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[YesAuto News] If you summarize the top ten hot words for the auto market in 2019, “Electric Vehicle Spontaneous Combustion” will definitely be on the list. According to the “New Energy Vehicle National Supervision Platform Big Data Safety Supervision Achievement Report” released by the National Big Data Alliance of New Energy Vehicles, from May to mid-August this year alone, the National Supervision Platform for New Energy Vehicles found 79 safety accidents involving There are 96 vehicles, and an electric vehicle spontaneous combustion accident occurs almost every 1.5 days.

“The Truth” is an original hot event investigation column of Auto House. We hope that through objective and in-depth reports, we can restore the truth about hot events in the automotive industry. If you are interested in or have doubts about the news events in which car circles, you are also welcome to leave us a message below the article, and we will try our best to find the truth and answer your doubts.

In April this year, everyone's attention and discussion on the spontaneous combustion of electric vehicles reached a climax. On April 21, Tesla spontaneously ignited rapidly in a few seconds without charging in a basement in Shanghai. And just one day later, another “net celebrity” model Weilai ES8 in the field of electric vehicles also burned in its own service center.

Although, the National Highway Traffic Safety Administration of the United States believes that the probability of fires in electric vehicles is lower than that of diesel and gasoline engine vehicles. The fire emergency management department of our country also expressed similar views. But people are obviously paying more attention to the fires of new energy vehicles. Among them, one of the reasons is that people are more concerned about new things, and more is the “fear” of unknown things.

As Wang Zidong, Deputy Secretary-General of China Power Battery Innovation Alliance and Deputy Secretary-General of China Electric Vehicle Charging Infrastructure Charging Alliance, said, we can find a clear cause for the spontaneous combustion of fuel vehicles and take measures to prevent problems before they occur. . But for electric vehicles, not every spontaneous combustion, we can find the exact reason. Because it is impossible to know the definite cause and when it will happen, it is difficult to completely avoid it.

The frequency of spontaneous combustion and the uncertainty of the cause of the fire may lead to a decline in consumer acceptance. From the corpus analysis data of Autohome, we found that compared with the same period in 2018, the number of mentions of “spontaneous combustion” and other safety accidents in the forum by Autohome netizens has increased by 14 times in 2019. Correspondingly, consumer acceptance of new energy vehicles has dropped from 71.9% in 2018 to 50.4%, and acceptance has fallen by more than 20%.

In this issue, we are not going to “wash the ground” for electric cars, telling everyone to buy them with confidence, it's okay; we also don't want to demonize electric cars, because spreading fear or selling anxiety may be able to quickly gain traffic, but it is useful to solve the problem of netizens and consumers. Doubts are not helpful. The first issue of the “Truth” column chose to start with the investigation of “electric vehicle spontaneous combustion” because it hopes not to beautify and demonize, to restore the truth of the spontaneous combustion incident, and to solve the “cloud of suspicion” in the minds of consumers.

Causes of spontaneous combustion of electric vehicles

Data speak

What are the reasons behind so many spontaneous combustion accidents? battery? Charging pile? Or the vehicle itself?

According to data from the National Big Data Alliance for New Energy Vehicles, vehicles that have experienced accidents since May this year are mainly vehicles produced in 2018. Among them, 86% of the accident vehicles were ternary lithium batteries, 7% were lithium iron phosphate batteries, and 7% were uncertain.

This is basically consistent with the views put forward by Yang Yusheng, an academician of the Chinese Academy of Engineering. In his view, excessive pursuit of long cruising range is one of the reasons for the frequent occurrence of spontaneous combustion at this stage. Because in general, a longer mileage means a higher energy density battery or a larger battery pack, which is more likely to cause spontaneous combustion.

At the same time, according to the post-analysis of the safety of new energy vehicles, 41% of the vehicle status at the time of fire has been found to be 41% in the driving state, 40% in the resting state, and 19% in the charging state; Among the causes of fire, battery problems accounted for 58%, collision problems 19%, water immersion problems 7%, usage problems 7%, other component failures 3%, and external causes 3%.

From the above three sets of data, it is not difficult to see that among the causes of fires in electric vehicles, battery problems account for the highest proportion, and models with ternary lithium batteries are the main ones. So why do today's batteries seem to be more and more “flammable”? Are ternary lithium batteries really more “dangerous”? Let's continue the analysis below.

