Inductive Charger from Scratch

I have a project, actually, a business that needs custom wireless charging and wireless comms. I thought I would document that process here - one, so that I can remember where I left off, and two, it might be interesting.

26/02/26

Project started today!

I needed to make an Amazon order today, so I wanted to add some stuff instead of ordering a small basket. I remembered I would need an inductive charging solution, so I started thinking, what would I need?

Requirements

The project has some interesting requirements:

Must wirelessly charge a device that’s shaped as a ball
Must be able to sustain the operating power of the ball device (a few watts, probably).
Must be able to also charge during operation - let’s say, 10W target power on the receiver side
The ball must be removable from the device
We must be able to transmit, either higher level, or lower level, motor commands to the base
The motor commands shall be:
- LEFT_100MS
- RIGHT_100MS
- DOWN_100MS
- UP_100MS
The ball must be magnetically attached to the base, with sufficient force to not be removed during operation at extreme (180 deg) angles.

So, with that, I thought: “Okay, I can’t use a traditional flat coil”. The reason being that my ball in spherical. If I want a decently sized coil (the radius is proportional to the distance at which the transmitter and receiver can be apart, roughly, the distance at which the inductive coupling can transmit power with reasonable efficiency is \(\frac{1}{2} \cdot d mm\) where \(d\) is the diameter of the coil). The math on that is pretty interesting, but I don’t have my apple pen to draw out diagrams nicely, when I do get that back, I will update this. But, essentially, at distances of one \(r\) then the maximum possible power transfer is a measly 12.5%.

That means, I need to keep the distance that separates the two coils as small as possible. BUT, the other issue, is that the inductance of the coil (more inductance means greater power transmission)