Near Field Magnetic Induction Charging

Mike Huselton

I want to charge lithium-ion batteries using magnetic coupling. This occurs in cordless toothbrushes that rest on a waterproof charging base.

I want to charge a 3.6 volt lithium polymer cell. I know all about the physics of inductive coupling and transformers.

What do you recommend for coil in the fixed base and portable unit? I need recommendations on frequency, inductance, coil turns and such.

I want magnetic coupling for safety reasons and easy-of-use. I've got plenty of circuits to charge lithium polymer batteries.

But the magnetics I'm not so sure of. Damn the inverse square law!

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mctrivia

obviously because of inverse square law you want as physically close together as possible.

As for frequency it depends on the coil size but uully higher frequencies are better. 200kHz is probably a good starting point.

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Mike Huselton

200 khz sounds like a good start. High frequency efficiencies, Q-factors and all that...
I'll wind the coil on a plastic sewing-machine bobbin, then try different caps to zero in on the resonant tank freq.
Light weight is also a freebie, since the bobbin is air core. If need be, I can insert a ferrite slug if things don't work out.
Maybe a Colpitts oscillator or just a 555 driving a power FET or 3055 junction transistor will do.

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Post Edited (James Michael Huselton) : 2/28/2009 3:16:34 AM GMT

Carl Hayes

Ferrite cores are wonderful for confining a magnetic circuit so that it will not couple to nearby objects. But that seems to be the opposite of what you want to do, eh? You want to couple the magnetic circuit in the base, and the magnetic circuit in the portable object, so that they become a single magnetic circuit.

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· -- Carl, nn5i@arrl.net

Mike Huselton

Yup. Transformer 101. I need to obtain some #30 magnet wire. Or scavenge the junk box.

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heater

Don't be so hard on the inverse square law.

It saves us from, well just about everything. Noise, heat, light, xrays, alpha, beta, gama, mother-in-laws....[noparse]:)[/noparse]

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For me, the past is not over yet.

Mike Huselton

In a five-dimensional universe, the force of gravity would be enough to orbit small objects around a fat person.
Stars would collapse around a mile across and would last one year. Sound would propogate very weirdly.

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Post Edited (James Michael Huselton) : 2/28/2009 4:29:48 AM GMT

Leon

RF in the near-field obeys an inverse cube law.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Philldapill

Ah ha! I was thinking that, Leon...

(I'm in my first of 3 Electromagnetics classes at my University)

Ken Peterson

if you want to maximize coupling, put one coil inside of the other. That's probably what happens in the toothbrush charger. When you load down your circuit, it will tend to reduce your resonant frequency. So tune it under load, i.e., with the battery charging.

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Mike Huselton

Good suggestion.

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