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Electric vehicle battery fires, thermal runaway is the main reason

In the past decade of vigorous development of new energy vehicles, the increasing base number has brought more reports of Electric vehicle battery fires accidents, and the safety situation of new energy vehicles has become increasingly severe.

Follow up relevant statistics ,sixty percent of Electric vehicle battery fire accidents are caused by the thermal runaway of the battery itself, 30% are charging accidents, and only about 3.6% are due to collisions in driving accidents.

1. Electric vehicle battery fires Case

In October 2013, after a Tesla Model S hit the bottom, the on-board system issued a warning (45-60 seconds after hitting the bottom), and the owner fled in time (a few more seconds), and the car quickly burned out (120 Seconds later after getting out of the car), Tesla shares fell 6%.

In the initial stage of the industry when new-type lithium-ion batteries are successively equipped with new energy vehicles, every electric vehicle battery fires accident can become a hot news at home and abroad. The distrust of new energy vehicles has never ceased (Semmelweis Reflex). The country’s enthusiasm for promoting new energy vehicles has become more “rebellious.”

Electric vehicle battery fires
Tesla catching fire 2013

Battery thermal runaway, the damage caused will be devastating. According to the description of @姚昌晟, Ph.D. in Power Engineering and Engineering Thermal Physics of Tsinghua University: A battery pack composed of 100 cells with a charge of 100Ah has an out-of-control energy of 240 million J and 57 kilograms of TNT explosives. So once we say that the battery is “TR”, we can basically post this picture.

However, you don’t have to worry too much. The fire rate of new energy passenger vehicles is decreasing year by year. In 2019, it was 0.38 per 10,000, and in 2020 it was 0.18 per 10,000. In addition, there are two important points to mention: the standard of commercial vehicles is lower, the quality is worse, the total energy of the battery is higher, and the burning is more terrible; the small car company with a cumulative production of less than 50,000 new energy vehicles (calculated according to the connected vehicles) , The ignition rate is five times that of a large car company with more than 100,000 vehicles.

2. The mechanism of battery thermal runaway


Thermal runaway is the main reason of electric vehicle battery fires. Therefore, in the following content, we will focus on analyzing the causes of battery thermal runaway

The mechanism of thermal runaway (generally referred to as “abuse” in academic circles) can be divided into two major directions: physics and electrochemistry. In order to make readers understand better, we divide it into more detail:

① Puncture

This is a physical attack that doesn’t talk about military morality. Insert a conductor (such as a steel needle with a diameter of 3mm) into the power battery, and the positive and negative poles are directly short-circuited. The thermal runaway speed is super fast, and flames are sprayed to the outside almost instantly.

The mechanism of the puncture is an internal short circuit. This situation is like a submarine being physically penetrated underwater, and there is no other possibility other than the death of all members.

Electric vehicle battery fires

The new national standard GB 38031-2020 “Safety Requirements for Power Batteries for Electric Vehicles” implemented on January 1, 2021 does not introduce puncture tests. The existing tests are divided into several categories, among which the battery cells have been tested Discharge, overcharge, external short circuit, heating, temperature cycle, extrusion test, battery pack or system vibration, simulated collision, extrusion, damp heat cycle, water immersion, thermal stability, temperature shock, salt spray, high altitude, over temperature Protection, over-current protection, external short-circuit protection, over-charge protection, over-discharge protection test.

Although I couldn’t find a puncture test, some self-media made a BYD lithium iron phosphate blade battery (CTP) before. In the end, there was no fire, but the eggs were cooked. After the face-slapped Ningde era, it also released a new type of safety battery, claiming that “only smoke can not fire”.

The more the arms race is beneficial to the safety of car owners, the more the better.

② Internal short circuit and lithium dendrites

The internal short circuit is well understood, that is, the positive and negative poles of the single battery are short-circuited, which is equivalent to suicide. Suicide is also divided into two types: rapid suicide and chronic suicide. Puncture is actually a rapid suicide among internal short-circuits. The most dangerous type is immediate retribution and directly destroys everything.

