Recently I encountered the following idea: that a mobile device manufacturer is not interested in including wireless charging in his devices because that prohibits him from using metal cases in the future (without excluding wireless charging from future devices of course). Sounds reasonable but...

Wireless charging uses the same technique as the transformer and the transformer includes a core. Perhaps the metal case could act as such core.

Is wireless charging indeed impossible for a device in a metal case like for example a mobile phone? Would it be possible to perhaps leave a small window permanently covered with plastic and use it for wireless charging?

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    \$\begingroup\$ Actually they're much more interested in wireless charging than in metal enclosures! \$\endgroup\$ – stevenvh Sep 21 '12 at 12:17
  • \$\begingroup\$ @stevenvh: Whatever, let's pretend that metal cases are top priority. \$\endgroup\$ – sharptooth Sep 21 '12 at 12:23

Wireless charging is currently most often done using a magnetic field (aka "near field" or inductive power transfer) and I'll assume that is what you mean.

If you can provide a metal free window then you can charge via it. Metal which intersects the field will have flux induced in it and there will be eddy current losses. With a window and suitable care you could minimise the field strength at the metal edges and keep losses low.

You can transfer power inductively through metal - you will incur eddy current losses whose magnitude will relate to the resistivity of the metal and it's thickness. Very thin metals and those of lower conductivity / higher resistivity will incur lower losses.

The transmit and receive coils are almost invariably resonated and voltage levels are far higher in the intended circuits than in the spurious ones.

Despite various claims and patents, this system is far from new. If we ignore Tesla and a century + of transformers - I saw IPT demonstrated at a power level of hundreds of Watts in 1972 or 1973 - just under 40 years ago. Professor Don Otto of auckland University had a number of related patents 'way back then' and many modern claims to originality seem to be hopeful of spurious.

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  • \$\begingroup\$ Will it be fair to say that charging through a full metal case will be less efficient yet still practical? \$\endgroup\$ – sharptooth Sep 21 '12 at 13:20
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    \$\begingroup\$ In the transformer the field lines are with the core; in the enclosure they would be perpendicular to it. A transformer core typically has a fine gap to prevent complete circle eddy current paths, or may alternatively be made of powdered iron suspended in an electrically insulating binder. The answer is thus that both the geometry of the situation and the material properties are different. \$\endgroup\$ – Chris Stratton Sep 21 '12 at 15:28
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    \$\begingroup\$ The transformer uses the field in the core as a means of coupling two inductors. But note the use of laminations in the core - these separate the layers so that current cannot flow between them. As Chris notes - powdered iron cores do the same thing but more so by further reducing the available current paths. [Transformer air gap is used to reduce flux in core to prevent saturation . Effect on losses is not primaty reason for having it.] \$\endgroup\$ – Russell McMahon Sep 21 '12 at 17:14
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    \$\begingroup\$ ... use (resistivity, frequency & thickness arranged so that losses are low) then useful power will be available beyond the metal (my brain says - as long as it's not asleep it may be right :-) ). | Modern induction heaters only work on certain materials as the maximum sensible frequencies that can be used will not produce adequate eddy current losses in eg copper saucepan bases. When/if switching devices allow power levels at higher frequencies economically this will change (I'm told). I imagine a microwave pan heater may work but also may "have issues" [tm] for other reasons. ... \$\endgroup\$ – Russell McMahon Apr 24 '17 at 4:23
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    \$\begingroup\$ ... | I read long long long ago about a company that established an illegal radio link at VHF by installing a yagi INSIDE a "tin" (corrugated iron) roof with the spacing set approprriately. Presumably they were aiming for a current null at the roof material boundary - some head scratching and/or experiment may be needed as probably very much near field. \$\endgroup\$ – Russell McMahon Apr 24 '17 at 4:23

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