Since August, the successive fire incidents of Tesla's pure electric vehicles in the United States have made some people's doubts about this ocean-side automobile company seem to have found evidence. Affected by the fire accident, Tesla's stock price has fallen sharply by nearly 30%. On September 8th, a stockholder even sued Tesla, accusing Tesla of publishing false and misleading statements, believing that Tesla was concealed of the security risks of the product and was suspected of being securities.

With batteries on fire, external doubts, and stock declines, Tesla, which has been consistently touted, seems to be in big trouble. Tesla CEO Musk said that the fire incident was caused by external factors and has nothing to do with the vehicle itself. At the same time, he insisted that "the company will definitely not recall electric vehicles because there is no reason for the recall."

While questioning Tesla, people once again began to discuss the safety of pure electric vehicle batteries: Is it common for electric vehicles to catch fire after an accident? Before it has been popularized, can a thorough solution be found to this security risk? What is the root cause of the battery fire? Whether this series of questions can be answered has once again become a stumbling block in the road to marketization of pure electric vehicles.

Regarding the cause of the fire in the accident vehicle, Tesla CEO Elon Musk explained on his blog: ModelS collided with a large metal object at the time. During relative movement, the object penetrates the bottom of the car and pierces the ModelS body through the leverage effect, forming a hole up to 3 inches in diameter. The thickness of the car body protecting the car body is only a quarter of an inch, which cannot stop the weight of up to 25 tons. Penetrating power. The lithium-ion battery caught fire under heavy impact.

Wang Zidong, director of the 863 Electric Vehicle Major Special Power Lithium-ion Battery Testing Center, expressed doubts about Musk’s 25-ton penetration power. “The accident itself is difficult to reproduce. How did the 25-ton penetration power be obtained? In Musk’s relevant remarks, it’s really impossible to derive directly. In addition, Musk did not explain how many battery compartments were damaged by the impact. From the fire, the impact is still relatively serious. I think the reason is still the battery caused by external force. A short circuit, which in turn caused the vehicle to catch fire."

Regarding this view, Wang Liusheng, chief analyst of China Merchants Securities Auto, said in an interview: "The outside world does not have the investigation data of Tesla's vehicles on fire. Whether these accidents are caused by battery short-circuit fires has not been confirmed."

Failed to cover the line of fire

As one of the core components of electric vehicles, Tesla's battery system was once questioned. Many industry experts called it "old battery technology" and "no core competitiveness." The reason is that Tesla is the only company that uses 18650 lithium cobalt oxide batteries. The so-called 18650 refers to the battery with a diameter of 18mm and a length of 65mm. It is often used as a battery cell for notebook computers. It was once considered difficult by the outside world. The "great house" of electric vehicles.

"If the external voltage of the 18650 battery is lower than 2.7V or higher than 3.3V, the symptoms of overheating will occur. If the battery pack is large and the temperature gradient in the group is not well controlled, there will be a great risk of fire", November On the 15th, Wang Zidong said to a reporter from the Auto Business Daily.

"Tesla's research and development phase began in 2006, when the lithium-ion battery for cars was still in its infancy. Under the circumstances at that time, Tesla's choice of 18650 battery was also a measure of equity," a person close to Tesla told Reporter, "In addition, in the early days of Tesla's establishment, Panasonic was also one of its important shareholders. In the early days of seeking capital injection, this was one of the reasons why Tesla chose Panasonic 18650 batteries."

In order to solve the high temperature instability of the 18650 battery, the Tesla R&D team began to find a solution to the problem on the battery control system. First of all, Tesla will minimize the decomposition of the battery in order to achieve the maximum stability of the battery. In order to solve the problem of overweight car body caused by too many batteries, Tesla also deliberately chose the lightest car body structure of Lotus. At the same time, Tesla's investment in battery safety control system has reached 25% of the vehicle cost. .

"The key to battery technology is precisely the voltage, current and heat control. Tesla's battery management system does a good job, but it's like putting on armor for a patient. It doesn't work in the bones. No matter how thick the armor is, it's useless", November On the 14th, a director-level professor at the National Engineering Laboratory of Automotive Electronic Control Technology revealed to a reporter from the Auto Business Daily.

Short circuit = the life gate of electric vehicles?

A person in charge of a power lithium-ion battery production company revealed to a reporter from Auto Business Daily that lithium-ion batteries have different development directions around the world due to different technical routes. Although the batteries and electronic control systems are not the same between companies, neither the lithium iron phosphate battery nor the ternary battery essentially solves the safety problem of the battery. "Because in principle, the energy density, discharge power, and the packaging of the battery pack and the high-voltage electric volts caused by collisions in the use process are all important factors that threaten the safety performance of electric vehicles, but until now, the world No technology can solve these problems fundamentally and systematically."

Song Jian, executive vice president of Tsinghua University’s Automotive Engineering Development and Research Institute, also said in an interview, “Solving battery short circuits is nothing more than reducing the voltage, increasing the current, and using a thicker fuse. But this strategy can be used in some wire plugs or other fixings. It can also be used on type batteries. For large battery packs like car batteries, it is impossible to add fuses to all batteries or reduce the current, and it is also impossible to add fuses between the positive and negative poles of the battery column, so the short-circuit problem is not good at present. solve."

"The electronic control system that Tesla has spent a lot of money to research and develop can only be used to ensure that the burning area does not spread too quickly after the battery is burned. It can be said that it can only be used for passive safety in the end." Interview Towards the end, Wang Zidong emphasized again, “This shows that no matter how powerful the electronic control system is to alleviate the spread of combustion. The problem of accidental short-circuit of the battery itself cannot be overcome, and the accident of burning caused by accidental short-circuit of electric vehicles will likely be staged again. ."

The conclusion of the Tesla fire incident should not be too early

Compared with the current widespread use of large lithium-ion batteries in the electric vehicle industry, the small batteries used by Tesla are relatively low in terms of manufacturing costs and unknown safety risks. The reason is that mature small batteries can use high-energy ratio cathode materials, high battery capacity, light weight and high cruising range.

You can see from the Tesla’s fire video that after Tesla’s battery burned, the fire was restricted to the front of the car body’s cockpit and did not cause the entire car body to burn. At the same time, Tesla products have also received a five-star safety rating in the previous National Highway Traffic Safety Administration (NHTSA), which is known as the world's most demanding safety crash test. This shows that it is doing well in the current car battery and electronic control.

In addition, the outside world does not have the investigation data of Tesla's fired vehicles. Whether these accidents are caused by battery short-circuit fires has not been confirmed. Therefore, before the investigation report of the National Highway Traffic Safety Administration is issued, we do not have to make a decision on the safety of Tesla and even the electric vehicle industry prematurely.