The internal short circuit of chronic suicide is to mention the “lithium dendrites” that we have only talked about in the last two years. In fact, this problem has been plagued by the battery industry for decades. Regarding the reasons for its formation in lithium-ion batteries, the author points out A few steps to talk about:

a. When the lithium ion battery with liquid electrolyte is charged and discharged for the first time, the electrode material and the electrolyte undergo a very complicated reaction at the solid-liquid interface to form a passivation layer covering the surface of the electrode material, in which the SEI film on the negative electrode (Solid Electrolyte Interphase) has a greater impact on the battery. This SEI film is insoluble in organic solvents. It is an electronic insulator so e- cannot pass. It is also a good ion conductor so Li+ passed happily.

b. The credit of SEI is so great, the result is that there is something called “lithium dendrites” to destroy it. When lithium ions are unevenly deposited on the surface of the negative electrode, they will become a reactionary small group of lithium dendrites (the conversion of lithium caused by the Li Plating effect of lithium evolution). The most frightening thing is that it will continue to grow unevenly and become longer and longer. Thicker and sharper, “popping up” in general.

c. When the spring bamboo shoots grow to a certain extent, they will be harvested naturally (dissolve near the negative electrode), and the lithium dendrites will separate from the electrode and become “dead lithium” that loses electrochemical activity, and the battery capacity is reduced.

d. Derating is only the smallest thing, the big deal is more frequent charging. The most silent thing is that these lithium dendrites will also pierce the battery SEI diaphragm (theories deny this statement), the internal insulation is broken, the internal short circuit, spontaneous combustion, release the small universe, and then burn the camp.

P.S. The growth rate of lithium dendrites is positively correlated with the Li+ offset speed, so it is easier to generate lithium dendrites in fast charging, and there are more lithium dendrites with more battery cycles (old electric vehicles are more prone to spontaneous combustion)

③ Fast charge

The reason why fast charge is fast is to force lithium ions to be inserted from the positive electrode and inserted into the negative electrode quickly, increasing the flow and speed of lithium ions. Twisted melons are not sweet, and fast charging will affect the stability of SEI, and will also bring a relatively large amount of heat in a short time, and the heat will be uneven.

At this time, we have to mention three temperatures in the battery field: T1 self-generated heat starting temperature, T2 thermal runaway initiation temperature, and T3 thermal runaway maximum temperature. T2 is a very critical temperature, and its mechanism has not been thoroughly studied in academic circles (excerpted from the speech by Academician Ouyang Minggao).

Of course, because there is a cooling system (air-cooled or liquid-cooled), and the battery’s energy conversion efficiency is particularly high (the thermal efficiency is not less than 95%), the battery is not so easy to reach T2.

At this time, the most magical thing is coming: the lithium analysis effect can greatly reduce T2.

In conjunction with the “quick charge makes the battery heat high and uneven”, the inside of the battery accidentally reaches the T2 thermal runaway temperature, which is over.


④ Over charge

During the charging process, Li+ escapes from the positive electrode compound and reaches the negative electrode lattice. The positive electrode is in a high-potential lithium-poor state, and the negative electrode is in a low-potential lithium-rich state. In order to balance the charge, the same amount of e- is removed from the negative electrode and inserted into the positive electrode. The discharge process can be reversed.

If overcharge (full charge and continue to charge), there will be excessive Li+ embedded in the negative electrode, and the positive electrode will collapse (heating + oxygen release) due to the excessive de-intercalation of Li+, and the release of oxygen will further cause the electrolyte to decompose. , The internal pressure of the battery increases, and the risk of thermal runaway greatly increases.

When playing a four-wheel drive as a child, most children would choose to overcharge to get better results. Only after overcharging, the internal structure of the Ni-MH battery will collapse and eventually “burst.”

Other studies have shown that the temperature of the negative electrode is always higher than that of the positive electrode during overdischarge. When an internal short circuit occurs, heat accumulates in the coil, which increases the risk of thermal runaway.

⑤ Over discharge

Many friends should have encountered that when the lithium-ion batteries of smartphones and laptops use about 3-7%, the system will automatically shut down/enter standby. This is because the system is preventing the battery from over-discharging (over discharge, continuing to discharge below the threshold voltage).

If the over-discharge occurs too violently, the battery with the lowest voltage will be “reversed”. This weakest and helpless battery will be reversely charged by other batteries in series. The voltage is negative and the active material structure collapses. It is equivalent to a resistor, abnormal heating, Game Over.

⑥ Vibration/squeeze/collision/fire, etc.

Here are four types of abuse, which are all physical attacks on the battery pack from the external environment. The following describes what GB 38031-2020 will do with the battery pack under test:

a. Vibration: SOC above 50%, vibration in x, y, and z directions for 1 hour each according to the standard frequency to simulate vehicle bumps.

b. Extrusion: The test method is to squeeze 30% of the overall size to simulate the vehicle being squeezed by force.

c. Collision: The test object with a vehicle mass of less than 3.5 tons has a maximum acceleration of 28g in the x direction and a maximum of 15g in the y direction, which simulates a collision of the vehicle.

d. Fire: At 0℃, preheat with a gasoline fire plate for 60 seconds, burn directly for 70 seconds, burn through a refractory partition for 70 seconds (or continue to burn directly for 60 seconds), and stand for 2 hours to simulate the vehicle being burned. 

