Stanford scientists develop wireless charger for cell phones, electric cars

Scientists at Stanford University in the US have developed a device that can wirelessly charge a moving object at close range.

The technology could one day be used to charge electric cars on the highway, or medical implants and cellphones as you walk nearby.

"In addition to advancing the wireless charging of vehicles and personal devices like cellphones, our new technology may untether robotics in manufacturing, which also are on the move," said Professor Shanhui Fan.

According to the study, published in the journal Nature, wireless charging would address a major drawback of plug-in electric cars -- their limited driving range. A charge-as-you-drive system would overcome these limitations.

"In theory, one could drive for an unlimited amount of time without having to stop to recharge. The hope is that you'll be able to charge your electric car while you're driving down the highway," Fan explained.

He noted that a coil in the bottom of the vehicle could receive electricity from a series of coils connected to an electric current embedded in the road.

"We can rethink how to deliver electricity not only to our cars but to smaller devices on or in our bodies. For anything that could benefit from dynamic, wireless charging, this is potentially very important," Fan said.

Mid-range wireless power transfer is based on magnetic resonance coupling.

"Just as major power plants generate alternating currents by rotating coils of wire between magnets, electricity moving through wires creates an oscillating magnetic field which causes electrons in a nearby coil of wires to oscillate, thereby transferring power wirelessly," the study said.

The team transmitted electricity wirelessly to a moving LED light bulb but the demonstration only involved a one milliwatt charge, far less than what electric cars require.

The scientists are now working on greatly increasing the amount of electricity that can be transferred, and tweaking the system to extend the transfer distance and improve efficiency.

"We still need to significantly increase the amount of electricity being transferred to charge electric cars, but we may not need to push the distance too much more," Fan added.

According to the research, the transfer efficiency can be further enhanced if both coils are tuned to the same magnetic resonance frequency and are positioned at the correct angle, but scientists found that was a complex process.

To address the challenge, the radio-frequency source in the transmitter was eliminated and replaced it with a commercially available voltage amplifier and feedback resistor.

"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," lead author of the study Sid Assawaworrarit said.