Reprinted from the "Vogel Industry Media" website, published on 2019-12-02, original URL: https://www.vogel.com.cn/top/2016/qcjs_xnyqc/article_view.html?id=581433
At this point, you will surely notice that the three-electric system is almost the entirety of an electric vehicle. But do you know what the three-electric system actually is? Today, the editor will give you a brief but not oversimplified introduction.
In recent years, the development of electric vehicles has become a trend in the automotive industry, and more and more automakers are gradually shifting their R&D focus to electric vehicles (electric and hybrid). Electric vehicles represent the future of the automotive industry, and this has long been an industry consensus.
What makes an electric vehicle is inevitably its three-electric system. In many references, the three-electric system is described as the absolute core of an electric vehicle. In fact, the three-electric system is not only the absolute core of an electric vehicle but also the foundation of the electric vehicle. According to authoritative industry institutions, the cost of the three-electric system accounts for more than 75% of the total cost of a pure electric vehicle, and more than 90% of pure electric vehicle component failures are also caused by the three-electric system.
At this point, you will surely notice that the three-electric system is almost the entirety of an electric vehicle. But do you know what the three-electric system actually is? Today, the editor will give you a brief but not oversimplified introduction.
What Is the Three-Electric System?
The three-electric system is the core technology that distinguishes pure electric vehicles from traditional fuel vehicles. It is a power system created to replace the engine system of fuel vehicles. Developing a three-electric system requires a large amount of time-consuming and labor-intensive foundational work. The three-electric system consists of: battery, motor, and electronic control system.
However, this is only a general classification, because under the battery, motor, and electronic control, there are still a large number of technical challenges. Among these three technologies, the battery is the most important, as it affects the driving range per charge and the production cost of the vehicle. Motor and electronic control technologies are relatively more mature.
(I) Battery of the Three-Electric System
Generally speaking, we call the high-voltage battery (as distinct from the low-voltage battery on the vehicle) the power battery. Similar to the fuel tank of a traditional fuel vehicle, it serves as the energy source for new energy vehicles. The power battery system usually consists of cells, battery packs, battery management system, cooling system, high- and low-voltage wiring harnesses, protective enclosure, and other structural components. The key lies in the cell group, and the composition of the cell group is as follows:
A cell is the smallest unit of a battery system. Multiple cells form a module, and multiple modules form a battery pack. This is the basic structure of an automotive power battery. A battery is like a container for storing electrical energy; how much capacity it can store is determined by the amount of active material carried on the positive and negative electrode sheets. The design of the positive and negative electrode sheets needs to be tailored to different vehicle models. The gram capacity of the positive and negative electrode materials, the ratio of active materials, electrode sheet thickness, and compaction density are also crucial to capacity. In other words, the most important material in the cell group is the cathode material inside the cell, which determines the power performance, driving range, cost, and safety performance of the battery.
Looking at the history of power battery development for new energy electric vehicles, lead-acid batteries were the earliest used as power batteries, followed by nickel-metal hydride batteries used in Japanese hybrid models, and now the mainstream lithium batteries. In other words, there are currently three main energy sources for new energy electric vehicles: lead-acid batteries, nickel-metal hydride batteries, and lithium-ion batteries.
(1) Lead-Acid Battery
Lead-acid batteries, also known as lead-acid storage batteries, were invented by G. Planté in 1859. They were used in early electric vehicle models. Their advantages include low price, readily available raw materials, and easy recycling, but lead-acid batteries have low energy density (which can be understood as being too bulky).
(2) Nickel-Metal Hydride Battery
Nickel-metal hydride batteries are batteries synthesized from hydrogen ions and metallic nickel. Toyota's currently sold hybrid models mainly use nickel-metal hydride batteries. This type of battery has advantages such as high stability, low production cost, good low-temperature performance, and high recycling value, but its disadvantages are also obvious: similarly low energy density, and cycle life significantly shorter than lithium batteries.
(3) Lithium-Ion Battery
Lithium-ion batteries, also known as Li-ion batteries, were first successfully developed by Sony Corporation of Japan in 1990. Compared with the above two types of batteries, lithium-ion batteries have high storage energy density; correspondingly, their manufacturing cost is also higher. Currently, mainstream electric vehicles all use this type of battery.
