A major advance in wireless charging technology could pave the way to a new era of limitless range electric cars.
Scientists have demonstrated in principle a practical method of transmitting electricity wirelessly to a moving object.
Devices such as electric toothbrushes and mobile phones can already be recharged by magnetic induction, without any physical connection to a power source.
But until now a major technical hurdle has prevented the same kind of wirelesssystem being used to power up moving objects, such as cars.
If the new research published in the journal Nature lives up to its potential, electric vehicles could in future become the standard form of transport on the roads, the US team believes.
Currently even the most efficient electric cars cannot travel more than about 200 miles without a plug-in battery recharge, which can take several hours.
A wireless charge-as-you-drive system would overcome these limitations, making it possible to motor any number of miles in an electric car without stopping.
Lead scientist Professor Shanhui Fan, from Stanford University in the US, said: “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.
“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.”
Wireless charging exploits the way electricity moving through a wire creates an oscillating magnetic field.
The field will induce electrons in a wire coil placed nearby to oscillate too, thereby generating a current.
But while the “magnetic resonance coupling” technique works well for static devices such as electric toothbrushes, there is a major problem associated with transferring power to a moving object.
With every tiny movement, the device’s circuit has to be retuned to match the powersource and maintain the flow of electricity. This continual tuning is such a complex process that it becomes impractical.
Prof Fan’s team devised a new kind of feedback circuit, incorporating a commercially available voltage amplifier, that effectively tunes itself as the gap between transmitter and receiver varies.
The system was tested by placing an LED bulb on the receiving coil. Without self-tuning, the light would normally become less bright with increasing distance. Instead, the light remained constantly bright as the receiver moved up to three feet away from the power source.
Prof Fan added: “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.
“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.”