The users of smartphones, MP3 players and laptops share the same lot.

You have to recharge your small battery-powered devices over and over again.

This is not only time-consuming, but also annoying, especially when you are on the go and no power outlet is available.

It would be more practical if the batteries would charge themselves when they are empty.

And that without a charging cable and over the greatest possible distance.

A Japanese-American research group has now come a little closer to this vision.

Their method makes it possible to charge a cell phone contactlessly anywhere in a living room.

Manfred Lindinger

Editor in the “Nature and Science” section.

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The researchers led by Takuya Sasatani from the University of Tokyo use the principle of magnetic coupling, which is also used in transformers and modern induction hobs.

An alternating magnetic field, which is generated in a coil through which current flows, induces an electric current in an adjacent coil.

The electrical energy is transmitted by the alternating magnetic field.

The transmission medium is air.

The inductive coupling usually only works over distances of a few centimeters, which is already used in some wireless charging stations for cell phones and also with electric toothbrushes.

A few years ago, researchers led by André Kurs at the Massachusetts Institute of Technology demonstrated that magnetic induction can even transmit electrical energy over a distance of two meters by wirelessly lighting up a 60-watt light bulb.

The prerequisite for the success of the experiment was that the 60 centimeter receiver coil, which was connected to the light bulb, was precisely tuned to the frequency of the transmitter coil of the same size.

Contactless charging with no health risk

However, both coils had to be firmly fixed. If only one was moved back and forth, the resonance frequency changed and the efficiency of energy transfer decreased. With a self-regulating voltage amplifier that looked for the optimal energy transfer for every distance, colleagues from Kurs were later able to remedy the weak point. The transmitter and receiver coils could now be moved against each other over a distance of 20 to 120 centimeters without changing the brightness of the LED.

Takuya Sasatani and his colleagues have gone one step further. You have transformed a 3 by 3 by 2 meter room into a three-dimensional charging station, so to speak. A smartphone can be charged anywhere there. This is made possible by a special resonance effect in which the room itself becomes a resonator. In their experiment, the researchers lined the walls with a metal surface. The magnetic transmitter coil was positioned in the middle.

Measurements with sensors showed that a uniform magnetic field developed within the room. A cell phone, a lightbulb and a fan, each equipped with a small receiver coil, could be operated wirelessly at any point in the room. The power transfer was up to 50 percent, as Sasatani and his colleagues

write

in the journal

Nature Energy

. However, the transmitter coil must be aligned as perpendicular as possible to the field lines during charging so that optimal transmission is achieved.

The researchers also examined whether the fields pose a health risk. Using a human model, they determined how much energy biological tissue absorbs when a person is in the room. The researchers have no concerns. Your measurement results are within the limits set by the American Federal Communications Commission and the Institute of Electrical and Electronics Engineers (IEEE). However, patients with a pacemaker should be more careful.