What is wireless charging technology for electric cars?

Nikola Tesla invented the “Tesla coil” in 1891 and used these wire diagrams to conduct a series of experiments that pioneered wireless power transmission. Tesla has said that wireless charging will be the ultimate way to make electricity prosper.

In 1901, Nikola Tesla received funding from financier John Pierpon Morgan to build a 187-foot (about 57 meters) high wireless charging tower, the Wardencliff Tower, on Long Island, New York. Prior to this, Nikola Tesla had conducted field trials in Colorado, successfully lighting 200 lights 25 miles (40 kilometers) away.

Although the Tesla coil was not implemented at the time, later generations have completely confirmed the feasibility of this scheme in theory. After years of research, scientists believe that wireless power transmission is possible. Wireless power transmission is a special power supply method that is different from wired power transmission. Its principle is also very simple. The transmitting end converts electrical energy into electromagnetic waves and transmits them. After the receiving end receives the electromagnetic waves, it is converted into electrical energy to the load. powered by. At present, there are three different implementations of wireless power transmission: electromagnetic induction, microwave transmission and magnetic resonance, each of which has its own advantages and disadvantages.

The electric car wireless charging device does not need to connect the car with the power supply system with a cable, and can directly charge the power battery of the electric car quickly. Wireless fast charging devices can be arranged in parking lots, residences, roadsides and other places, and can provide charging services for various types of electric (including externally charged hybrid) cars, making it possible to charge electric cars anytime, anywhere. For buses, charging facilities can be arranged at the terminal station, Jiaoniao station, transfer station, etc., and fast charging can be completed with a short parking time.

As soon as the wireless charging technology came out, it has received widespread attention from all over the world. Compared with the construction investment of conventional charging stations and charging piles, the wireless charging device has a lower cost, and eliminates the operation and waiting time required for wiring. Flexible, easy to use, safe and reliable. The schematic diagram of wireless charging of electric cars is shown in Figure 1.

What is wireless charging technology for electric cars?
Figure 1 Schematic diagram of wireless charging for electric cars

In general, the basic principles of electromagnetic induction, magnetic field resonance, and microwave transmission wireless charging technologies are the same, that is, the use of electromagnetic induction of alternating electromagnetic fields to realize wireless transmission of electrical energy.

For electric cars, the ideal situation is to charge while the car is cruising on the road, so-called mobile charging (MAC), and so on. Electric car users do not have to search for charging stations, park their cars and spend time charging. The MAC system is buried under a section of road, the charging area, and does not require additional space.

Both contact and inductive MAC systems can be implemented. For the contact type MAC system, a contact arch needs to be installed at the bottom of the car body. By contacting the charging element embedded in the road surface, the contact arch can obtain instant results. high current. When the electric car cruises through the MAC zone, its charging process is pulse charging. For inductive MAC systems, the on-board contact arches are replaced by inductive coils, and the charging elements embedded in the road surface are replaced by high-current windings that generate strong magnetic fields. It is obvious that the contact type MAC is not very attractive to people due to factors such as mechanical loss and the installation location of the contact arch.

In July 2009, Nissan and Showa Aircraft Corporation announced the electromagnetic induction wireless charging system, the transmission distance is about 100mm, and the transmission efficiency can reach 90%. However, when the parking position deviates, resulting in a large error between the sending and receiving trays, it will seriously affect the power transmission efficiency. At present, we are working on the research that the horizontal and vertical deviation of parking is in the range of 200-300mm, and it also ensures that it has a transmission efficiency of more than 90%. The electromagnetic induction wireless charging device developed by Showa Aircraft Company is shown in Figure 2.

What is wireless charging technology for electric cars?
Figure 2 The electromagnetic induction wireless charging device developed by Showa Aircraft Company

In addition, the two companies have also studied the adverse effects of animals and metal fragments entering between transmission and reception, because such foreign objects can create eddy currents between the two, which can cause heat and affect the efficiency of power transmission.

Nagano Japan Wireless Corporation announced in August 2009 that it has developed a charging system based on magnetic field resonance. Compared with the electromagnetic induction type, the magnetic field resonance method has the advantages of long transmission distance and low parking error requirements. It can ensure 90% transmission efficiency within the transmission distance of 600mm. However, the current transmission power is still relatively small (about 1kW). It is planned to enter the market from forklift trucks and other applications. With the improvement of technology maturity and transmission power, it is expected to enter the field of electric car charging soon.

The microwave transmission wireless charging system developed by Mitsubishi Heavy Industries uses a group of 48 silicon rectifier diodes as the receiving antenna. And can achieve 1kW power output. The advantage is that the cost is low (the whole set costs about RMB 20,000). The disadvantage is that the transmission efficiency is low. The current transmission efficiency is only 38%. In this regard, Mitsubishi Heavy Industries believes that although it is not suitable for fast charging, as a night valley charging, the electricity cost is only 10% to 20% of the traditional fuel cost. If the magnetron with excessive heat is cooled with water, the overall efficiency can reach 70%. In addition, in terms of safety, there is a microwave leakage prevention device, which will not affect the operation of the electronic equipment on the car and endanger the bodies of nearby people during use.

At present, the developed electromagnetic resonance wireless charging technology is to install an electromagnetic coil with a variable current in the parking space on the road or the parking space of the parking lot as a transmitter for wireless charging, and install an electromagnetic coil as a receiver under the car. When driving to the corresponding position, the transmitter and receiver resonate through wireless induction to generate current, and the current is converted to charge the power battery of the electric car. The schematic diagram of the electromagnetic coil on the road parking space or the parking space in the parking lot and the electromagnetic coil under the car resonantly transmits electric energy is shown in Figure 3.

What is wireless charging technology for electric cars?
Figure 3. Schematic diagram of the resonance transmission of electric energy between the magnetic coil on the road parking space or the parking space in the parking lot and the electromagnetic coil under the car

Recently, Delphi, Infiniti, Nissan, Qualcomm, Rolls-Royce and WiTricity have all set out to study wireless charging technology for electric cars, that is, placing the charging device in the parking lot without additional connecting equipment, such as wires. In the parking space, the driver only needs to park the car in the corresponding position to leave. The permanent magnet coil fixed on the road is shown in Figure 4.

What is wireless charging technology for electric cars?
Figure 4 Permanent magnet coil fixed on the road

Transmission efficiency is a problem faced by all wireless charging, especially for “electrical appliances” with higher charging power such as electric cars. Electric energy is first converted into wireless electromagnetic waves, and then converted into electric energy by wireless electromagnetic waves, both of which will be lost. A lot of energy, which seems a bit out of place for an electric car that is green and environmentally friendly.

Electromagnetic compatibility is also one of the technical bottlenecks that wireless charging needs to solve. As we all know, electromagnetic waves are easy to leak. When high-power wireless charging equipment for cars is running, it will also affect the surrounding biological and electronic equipment, and even endanger human health. People are very sensitive today when people talk about radiation discoloration. Therefore, how to deal with this aspect is also a problem that needs to be solved to realize the engineering of wireless charging of electric cars.

In addition, wireless charging will still face problems such as electrical standards, which also need to be solved jointly by wireless charging system developers and car manufacturers. It is believed that after the key technical problems are solved, these problems will also be solved under the general trend.

At present, the bottleneck hindering the large-scale adoption of wireless inductive charging technology is mainly radiation concerns, because wireless charging generates strong magnetic fields. Electromagnetic injury is possible when a person or animal is located between the electric car and the charging unit. Therefore, ensuring the safety of wireless charging systems is also a key point. In this regard, companies still need a lot of testing and improving related technologies.