I use a QI charger with my phone. It is so easy to use compared to fiddling with a micro USB 
Thanks for the excellent write up and explanations. Much appreciated!
How does the receiver attach to your phone? Tablet?
I love this idea. I have broken more screens and USB connectors forgetting they are plugged in, or someone walking by and snagging the charge wire. This looks like a good solution for the accident prone
Depends on the phone:
- Some phone has already one build in.
- The above patch will fit between the back cover and the phone and the phone has a connector/contact points for it.
- You get a new back cover with it build in and it might fit a connector/contact points inside the phone.
- You buy a patch and glue it to the back of the phone, it is connected to the usb input
Try do a google search on you phone model and QI to see if anything pops up.
Do not expect anything to work if the phone has a metal back cover. This is a problem for me, I love phones with metal covers, but would like to get QI on my next phone.
Great timing, I just ordered some QI chargers and receivers a few days ago. The one thing that concerned me from the reviews were reports of batteries getting overheated, but it wasn’t clear whether it was too much charging current to the batteries or just trapped heat from the receiver. Looking at your thermal shot of the receiver, it looks like they need to do some more work to keep the electronics cooler.
KuoH
Very nice little look into qi charging. A while ago I got one of these chargers and glued 4 magnets inside the puck directly adjacent to the coil and mounted the puck on an incline as a desk charger. My phone has strategicly placed steel plates in the back around the antenna so I just place my phone on the puck and it sticks to it and charges. Never measured current draw before and after but now I’m thinking I will do so. Also I did not come up with the idea for the dock, there’s a thread on xda about it. My phone is the nexus 5.
I hope the next Nexus will have qi. I want to uprade from my Optimus G which is the bastard sibling of the Nexus 4.
Very informative write up.
I had a Samsung Note3 with the Qi backplate and charger base, and despite being heavy on current draw (1.6A 5V) It didn’t charge my phone nearly half as fast as with a 2A charger connected. It also got very hot…
There is plenty of room for improvement for this technology, I’m sure one day it will have close to full efficiency and probably replaces the physical connection in all the devices. The true “wireless era” is only a few years away.
I did not thing about the heat, I have just added a chapter more to the article about it.
No need to plug to charge?
I like the idea but only few mobile phone support that function 
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:
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.