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Wireless charging of medical devices

Posted: 14 Mar 2012     Print Version  Bookmark and Share

Keywords:Wireless charging  electric vehicles  electro dynamic induction 

Electromagnetic energy from radio towers, mobile phones, Wi-Fi, routers, and now wireless charging, are categorised as non-ionizing radiation, and are believed to be harmless. Ionizing rays from x-rays, on the other hand, have been shown to cause cancer. As the number of non-ionizing devices increases, people begin to question safety. Regulatory authorities are waiting for evidence and will only impose restrictions if a health risk can be scientifically proven. Meanwhile, parents object to schools installing Wi-Fi, and homeowners protest about electric meters that transmit data without wires. Radiation from wireless chargers may be seen as harmless because they don't transmit intelligence. In most cases, the radiation in hospitals or care homes is low enough not to worry, but it's the field strength and close proximity to the source that could add to potential harm.

Charging EVs without a plug or cable offers the ultimate in convenience as the driver simply parks the vehicle over a transmit coil. Engineers talk about embedding charging coils into highways for continuous charging while driving or when waiting at a traffic light. While this is technically feasible, cost, efficiency, and radiation issues at these higher powers are insurmountable challenges.

At a transfer efficiency of 80% to 90%, 10% to 20% of the power is lost. This reflects in a substantial energy cost to the user and should be calculated as a decrease in drivable distance per watt. Applied to a large vehicle population, this goes against the efforts to conserve energy. Daimler's Head of Future Mobility, Professor Herbert Kohler, says that inductive charging for EVs is at least 15 years away and cautioned about safety. The potential radiation of EV charging is higher than Wi-Fi or talking on a mobile phone; it could also endanger people wearing a pacemaker.

Besides low efficiency and radiation concerns, wireless charging offers decisive advantages in industry. It allows safe charging in a hazardous environment where an electrical spark through charge contacts could cause an explosion, or where heavy grease, dust, and corrosion would make electrical contacts impractical. Eliminating contacts also helps in sterilizing surgical tools, as well as preventing breakage of contacts on multiple insertions. There is, however, a cost premium and this is especially apparent in custom devices that can't take advantage of cost reductions through mass production.

Currently, a wireless charging station will cost about 25% more than a regular charger. A 25% premium also applies to the receiver. If the portable device can't be charged with the battery installed, as is possible with a mobile phone or wireless patient monitor, then each battery would need its own receiver and the battery pack would bear the added cost. Unless wireless charging is necessary for convenience or environmental reasons, charging through battery contact continues to be a practical alternative.

About the author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. For three decades, Buchmann has studied the behaviour of rechargeable batteries in practical, everyday applications, and has written articles and books, including "Batteries in a Portable World." Cadex specialises in the design and manufacturing of battery chargers, analysers and monitoring devices.

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