It is common practice in the field of medicine to implant electronic devices inside the human body in order to overcome various problems, for example a pacemaker, a defibrillator or an infusion pump. Such devices require a power source for the device to monitor the body and optionally, perform a remedial function. For some devices, power for the device is supplied by a rechargeable battery that can be recharged transcutaneously so that additional operations on the patient are not needed in order to replenish the electrical power source.
Some types of rechargeable batteries (e.g. Nickel-Cadmium) suffer from memory effects, which require charging them when they reach a certain level of depletion, in order to prevent damage to their ability to hold a charge. Newer batteries (e.g. Lithium-Ion) can be charged at the convenience of a user (either a patient, or someone else responsible), for example periodically. In either case the user needs to keep track of the charge status of the battery to assure that the battery has enough charge to function if needed. If the user overestimates the available charge the result could be fatal, depending on the application.
Implanted devices are typically placed in an enclosure made of a bio-compatible metal material, to protect the device, and to protect the patient from leakage from the device. One problem with using such a metal enclosure is that it may heat up as a result of eddy currents caused by a time-varying electromagnetic field, for example when charging the device. Generally the heat dissipates through surrounding tissue and the blood flow. However when transcutaneously charging a battery by induction, the implant may heat up faster than it can dissipate the excess heat. High temperatures (e.g. above 42° C.) can damage surrounding tissue. As a result, charging the battery is typically performed at a low rate to prevent harm to the patient.
U.S. Pat. No. 5,991,665 describes a self cooling transcutaneous energy transfer system, wherein a fan is used to cool the skin at the charging point in order to allow a faster charge rate.
US 2005/0075696 describes an inductive charger for an implant, in which various means are used to detect when the primary and secondary coils are not optimally aligned, including measuring the power load on the primary load.