Finding the wireless charging coil

[metrologist]

My new phone has wireless charging and I happen to have a battery bank that supports wireless charging.

I'm having a hard time getting it started and sometimes it decouples. I can find images of where the coil is on my phone, but not the battery bank, so how can I locate the coil without ripping the case apart?

I thought of using a near field probe, probably the large loop, and look for the strongest signal on an o-scope or SPA.

I also thought of buying some magnetic field film, but not sure that would work.

Another interesting thing is they put small square metal plate on the back of the rubber case and that thing gets very hot when charging, and it actually slows the charge way down and drains the battery, even when plugged into a USB charger.

[Infraviolet]

Solder together an LED, a coiled inductor and a capacitor in parallel. Ideal values for them will be set by the frequency of the wireles charger. Place this little thing around the battery pack and see where it glows brightest. There is a standard frequency for the common wirles charging methods, but I can't rememebr what. The product L*C needs to be set appropriately for that frequency so it will resonate.

Look up "wireless LEDs" online to see example people have made and bought.

Note that if there are previous negotiation communications (either via the wireless charging fields or over some other medium, wi-fi?) then the wireless LED might not have a changing field available to light up from until this negotiation is complete between the phone and battery bank. If the negotiation is done vi the wireless charging coils then you'll probably see the LED making occasional pulses each time the battery pack sends out an "is there anything near me to be charged" query.

[amyk]

You can also ask someone to x-ray it.

[Andy Chee]

Another interesting thing is they put small square metal plate on the back of the rubber case and that thing gets very hot when charging

There's your problem!  You don't have a battery charger, you have an induction cooktop hotplate!

[Jwillis]

Wireless chargers are very simple concepts . Essentially the charger is a Primary coil with AC  passing though it . Add a plate (Core) and a Secondary coil in your phone and you have a transformer. The core between the charger and phone coil directs and confines the alternating field. This will get warm. The secondary coil in the phone will connect to a rectifier and charging circuit .
A coil of wire connected to the oscilloscope probes will act as a secondary. You can locate the Primary coil of the charger and see if it is operating. You may need a resistor in parallel to give some load to the secondary coil on  the probes. You may also need to place a piece of ferrous material between the charger and coil on the oscilloscope. You should see a wave . The wave will likely be a square wave.
The charger will also have a power rating . If that power rating is exceeded it will shut down due to internal protection.
Like any transformer the core will over heat if the current drawn is to high.