High energy density battery back pot?

In fact, how to strike a balance between high energy density and improved safety has always been a topic of concern to the industry and an urgent problem to be solved. Data shows that most of the ternary lithium batteries carried by the accident vehicles in spontaneous combustion accidents are mainly 523 systems. In the future, there will be 811 (80% nickel, 10% cobalt and 10% manganese) high-nickel system power batteries in the future. The application, or will bring greater risks.

The basis for the judgment is that, relatively speaking, the 811 high nickel material has certain advantages in energy density, but it does not have advantages in material stability and cycle life.

Not only that, with regard to the safety of lithium-ion power batteries, Wang Zidong added that lithium-ion power batteries (especially the 811 ternary lithium battery used on a large scale today) have relatively active chemical properties and are very sensitive to temperature. In short, it is afraid of heat, cold, and “deep release.”

First of all, the electrolyte in the ternary lithium battery cell with high energy density commonly used today is flammable. Under high temperature conditions, the ternary material will precipitate oxygen, which means that even if the ternary lithium battery is placed in a In a vacuum environment completely isolated from oxygen, once abnormally high temperature occurs, the battery may still ignite spontaneously, and it is difficult to extinguish it by current technical means (isolated from oxygen), and it can only be left to “fend and destroy.”

Secondly, in a low temperature environment, the lithium ions inside the cell will precipitate into a metallic state, which will reduce the capacity of the cell on the one hand. On the other hand, the precipitated metallic lithium will gather and plate on the negative electrode surface of the cell to form lithium dendrites. Once the dendrites pierce the diaphragm and connect the positive and negative electrodes of the cell, it will cause a short circuit and cause danger.

And the power battery using ternary material as the cathode material, if left for a long time without charging and discharging, the battery's own capacity will also be affected. This is because the ternary material is a synthetic chemical substance, and its own chemical bond is not stable. If it is left for a long time, it will have a certain chemical reaction by itself, resulting in a decrease in capacity.

The cause and process of thermal runaway

There have been many reports that the spontaneous combustion of pure electric vehicles is caused by thermal runaway of the power battery pack. However, it should be clear here that thermal runaway is a phenomenon of problems with the power battery pack, not the real cause of the problem. What we call thermal runaway is an uncontrollable temperature rise inside the cell, but how does the temperature rise inside the cell? When did it happen? At present, we have not fully grasped the law of it.

On the one hand, the spontaneous combustion of pure electric vehicles is spontaneous, and the “crisis scene” is often severely damaged afterwards; on the other hand, the spontaneous combustion of power batteries cannot be “recovered” through technical means. In addition, we currently lack scientific and systematic research and understanding of the chemical properties of lithium-ion batteries. Therefore, for many spontaneous combustion accidents of pure electric vehicles currently occurring, the final survey results are often unsatisfactory.

From the known causes of thermal runaway, it can be roughly divided into external causes and internal causes. External causes such as chassis impact, squeeze, water immersion, etc. are relatively common; internal causes such as battery overcharging, overdischarging, battery aging, dendrites, etc., are also considered to be the “culprits” of many spontaneous combustion incidents this year. .

For the process of thermal runaway, we can look at it in three stages:

The first stage is the stage of heat accumulation. At this stage, the inside of the cell generates abnormal heat due to the above-mentioned reasons, which causes the SEI film and diaphragm inside the cell to decompose. With the decomposition of the SEI film and diaphragm, the inside of the cell It will accelerate the heating process. If an abnormality can be detected in time at this stage, it is the best time to avoid thermal runaway or escape.

The second stage is the thermal runaway stage. As the temperature in the cell continues to rise, it eventually leads to a large-scale short circuit. The electrolyte in the cell starts to burn, causing the single cell to burn completely. The battery cells are closely arranged in the battery pack. Once a battery cell burns out of control, it will quickly ignite other battery cells. At this point, the combustion of the battery has basically entered an irreversible thermal runaway state.

The third stage is often the scene we saw in some videos of spontaneous combustion accidents of electric vehicles. A large number of electric cores are ignited. In order to prevent the explosion of the battery pack, a large amount of gas is ejected from the bottom of the vehicle, sometimes accompanied by flames. The fire will continue to burn until the entire battery pack and vehicle are burnt out.

The battery management system is negligent?

