Overview of wireless charging technology and applications in EV

In the conductive charging system of electric vehicles, the charging pile and the on-board interface are connected by cable assemblies to realize the energy replenishment of electric vehicles.

1. Wireless charging basics

The development of wireless charging technology has brought a new way to replenish the energy of electric vehicles – EV wireless power transfer (EV WPT).

The wireless charging system of electric vehicles is divided into two parts: the ground transmitter (referred to as the ground end) and the vehicle receiving end (referred to as the vehicle end), and the ground end and the vehicle end transmit energy through the air.

The ground end adopts an energy conversion unit to convert the power frequency AC energy input by the power grid into high frequency AC energy, and wirelessly transmit the power to the vehicle end. The power conversion device then transmits power to the battery, and the ground end and the vehicle end can use wireless communication for information exchange.

2. Wireless charging technology classification

WPT has many technical solutions and different working mechanisms. However, the system structure is essentially the same, and the energy is transmitted wirelessly through the transmitter power transmitting unit and the receiver power receiving unit.

● Transmitter

The transmitter converts the power supply energy through the power conversion circuit into the form required by the power transmitting unit, such as electric field, magnetic field, radio frequency or laser.

● Receiving unit

The power receiving unit at the receiving end receives electric field, magnetic field, radio frequency or laser, etc., and then converts it into the energy supply required by the load through the power conversion circuit. From the perspective of energy transmission distance, EV wireless charging can be divided into two categories: short-range wireless charging and long-distance wireless charging.

The magnetic field coupling method and the electric field coupling method belong to short-range wireless charging. RF microwave and laser methods are usually used for long distance wireless charging.

① Short-range wireless charging

It is mainly divided into two types: magnetic field coupling type and electric field coupling type.

● Magnetic field coupled type

The principle of magnetic resonance or magnetic induction is used to transfer energy from the transmitter to the receiving end, and the transmitting coil and the receiving coil are used in this method. The distance between the transmitting coil and the receiving coil is generally a few centimeters to tens of centimeters, and the coupling coefficient between the coils is generally more than 0.1.

There is also a near-field coupled wireless charging technology based on magnetic gears. At the transmitter end of the corresponding system, the motor is used to drive the magnet to move, influence car battery voltage and the magnet at the receiving end will move with the interaction of the force between the magnetic fields. And then drive the generator at the receiving end to generate electricity, so as to realize the wireless transmission of energy from the transmitter to the receiving end.

Compared to techniques based on magnetic resonance or magnetic induction, this scheme operates at a lower frequency, usually several hundred hertz, with a charging distance of twenty or thirty centimeters, but the operating magnetic field is stronger.

● Electric field coupled type

Generally, two separate plates are used to form a coupling capacitor. The size of the capacitance value is related to the area directly opposite the plates, the distance between the plates and the size of the dielectric constant.

In order to achieve a larger power transmission, a larger plate surface area, a smaller transmission distance and a higher operating frequency are required. And it is difficult to meet these conditions in practical application scenarios, resulting in less application of electric field coupling in practice.

② Long-distance wireless charging

For long-distance wireless charging using RF and microwave, antennas are required at both the transmitter and receiver. The antenna emission may be multidirectional or directional. It is easier to achieve one-to-many charging, and the higher the frequency, the farther the charging distance.

Laser wireless charging is based on laser as the carrier to realize wireless transmission of energy, convert electrical energy into laser at the transmitting end and emit, and convert laser into electrical energy through photoelectric conversion at the receiving end.

At present, long-distance wireless charging technology is mostly in the stage of theoretical research and technical verification, and it still faces great challenges in energy transmission efficiency and safety, and there is still a certain distance from the real large-scale commercial application.

3. Typical applications for wireless charging

Wireless charging technology has been widely used in low-power fields, such as lifepo4 battery in mobile phones, watches, electric toothbrushes, drones and other fields.

