A rechargeable battery system and method are disclosed, in which an implantable medical device (IMD) regulates its transfer of energy from a separate charger unit. For recharging, a charger unit is brought into proximity to the implanted device. An oscillating current is generated in a primary coil, located in the charger. By inductive coupling through an oscillating magnetic field, an alternating current is generated in a secondary coil, which is implanted in or near the implanted device. The alternating current then passes through a half-wave or full-wave rectifier to form a one-sided current, then passes through a regulator to form an essentially direct current, which is in turn directed to the rechargeable battery in the implanted device. The secondary coil has a controllable damped resonant frequency, which can be dynamically tuned away from the driving frequency of the primary coil by a variable resistor and/or by varying a duty cycle of a rapidly switched electrical element. If a control loop in the implant senses that more power is being received at the second coil than is actually being used to recharge the battery, the control loop temporarily changes the variable resistance. When this happens, the resonant frequency of the secondary coil is detuned slightly away from the driving frequency, so that less of the incoming power is absorbed by the secondary coil. Alternatively, the secondary coil may be temporarily short-circuited. With less or no excess power entering the circuitry of the implant, the problem of overheating is mitigated.
