The time is right and the place is right, and all Chinese electric vehicle companies are occupied. China seems to have become the center of the world’s electric vehicle industry.
In fact, in Germany, if your unit does not provide charging piles, you may need to buy one yourself. on the doorstep. However, we are always discussing why so many excellent German car companies can’t make Tesla, and it is not difficult to find the reasons now.
In 2014, Professor Lienkamp of the Technical University of Munich published a new book “Status of electrical mobility 2014″, which is free and open to the society, and said: “Although electric vehicles have various defects, I have never seen a car that already owns an electric mobility. The driver of the car, re-enter the embrace of the traditional car. Even the most common electric car brings you the joy of driving, which is unmatched by a gasoline car.” Such a car can really make the car owner not renew Throwing back into the arms of traditional cars?
As we all know, the heart of an electric vehicle is the battery.
For an ordinary electric vehicle, under the European standard test, the energy consumption per 100 kilometers is about 17kWh, that is, 17 kWh. Dr. Thomas Pesce studied the energy consumption of compact vehicles under the optimal configuration. Without considering the cost, the optimal energy consumption per 100 kilometers obtained by using the existing available technology is slightly more than 15kWh. This means that in the short term, trying to reduce energy consumption by optimizing the efficiency of the car itself, even without considering the additional cost, the energy saving effect is relatively small.
Take Tesla’s 85kWh battery pack as an example. The nominal driving distance is 500km. If the energy consumption is reduced to 15kWh/100km through various efforts, the driving distance can be increased to 560km. Therefore, it can be said that the battery life of the car is proportional to the capacity of the battery pack, and the proportional coefficient is relatively fixed. From this point of view, the use of batteries with higher energy density (both energy Wh/kg per unit weight and energy Wh/L per unit volume need to be considered) is of great significance to improve the performance of electric vehicles, because in electric vehicles, the battery occupies a large part of the total weight.
All kinds of lithium-ion batteries are the most anticipated and the most widely used batteries. The lithium batteries used in automobiles mainly include nickel cobalt lithium manganate ternary battery (NCM), nickel cobalt lithium aluminate battery (NCA) and lithium iron phosphate battery (LPF).
1. Nickel-cobalt lithium manganate ternary battery NCM is used by many electric vehicles abroad because of its low heat production rate, relatively good stability, long life, and energy density of 150-220Wh/kg.
2. NCA nickel-cobalt aluminate lithium battery
Tesla uses this battery. The energy density is high, at 200-260Wh/kg, and is expected to reach 300Wh/kg soon. The main problem is that only Panasonic can produce this battery at present, the price is high, and the safety is the worst among the three lithium batteries, which requires high-performance heat dissipation and battery management system.
3. LPF lithium iron phosphate battery Finally, let’s look at the LPF battery most used in domestic electric vehicles. The biggest disadvantage of this type of battery is that the energy density is very low, which can only reach 100-120Wh/kg. In addition, LPF also has a high self-discharge rate. None of this is desired by EV makers. The widespread adoption of LPF in China is more like a compromise made by domestic manufacturers for expensive battery management and cooling systems – LPF batteries have very high stability and safety, and can ensure stable operation even with poor battery management systems and longer battery life. Another benefit brought by this feature is that some LPF batteries have extremely high discharge power density, which can improve vehicle dynamic performance. In addition, the price of LPF batteries is relatively low, so it is suitable for the current low-end and low-price strategy of domestic electric vehicles. But whether it will be vigorously developed as the battery technology of the future, there is still a question mark.
How big should the battery of an average electric car be? Is it a battery pack with thousands of Tesla batteries in series and parallel, or a battery pack built with a few large batteries from BYD? This is an under-research question, and there is currently no definite answer. Only the characteristics of the battery pack composed of large cells and small cells are introduced here.
When the battery is small, the total heat dissipation area of the battery will be relatively large, and the temperature of the entire battery pack can be effectively controlled through a reasonable heat dissipation design to prevent the high temperature from accelerating and detracting from the life of the battery. Generally, the power and energy density of batteries with smaller single capacity will be higher. Finally, and more importantly, generally speaking, the less energy a single battery has, the higher the safety of the entire vehicle. A battery pack composed of a large number of small cells, even if a single cell fails, it will not cause too much problem. But if there is a problem inside a battery with a large capacity, the safety hazard is much greater. Therefore, large cells require more protection devices, which further reduces the energy density of the battery pack composed of large cells.