Although lithium-ion batteries have stood out from external competition and become mainstream, within the lithium-ion battery category there is also fierce competition among peers. Depending on the materials used, current lithium-ion batteries are mainly divided into four types: lithium cobalt oxide, lithium nickel manganese cobalt oxide (NMC), lithium manganese oxide, and lithium iron phosphate. However, the current new energy power battery market is split between lithium iron phosphate batteries and NMC lithium batteries. The electric bus market, which has higher safety requirements, uses more lithium iron phosphate batteries, while the passenger vehicle market, which has higher energy density requirements, has a higher proportion of NMC lithium batteries. So what are the differences among these four types of batteries?
(1) Lithium Cobalt Oxide Battery Advantages: stable structure, high specific capacity, outstanding overall performance Disadvantages: insufficient stability, poor safety, very high cost; actually not suitable as a power battery Representative model: Tesla Roadster
(2) NMC Lithium Battery Advantages: high energy density, can exceed 200 Wh/kg, outstanding overall performance Disadvantages: low output efficiency, fast capacity fade, short life, poor safety Representative model: Tesla Model S
(3) Lithium Manganese Oxide Battery Advantages: lowest price, safe and environmentally friendly, and no patent restrictions; largest market share; the mainstream electric battery in Japan, South Korea, Europe, and the United States; adopted by enterprises in Japan, South Korea, Europe, the United States, and China. Disadvantages: no outstanding performance in any aspect Representative model: Nissan Leaf
(4) Lithium Iron Phosphate Battery Advantages: high-efficiency output, high cycle count, long life, good safety performance, abundant reserves of iron phosphate as cathode material, and many R&D and manufacturing companies for lithium iron phosphate batteries. Disadvantages: low energy density, only about 150 Wh/kg; massive capacity fade at low temperatures (capacity at -10°C is about 55% of that at 25°C); higher production cost than lithium manganese oxide; further research is needed to reduce costs. Representative model: BYD
Through the above comparison, you should have a general understanding of each type of lithium-ion battery. Currently, there are two mainstream designs for electric vehicle batteries. One is represented by Tesla, which uses NMC (nickel Ni, cobalt Co, manganese Mn, or nickel Ni, cobalt Co, aluminum Al) lithium batteries plus a highly intelligent battery management system, fully leveraging the advantage of high energy density of NMC lithium batteries. The other is represented by BYD, which uses lithium iron phosphate batteries plus a relatively simple battery management system, fully leveraging the durability of lithium iron phosphate batteries. Regardless of which route automakers choose, as long as it can promote innovation in the lithium battery industry (after all, batteries are the bottleneck constraining the development of new energy vehicles), I think it is the right one.
(II) Electronic Control System of the Three-Electric System
When it comes to the electronic control system, the first thing that comes to mind is the ECU. But the electronic control system of an electric vehicle is different from that of a fuel vehicle. The electronic control system of an electric vehicle is the brain of the electric vehicle, composed of various subsystems. Each subsystem generally consists of sensors, signal processing circuits, electronic control units, control strategies, actuators, self-diagnostic circuits, and indicator lights. There are some differences in electronic control systems among different types of electric vehicles, but generally speaking, they all include an energy management system, regenerative braking control system, motor drive control system, electric power steering control system, and powertrain control system. The functions of each subsystem are not simply additive; rather, they comprehensively control the electric vehicle by integrating the functions of each subsystem. These control systems are consolidated into a control box, commonly called the vehicle control unit.
The pure electric vehicle control unit basically has the following functions: (1) vehicle driving control; (2) network management of the entire vehicle; (3) regenerative braking energy feedback control; (4) vehicle energy management and optimization; (5) vehicle status monitoring and display; (6) fault diagnosis and handling; (7) external charging management.
The following is a brief description of each functional module:
1. Digital Input Adjustment Module Used for level conversion and shaping of switch input signals; one end is connected to multiple switch sensors, and the other end is connected to the microcontroller.
2. Relay Drive Module Used to drive multiple relays; one end is connected to the microcontroller via an optocoupler, and the other end is connected to multiple relays.
3. High-Speed CAN Bus Interface Module Used to provide a high-speed CAN bus interface; one end is connected to the microcontroller via an optocoupler, and the other end is connected to the system high-speed CAN bus. It mainly handles signals with high real-time requirements, such as motor speed signals.
4. Low-Speed CAN Bus Interface Module Used to provide a low-speed CAN bus interface; one end is connected to the microcontroller via an optocoupler, and the other end is connected to the system low-speed CAN bus. It mainly handles signals with lower real-time requirements, such as lighting control.
5. Power Module The power module can provide isolated power to the microprocessor and each input and output module, and monitors the battery voltage; it is connected to the microcontroller.
