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 cell phones as you walk nearby. In addition to advancing the wireless charging of vehicles and personal devices like cell phones, new technology may untether robotics in manufacturing. According to study wireless charging would address a major drawback of plug-in electric cars, which is 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 one will be able to charge your electric car while driving down the highway. Researchers 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. 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 wireless.

According to the researchers, mid-range wireless power transfer is based on magnetic resonance coupling. The continuous flow of electricity can only be maintained, if some aspects of the circuits, such as the frequency, are manually tuned as the object moves. To address the challenge, the Stanford team eliminated the radio-frequency source in the transmitter and replaced it with a commercially available voltage amplifier and feedback resistor. This system automatically figures out the right frequency for different distances without the need for human interference. The team transmitted electricity wirelessly to a moving LED lightbulb but the demonstration only involved a one milliwatt charge, far less than what electric cars require.