Wireless Add-On Charger

Thread Starter

MrAl

Joined Jun 17, 2014
8,508
Hello,

Yesterday i got one of those wireless charger add-ons that you add to your phone.
The coil plugs into the USB port and the coil part sits flat against the underside of the phone. When the coil gets close to the transmitting plate, the phone beings to charge. The spec was 15 watts but it only charges at about 9 watts max, but that's not the real problem.

The real problem is that the coil never stops drawing current even when the phone is fully charged. The current stays at about 300ma. That means the coil is being heated with about 1.5 watts of power which means it gets warm. I would say it even gets very warm and stays that way all night long. The silicone phone case acts as a heat insulator too so the heat mostly goes into the phone back, where the battery is. Needless to say, it keeps the battery warmer than it should be when not being charged.

The problem seems to be with the thickness of the case material. The case is silicone about 1/16 inch chick, which unfortunately ends up between the coil and the charging plate. That means it adds distance between the transmit coil and the receive coil. Distance is very important in magnetic circuits, and even this small distance makes a difference.
If i take the case off and allow the receive coil to rest right on top of the charge plate the current will drop to around 90ma which isnt to bad even though the phone only draws 30ma or so with the regular wall wart (no wireless). If i add two or three index cards between the charge plate and the receive coil the current goes up. This confirms that the reason for the current is because the case keeps the receive coil too far from teh charge plate top.

At first this doesnt sound too bad, but when you feel the back of the phone you can note that it should not be that warm. Sure if you are in a cold environment that could actually help the battery, but with normal room temperature it is not good because any extra heat shortens the life of the battery and also increases the self discharge. How much is hard to tell, but the warming of the battery is not good and besides that it also eats up power when it should not require any power or very little.
But even still that' not the only problem.

This other problem is somewhat secondary but could be very important depending on how the phone USB port is to be used in normal operation. The problem is that once you plug in the receive coil, it takes up the only USB power on the phone, so you can not plug in a USB drive or keyboard or mouse or anything else. If you think you can just unplug the receiver coil to get the USB port free again, guess again. Once installed it is difficult to unplug the coil because the case cover it so you would have to take the case off first, then very carefully try to grab the USB connector of the coil to pull it out, and that is not easy because they designed it so that it is nearly flush with the bottom. That's very good if you never intend to remove it, but bad if you do because it takes time and effort to get it out.

I could probably put up with the loss of the use of the USB port but i cant put up with the extra heat. The batteries on new phones are not removable (or at least not easy at all) so it is harder to replace once it goes bad.

My guess is that when the designed the receive coil they tested it when it was on the back of the phone but with no case. The only way to get that to work is to glue the receive coil to the back of the phone, or use tape. That really puts the USB port out of use forever, but if you dont need it you would be ok and the extra heat would be minimal (almost no heating).

So in conclusion, you may be able to use one of these but only if you dont use a case and dont need the USB port for anything else.

A question that comes up is, could there be something wrong with the transmit charge plate? Perhaps it just can not sense the charge cycle is over and therefore keeps supplying current.

If anyone else here uses one it would be good to ehar from you.

Thanks and take care.
 

Alec_t

Joined Sep 17, 2013
12,264
Do you know if the phone's battery management system relies on a change in cell temperature to determine the end-of-charge point? If it does, that might be adversely affected by the heat generated by the charger.
 

Thread Starter

MrAl

Joined Jun 17, 2014
8,508
Do you know if the phone's battery management system relies on a change in cell temperature to determine the end-of-charge point? If it does, that might be adversely affected by the heat generated by the charger.
I dont know but that would not bother me as much as the fact that it keeps the bottom of the cell phone warm all night long, and long after the charge has terminated. With my wall charger plugged directly into the USB port the current drops down to maybe 30ma or less. That never happens with the coil, but also the phone does stop charging in both cases it does not keep charging it is just that the power to the coil keeps going and that heats up the coil with no electrical effect such as charging going on then.
 

Thread Starter

MrAl

Joined Jun 17, 2014
8,508
UPDATE

I tried a different base unit (the charging pad).

