Why do electric cars self-ignite?

In 2018, the electric car is all a bit of a "fire". According to incomplete statistics, in the first half of 2018, the electric car 10 burning accident happened. Why are electric cars on fire? Car companies existing technical level can't prevent it from fire? Or car companies didn't spend enough to prevent the fire cost?

The other day, the author of three spontaneous new energy cars apart., and based on the analysis of dismantling the battery and the mechanism of thermal runaway, the author thinks that the electric car spontaneous combustion, reason is not the technical level to reach, but is involved power battery enterprises and electric cars, do not take the security, in order to reduce costs, the batteries quality control is lax, BMS design requirements is too low, there is no design to protect the safety of the power battery pack enough, etc., so that the light.

Bat light make motive force battery and new energy automobile enterprises. Departments in the admittance threshold of many links, now seeing so many accidents, also do not move to?

The electric car on fire cause analysis

The dismantling of the electric car spontaneous combustion occurs, one in charge; A vehicle in a stopped state; Also found abnormal, in the process of driving a car stopped and burst into flames.

First car (charging the electric car of the fire), from the testing data before the fire, the battery in the process of charging, the large pressure difference, but the BMS battery did not stop. Until the temperature rising rapidly for 10 seconds to 45 ℃ threshold, stop charging. At that point the battery has more than 500 mv static pressure differences, and then the communication interrupt spontaneous combustion occurred.

The second car (stopped status of the fire vehicle) is apart from the spot, internal with traces of wading. Sealing strip serious deformation of the battery pack (supposedly design problem), seal did not achieve the desired effect. At the bottom of the battery pack, there is a clear three arc breakdown. Before this car accident site in Shenzhen, it was heavy rain, are consistent with the judgment. The seal failure, resulting in vehicle wading short circuit caused by spontaneous combustion.

The third car (a fire in the process of driving the car), the first in the process of driving, the driver found abnormal, pull over, then the spontaneous combustion. From the monitoring data of internal temperature of the battery pack for 20 seconds, up from 34 ℃ to 113 ℃, then communication interrupt. After the dismantling, preliminary judgment is individual batteries explosion happened, cause the main power line overheat, insulation skin damage, and battery shell and internal fixator lap, short-circuit.

In addition, according to a fire fighting unit, new energy vehicles combustion basically has the following four scenarios:

In the process of charging combustion;

Batteries or place caused in the process of combustion;

Collision accident caused by burning;

Wade caused by burn.

The four kinds of scenario, the combustion is most common in the process of charging.

In the process of the electric vehicle charging, charging pile and the electric car of the BMS (battery management system) communications "handshake", to control the charging condition. That is to say, the BMS will be state of the battery, and the judgment, so as to give a reasonable charging scheme. Question, however, the BMS is how to know the status of the battery? Is achieved by different sensor feedback signal.

Batteries, BMS, sensors

First of all from the batteries. Each of the battery has a different "constitution". Specific performance in such as internal resistance, self-discharge rate, attenuation, polarization parameters. Although, professional and technical personnel will be for grouping the "constitution" of the battery, to reduce the individual differences, but the "constitution" of the battery and, over time, changes. Good quality batteries, "physical" relatively small differences, to do this, select material better consistency, production process automation level is higher, the quality standard is higher, the cost is high. On the other hand, the poor quality batteries, low cost, individual differences, there is great potential safety hazard. For example, in the process of charging individual batteries overheating is on fire.

But batteries can never be completely consistent, the need to BMS involvement, is responsible for the management strategy of the battery. First of all, I personally think, each kind of battery has different features. BMS should be separate development management strategy, rather than general design.

BMS for battery management before, first of all to grasp the information of the battery, that it is only through sensor monitoring. That is to say, the more sensors, the higher the precision of the sensor, the feedback data more comprehensive, BMS for judgment of the battery will be more accurate. The corresponding, the higher the cost is.

In the process of charging, batteries, BMS, sensors, bad cooperation in three stages, spontaneous combustion can occur.

If the BMS failure? Imagine our smartphone tiny probability failure need to restart the scene, I believe that any electronic equipment, has a certain probability of failure. And if the BMS fails, or even a short crash, the consequences are more serious than the phone fault. What to do when this happens, and can have a second set of spare battery management plan? I believe that when "emergency device" exist in the system, the system will more secure, but also increase my cost.

In addition to charging scenarios, battery also produces in the process of moving or stopped burning. The road, is the battery discharge process, in the process of working, the battery problems, easy to understand, but in the process of stopped, why will burn?

Offend nature from "sequela"

Recently taken off for a couple of new energy car case study found that new energy car stopped before, has been overloaded or longer driving experience. And stopped in the process of spontaneous combustion, is actually the sequela, the sequela is how cause?

Reason 1: car in the process of driving because the air flow, the battery is in a state of heat dissipation. The car stopped, engine, cooling system and stop working. While the battery heat may not fully dispersed, heat in local cluster, it will result in high temperature and combustion.

Solution is very simple, after the car stopped, the cooling system should continue to work (this requires active radiator), I remember one of my fuel car, parking, in front of the cooling fan (then called electronic fan) will continue to be easy for a while. Early turbo engine has a similar requirement, trains cannot stop immediately remove from heat. For new energy vehicles, this is not a difficult technology. But it does have added some cost, hush, may sacrifice some energy density.