However, with Tesla’s solution, the disadvantages are also obvious. Thousands of batteries require an extremely complex battery management system, and the additional cost cannot be underestimated. The BMS (Battery Management System) used on the Volkswagen E-Golf, a sub-module capable of managing 12 batteries, costs $17. According to the estimation of the number of batteries used by Tesla, even if the cost of self-developed BMS is low, the cost of Tesla’s investment in BMS is more than 5,000 US dollars, accounting for more than 5% of the cost of the whole vehicle. From this point of view, it cannot be said that a large battery is not good. In the case that the price of BMS has not been significantly reduced, the size of the battery pack should be determined according to the positioning of the car.
As another core technology in electric vehicles, the motor often becomes the core of discussion, especially Tesla’s watermelon-sized motor with sports car performance, which is even more astounding (the peak power of the Model S motor can reach more than 300kW, The maximum torque is 600Nm, and the peak power is close to the power of a single motor of a high-speed EMU). Some researchers in the German automotive industry commented as follows:
Tesla uses almost nothing except conventional components (aluminum body, asynchronous motor for propulsion, conventional chassis technology with air suspension, ESP and a conventional brake system with electrical vacuum pump, laptop cells etc.)
Tesla uses all conventional parts, aluminum body, asynchronous motors, conventional car structure, brake system and laptop battery etc.
The only genuine innovation lies in the technology linking up the battery cells, which uses bonding wires that Tesla has patented, as well as battery management system that can be flashed “over the air”, meaning that the vehicle no longer needs to drive to a workshop to receive software updates.
Tesla’s only genius invention is in their handling of the battery. They use a special battery cable, and a BMS that enables direct wireless networking without the need to go back to the factory to update the software.
In fact, Tesla’s high power density asynchronous motor is not too new. In Tesla’s earliest Roadster model, the products of Taiwan’s Tomita Electric are used, and the parameters are not too different from the parameters announced by Model S. In the current research, scholars at home and abroad have designs for low-cost, high-power motors that can be quickly put into production. So when looking at this field, avoid the mythical Tesla – Tesla’s motors are good enough, but not so good that no one else can build them.
Among the many motor types, the ones commonly used in electric vehicles are mainly asynchronous motors (also called induction motors), externally excited synchronous motors, permanent magnet synchronous motors and hybrid synchronous motors. Those who believe that the first three motors have some knowledge about electric vehicles will have some basic concepts. Asynchronous motors have low cost and high reliability, permanent magnet synchronous motors have high power density and efficiency, small size but high price, and complex high-speed section control. .
You may have heard less about hybrid synchronous motors, but recently, many European motor suppliers have begun to provide such motors. The power density and efficiency are very high, and the overload capacity is strong, but the control is not difficult, which is very suitable for electric vehicles.
There is nothing special about this motor. Compared with the permanent magnet synchronous motor, in addition to the permanent magnets, the rotor also adds an excitation winding similar to the traditional synchronous motor. Such a motor not only has the high power density brought by the permanent magnet, but also can adjust the magnetic field according to the needs through the excitation winding, which can be easily controlled at each speed section. A typical example is the HSM1 series motor produced by BRUSA in Switzerland. The HSM1-10.18.22 characteristic curve is as shown in the figure below. The maximum power is 220kW and the maximum torque is 460Nm, but its volume is only 24L (30 cm in diameter and 34 cm in length) and weighs about 76kg. The power density and torque density are basically comparable to Tesla’s products. Of course, the price is not cheap. This motor is equipped with a frequency converter, and the price is around 11,000 euros.
For the demand for electric vehicles, the accumulation of motor technology is mature enough. What is currently lacking is a motor designed specifically for electric vehicles, not the technology to make such a motor. It is believed that with the gradual maturity and development of the market, motors with high power density will become more and more popular, and the price will become more and more close to the people.
For the demand for electric vehicles, there is currently only a lack of motors specially designed for electric vehicles. It is believed that with the gradual maturity and development of the market, motors with high power density will become more and more popular, and the price will become more and more close to the people.
The research on electric vehicles needs to return to the essence. The essence of electric vehicles is safe and affordable transportation, not a mobile technology laboratory, and it does not necessarily need to use the most advanced and fashionable technology. In the final analysis, it should be planned and designed according to the needs of the region.
The emergence of Tesla has shown people that the future must belong to electric vehicles. What the future electric vehicles will look like and what position China will occupy in the electric vehicle industry in the future are still unknown. This is also the charm of industrial work: unlike natural science, even the inevitable result indicated by the laws of social science requires people to achieve it through arduous exploration and effort!
(Author: PhD candidate in electric vehicle engineering at the Technical University of Munich)
Post time: Mar-24-2022