Quote:
Originally Posted by HarryT
The efficiency will fall off with the inverse square of the distance between the device and the charger, so a case of any significant thickness will probably noticeably increase the charge time.
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I don't think it's necessarily an inverse-square relation. (If nothing else, inverse square relationships tend to infinity as separation approaches zero, and efficiency clearly has a definite upper bound so that can't happen!).
Inverse square laws tend to be for point sources (or where the separation between the elements is very large with respect to the size of those elements) - for example the electric field generated by a charged plane doesn't fall off with an inverse square so long as we're close to that plane (where close means distances similar to the size of the plane) - an infinite charged plane will generate a uniform field that is independent of the distance from it.
In this case we're looking at coupling between two planar inductive coils of reasonable sizes. IIRC, the transfer efficiency of such an arrangement remains pretty good so long as the distance between the coils is smaller than the diameter of the coils (the smaller coil in the case where the transmitter and receiver coils are different sizes).
I don't know the size of the coils in the case of Qi, but looking at the size of device I'd say it would likely be larger than the thickness of most cases.
Of course, the electromagnetic properties of the materials of the case could make it worse than we'd get just by increasing the distance.
/JB