I have a project that requires wireless charging of a small hand-held controller with a single 1S lithium-ion cell. People's initial thought was to use the standard Qi mobile phone chargers to save reinventing the wheel, however they appear to need large, flat coils which isn't ideal for the form factor we have.
What IS ideal is the system used in electric toohbrushes which have a smaller base that drops onto a little charging dock - every teardown I've seen online they appear to use a very similar if not identical little coil of wire and very simple charging circuit, which is perfect.
However, I've not seen any great detail on how they work, if there's a standard system or chipset in use or if it's just something that one manufacturer came up with and everyone's kinda copied.
As this is not my area of expertise it would be really helpful if there's any sort of appnote or design / standard out there for this application, and indeed any off-the-shelf components such as standardised coils that could be incorporated in a design.
Ken Shriff's teardown shows the circuit and describes it briefly, all the others seem to gloss over it.
To charge the toothbrush, it is set on a stand and charges inductively without physically being plugged in. A coil in the stand is magnetically coupled to a coil in the toothbrush, transmitting the power wirelessly. You can see the coil at the bottom of the toothbrush. When set on the stand, the coil picks up about 12 volts, which is used to charge the battery. The power is transmitted at high frequency (80kHz) for efficiency.
The coil is connected to a diode bridge that converts the power to DC. It then goes through a transistor circuit that regulates the charging, as directed by the microcontroller.