How Wireless Charging Works
The fact that over 200,000 people have downloaded one of the various “shake to charge” apps, now available from Google Play, indicates our willingness to suspend any form of practical reasoning in pursuit of the dream of wireless charging. A quick investigation of the source code would likely reveal these apps do little more than to link the interrupt signal from the accelerometer to a progress bar indicating an alleged battery charge. A piezoelectric  accelerometer could generate a small voltage secondary to deformations induced by rapid motions applied to it, however trying to use that millivolt signal to charge a battery would not be practical. In order words, shaking your smartphone isn’t going to do anything but get your arm tired.
During any energy conversion there will be losses in going from one form to another. The magnitude of those losses is what dictates the practicality of any type of wireless charging. Magnetic or inductive charging, in particular has been effectively used for some time to power various kinds of biomedical implants. Presently it is the safest and most enduring method to accomplish the job of transferring power to the inside of the body. In these systems, oscillating current in an external coil of wire generates a changing magnetic field which induces a voltage inside an implanted coil. The current resultant from this voltage can charge a battery or power the device directly.
While a moving magnet might just as well be used to externally generate the field, an external coil is simply more practical. Apple has just filed a patent for hardware which could make the shake to charge concept a reality, at least in theory. They claim a unique design incorporating internal moveable magnets, and a flat printed circuit board coil. Current chip efficiencies will however preclude practical implementation of this scheme for some time.
Many smartphone users will be wondering wonder whether their near field communication (NFC ) chip can be used to harvest power from a dedicated external source, or perhaps an ambient electromagnetic source like WiFi. In theory it is possible and such systems are on the market already, however not every NFC chip would be up to the task. To achieve maximum efficiency the system should be optimized for a use at a particular separation distance, angle of incidence, phase, and frequency such that it is in a resonant condition.
Resonance in an electromagnetic system can be likened to pushing a child on swing only when the swing is at the high point. Anywhere else and the energy transferred to the child will be reduced. If the separation distance is no more than a quarter of the wavelength, such a system can operate at efficiencies up to 35% .
One thing to keep in mind when considering wireless charging: If your charging system is throwing away nearly all of the 10 or so amps available from your wall outlet just to provide you with convenient at-a-distance charging, not only will charging be wasteful but it will be slow. Other wireless charging technologies relying on ultrasound or solar power are being developed, for example by Ubeam . For the time being, however, magnetic inductive charging technologies — spearheaded by the Qi consortium and smartphones like the Nokia Lumia 920  — such have taken the stage.
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October 16, 2012