Researchers at Stanford University have developed a technique to transfer power wirelessly at a few feet and in motion.

Source: Stanford University/YouTube

Wireless charging can be great convenience, cutting out the need to carry cords and plugs around all day in case your phone's battery dies. Today, though, it requires a phone be stationary and in contact with a charging pad.

Wireless charging at a distance could make life easier not just for people with smartphones, and for drones and electric vehicles, but also for patients with electronic medical implants.

Researchers at Stanford have now developed a highly efficient technique that enables wireless power transfer at a few feet and in motion.

The work builds on "magnetic resonance coupling" in which electricity passes through an oscillating magnetic field created by a pair of transmitting and receiving coils. The best result can be achieved if each coil is at a fixed distance and positioned at an optimal angle.

However, as the Stanford researchers detail in a new paper on nature.com, their new "robust" wireless power transfer system enables a steady charge at variable distances of up to few feet and, impressively, doesn't require manual tuning as the distance and angles between the two change.

The technique was developed by Stanford electrical engineer researchers Sid Assawaworrarit, Xiaofang Yu and Shanhui Fan.

While it has the potential to bring major improvements to electric vehicles, as Ars Technica notes, there are a number of limitations. The researchers have only demonstrated it working at a charge sufficient to keep a tiny LED lit, and the demonstration system is not a practical size.

The demo video shows a barbell-like contraption with two large disc-shaped cardboard boxes with a transmitter and receiver inside each. The LED attached to the receiver stays lit as the receiver slides further away from the transmitter and starts to fade after 75cm/29.5 inches. The power transfer stops fully at about a meter away.

To bypass tuning obstacles, the researchers switched the transmitter's radio-frequency source for a voltage amplifier and feedback resistor, according to Stanford.

"Adding the amplifier allows power to be very efficiently transferred across most of the three-foot range and despite the changing orientation of the receiving coil," explained Assawaworrarit.

"This eliminates the need for automatic and continuous tuning of any aspect of the circuits."

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