The above four working conditions are likely to be encountered during driving. Among them, vehicle bumps are the most common. If the mechanical performance of the battery itself is too poor, or the module is not made strong, there are certain risks. Of course, this possibility is very low. In the past, new energy vehicles were recalled because the wiring harness was not firmly fixed, and it was not the internal wiring harness of the battery pack.

Squeeze, collision, and fire are all possible abuses in a vehicle collision, which may cause problems such as the collapse of the battery structure, the tearing of the SEI film, the leakage of electrolyte, and the internal short circuit.

⑦ External short circuit

When we were in junior high school, we learned about external short circuits. The positive and negative poles were connected into a path when the resistance was very small. Because I=U/R, the current was too large, the heat was too much, and the battery was damaged. We usually hold the positive and negative poles of the AA battery, and the battery will not be destroyed because of the resistance of the human body as large as 100000Ω.

In the test of the national standard, the battery must be short-circuited in the external circuit of only 5mΩ for 10 minutes before it can be put on the market.

There are many reasons for the external short circuit, in addition to the collision just mentioned, there are conductor pollution, water immersion and so on. Flooding is a big problem. In 2012, 16 Karma in Fisker, USA were flooded to the top by the secondary seawater of Hurricane Sandy in the Port of Newark, and then spontaneously burned into scrap iron. Who says fire and water are incompatible? Batteries can turn floods into fires.

Water immersion will also cause the BMS to fail. After the electronic control fails, it is like the aircraft carrier is knocked off the island. Don’t be too surprised if you get nervous next. Everything is possible.

The culprit of the external short circuit is heat, so if you pinch off the short circuit before reaching T2, there may be a rescue. If the electric bicycle is charging and catching fire on a rainy day, then don’t save it. Hurry up, pull the brakes, put out the fire, call the police, and escape.

The insulation and BMS of electric vehicles are much better. There is no problem with charging in rainy days. Just don’t let the charging plug be filled with water. At present, the battery packs of major manufacturers have an IP67 protection level. 7 means that the battery pack can be soaked in 1 meter of water for 30 minutes without failure, but it is highly recommended that you do not use it for wading. The battery pack does not enter the water and other parts are soaked Even with such dirty water, the condition of the car will deteriorate rapidly.

So those friends who think that they can wade through the water at will without fear of turning off the flame after driving on an electric car, please take a moment to relax next time.

⑧ Chain reaction

The thermal runaway of single battery (cell) is not terrible. The terrible thing is that lithium ion is equipped with its own combustion conditions. There are reducing agents and oxidants in each single battery. Everything is ready, even the east wind is not needed. “Chain reaction”.

⑨ Indigenous modification

At present, most electric car modification is not “car modification”, it should belong to the lowest “car makeup”. If the electrical circuit is not moved, it will not have a big impact, but the wind resistance is greater and the battery life is lower, and it is basically pollution-free.

The most troublesome native method modification is the PHEV plug-in hybrid model “slow charge to fast charge”. The principle of this grassroots modification method is to use the electronic controller interface in the cabin to connect to the kinetic energy recovery system, and use the only high-power kinetic energy in the whole vehicle. The recovery line is used for fast charging, and the maximum charging power can be changed to 30kW.

Regardless of whether the power battery has fast charging capability or not, it actually has the ability to charge and discharge at a high rate, but the damage condition is unclear. A battery with fast charge capability can be charged and discharged more times quickly, and it has no fast charge capability. Of batteries can only “break life”

3. Electric vehicle battery fires video

4. Disadvantages of electric cars

As the author just said, on the one hand, we need the car to charge faster and accelerate faster, on the other hand, the car is not easy to spontaneously ignite, then the power battery formula must not be unstable when you need it to be lively, and it must not be stable when you need it naughty. Even if there is a solution, it is certainly not cheap at this moment.

With a limited budget and technology, what we can do at present is: try to buy big-brand power batteries and new energy vehicles, avoid overcharge and overdischarge, and don’t wading.

At the same time, we can install an automatic aerosol fire extinguishing device in the battery pack/ compartment to prevent early fires and reduce the losses caused by the fire. For more details, you can read this article: Automatic fire suppression system for vehicles.