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Comment by mjsweet | Hacker News Reader

Is it my understanding that induction rings can't overlap?

You can't just plonk a phone, watch and AirPods case anywhere on a charging pad and expect them to start charging right?

The coil in each device needs to be fairly well aligned with the coil in the Qi charging pad. The proper way to do this is to have three individual coils with visual location targets for each device to charge.

I wonder if Apple was trying to make it a bit too magical (judging by the pre-render shots) allowing the user to throw a device onto the pad anywhere and expect them to charge.

Do the laws of physics apply here?

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Check out "Fig 2" on this page (coil diagram): https://ifixit.org/blog/14883/what-finally-killed-airpower/

Or "Fig 5" on this page: https://appleinsider.com/articles/19/02/19/airpods-wireless-...

P.S. Glad to finally find a technical comment, after scrolling through 200 marketing/business comments. :)

> I wonder if Apple was trying to make it a bit too magical (judging by the pre-render shots) allowing the user to throw a device onto the pad anywhere and expect them to charge.

That was exactly the sales pitch.

Following up on this, why is it impossible to use just one coil that's more powerful? IIUC To transmit power, you just need a temporally varying EM field to induce current on the other end, so why not have one that produces enough power? (My Electromagnetics course was 2-3 years ago, so forgive me if I'm missing something basic.)

A larger (diameter) coil would require much more power to create a magnetic field capable of inducing a (useful) current in the device's battery charger. Also remember that induction requires AC. A high-power AC circuit creates a lot of RF noise.

Qi includes communications between the charger and the charged device, it's not just a "dumb" magnetic field. I'm not sure what it's negotiating, but if it's something like the power level that could cause conflicts between multiple devices.

> Is it my understanding that induction rings can't overlap?

They technically can and that is what Apple was trying to do but the overlap creates a lot of weird RF noise and makes stable induction difficult.

> Do the laws of physics apply here?

Yes, it's just a much harder physics problem.

If you send power to one coil, some % will be induced in adjacent coils, right? And I guess that radiates interference like an antenna?

So question: could an inverse signal be applied to adjacent coils to cancel out the interference (from induction)?

I believe that is what Apple was trying to do, but again, it is a very hard, non-linear problem. By attempting to counteract that noise, you get a bunch of competing, variable magnetic fields.

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