Some people are also questioning whether it is due to the “dereliction of duty” of the battery pack's BMS (battery management system) that the failure to monitor the battery pack's problems in time and take measures has led to the occurrence of the problem? From this point of view, Wang Zidong said that BMS monitors the “external enemy”, and it is difficult to detect problems inside the battery cell in advance.

We know that BMS can monitor the entire battery pack, including the voltage and current of the cells, and the temperature of some cells, to prevent overcharging or overdischarging, and to ensure the healthy operation of the battery pack. However, for the health problems hidden inside the batteries, BMS is somewhat powerless. And once the abnormal temperature of the battery cell is detected externally, it is likely that “thermal runaway” has occurred internally, and even cutting off the power of the entire battery pack will not help.

Charging may be the cause

Although from the data point of view, the proportion of burning in the charging state is the least, and the reason for the battery is the most. However, Wang Zidong pointed out that among the spontaneous combustion accidents of pure electric vehicles that have occurred, more than half of the models have just been charged or are being charged. The charging process is likely to be a cause of spontaneous combustion of pure electric vehicles.

In this regard, the charging pile operator Xingxing Charging has a different view. Li Hongqing, general manager of operation and maintenance of Xingxing Charging Group, said that according to the public opinion of Professor Ouyang Minggao from the Department of Automotive Engineering of Tsinghua University in 2018, the main cause of spontaneous combustion during charging is insufficient battery product testing and verification; reliability deterioration during vehicle use; charging safety The management technology is low; the battery system heat spread suppression technology is limited and other reasons.

Li Hongqing believes that the current in the charging process is output by the pile end after comprehensive judgment based on the vehicle end demand. National standard requirements and enterprise equipment implementation have multiple control mechanisms for the pile end current output, which can ensure that the output current is less than or equal to the vehicle end demand current. .

However, not all charging pile companies can be so confident. According to CCTV News, recently, Guangdong Product Quality Supervision and Inspection Institute announced the risk monitoring results of electric vehicle charging pile products for the first time. The results show that 70% of the tested samples have safety hazards. In this risk monitoring, a total of 10 batches of electric vehicle charging pile products from 9 manufacturers were collected, of which 7 batches did not meet the requirements of the national standard. Risk monitoring found that there were four test items in the samples that did not meet the requirements of the national standard, and even one batch of samples and 3 test items did not meet the national standard.

The reasons for non-conformity are also “various”, including the charging pile output voltage is generally too high, far exceeding the household 220V voltage, people who do not have the common sense of professional electricians will have the risk of electric shock; 70% of the samples do not meet the national standard for electric shock prevention, the main problem Including metal materials are not grounded, there is no warning if there is a failure, etc.; the electronic lock of the charging gun is not reliably locked, and the gun head will be charged, and the user is prone to danger when plugging or unplugging; the charging pile parts have poor rust resistance, moisture and conductive parts Contact will cause corrosion or external conduction.

When we see that an electric car ignites spontaneously, we often just attribute the blame to the car manufacturer. In fact, whether the battery can be used safely and stably is closely related to every charge and discharge. A number of tests have proved that frequent overcharge and overdischarge will cause the thermal stability of the battery to deteriorate, and plant the seeds of spontaneous combustion for electric vehicles.

● Recommendations for the use of electric vehicles

What should I do if the electric car ignites spontaneously

Compared with gasoline vehicles, electric vehicles are more dangerous during spontaneous combustion and are difficult to extinguish with conventional means. Therefore, once the vehicle alarms or emits smoke, you should leave the vehicle as soon as possible and wait for firefighters to deal with it.

Compared with gasoline vehicles, the characteristics of electric vehicles on fire are very different:

First of all, the burning of the battery will produce a chemical reaction, releasing a large amount of toxic gas, which will cause instant suffocation and poisoning after inhalation; secondly, the battery will burn at a high temperature and fast. 60 seconds after the fire, the temperature can reach 200 ℃, after 180 seconds, The flame temperature will reach over 1000℃, and there is a risk of explosion; third, due to the limitation of the battery protective shell, the combustion flame is often spray-like, and the spray strength is large, the distance is long, and the surrounding vehicles are more likely to ignite; fourth, the location of the ignition point It is relatively concealed, and cannot be extinguished by traditional air isolation methods, which is difficult to put out and there is a possibility of re-ignition.

Who will pay for the spontaneous combustion of the car?

