At the beginning of December, a pure electric vehicle parked on a certain roadside in Shanghai spontaneously ignited. Since the development of new energy vehicles, every accident case related to it has received much attention from the society. According to incomplete statistics, there were 35 new energy vehicle fire accidents in the world in 2016, including 6 foreign countries and 29 domestic ones. The country is the hardest hit area of ​​fire accidents.

As a breakthrough in the transformation of the entire automotive industry, new energy vehicles have been repeatedly written into major policy guidelines such as “Made in China 2025” and “Long-term Development Plan for the Automotive Industry”. Although the current number of new energy vehicles is low, the fire accident rate of new energy vehicles is far lower than that of traditional fuel vehicles. However, it is undeniable that the alarm of new energy vehicle safety has been ringing. In order to maintain social public safety and ensure the healthy development of the industry, research is underway. At the same time as the development route of new energy vehicle technology, it is necessary to conduct special discussions on new energy vehicle fires to improve the safety design of new energy vehicles and prevent them from happening.

The cause of the fire is mostly electrical failure.

Liu Zhengang, deputy director of the Tianjin Fire Evidence Identification Center of the Fire Department of the Ministry of Public Security, said in an interview with this reporter that the new energy vehicles are still dominated by electric vehicles. The fires caused by their own faults are mainly concentrated in the electrical system. The electrical system of an electric vehicle includes a high voltage electrical system, a power battery system, a thermal management system, and a low voltage electrical system.

According to Liu Zhengang, the main faults of the high-voltage electrical system are direct short circuit of the line, battery fixed strip and battery discharge fuse, poor contact of high-voltage line terminals and defects. The danger of the power battery mainly comes from the internal short circuit and external short circuit of the battery. Among them, the short circuit inside the battery occurs in the case of overcharging and discharging, and it is difficult to completely avoid it. The thermal management system will reduce the battery charging and discharging cycle efficiency and affect the power and energy of the battery when the battery pack temperature is uneven, the battery module, the internal resistance and capacity of the battery are inconsistent, and the heat dissipation capacity of the engine room is reduced. This leads to thermal runaway, affecting the safety and reliability of the battery, and even causing a fire. Fire during charging is also a major cause of fire in electric vehicles. From the current case of electric vehicle fires, it is related to charging, which is characterized by the matching problem between battery management system and charging system, internal problems of charger, and short circuit. Problems, etc. The low-voltage electrical system of an electric vehicle is the same as that of a conventional fuel vehicle. It should be pointed out that the hybrid electric vehicle has a more complicated fire risk due to the addition of a pure electric part to the conventional automobile.

Preventing battery thermal runaway is the key

Recently, the China Automotive Safety and Recall Technology Forum jointly organized by the AQSIQ Defective Product Management Center and the Ministry of Public Security Fire Department Tianjin Fire Evidence Identification Center was held in Tianjin. The theme of the forum was “In-depth Research and Prevention of Vehicle Fire Accidents”. This forum is the first academic conference on the in-depth discussion of vehicle fire accidents in China. The seminar focused on the topic of “new energy vehicle fire safety research” and has received extensive attention from the industry.

According to Ye Lei, deputy chief engineer of the New Energy Vehicle Testing Center of Chongqing Vehicle Inspection and Research Institute, 29 new energy vehicles have been reported to be fired by domestic relevant media. From the perspective of train type, there are 18 passenger cars, which is a fire accident of new energy vehicles. The main force; followed by passenger cars, a total of 10. Judging from the cause of fire, spontaneous combustion caused by self-ignition was the primary cause of fires in new energy vehicles; six failures caused by parts and components; three charges for charging and immersion. From the perspective of new energy vehicle types, pure electric vehicles are the type of vehicles with high fire accidents, a total of 23 vehicles; followed by hybrid electric vehicles, a total of three.

Lan Fengchong, a professor at South China University of Technology and vice chairman of the Guangdong Automobile Engineering Society, believes that based on the principle of thermal runaway of single cells and its propagation laws, the final fire-explosion of battery packs that cause power is the basic principle of electric vehicle fire accidents. Theoretical research and experimental evidence; the relationship between the factors affecting thermal runaway, the analysis of thermal runaway mechanism, the study of thermal runaway propagation behavior and the thermal runaway blocking technology are measures to prevent fires from the root cause, but fundamentally Probability is still very difficult.

In fact, the factors affecting the safety performance of power batteries run through the entire life cycle of a power battery from the selection of batteries to the end of use, which is the focus of the industry. Zhang Yu, secretary-general of the Power Battery Application Branch, said in an interview with this reporter: "The intrinsic orbital energy, crystal structure and traits of the material itself determine the intrinsic safety performance of a cell; each process in the manufacturing process of the cell The degree of excellence, automation and formation conditions determine its cycle performance and safety performance, affecting its lithium insertion stability and thermal stability; battery design in terms of BMS and safety can effectively ensure battery safety However, the manufacturing and use conditions of the battery cannot always be in an ideal state. Manufacturing errors and abuse conditions are difficult to avoid. Under this realistic condition, various pre-planning designs for the thermal runaway of the battery are particularly important."

Safety standards are mostly recommended standards

Nowadays, new energy vehicles are in a huge turmoil. Whether it is policy guidance or interest, with the increase of capital and the increase of possession, a series of problems such as excessive pursuit of energy density and cruising range and low-end surplus have gradually surfaced. Promoting product quality improvement is a common problem facing the entire industry.

