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Why are new energy vehicles self ignition?

TechWeb @ 2020/05/22


This article is authorized to be reproduced from the public No.

ID rancaijing
Author Shi Zhou Jifeng

The spontaneous combustion and fire of new energy vehicles is no longer a new topic, but this time they joined the new players.
 
In May 8th, an ideal ONE new energy vehicle took place in the streets of Changsha, Hunan. According to the live video circulated on the Internet, a lot of smoke and fires were found in the front engine compartment of the accident. After the open fire was extinguished, the front face and front cover of the vehicle were almost destroyed. Fortunately, no one was injured. "Congratulations on the ideal ONE entering the new energy family." After the incident, some netizens laughed.
 
After the event, the ideal automobile official micro-blog responded quickly: after on-site testing, there was no problem in the vehicle battery system. At present, the vehicle has been moved to the safety area for testing, and the follow-up investigation results will be released in time.
 

In reply to the official APP, the owner of the car said that the cause of the fire was not the problem of the range extender and the oil circuit, nor the battery problem.
 
For the first time, the relationship between the spontaneous combustion and the battery was cleared up. This is not surprising. In recent years, the spontaneous combustion of new energy vehicles is common, many of which happen on batteries.
 
Under the background of the subsidy policy for new energy vehicles in China, sales of new energy vehicles have gone all the way. According to the statistics of the new energy vehicle national big data alliance, as of the end of 2019, the number of new energy vehicles in the country reached 3 million 810 thousand, accounting for 1.46% of the total vehicle volume, and the new energy vehicle increased by more than 1 million vehicles for two consecutive years, showing a rapid growth trend.
 
Insiders said that the mileage, power and configuration became the focus of attention of consumers and businesses, but the safety aspect was ignored. On the other side, new energy vehicles have frequent safety incidents.
 
According to the "new energy vehicle national regulatory platform big data safety regulation results report", since May 2019, the new energy vehicle national regulatory platform has found 79 new energy vehicle safety accidents, involving 96 vehicles. Among them, 47 accident vehicles were connected to the national regulatory platform, and 28 days before the accident occurred, the national regulatory platform had alerted them for 10 days.
 
For car companies, the hidden danger of new energy vehicles has become a short board restricting their development. 。 For example, last year, due to a number of spontaneous combustion incidents, Wei Lai had to recall 4803 vehicles at a stretch and suffered heavy losses.
 
The ideal automobile self ignite once again pushed the new energy vehicle to the cusp of the storm. People have to re-examine the quality and safety of new energy vehicles. Why are new energy vehicles always self ignition? Is there any safety guarantee for buying new energy vehicles?
 
New energy vehicles love spontaneous combustion, for three reasons.

From the spontaneous combustion incidents of new energy vehicles since 2019, we selected 18 samples with clear information about the accident. In terms of brand, there are many fires in the new car manufacturers in China, and the leader of Tesla is not immune. The biggest fermentation is Tesla's "421 fire case". In April 21, 2019, in the underground garage of the TAD Garden District, Yu De road, Xuhui District, a car on the Tesla suddenly appeared white smoke, and then started to burn. The fire also brought disaster to other vehicles parked around. The whole process of spontaneous combustion was recorded by the monitor, and a time to talk about Tesla color change.
 
From the battery type, quite a number of accident vehicles were equipped with three yuan lithium battery; from the state of the accident, the driving state was four, accounting for about 22%, five in charge state, accounting for about 28%, seven in the static state, accounting for about 38%, and two in the unknown state. In addition, the spontaneous combustion accidents occurred mostly in summer, and the accidents of passenger cars were much higher than that of special vehicles.

Drawing / burning finance and Economics

 

Liu Peng, an associate professor of Beijing Institute of Technology and Deputy Secretary General of the new energy automotive national big data alliance, has also verified this in a sharing of micro classrooms. Charging process and the static state after full electricity are the main components of the fire accident. Most of the new energy vehicle safety accidents happen in high SOC (State of Charge, that is, "residual power"). State; in the national vehicle safety accidents, the proportion of passenger car accidents was higher than that of special vehicles, reaching 56.9%, of which three yuan accounted for the highest proportion of lithium batteries, reaching 88.89%.
 
