parameter, and a sensor in communication with the modulation circuit and capable of sensing a condition indicating a need to adjust the charging parameter during a charging process. The parameter may be varied so that data sensed by the sensor meets a threshold requirement, which may be based on a patient preference, a government regulation, a recommendation promulgated by a health authority and / or a requirement associated with another device carried by the patient. In one embodiment, the regulation dictates maximum magnetic field exposure, and a field limiting circuit is employed to adjust the charging process.