At present, there are 128 effective standards for new energy vehicles, covering the whole vehicle, key systems and components, interfaces and charging infrastructure. According to the requirements of the Ministry of Industry and Information Technology No. 39, new energy vehicle products are subject to 39 standards and 1 technical condition, including 31 safety technology standards for new energy vehicles and key components, accounting for nearly 80%. For example, GB/T31485-2015 "Safety Requirements and Test Methods for Electric Vehicle Power Battery" has strictly regulated 10 safety requirements for over-discharge, over-charging and short-circuit of battery cells and modules; GB/T31498-2015 "Electric Vehicle Collision" After the safety requirements, there are clear requirements for protection against electric shock, electrolyte leakage, etc. If within 30 minutes of the end of the collision, REESS should not cause fire or explosion.

"At present, some problems are also reflected in the standard setting." Ye Lei pointed out: "First, the safety standards for electric vehicles are currently recommended standards, second, the safety standards in the commercial vehicle field are ignored, and the third is the lack of passive safety experimental projects. To this end, the National Standards Committee is also organizing relevant units to supplement and improve, GB "Safety Requirements for Lithium Ion Power Battery for Electric Vehicles" is being developed, including the addition of battery packs, system thermal runaway expansion, system low temperature protection, overcurrent Protection and other experimental projects, the standard was established in August 2016, after seven discussions, the second draft of the official draft has been released, in the stage of soliciting comments. In addition, GB "Safety Technical Conditions for Electrician Buses" has also formed the first draft, and is seeking advice from the community.

Car companies should be safe

For new energy auto companies, safety design is a systematic project. At the beginning of design, the safety design of battery cells, the safety design of battery modules, the safety design of high-voltage systems, and the safety design of complete vehicles should be considered. Physical protection from hardware and policy protection from software.

Liu Zhengang suggested that, first of all, collision safety is an important indicator to investigate the safety of new energy vehicles. Power batteries and thermal management systems must be placed in the interior of the vehicle that is not vulnerable to collision damage. At the same time, necessary design must be used for collision protection. Measures such as ribs; secondly, the battery compartment should be physically separated from the passenger compartment to avoid the fire quickly spreading to the passenger compartment after the battery compartment is fired. Third, the power battery is in a closed battery compartment, for power. In the initial stage of battery fire, it is considered to set up an automatic fire extinguishing device. Fourth, electric vehicles use high-voltage power. No matter the use, maintenance, fire fighting and rescue, there is a danger of electric shock. Therefore, electric vehicles should pay special attention to the protection of high-voltage power. .

"New energy vehicle manufacturers have negative responsibility for product safety, and recall of defective products is also an effective means to avoid major safety accidents." Ye Lei pointed out. According to statistics, from 2004 to 2016, China implemented a total of 1,296 defective car recalls, involving 36.69 million vehicles, of which 313 were related to fire defects, involving 10,700 vehicles, accounting for 24.2% and 27.4 of the total number and number of vehicles respectively. %. Although traditional fuel vehicles are the mainstays, with the large-scale production and application of new energy vehicles, it is an important part of post-market regulation to eliminate potential safety hazards for consumers and achieve quality improvement through recalls.

Battery companies need to improve quality and efficiency

In addition to the safety performance of the power battery product itself, the safety of the new energy vehicle is also closely related to the maintenance of the later operation. Zhang Yu pointed out that it is not appropriate for the current public opinion to blame the fire battery entirely on the power battery, which is not conducive to the development of the industry. However, through the hot investment environment of the power battery industry, some serious problems are also highlighted. For example, the product performance is uneven and the low-end capacity is excessive. Although the lithium battery is recognized as the ideal energy storage component, it is still unable to meet the high safety of the whole vehicle. Sexuality, high consistency requirements, etc.

Zhang Yu believes that power battery companies should actively change their development mode, from capacity expansion to quality improvement. We must persist in relying on technological innovation to accelerate the intelligent manufacturing of batteries. It is necessary to develop jointly with scientific research institutes, closely integrate with upstream and downstream industries, accelerate the structural adjustment of power batteries, and meet the requirements of new energy vehicles for cruising range, high safety and high consistency. At present, the power battery industry has entered the “shuffle period”. The research on the safety of power batteries still has a long way to go. Only by combining theory with actual innovation can we usher in the true meaning of high-energy and high-power applications. brilliant.

Consumers need to strengthen the use of electricity common sense education

For new energy owners, proper use is also an effective measure to prevent malfunctions, such as complying with the charging specifications and procedures specified in the user manual, regular inspection and maintenance, and not parking at will.

"We must strengthen the education of consumers' common sense of electricity, especially the education of slow charging. Try to use charging piles, do not charge the lines for charging. Do not use extension cords to put the mobile power sockets connected to the mains in the trunk. Charging. To take a step back, if you use the extension line for slow charging, you must use the qualified high-quality wire that meets the power requirements and not wrap. The consumer can not disassemble the parts at will, and pay special attention to prevent the occurrence of electric shock accidents." Liu Zhengang suggested.

In addition, after discovering that there is a problem with the vehicle, the consumer should contact the professional repair factory to eliminate the safety hazard, especially the fire hazard, in the bud, and also report the safety hazard information to the AQSIQ Defective Product Management Center and the enterprise. To help car companies find defects in time and take measures to improve product quality.

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