The development of electric vehicles requires addressing four key technologies: battery technology, motor drive and control technology, electric vehicle technology, and energy management technology.
1. Battery Technology
Batteries are the power source of electric vehicles and have always been a key factor restricting their development. The main performance indicators of batteries used in electric vehicles are specific energy (E), energy density (Ed), specific power (P), cycle life (L), and cost (C). To enable electric vehicles to compete with gasoline-powered vehicles, the key is to develop high-efficiency batteries with high specific energy, high specific power, and long lifespan.
To date, electric vehicle batteries have undergone three generations of development, achieving groundbreaking progress. The first generation was lead-acid batteries, primarily valve-regulated lead-acid (VRLA) batteries. Due to their high specific energy, low price, and high-rate discharge capability, they were the only batteries suitable for mass production in electric vehicles. The second generation was alkaline batteries, including nickel-cadmium (NJ-Cd), nickel-metal hydride (Ni-MH), sodium-sulfur (Na/S), lithium-ion (Li-ion), and zinc-air (Zn/Air) batteries. Their specific energy and specific power were higher than lead-acid batteries, significantly improving the power performance and driving range of electric vehicles, but their price was higher. The third generation is primarily based on fuel cells. Fuel cells directly convert the chemical energy of fuel into electrical energy, exhibiting high energy conversion efficiency, high specific energy and specific power, and controllable reaction processes. The energy conversion process can be continuous, making them ideal for automotive batteries. However, this generation is still under development, and some key technologies require further breakthroughs.
2. Electric drive and its control technology
The electric motor and drive system are key components of electric vehicles. To ensure good performance, the drive motor should have characteristics such as wide speed range, high speed, large starting torque, small size, light weight, high efficiency, strong dynamic braking, and energy regenerative braking. Electric vehicle motors mainly fall into four categories: DC motors (DCM), induction motors (IM), permanent magnet brushless motors (PMBLM), and switched reluctance motors (SRM).
In recent years, electric vehicles driven by induction motors have almost exclusively adopted vector control and direct torque control. Direct torque control is highly suitable for electric vehicles due to its direct control method, simple structure, excellent control performance, and rapid dynamic response. Electric vehicles developed in the United States and Europe primarily use this type of motor. Permanent magnet brushless motors can be divided into brushless DC motor systems driven by square waves (BLDCM) and brushless DC motor systems driven by sine waves (PMSM). Both have high power density, and their control methods are basically the same as induction motors, thus they are widely used in electric vehicles. PMSM motors have high energy density and efficiency, small size, low inertia, and fast response, making them very suitable for electric vehicle drive systems and showing excellent application prospects. Electric vehicles developed in Japan mainly use this type of motor.
Switched reluctance motors (SRMs) offer advantages such as simplicity, reliability, high efficiency across a wide speed and torque range, flexible control, four-quadrant operation, fast response, and low cost. However, practical applications have revealed drawbacks, including large torque ripple, high noise levels, and the need for position detectors, which limits their application.
With the development of electric motors and drive systems, control systems are becoming increasingly intelligent and digital. Nonlinear intelligent control technologies such as variable structure control, fuzzy control, neural networks, adaptive control, expert control, and genetic algorithms will be applied individually or in combination to the electric motor control systems of electric vehicles.
3. Electric vehicle technology
Electric vehicles are high-tech, comprehensive products. Besides batteries and electric motors, the vehicle body itself incorporates many advanced technologies, and some energy-saving measures are easier to implement than simply increasing battery energy storage capacity. Using lightweight materials such as magnesium, aluminum, high-quality steel, and composite materials, and optimizing the structure, can reduce the vehicle's weight by 30%-50%; energy recovery can be achieved during braking, downhill driving, and idling; high-pressure radial tires made of highly elastic materials can reduce rolling resistance by 50%; and a more streamlined vehicle body, especially the undercarriage, can reduce air resistance by 50%.
4. Energy Management Technology
Batteries are the energy storage and power source for electric vehicles. For electric vehicles to achieve excellent power characteristics, they must have batteries with high specific energy, long service life, and high specific power as their power source. Furthermore, to ensure good performance of electric vehicles, systematic battery management is essential.
The energy management system is the intelligent core of an electric vehicle. A well-designed electric vehicle, in addition to good mechanical performance, electric drive performance, and a suitable energy source (i.e., battery), should also have an energy management system that coordinates the operation of various functional components. Its function is to detect the state of charge of individual batteries or battery packs and, based on various sensor information, including force, acceleration/deceleration commands, road conditions, battery operating conditions, and ambient temperature, rationally allocate and utilize the limited onboard energy. It can also select the optimal charging method based on the battery pack's usage and charge/discharge history to extend battery life as much as possible.
Research institutions of major automakers worldwide are conducting research and development on-board battery energy management systems for electric vehicles. Knowing how much energy the electric vehicle battery currently holds and how many kilometers it can still travel are crucial parameters that must be known during operation, and are also essential functions that the electric vehicle energy management system should perform. Applying an on-board energy management system allows for more accurate design of the electric vehicle's energy storage system, determining an optimal energy storage and management structure, and ultimately improving the performance of the electric vehicle itself.
The challenge of energy management in electric vehicles lies in how to establish a relatively accurate mathematical model to determine how much energy each battery has remaining, based on historical data of voltage, temperature, and charge/discharge current collected for each battery.