6. Analog Input and Output Module Can acquire 0–5V analog signals and output analog voltage signals of 0–4.095V.
7. Pulse Signal Input and Output Module Can acquire and adjust pulse signals, range 1 Hz–20 kHz, amplitude 6–50V; output PWM signals, range 1 Hz–10 kHz, amplitude 0–14V.
8. Fault and Data Storage Module The memory can store calibrated data, fault codes, vehicle characteristic parameters, etc., with a capacity of 32K.
The parameter matching and selection of the electronic control system of a pure electric vehicle has a great impact on its power performance and economy. Generally, during the development stage of a new pure electric vehicle, engineers will, according to the vehicle design goals, use drive motor parameter and power battery parameter matching simulation methods and design vehicle control strategies so that the "battery + motor + electronic control" three-electric system of the pure electric vehicle can be better integrated in the early stage of power matching development, achieving the final design goals and realizing related performance and functions.
(III) Electric Drive System of the Three-Electric System (Drive Motor)
The electric drive system also consists of three parts: transmission mechanism, motor, and inverter. As for the transmission mechanism, currently domestic electric vehicle transmission mechanisms are all single-speed reducers, meaning no clutch and no gear shifting. The transmission mechanism can increase its complexity to reduce the demand for the motor and motor inverter, improve performance, and reduce cost. The motor system is the high-voltage motor that provides torque to the vehicle (that is, provides force to the vehicle). A vehicle can be equipped with several motors. Torque determines the acceleration speed of a new energy vehicle, and the cost of the battery is greater than that of the motor, so the efficiency and performance of the motor are crucial.
The mechanical energy required by an electric vehicle—that is, the kinetic energy for moving forward and backward—all comes from the motor. The motor replaces the internal combustion engine by converting the electrical energy of the battery into mechanical energy. There is much to learn about motors on electric vehicles. The drive motors used in electric vehicles fall into four categories: DC motors, asynchronous motors, permanent magnet synchronous motors, and switched reluctance motors.
However, mainstream electric vehicles currently on the market mainly use the following two types: AC asynchronous motors and rare-earth permanent magnet synchronous motors.
Asynchronous Motor (Induction Motor) The working principle is that the rotating magnetic field of the stator induces current in the rotor, generating electromagnetic torque; the rotor does not directly generate a magnetic field. Therefore, the rotor speed must be lower than the synchronous speed (without this difference, i.e., slip, there would be no rotor induced current), hence the name asynchronous motor.
(Asynchronous motor used by Tesla)
Synchronous Motor The rotor itself generates a magnetic field in a fixed direction (generated by permanent magnets or DC current), and the stator rotating magnetic field "drags" the rotor magnetic field (the rotor) to rotate. Therefore, the rotor speed must equal the synchronous speed, hence the name synchronous motor.
(Permanent magnet synchronous motor)
The biggest advantage of permanent magnet synchronous motors is their high power density and torque density. Put plainly, compared with other types of motors, under the same mass and volume, permanent magnet synchronous motors can provide the greatest power output and acceleration for new energy vehicles. This is also the main reason why permanent magnet synchronous motors have become the first choice in the new energy vehicle industry, which has extremely high requirements for space and weight.
Compared with permanent magnet synchronous motors, asynchronous motors have the advantages of low cost, simple process, reliable and durable operation, convenient maintenance, and the ability to withstand large operating temperature changes. Conversely, large temperature changes will damage permanent magnet synchronous motors. Although asynchronous motors are not advantageous in terms of weight and volume, their wide speed range and peak speed of around 20,000 rpm mean that even without a two-stage differential they can meet the speed requirements for high-speed cruising of vehicles in this class. As for the impact of weight on driving range, high-energy-density batteries can "cover" the disadvantage of motor weight.
As important components of electric vehicles, the choice of different motors will determine the production cost and usage of the electric vehicle. At present, the most widely used are still permanent magnet synchronous motors. The biggest advantage of permanent magnet synchronous motors is their high power density and torque density. Put plainly, compared with other types of motors, under the same mass and volume, permanent magnet synchronous motors can provide the greatest power output and acceleration for new energy vehicles. This is also the main reason why permanent magnet synchronous motors have become the first choice in the new energy vehicle industry, which has extremely high requirements for space and weight.
Summary:
At present, most domestic independent brands have only mastered the vehicle control unit and three-electric integration technology, while three-electric component technology is still hovering at the introductory stage. After all, technological accumulation is not something that can be achieved overnight.
It can be said that whoever makes strong improvements in three-electric technology, masters the core three-electric technologies, improves performance, and reduces costs will be the winner at the critical stage of the game between technology and market.