Specific to the cause of the accident, Cui Dongshu, Secretary General of China Federation of telecommunications, said there are mainly several situations in the spontaneous combustion of new energy vehicles:
 
First, when the chassis collides, the core or the high voltage device in the battery pack is squeezed or punctured, and other parts inside the battery pack are also short circuited or even exploded in the collision, which is likely to cause a fire.
 
Take the ES8 accident in April 22, 2019 as an example. A ES8 under repair in Xi'an suddenly caught fire. The survey results showed that the chassis of the power battery pack had a large deformation due to the serious impact of the chassis before the accident.
 
Subsequently, the spontaneous combustion occurred in a short time. At last, according to the investigation by Wei Lai and the expert group, the cause of the vehicle defects is that there is a problem in the direction of the inner direction of the power battery module, which leads to the friction between the wiring harness and the battery frame, and is extruded by the cover plate. In the extreme case, the insulation layer of the line will be worn, resulting in a short circuit, and the fire will be out of control after overheating.
 
For this reason, Wei Lai had to recall the ES8 electric vehicle powered by power battery from April 2, 2018 to October 19, 2018, which had 4803 cars.
 
Second, the lack of thermal management in the early stage of the battery, coupled with the high temperature in the high temperature season, is likely to cause the thermal runaway of the battery.
 
For example, in March 2019, a number of 407EV spontaneous combustion incidents occurred in Shenzhen, and the accident confirmed that there were quality problems in the three yuan lithium battery pack of a group of 43.5kWh used by Beiwang Wei Wei 407EV, resulting in frequent thermal runaway incidents.
 
Third, it is also the most common situation, that is, in the charging process of new energy vehicle batteries, especially under the fast charging mode, the battery risk increases.
 
At present, the power batteries of electric vehicles are mainly lithium batteries, which are divided into lithium manganate, lithium iron phosphate, nickel cobalt manganese oxide lithium, cobalt lithium and so on. Among them, lithium iron phosphate has poor density, low endurance and few applications. Lithium manganate has low cycle life and low energy and is not suitable for power vehicles. At present, the most favorite electric vehicle business is nickel cobalt manganese acid lithium battery, that is, three yuan lithium battery, and its energy density is higher than that of lithium manganese oxide. It can reach 200 to 300wh/kg, and the mileage is longer.
 
Three yuan lithium high endurance, high energy density, high activity, but also easy to heat failure. "Once overcharged, that is, the charge is too full, it is easy to produce crystallization, puncture the battery separator, short circuit, or generate a lot of residual heat to burn." Wang Jingzhong, executive vice president and Secretary General of China Battery Industry Association, explained to China News Weekly.
 
Wang Pengbo, a senior expert in the field of new energy vehicles, said, "basically, a fire accident is either a heat failure caused by charging, or an accident that has been hit or been hit, and it is extremely rare to fire itself in a completely static state."
 
Behind the high endurance lies everywhere.

"In the past few years, the whole industry has grown brutally. On the one hand, the standard of new energy vehicle promotion and application of financial subsidy policy has increased year by year, and the cost of the whole industry is increasing. Long term observation of new energy automotive industry analyst Wang Xu said.
 
Compared to 2017, the subsidy policy in 2018 increased the threshold of endurance mileage, battery energy density and energy consumption level, such as the quality and energy density threshold of power battery system, from the former 90Wh/kg to 105Wh/kg, and the mileage threshold increased from 150km to 150km. That is to say, cars with less than 150km mileage will not be able to get subsidies.
 
The subsidy policy in 2019 further raised the subsidy threshold, and the mileage of pure electric passenger vehicle operating mode should not be less than 250 km, which improved by 100 km compared with the minimum endurance requirement of last year. At the same time, the subsidy standard of new energy passenger cars, new energy buses and new energy trucks was reduced, and the rate of subsidised slope was over 50%.
 
Consumers at the other end also value the mileage. More than one customer who buys or prepares to buy new energy vehicles says that in addition to licences, the mileage is the first factor to consider.
 

