Method Recharges an Electric Car as it Drives

This practical technique uses magnetism to transmit electricity wirelessly to recharge electric cars, robots, or drones.

Although wireless charging pads already exist for smartphones, they only work if the phone is sitting still. For cars, that would be just as inconvenient as the current practice of plugging them in for an hour or two at charging stations.

Engineers have demonstrated a technology that could one day be scaled up to power a car moving down the road at high speed. In the nearer term, the system could soon make it practical to wirelessly recharge robots as they move around in warehouses and on factory floors, eliminating downtime and enabling robots to work almost around the clock.

Wireless chargers transmit electricity by creating a magnetic field that oscillates at a frequency that creates a resonating vibration in magnetic coils on the receiving device. The problem is that the resonant frequency changes if the distance between the source and receiver changes by even a small amount.

Previously, the researchers developed a wireless charger that could transmit electricity even as the distance to the receiver changes. They did this by incorporating an amplifier and feedback resistor that allowed the system to automatically adjust its operating frequency as the distance between the charger and the moving object changed. But that initial system wasn’t efficient enough to be practical. The amplifier used so much electricity internally to produce the required amplification effect that the system only transmitted 10% of the power flowing through the system.

In the new work, the researchers boosted the system’s wireless-transmission efficiency to 92%. The key was to replace the original amplifier with a far more efficient switch mode amplifier. Such amplifiers aren’t new but they will only produce high-efficiency amplification under very precise conditions.

The new prototype can wirelessly transmit 10 watts of electricity over a distance of two or three feet. There aren’t any fundamental obstacles to scaling up a system to transmit the tens or hundreds of kilowatts that a car would need and the system is more than fast enough to resupply a speeding automobile. The only limiting factor will be how fast the car’s batteries can absorb all the power.

The wireless chargers shouldn’t pose a health risk because even those that are powerful enough for cars would produce magnetic fields that are well within established safety guidelines. The magnetic fields can transmit electricity through people without them feeling a thing.

Though it could be years before wireless chargers become embedded in highways, the opportunities for robots and drones are more immediate. It’s much less costly to embed chargers in floors or on rooftops than on long stretches of highway.

For more information, contact Shanhui Fan at This email address is being protected from spambots. You need JavaScript enabled to view it..