Is it possible to wirelessly charge a battery?

14 March 2010

Question

Is it possible to wirelessly charge a battery? 

Answer

Well yes, it definitely is and people are doing exactly that with things like toothbrushes. You might have seen these toothbrush devices where you plug the toothbrush device into some kind of base station or holder, but there's no obvious electrodes. And what's going on there is it's using a magnetic field to convey the energy from the base station into the handset and then from there, into the battery. So what you do is have a coil in the base station which passes an electric current through it, generating a magnetic field and you pass an alternating current through it so that the magnetic field is changing. You then have in the base of the object you want to charge, another coil which you put into the magnetic field which is created by the base station. That's what's happening when you're docking one into the other. The magnetic coil in the device therefore sees this changing magnetic field which induces an electrical current in the handheld device. That current is then fed into some kind of rectifier, to turn an alternating current into a direct current and it's then used to charge a battery. And that happens with things like toothbrushes, there are certain shavers that work the same way. Doing it over a greater distance would be really, really inefficient because the magnetic field decays as 1/r2 so, inverse square law over distance and therefore the amount of energy you'd have to put into the coil would be huge in order to charge things at a remote distance. So it's useful over small distances, it's not very good over long distances.

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