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I have this wireless charger. I put the transmitter in a 3D printed enclosure and the receiver in another 3D printed enclosure. On top of the receiver, I've put 8x INR18650-30Q 3000mAh batteries.

I've done some experimenting and have ultimately decided that the batteries get hot when they are not connected to anything and the wireless charger is working.

Am I correct?

Is there a way to stop the heating?

Edit:

Am I correct? Yes, @John Doe answered that this kind of power transfer is used in induction cookers. This effect seems to cook my batteries while charging them.

Is there a way to stop the heating? I did some more experiments.

As @Neil_UK suggested, I added a copper sheet between the batteries and the wireless receiver coil.

enter image description here

In the above image, two batteries are on top of the receiver. One of them is coated with a copper sheet, while the other isn't. After a minute, the naked battery is hot, while the coated is barely warm.

enter image description here

I added a plastic separator of 10 mm between the coil and the battery pack of 10 batteries in the above image. The current is 0.347A. Almost twice the nominal of 0.158A.

enter image description here

I added a copper sheet inside the battery pack just below the batteries. The current dropped to 0.187A. Still above nominal. So I added another sheet and it dropped to 0.142A.

After 10 minutes of running the wireless charger and two copper sheets below the batteries, they didn't get hot. Thank you all for helping out.

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  • \$\begingroup\$ A battery charger and protection circuit? \$\endgroup\$
    – Passerby
    Jun 28, 2021 at 18:34
  • \$\begingroup\$ What happens if you replace the batteries with a block of wood? (i.e. -- is it the batteries or the charger?) \$\endgroup\$
    – TimWescott
    Jun 28, 2021 at 18:43
  • \$\begingroup\$ @TimWescott There is a layer of PLA (plastic) of 5 mm between the receiver and the batteries. It also warms up but not as much as the batteries. \$\endgroup\$ Jun 28, 2021 at 20:54
  • \$\begingroup\$ Screen the lossy battery cases with a less lossy copper sheet. \$\endgroup\$
    – Neil_UK
    Jun 28, 2021 at 20:58
  • \$\begingroup\$ @Neil_UK I actually have some old copper sheets at hand. I will try that, thanks. \$\endgroup\$ Jun 28, 2021 at 21:00

2 Answers 2

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Yes you are correct. The wireless charger in this case acts exactly like an induction cooker. Even though the battery is not connected, the changing electromagnetic field produced by the wireless charger induces an eddy current in the metal components (such as electrode plates) inside the battery, causing them to heat up. The battery in this case acts like a metal pot placed on an induction cooker.

Both induction cookers and wireless chargers transfer energy through electromagnetism. The energy is reproduced as heat by chaotic eddy current in the cookware, but as ordered and filtered usable energy in the receiver coil in a wireless charging receiver.

Below is an image of the inside of an induction cooker. As can be seen it has the same construction as a wireless charger.

Image from Wikipedia courtesy of Wdwd, CC BY-SA 3.0 View citation

Inside view of an induction cooker: the large copper coil forms the magnetic field, a cooling fan is visible below it, and power supply and line filter surround the coil. In the centre of the coil is a temperature sensor, covered in white thermal grease

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It is possible that the magnetic field goes up to the batteries and heats them by induction; the battery body is usually some kind of steel so it's quite susceptible. You should also see the batteries as load in the transmitter if it has such a measurement interface.

The obvious way would be to move the batteries, if possible; otherwise you could try to work on the transmitter coil to see if you can reduce the range (without reducing the power capability); I'm not an expert in the field but technically it's a magnetic antenna so it has some radiation pattern.

Have you looked for application notes regarding your power transmitter?

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  • \$\begingroup\$ I can't move the batteries because they need to be at the base of the robot for the balance problem. I tried to put a ferrite sheet between the receiver and the batteries but the receiver didn't like that and started smoking. The only important application note is to not make the transmitter and receiver coils closer than 7 mm, which I believe is so. \$\endgroup\$ Jun 28, 2021 at 20:55

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