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I have a question regarding wireless charging. Traditionally, you have two coils, the charger and the receiver for wireless charging with Qi (from what I understand).

Would you be able to have an additional coil in between the two and extend the range from charger to receiver by a few millimeters? I'm very unsure of the science here, so I thought I'd ask here.

Here's a really garbage diagram of what I'm thinking: https://imgur.com/a/ncjwxyh

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It's hard to say without doing a very detailed analysis. The magnetic field falls off with the distance cubed, but a coil in the middle with have resistive losses associated with the coil. The coils are also most likely tuned to certain frequency and adding in extra inductance may also change the resonance of the charger and also create loss. I would think that for a few mm, adding in an extra coil will create more losses than efficiency gains .

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  • \$\begingroup\$ That's very helpful. I was hopeful that by just adding in the coil it could help alleviate the distance cubed factor by making the distance between the extender and receiver the new distance variable. Is there any way to test this out with putting a coil in between that was bought online as a trial? Thank you for your response! \$\endgroup\$ – Graham Bailey Aug 15 '18 at 22:44
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Would you be able to have an additional coil in between the two and extend the range from charger to receiver by a few millimeters?

Using a single extra coil, no unfortunately (resonant tuned or not).

You can use two extra coils where one coil couples closely with the transmit coil and the other couples closely with the receive coil but you have to connect the 1st extra coil (regard it as a secondary to the transmit coil) to the other extra coil and this then becomes primary to the receive coil.

In other words, the relatively strong magnetic field close to the transmit coil is turned back to electricity and transported the extra distance on wires whereupon it is fed to the 2nd extra coil.

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  • \$\begingroup\$ If properly optimized, the maximum efficiency of the three coil system can be higher than the maximum efficiency of the two-coil setup. It is analytically proven. see this paper \$\endgroup\$ – Pojj Sep 1 '18 at 20:47

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