(Editor's note: Also of interest by this author are "Risk management in batteries for medical (and other) applications"; and "Battery fuel gauge: factual or fallacy?")
Wireless charging may one day replace plugs and wires similar to how Wi-Fi and Bluetooth have modernized personal communication. Wireless charging with inductive coupling uses an electromagnetic field that transfers energy from the transmitter to the receiver, and this technology is a suitable method to charge medical devices.
How does wireless charging relate to radio transmission? Both are similar in that they transmit power by electro-magnetic waves. Wireless charging operates in a near field condition in which the primary coil produces a magnetic field that’s picked up by the secondary coil in close proximity. The radio transmitter works on the far-field principle by sending waves that travel through space. While the receiving coil of the wireless charger captures most of the energy generated, the receiving antenna of the radio needs only a few microvolts to raise the signal above the noise level and receive clear intelligence when amplified.
Types of wireless charging
Wireless charging is classified into three categories: 1) radio charging, 2) inductive charging, and 3) resonance charging.
Radio charging will serve low-power devices operating within a 10-m radius from the transmitter to charge batteries in medical implants, hearing aids, watches, and entertainment devices. Radio charging can also activate advanced RFID (radio-frequency identification) chips through resonantly enhanced induction. The transmitter sends a low-power radio wave at a frequency of 915 MHz (frequency for microwave ovens) and the receiver converts the signal to energy. The radio-charging method is closest to a regular radio transmitter; it offers high flexibility but has low power capture and exposes people to electro-smog.
Most of today’s wireless chargers use inductive charging, featuring a transmit coil and a receive coil in close proximity. Electric toothbrushes were one of the first devices to use this charging method, and mobile phones are the largest growing sector to charge without wires. To retrofit an existing mobile phone for mobile charging, simply attach a “skin” that contains the receiver and provides interconnection to the charger socket. Many new devices will have this feature built in.
For larger batteries, such as those in electric vehicles (EVs), resonance charging, or electro dynamic induction, is being developed. Resonance charging works by making a coil ring. The oscillating magnetic field works within a 1-m radius; the distance between the transmit and receive coils must be well within the 1/4 wavelength (915 MHz has a wavelength of 0.328 m). Currently, resonance charging in trials can deliver about 3000 W at a transfer efficiency of 80% to 90%.
The success of wireless charging was subject to adapting a global standard, and the WPC (Wireless Power Consortium) accomplished this in 2008. With the “Qi” norm, device manufacturers can now build charger platforms to serve a broad range of Qi-compatible devices. The first release limits the power to 5 W.
"Charge medical devices without wires" looks at how wireless charging and charging mats may be beneficial for medical devices, as well as some of the drawbacks of this approach; it was previously posted at Medical Electronics Design.
About the author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. For three decades, Buchmann has studied the behavior of rechargeable batteries in practical, everyday applications, and has written articles and books, including “Batteries in a Portable World.”
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