Pictures from the Internet


Under "double pressure", In order to increase the mileage, attract customers and catch up with the policy, the car companies are willing to take risks, reduce the R & D cycle, reduce the safety test, and leave behind many hidden dangers. " Wang Xu said.
 
Car industry analyst Ren Wan Fu further said, "to increase the energy density of the battery to a certain level requires 12-24 months of verification cycle. If the safety durability test is artificially cut down, commercial verification has not yet been fully validated, and the security risks will naturally be left behind." He believes that more than batteries, the new energy vehicle "quick success and instant benefit" behind, hidden many hidden dangers.
 
Cui Dongshu also revealed that the core reason for the recent series of spontaneous combustion incidents is " Unreasonable design " Not just the battery itself. "Blame the three yuan lithium battery is an ulterior motive, or to protect backward production capacity. " He said.
 
Wang Pengbo also said that the activity of the three yuan lithium battery has a large core activity, but its safety is also related to the insulation materials which are insulated from the core, the BMS management strategy of the battery, the design of the cable line, the design of the heat dissipation system, the structural design of the battery shell, and the structural protection of the whole vehicle.
 
First, in order to improve the mileage, most of the current pure electric vehicles are placed under the bottom of the power battery, the battery area is large, almost covers the whole floor, and the distance from its periphery to the body edge is usually not large, so it is easy to be squeezed in collision or underpinning. Second, in recent years, the energy density of battery monomers has increased too fast, which is accompanied by the reduction of core stability and the risk of thermal runaway under mechanical loading. Because the subsidy policy has a demand for the power density of the power battery system, in order to improve the whole packet energy density, the domestic pure electric vehicle tends to make the battery pack shell structure as light as possible, resulting in insufficient protection for the internal modules and high-voltage devices.
 
Can life and safety be shared?

"From the recent policy, we can see that the country's technical requirements for new energy vehicles are slowing down gradually." Ren Wanfu said.
 
In April 23rd, the Ministry of Finance and other four ministries and commissions issued the notice on improving the fiscal subsidy policy for the promotion and application of new energy vehicles, and the 2020-2022 year subsidy standards were 10%, 20%, 30% on the basis of the previous year. The new deal also revised the mileage of subsidies for new energy electric vehicles, which is greater than or equal to 300, and the subsidy is not less than 400 kilometers, 16 thousand and 200 yuan, or 22 thousand and 500 yuan more than 400 kilometers.
 
Ren Wanfu told us, "a few years ago, the average mileage of new energy vehicles was more than 100 kilometers. Now the average mileage has reached 300 kilometers, and the endurance of some products can reach 700-800 kilometers. but Policy requirements for the mileage stay at 300 km, basically the average level of the industry. "
 
Against this background, the charging mode has also been called for improvement by some people in the industry. Compared with direct charging, The electricity exchange mode can get a better understanding of the safety and usage status of batteries, but more than one industry insiders say they are not optimistic about the mode.

Among them, Mr. Song Jin, an analyst at Analysys, said that the electricity exchange mode can really achieve energy supplement and reduce life anxiety, but there are still some uncertainties at the present stage. First of all, the standardization of power battery is not perfect, and the specifications are quite different. Secondly, in 2019, the sales volume of electric vehicles in China was 1 million 200 thousand, to the specific size of each type, and the scale was not large, so it was difficult to make profits temporarily.
 
"In fact, the most important thing is the progress of technology. Recently, enterprises have begun to pay attention to safety issues, such as strengthening the thermal management system of batteries and increasing vehicle protection for battery packs." Wang Pengbo said.
 
The core itself has improved. Wang Xu said that at present, technology continues to progress, such as BYD's recently developed "blade battery", compared to the traditional lithium iron phosphate battery, the volume specific energy density increased by 50%, all aspects of the characteristics can be higher than the three yuan lithium battery.
 
"In fact, after the country put forward higher requirements for energy and endurance mileage, the security risks did not increase because the old ones were not new products." Cui Dongshu said that at present, the solution to the problem depends on the upgrading of product technology. The subsidy policy should also guide technological upgrading and promote the progress of the industry, rather than simply protecting backward products.


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