Researchers at Stanford University have designed a system that would let electric vehicles wirelessly recharge their batteries while driving on a highway.

The technology involves the use of magnetic fields to wirelessly transmit large electric currents between metal coils placed several feet apart along the highway. The electric vehicle, which would also have a metal coil attached underneath, would wirelessly draw power from the coils in the roadbed.

The long-term goal is to develop an all-electric highway that rechargesvehicles as they move along the road.

The research results were published in the journal Applied Physics Letters and a summary was provided by the Stanford University News Web site.

"Our vision is that you'll be able to drive onto any highway and charge your car," said Shanhui Fan, an associate professor of electrical engineering.

The technology addresses the limited range of full battery-powered electric vehicles, one of the major concerns of the fledgling EV industry. At present, state-of-the-art lithium ion batteries give anEV such as the Nissan Leafa range of around 150km.

Though carmakers say their customer research shows that range is sufficient in most everyday use, they are also working on technologies to make EVs suitable for longer journeys. Some EVs, such as the Opel Ampera/Chevrolet Volt are equipped with a small range-extending combustion engine. Other carmakers are working to shorten battery recharging times.

The Stanford University research project has come up with a solution that wouldn't require an EV to stop anywhere to recharge.

"What makes this concept exciting is that you could potentially drive for an unlimited amount of time without having to recharge," Richard Sassoon, the managing director of the Stanford Global Climate and Energy Project (GCEP), said in a press release. GCEP funded the research.

The research is based on so-called magnetic resonance coupling, where two copper coils are tuned to resonate at the same natural frequency. One coil is connected to an electric current, which generates a magnetic field that causes the second coil to resonate. This magnetic resonance results in the invisible transfer of electric energy through the air from the first coil to the receiving coil.

In 2007, researchers at the Massachusetts Institute of Technology used magnetic resonance to light a 60-watt bulb. That research also showed that the magnetic field between the two coils had no impact on humans, which is important from a safety point of view.

In the Stanford research project, a series of coils connected to an electric current would be embedded in a highway. Receiving coils attached to the bottom of the car would resonate as the vehicle speeds along, creating magnetic fields that continuously transfer electricity to charge the battery.

At the moment, the technology exists as a computer model, but the Stanford researchers recently filed a patent application for the wireless system.

In a next phase,the team wants to make sure the system doesn't affect people or the microcomputers that control various key functions in the car.

It is unclear whether the technology could be used on real highways, but the Stanford team considers its research a new approach to power delivery. Said researcher Fan: " Imagine that instead of wires and plugs, you could transfer power through a vacuum. Our work is a step in that direction."