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How does the battery of a new energy vehicle get out of control?

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On June 27, in response to the recall of NexteV, NexteV's battery module supplier, Ningde Times, issued a statement saying that the battery case body of this recall interferes with the module structure supplied by Ningde Times, and there may be a risk of short-circuit of low-voltage sampling wire harness under some extreme conditions, which poses a potential safety hazard. The batch module has a custom design that is only applicable to the 4,803 ES8 products recalled.
Nextev previously said in the recall announcement that the voltage sampling wiring harness in the module used in the accident vehicle was likely to be squeezed by the upper cover plate of the module due to individual improper alignment. In extreme cases, the extruded voltage sampling wire harness skin insulation material may wear, resulting in a short circuit, a safety hazard.
The Nextev recall is an episode in the development history of Electric vehicles in China.
In China, 2 million consumers of new energy vehicles are willing to try this new form of transportation, but they have not been able to get the investigation results of the fire incident from the authorities. Frequent spontaneous combustion incidents are bound to shake their confidence in purchasing and demonstration.
As of press time, no state authority has released findings on a series of new energy vehicle fires since 2018 to reassure the public.
If you don't know why, you can't solve the security problem. For China, the world's largest consumer of new energy vehicles, it will be a confidence disaster: for example, most vehicles will consciously stay away from electric vehicles when they stop; For example, many foreign companies advise employees to stay away from electric cars charging during safety Sharing before their daily meeting.
In May 2019, China produced and sold 112,000 and 104,000 new energy vehicles, up 16.9 percent and 1.8 percent year on year, respectively. Compared with the tens of percent year-on-year growth in 2018, China's new-energy vehicle production and sales growth has slowed this year.
But As the world's pioneer in the use of new energy vehicles, China is still speeding up the process.
In November 2018, The Society of Automotive Engineering of China issued the Implementation Plan of Eight Key Projects in the Medium and long term Development Plan of the Automotive Industry: it is planned to sell 7 million new energy vehicles annually by 2025, with the population reaching 20 million, promote 50,000 fuel cell vehicles, and take up 20% of the market share of hybrid electric vehicles.
It is not only China. Due to the policy pressure of energy conservation and emission reduction policies, new-energy vehicles have become an irresistible trend in the world.
Roland Berger predicts that by 2020, about 15% of the new vehicles sold in the world that year will be new energy vehicles. By 2030, sales of new energy vehicles will surpass those of traditional fuel vehicles, accounting for 57% of that year's sales to 59 million vehicles per year.
The systematic elimination of consumer panic is a prerequisite for achieving the sales forecast in a market with conservative estimates of no less than 10 trillion yuan of output.