Reason two: the environment temperature influence. In the environment temperature, mostly is heat reflection. How many degrees is the ground temperature exactly? According to the Tianjin meteorological bureau, the summer ground maximum temperature 64.7 ℃, the lowest temperature are more than 50 ℃. while lithium battery Most of the optimum operating temperature, not more than 50 ℃, while the lithium battery pack is often installed in the bottom of the car, and is very close to the distance of the ground, the ground a large amount of heat radiation is absorbed by the battery pack. If we accumulate and cause a problem, or stop for a long time, can make thermal runaway car battery, and cause burning accident.

Solution is easy, also is the battery pack do insulation design, such as inside the battery pack with a layer of insulation pad, this also is helpful for heat preservation in winter. But add insulation pad, and can bring three questions: first, the cost increase; Second, the natural cooling performance degradation, need active cooling system; Third, the energy density of the battery pack are down a bit.

Reason 3: new energy vehicles in the process of stopped, low-voltage electrical or at work, such as GPS signal, driving computer, remote control locking this a series of functions. the malfunction of the low-voltage electrical is also likely to be a safety hazard.

In addition to the batteries, BMS, Pack design, small part also cannot ignore the rest, such as waterproof IP67, such as the quality of the wiring harness and arrangement.

The above for the cause of the accident, solution all point to increase the cost. But the high cost, whether can avoid the accident? I also want to talk about tesla in the case of the accident. How to achieve 100% of security?

As is known to all, tesla is a rich man's "toy", costs shouldn't be a problem. In the first half of 2018, Internet to tesla there were three accidents:

In January, in Chongqing, tesla at no charge, no collision cases, the combustion;

In March, in the United States, tesla struck aspecial because it is over, the combustion;

In May, in the United States, tesla because of collision, spontaneous combustion happened again.

Tesla did not achieve 100% safety.

I want to here, should give security to the next definition.

New technology doesn't demanding a 100% no accident, but the consequence of the accident to a minimum. While on the new energy vehicles, the design considerations, in case of accident, 100% guarantee personal safety. We still look at tesla's examples. Above the tesla's battery pack, there is a layer of flame retardant of aluminum plate, this layer board, may not be able to completely block the burning, but at least I can give the driver and passengers to win a few minutes of time to escape.

Therefore, the safety of the so-called new energy vehicle is 100% of car is superior to personnel safety guarantee. Early warning gives the driver enough time to escape.

Safety is always relative, increase costs is simple, the key is to market recognition, customers are willing to pay. Who also don't want to buy a 10 km/h tank, although it should be safe. But I think that in order to reduce accidents, some cost or cost:

First, high quality batteries. The energy density is higher and higher, no doubt, to the quality requirement of the batteries is becoming more and more high. Ternary era, each recipe is what? Are three yuan real materials, or different formulations of doping? How consistent? Improve the quality of the batteries, from the source to reduce risk, the money to spend.

Second, the security Pack design. Some manufacturers of so-called Pack design, is actually Pack arrangement. What's the difference? For example, a Pack of temperature sensor should be placed where? Pack arrangement is installed, the battery module, where to find a "reasonable". And Pack design practices, is combined with the cooling system in the structure, design of air duct, preset temperature monitoring, further simulation and experiment, the final confirmation.

Third, the cushion design. As the name suggests, in the design, it is necessary to leave design margin. Such as said above, if the malfunction of the BMS has does not have a backup system can temporarily guarantee the safety of the system. The standby system may not be able to complete functions of BMS, but at least to ensure safety.

Fourth, the thermal management system. The thermal management system, if the security of the new energy vehicles should be enough. Battery energy density is higher and higher, to the requirement of thermal management, more and more is also high. Energy density increase, will cause the charging speed boost, I think at least a water cooling system is necessary.

Fifth, fault escape system. Whatever the reasons caused by the new energy vehicles on fire, will leave enough time to escape to the division by people. Especially the minivan or SUV have three rows of seats, as well as the electric bus. Therefore, for fault, early warning; In the system, there will at least delay the design of the fire, the maximum time to escape.

Sixth, safety training. Nutshell is to tell new energy automobile owners, in the event of accident, should be how to deal with. To give up property, for example, the first time to escape, should keep a safe distance, and accident car to stand on the tuyere and so on. For accident disposal is undeserved, also is the main reason that cause personal injury.

Seventh, punishing the responsible party, support huge claims. In addition to the human life, anything can take money to calculate. There is such a story: a car ford motor production, its design safe hidden trouble, but if the increased cost of $16, safety coefficient of the opportunity to improve a lot. Ford motor company knows this situation, but they calculated brushstroke zhang, if every car, plus the cost of a $16 baffle than they of accident compensation, so they choose the compensation. Understand from this Angle, the compensation is actually paying for product cost control, so spend money where an estimated each big factory to the reckoning.

In 2018, there are many fuel burning car accident, but does not seem to like new energy vehicles, have so much attention. Recently, new energy vehicles on fire events and subsidy policies to improve the energy density requirement, has a direct correlation. Based on the principle of relative safety that means there are more security safeguards; Include improving charging speed, also want to have more effective temperature control system.

But, clearly, power battery enterprises, new energy car companies did not do so. They like province ford $16 paddle, calculates the input and output. So, we are calling for power battery, new energy automobile enterprises own reflection, correction at the same time, more a call to departments, to punish the fires caused by churning out responsibility, especially for personal injury accidents caused by the responsibility of the enterprise, cancel the qualification of subsidies and announcement, ejected from the new energy automobile market. Otherwise, all the prospect of new energy vehicles industry and results may be destroyed overnight.

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