To be clear, if your phone does not natively support wireless charging then you have to buy two things:
1. The wireless adapter. This plugs into your phone USB port (USB C, Micro USB, etc.). It is a very very thin enclosed coil that folds back onto the back of the phone and it is surprising how thin it is so the case still fits on well. The cord is also a flat cable, and the USB connector is flat flush with the bottom of the phone. That part is well engineered i think.
2. The charging pad (the thing that sends power to #1 above). This plugs into your wall wart. The wall wart gives it power, the pad transmits power to #1 above via a changing magnetic field generated by the pad.

I got another charging pad made by Anker. This one is rated for 10 watts the other was rated for 15 watts but did not charge that high anyway. The previous one also would only do down to about 300ma after the charge was complete so it kept the back of the phone a little too warm even when not charging anymore. The Anker goes down to about 170ma when the charging is done which seems to be a lot better as the back of the phone is only slightly warm at that point.

The only mistake i made now was getting a charging pad that uses a micro USB itself, which is not my preferred USB connector i would rather have the USB C mini which the old charging pad had. But this new one seems to work better so i'll see how it goes and keep it if it works ok in the long run.

I got this new one mainly because it advertised that it works well even with a case thickness of 5mm and my case is only about 2mm thick. I am not even sure i would call it a case it is clear silicone and protects the edges and back and the front is protected via the raised edges that the case edges produce after installation.

I mainly wanted this so that i dont have to keep plugging and unplugging the USB connector in and out every time i go out and come back home. It's wear and tare on the connector.
I had previously tried magnetic USB connectors but they only lasted abotu 6 months then started to fail to connect.

I think the amount of back-phone-heating is due to the inefficiency of the coil on the back of the phone, but apparently the charge pad has something to do with it too.

I'll update again when i have more experience with it or find out more info.
 

Thread Starter

MrAl

Joined Jun 17, 2014
8,508
Quick update...

I measure now the max power at about 14 watts which is above the rated 10 watts advertised for this new charging pad.

The thing is though, i cant actually measure the battery current because there is no way to get into the phone. I cant even measure the charging port without unplugging and using a couple adapters to measure the current going into the actual phone. What i am measuring for now is the current and voltage going into the charging pad. Thus i am not yet accounting for the inefficiency of the pad itself and the coils.
I may do better measurements in the future. However, the charge time is not too bad so it cant be too low.

It's a little picky trying to get the phone lined up with the coil in the pad but lucky there is a blue LED on this one that stays lit when charging the phone so you know it made contact, and it stays lit. The older pad woudl have the LED go out after a couple seconds. This one light up as soon as it makes contact. Only problem with this one is that the LED is very very dim so hard to see even in the near dark
So there is a lot of variation in the way these things work. Be prepared to return one or two if you dont like them if you buy any.
 

Thread Starter

MrAl

Joined Jun 17, 2014
8,508
Another small update for those interested in getting one of these add on wireless chargers.

As i explained previously, one of the problems is you have to position the phone just right on the charging pad or it will not charge at all. I believe it is because the coils have to line up well enough to transfer the energy. This is a little hard to do and what happens is you end up moving the phone around a little as you wait for the blue light to come on or you wait for the phone to respond with a lit lock screen and a beep (you can set them to beep when the charger is plugged in).

I realized that it is not so much the lateral movement as the forward and backward movement that makes the difference. It is easy to judge the lateral position because the pad is just about the same width as the phone body. Unfortunately though the pad width (it is round in this case) does not extend the whole length of the phone (top to bottom of the screen) so you have to move it forward and back a few times to find the right position.

Since it is the forward and back movement that causes the problem, i put a rectangular weight just behind the pad so then when the top of the phone touches the weight the position is correct so there is no guesswork.
Just to try it out i used several small ceramic tiles piled on top of each other until the stack was high enough such that the phone top would touch it when it is in the right position. They have some weight to them so they dont move too easy.
In the future i will build a small platform with the necessary guide surfaces to guide the phone into place every time.
I suspect the reason the manufacturers of the pads dont do this yet is because the phone sizes vary so much. However, an adjustable platform would work for nearly every phone.
Of course when we travel we probably wont want to take the platform with us just the pad because it is easily packed while the platform will just take up more room. That's unless you dont mind taking that too.
 
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