Own car

If the car ignites spontaneously, compensation will be involved. On the one hand, it is the loss of one's own vehicle, on the other hand, it is the compensation of other ignited vehicles. Let's talk about our car first. We all know that there is an additional insurance in car insurance called spontaneous combustion loss insurance .

The insurance liability stipulates that: during the use of the motor vehicle insured with this insurance, due to the failure of the electrical appliances, wiring, fuel supply system of the vehicle and the cargo itself, it catches fire and burns, resulting in the loss of the insured vehicle and the insured’s In the event of an accident under this insurance, the insurer shall be responsible for compensation for necessary and reasonable rescue costs in order to reduce the loss of the insured vehicle.

It is important to note that the primary condition of insurance compensation is caused by non-human factors , and the current uncertain electric vehicle fires during charging is not clearly stated in the insurance clause, which means that the compensation after spontaneous combustion Gasoline vehicles are the same as electric vehicles-both require the fire department’s appraisal , that is, the root cause of the vehicle fire is determined by non-human factors and the cause of the fire is explained.

What if the car owner does not pay spontaneous combustion insurance for the vehicle? In spontaneous combustion accidents, if the vehicle is within the warranty period, the spontaneous combustion caused by the quality of the new car can apply to the manufacturer for compensation . It should be noted that compensation has the characteristic of non-repetition, which means that even if the manufacturer can indemnify, the insurance company can also indemnify, but they can only choose one of the two. But no matter what kind of compensation, the appraisal certificate provided by the fire department is very important, and it will directly determine whether the insurance company and the manufacturer can make compensation.

Then, without paying the additional insurance for spontaneous combustion losses, can I get compensation only after the vehicle's loss insurance spontaneously combusts? The answer is obviously no. There is a clear statement in the liability exemption clause of vehicle loss insurance-the insurer is not responsible for compensation for losses caused by spontaneous combustion and fire caused by unknown causes. Among them, the definition of spontaneous combustion is also clearly stated-spontaneous combustion refers to the fire and combustion caused by the failure of the vehicle's electrical appliances, wiring, and fuel supply system or the cargo itself.

Someone else's car

The affected vehicles must have suffered a disaster, so how do I apply for compensation? First of all, the three-part insurance for vehicles can be used to cover the losses of other vehicles . Third-party liability insurance refers to the amount of compensation that should be paid by the insured in accordance with the law for accidents that occur during the use of the insured vehicle by a qualified driver permitted by the insured, causing a third party to suffer personal injury or death or direct property loss , The insurer will pay compensation in accordance with the relevant provisions of the insurance contract.

Damaged car owners will undoubtedly face poor communication when applying for compensation from the party responsible for the accident. For owners of damaged vehicles who pay car damage insurance, they can apply to their own insurance company for subrogation , that is, their own insurance company will repair the vehicle first. /Send the compensation payment to the car owner, and then the insurance company will seek compensation from the party responsible for the fire.

In addition, some damaged car owners may think that they are unlucky, and they happen to have spontaneous combustion insurance. Can they be compensated? The answer shows that it is no. It is clearly stated in the spontaneous combustion insurance liability content that the cause of the fire was caused by its own cause, and the ignition by other vehicles is obviously an external cause, so it is not in the scope of spontaneous combustion insurance compensation.

● Will electric vehicles in the future be safer?

Although this issue talks a lot about the challenges faced by electric vehicles, we still firmly believe that the future of electric vehicles will become more and more safe. Because safety is the basic condition for the existence of electric vehicles.

So, how to ensure the safety of new energy vehicles to the greatest extent?

Relevant departments have gradually established a new energy vehicle national supervision platform system structure. On the one hand, through the supervision of new energy vehicle enterprise platform operation and real-time vehicle operation status supervision, timely and accurately grasp the technical status of vehicles; on the other hand, promote the implementation of the first responsible entity , Implement new energy vehicle safety early warning, accident alarm and accident tracing. At the same time, relevant departments will also publicly monitor macro-statistics, analyze and appraise accident responsibilities, and evaluate accident reports issued by enterprises, local governments and national authorities.

Corresponding early warning models are also being built. According to Sun Fengchun, an academician of the Chinese Academy of Engineering, the current national regulatory platform has established four early warning models and conducted experiments in 25 companies. In the future, they will all be opened to new energy vehicles and related power battery companies across the country for free.

