A look at QI (Wireless power transfer)

Love wireless charging on my Lumia phone!

Ordered two wireless chargers from Ebay, one was cheapest square one - didnt work, phone got very hot but was not charging, while the other one, cheapest round one, chargers very good!

I like the convenience of wireless charging - if your phone spends most of its time on the desk or at least significant time, you can always place it on charging pad and you will less likely have to worry about your smart phone running out of battery at the end of they day ;)!

Only drawback that on some chargers you need to place phone on them pretty accurately to make contact as well as external silicone shells might add 1mm or 2mm to the distance between charger and phone thus making important perfect placement of your phone on charging pad!

I bought two Qi chargers, one was heating up battery and not charging at all, the other is charging very well and the battery get only warm, not hot. I charge it with the cable pretty rarely so cant say for sure how much warmer the battery gets when Qi is being used, but it appears that it does get warmer, but not to the point of dangerously warm IMHO.

Qualcomm supposedly have found a way around it:

https://www.qualcomm.com/news/releases/2015/07/28/qualcomm-becomes-first-company-enable-wireless-charging-mobile-devices

This is finally the practical version of Nicola Tesla’s scheme of world broadcast power, without the disadvantages like burning down all trees that happened to be in resonance.
It was also my high school science fair project. I lighted a flashlight bulb with my train transformer with cardboard between the coils of wire. Not very neat, but it got me the extra points in my physics class.

If you guys are tired of plugging in usb cables to charge, check out Sony Xperia magnetic charging cables, they work great & just as fast as normal usb charging.

The only downside is they are only available on some models of the Xperia line. And unfortunately not on the latest (for sale ;)) Xperia Z3+

The problem of heating batteries during wireless charging can be attributed to two things. 1) The thickness of shield plate behind the receive coil. 2) Whether the transmitter uses an attractor magnet to help guide and center the phone on the charging base.

The shield plate is critical for operation as it provides a path for the magnetic flux, concentrating it through the coil (and not through the phone). Thicker is better, but thicker means the phone is thicker, and its hard to market a thicker phone. And because thin uses less material, thinner is cheaper. But a thin receiver shield can work well enough... except when it's used with a magnetic centering transmitter. This is because all magnetics have a saturation point at which they can no longer conduct magnetic flux, related to their volume (in this case thickness). The field from the magnet shifts the saturation point of the receiver causing it to be much less efficient.

The result is the shield begins heating and the flux finds a different path to follow, like through the battery. :(

Unfortunately the battery is not particularly good at magnetics, and also heats-up due to something called "eddy current losses". This is how inductive cooking works.

Because the wireless receiver is usually very close to the battery, heat generated in either will cause a bad affect on the battery.

Bottom line: choose a charging pad without a magnet. And hope the Qi receiver supports thermal management (it should). Display of the "Qi" logo implies adherence to the standard. (They would never lie about that , would they?)

That would be very interesting to me if I were interested in cell phones. Sounds like a good approach would be to calculate all that with the finite difference time domain method. It was first published by a man I worked with at Lockheed. It isn’t that time is evolved directly, it is that the saturation mixes frequencies so frequency domain methods are not directly applicable.

Do you know if the receiving coil/chip has a parasitic draw?

If you are thinking about current going from the phone into the patch, I have not measured it.

I do not believe it is relevant, the patch is not mounted directly across the battery and the phone is not supposed to boost battery voltage and then supply it to the patch.

I was confused as heck, but I think I’m figuring this - those red graphs marked “Input Current” on the legends are actually “output voltage”… right?

Ahh, okay. I was curious if the patch had to draw power for the electronics from the host to sense for the charger.

Oops, you are correct, I need to fix that.

No, the small pulses each second has enough power to start the receiver and let it send a request for power back.

I suppose it would not be a good idea to have this technology charging in your bed room?

This transmitter has a fairly low power and the low frequency has low effect on humans, it is not like mobil phone transmitters that can heat tissue.

People that lives near long wave radio transmitters receives more radiation.

The patch doesn't draw any power from the host, all the power it consumes comes from the transmitter. The patch has a blocking diode insuring current only flows out of the patch.

To get Qi certification, devices are supposed to be at least 70% efficient. I'm sure that's near impossible to enforce considering the volume of suppliers.

A QI charger contains two devices (Transmitter and receiver), what device has to be 70% efficient? and how efficient do the other one have to be?

Specifying that the total must be 70% gives a lot of wriggle room, i.e. you get the certification with one of the devices a super efficient design and the other a average design. The user can then buy two cheap average devices and the total efficiency is below 70%.

There is also the question if Chinese companies bother with the certifications.

The Qi spec for tansmitters is pretty well locked down, but still allows quite a few different designs, but the transmitters need to be tested to certify they meet the standard. I'm not sure if (but I think) the transmitters need to meet a certain efficiency level (I've only designed receivers). The idea here is that any receiver should work with any transmitter.The spec is more relaxed on receivers, if you put a poor receiver in your product, your product and reputation will suffer.

The overall system 70% doesn't give a lot of wiggle room. If the trans were 80%, the receiver would need to be over 87% to hit 70% total. The Qi standard need not be applied if the product doesn't bear the Qi logo. And there are many applications (like mine) that don't hit the 70% target. I use a very small coil, which causes a coupling mismatch, and I'm only transferring about 400mW, so the conversion losses eat up a lot of my efficiency.

It should be noted that the efficiency is tested at full load (about 10W). As the power transfer tapers off - as a battery reaches full charge - the efficiency will drop well below 40%.

But for putting power back into a sealed system it hard to beat wireless power transfer.

See http://www.wirelesspowerconsortium.com/

A watch perhaps? Could you share any pictures?

I really would not want those in my bed room despite all the assurances :stuck_out_tongue:

Here's a link to the product website:

Slides 3 and 5 show it on a charging puck. Slide 4 shows it worn on a finger.

The Kinesia product line monitors and assesses tremor and motion symptoms in patients with Parkinson's disease. The wireless recharging allows the unit to withstand the inevitable dunking in water, and the lack of connectors really improves ESD resistance.