In recent years, it has gradually begun to be commercialized in the field of high power, and the application range is becoming wider and wider, such as wireless charging for passenger cars, wireless charging for buses, and mobile wireless charging for trolleybuses.

4. The core technology of wireless charging in EV

The first task of the wireless charging system for EVs is to achieve efficient wireless energy replenishment of EVs. Therefore, efficient and safe power conversion and transmission are at the heart of wireless charging systems. In terms of power conversion circuits, there are mainly circuit optimization design technologies, which directly affect the overall characteristics of wireless power and the efficiency of wireless power charging systems.

In addition, in order to achieve automatic energy replenishment in all-weather scenarios, high-efficiency charging of EV wireless charging systems is only a basic functional requirement. At the same time, a number of auxiliary functions are required to ensure safety and accurately assist the vehicle in parking.

In wireless charging systems, ground-side and vehicle-side devices may be produced by different equipment companies, and there are many situations. In addition to meeting performance and safety requirements, ground-side and vehicle-side devices also need to meet interoperability requirements.

5. Composition of EV wireless charging system

Wireless charging of EVs belongs to the field of short-range wireless charging, and magnetic coupling is the mainstream implementation scheme at present. There is no cable connection between the ground side and the vehicle side.

● Ground end

Rectifier and voltage regulation circuitry on the ground side converts alternating current power frequency into direct current. Inverter circuits convert direct current into high frequency alternating current. The ground-side compensation network compensates for the transmission coil. The transmitting coil converts high-frequency alternating current into a high-frequency alternating magnetic field.

● On-board terminal

The receiving coil on the vehicle side receives magnetic field energy by magnetic coupling and reduces it to high-frequency electrical energy. The on-board compensation network compensates for the receiving coil.

The rectifier circuit converts high frequency alternating current into stable direct current to feed the load. In addition, the control signals between the ground end and the vehicle end interact via wireless communication, and commonly used wireless communication methods include WiFi, Bluetooth, Zigbee, etc.

6. Advantages of EV wireless charging technology

Compared to conductive charging methods, there are the following advantages:

● More secure and reliable

Conduction charging facilities require cables and charging plugs, charging plugs and cables are easy to wear out during continuous unplugging and use, and the connectors of battery hookup and cables need to be replaced regularly to prevent aging failures. Wireless charging can effectively avoid the above safety hazards and save operation and maintenance costs.

● More convenient in charging 

There is no need for the driver to manually drag and pull the plugging and unplugging cable, and the user experience is better. Can be used around the clock It can adapt to a variety of harsh environments and weather, even in thunderstorms without the risk of electric shock.

● Intelligent automatic charging

It can effectively support the unmanned operation of automatic driving, and is the best commercial charging solution with automatic parking.

● Space saving

The wireless charging energy transmitter is built under the parking space, which can effectively alleviate the occupation of land and space resources caused by the construction of a large number of charging stations.

7. Challenges in EV wireless charging technology

At present, the challenges of wireless charging technology for EVs in the promotion and application mainly come from the following aspects:

● The cost is higher

Compared to conductive charging, wireless charging is more complex than the system, coupled with the fact that large-scale mass production has not yet been achieved, and the industrial chain has not yet been perfected, resulting in a high cost of system equipment. However, with the continuous development of wireless charging technology and large-scale commercial use, the cost of products will be greatly reduced.

● System efficiency needs to be improved

Wireless charging has undergone multiple conversions of electrical energy, and there is currently a difference of about 3% compared to conductive charging. At present, mainstream technical solutions and products can achieve a maximum end-to-end charging efficiency of about 92%, which can meet the application needs.

● The standard has not yet been established

The companies adopt different technical solutions and the performance is different. Also the interconnection cannot be fully realized, which affects the promotion and use of EV WPT technology.

● Need to compatible with charging piles

Since the development of conductive charging methods predates wireless charging methods, the transformation of charging pile infrastructure may be involved in the promotion of wireless charging technology. How to achieve compatibility between the two charging methods is also one of the current challenges.

8. Conclusion

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