Strictly control the quality
As Ouyang Minggao, an academician at the Chinese Academy of Sciences and executive vice President of the China Association of 100 Electric Vehicles, pointed out, fires in new-energy vehicles are a matter of product quality, not technical route.
Electric vehicle fire is mostly caused by battery fire, battery fire is mainly caused by the battery heat out of control.
When the battery temperature is too high or the charging voltage is too high, there will be a chain chemical reaction inside the battery, which will lead to a sharp rise in the pressure and temperature inside the battery, which will cause the thermal control of the battery and eventually lead to combustion.
There are a variety of reasons for the battery heat out of control, it may be the battery pack itself temperature is not uniform, there are local area high temperature, short circuit outside, short circuit inside and so on causes of fire.
Lin Yuchun, head of Research and development for power battery technology company Vision AESC In China, told a reporter travelling with the company that the diaphragm design is one of the important factors affecting the occurrence of short-circuit in the battery. If the diaphragm design margin is insufficient or the design direction is wrong, the mechanical ductility and flexibility of the diaphragm will be affected, leading to shrinkage in the charging process, and then the positive and negative poles will contact each other, resulting in short circuit.
At the same time, in the production process, the control is not strict, the cell is mixed with metal particles, these impurities in the charging and discharging process will lead to electrode surface difference reaction, after continuous accumulation, will puncture the diaphragm, resulting in a short circuit.
Once there is a problem in one cell, such as short circuit, open circuit, etc., it will affect other cells in the battery pack, resulting in serious internal problems, and finally lead to safety problems.
Tesla's electric car, which USES a series circuit, is particularly dangerous. Xu Rongzhen, a marketing manager at SoftBank china-invested Taiwanese solid-state battery maker Huineng Technology Co., told a traveling reporter that Tesla has 4,416 lithium batteries, and if one link is not done well, there will be a chain reaction.
BMS technology needs to be improved
In addition to the efforts of battery factories and charging enterprises, in fact, the responsibility of the whole car factory is not light. Because the Battery Management System (BMS) is an important element in the overall architecture of electric vehicles, which is currently designed by the oEMS.
BMS is at the core of power battery system, and is the core component of battery protection and management. BMS should not only ensure the safe and reliable use of the battery, but also control the charge and discharge of the battery pack, and report the basic parameters and fault information of the power battery system to the vehicle controller, which can be described as the bridge between the battery, the vehicle controller and the driver.
A large lithium power plant in the middle to travel a reporter said that the thermal management system in the BMS is more important. The basic working principle of the thermal management system is to keep the temperature of the battery pack within a certain temperature range by means of cooling or heating to ensure the performance and life of the cell.
The thermal management system is divided into three categories: heating system, air cooling system and water cooling system. Different design schemes work in different principles, but all of them have the possibility of causing the battery pack heat out of control.
First of all, the water cooling system, it is through the liquid convection exchange, take away heat to reduce the temperature of batteries a thermal management way, but the water pipes are usually located at the bottom of the battery pack, installed in the vehicle chassis, the vehicle in the process of long-term operation of impact of abnormal water cold plate, the bottom, or water cooling design structure reliability failure for a long time, may make its coolant leakage occurs, resulting in battery insulation failure caused by the thermal runaway.
Second, the air-cooling system is a thermal management mode that USES air as the medium and USES heat convection to reduce the temperature of the battery cell. However, the air-cooling design will improve the sealing design difficulty of the battery system, and the sealing structure of the vehicle will fail during long-term operation, which will lead to the risk of thermal control due to the insulation failure due to water inflow during driving in cloudy and rainy days.
Finally, for electric vehicles mainly used in cold areas, the heating system will be used. The principle is to use the heating film to heat the battery pack, so that it can maintain a reasonable operating temperature range and ensure the performance of the battery cell. Unreasonable heating power design or assembly scheme design of the heating film, or its long-term reliability failure, may also lead to the failure of insulation of the battery pack, leading to thermal runaway events.
The middle-level managers of the lithium power plants mentioned above said that the thermal management problems have technical solutions at present, and whether these problems can be effectively solved is an indicator to reflect the advanced technology of each manufacturer.
Risk at high energy density
In order to solve the problem of ev range, it is the only way to improve the power battery energy ratio. To improve the energy ratio, it is necessary to adjust the ratio of materials related to lithium batteries.
At present, most of the power batteries of passenger cars are made of ternary system, namely, lithium batteries of lithium nickel-cobalt manganate or lithium nickel-cobalt aluminate are used as positive electrode materials. Lithium nickel-Cobalt manganate is used in China's ternary power batteries.
According to the different ratio of ni-Cobalt-manganese three elements, it can be divided into type 111, type 532, type 622 and type 811. As the nickel ratio increases, so does the energy ratio of the power battery, which means the range of the car will increase accordingly.
For lithium batteries, safety, life, cost and energy density are in a state of dynamic balance. If the energy density goes up, then there must be some problems with the other three.
Li Fengmei, vice President of The business management Center of Shenzhen Bike Battery Co., Ltd. said in an interview with a reporter that the higher the nickel proportion, the worse the thermal stability of the whole positive electrode material. In case of high temperature and external force impact, high-nickel batteries will have potential safety hazards. Swelling of high-nickel batteries caused by gas production during charging is also a problem.
At the same time, 811 battery once the problem of heat out of control, the consequences are more serious. In the past, lithium iron phosphate batteries would lose control of heat and only emit smoke. The 532 ternary lithium battery will burn out of control. If the type 811 ternary battery gets out of control, deflagration is likely to occur.
The pursuit of high energy density is an inevitable development, but Ouyang Minggao reminded a reporter, the development of high specific energy power batteries for electric vehicles, safety is always the first priority.



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