Not only the whole vehicle, but also the relevant policies and regulations on power batteries and charging piles are also improving. Among them, in terms of power batteries, whether it is the “Automobile Three Guarantees Regulations”, the “Regulations on the Recall of Defective Automobile Products”, and the “Regulations on Responsibilities for Repair, Replacement and Return of Household Automobile Products”, power batteries are included; in terms of charging piles, technology Standards and acceptance specifications are also being further strengthened.

Taking Xingxing charging as an example, the current charging piles have established an active safety protection system, including the safety protection of the charging facilities themselves, the active safety protection during the charging process, and the automatic safety alarm protection. The self-safety protection measures of the charging facilities use the intelligent networking platform to place the equipment operation status under the two-level safety protection of the local safety control and the platform policy safety control, combined with the intelligent operation and maintenance system to achieve 7*24 hours of uninterrupted active safety of the equipment control.

“This means that in the active safety protection of the charging process, through the charging process model big data algorithm, the active protection of abnormal BMS communication, abnormal power demand, abnormal temperature, etc. can be realized. By docking with the platform of the fire fighting unit, we can achieve The one-key automatic alarm function of stations with safety risks improves the speed of safety response.” Li Hongqing said.

The occurrence of any accident is not without warning, and so is the spontaneous combustion of new energy vehicles. Before the vehicle spontaneously ignites, there will still be some subtle changes inside the battery pack. The National Big Data Alliance of New Energy Vehicles analyzed a spontaneous combustion logistics vehicle afterwards and found that according to the accident site and monitoring data, it was confirmed that the battery was overcharged due to the battery consistency problem, which caused the accident.

In fact, about 10 hours before the occurrence of thermal runaway, the maximum voltage difference between the monomer with thermal runaway and other monomers reached 482mv; at the same time, its Shannon entropy Z score exceeded the range of ±4 many times, which can also be judged from the data It has a high risk.

This research also means that spontaneous combustion may be notified in advance. In response to this, the new energy vehicle national supervision platform has deployed separate arrangements before, during and after the accident. The national regulatory platform has also established a security incident handling mechanism. Formulate emergency plans, rescue plans and accident investigation plans for new energy vehicle accidents.

It is reported that the principle of the early warning model is mainly considered based on the time dimension, the single dimension, and the short-term transient dimension. One is to measure the volatility of the time dimension, using the fault diagnosis model based on volatility detection and the entropy diagnosis model; the second is to measure the consistency of the individual dimensions, using the data analysis model of the model threshold table and the elevator threshold table; the third is to measure the short-term Transient, using the pressure drop consistency judgment model.

According to data, as of August 15, 2019, 2.509 million vehicles have been connected to the new energy vehicle national supervision platform, with an average monthly number of nearly 150,000 vehicles, and a total operating mileage of 60 billion kilometers. The daily online rate is over 60%, the real-time online rate is nearly 22%, and the average daily accumulated mileage reached 70 million kilometers. With the continuous accumulation of data, the early warning model for battery failure warnings will be more accurate and timely.

● Write at the end

The internal combustion engine vehicle has a history of more than 130 years since its invention. Its safety and stability have also been developed step by step in this long process, and it is still being improved.

The mass production of purely electric cars has only taken more than ten years. We should see its potential, but we should also maintain sufficient patience. With the continuous improvement of battery energy density, we will be “tamed” to become a bigger and more dangerous beast. This requires battery management technology and charging technology to keep up with it, and one of the three is indispensable.

For the sustainable development of the new energy industry, ensuring safety is a prerequisite. Because whether it is driven by an internal combustion engine or an electric motor, it is driven by humans or automatically. Cars have always been a means of transportation for people, so ensuring people's safety is the most basic and most important thing.

When talking about the technical route of future automobiles, Wang Zidong also said that energy saving and emission reduction are the general direction of future automobile technology progress, and electric drive systems will be the trend of future automobile evolution due to their simple structure and high energy conversion efficiency. There is no final conclusion about what kind of energy source to use. Only a car that can meet the needs of future travel scenarios, provide consumers with safe, convenient and comfortable travel, and at the same time achieve energy saving and emission reduction, is an excellent “future car”. (Interview/Editor: Zhang Lianyi, Wang Xingyu, Liu Yuxin Design: Li Peng Special topic: Sun Mengyang)