Rechargeable batteries in cellular phones and other portable communication devices, such as NiCd, nickel-metal hydride (NiMH), Lithium-ion, and Lithium-Polymer batteries, can be recharged with household alternating current (AC) power coupled through a voltage reduction transformer, an alternating-to-direct current converter, and appropriate battery monitoring and charging circuits. They can also be recharged with a 12-volt cigarette lighter socket provided in an automobile coupled through a DC voltage reduction circuit and appropriate battery monitoring and charging circuits. However, in both cases, the portable communication device must be plugged into the household AC power source or into the automobile power source, limiting the mobility of the communication device.
Recently, wireless charging has become available for rechargeable batteries in cellular phones and other portable communication devices, using contact-less electromagnetic induction. A power source circuit in a wireless charging device drives a resonant frequency oscillator that produces a source alternating current in a frequency range between 50 kHz and 20 MHz, which is driven through a transmitting coil in the charging device. The alternating magnetic field produced by the transmitting coil inductively couples with a corresponding receiving coil in the cellular phone or other portable communication device, thereby producing a corresponding induced alternating current that drives an oscillator at its resonant frequency in the range between 50 kHz and 20 MHz to produce an output AC voltage. A conversion circuit in the cellular phone or other portable communication device, uses a transformer to adjust the output AC voltage, an alternating-to-direct current converter, and appropriate battery monitoring and charging circuits to produce an appropriate DC charging voltage for the rechargeable battery.