As the "heart" of pure electric vehicles, the power battery directly determines the safety, life and performance of the vehicle. In the process of continuously improving the energy density and performance of power batteries, safety has also become a topic of much concern and controversy. "There are many uncertainties in the safety level of the power battery system." As Wang Zidong, deputy secretary-general of the China Automotive Power Battery Industry Innovation Alliance, said, the safety of the battery pack composed of monomers and systems is also related to all aspects of the product. It involves a wide range of areas, but there is generally a lack of deep understanding of it inside and outside the industry, and some concepts may even be wrong, which aggravates security risks.
Previously, the "Electric Vehicle Safety Guidelines" (hereinafter referred to as the "Guidelines") compiled by the China Association of Automobile Manufacturers, China Automotive Power Battery Industry Innovation Alliance, and China Electric Vehicle Charging Infrastructure Promotion Alliance pointed out that design, production, inspection, transportation, etc. The link pays attention to safety, improves the safety awareness of lithium-ion batteries for new energy vehicles, and improves the level of safety design, manufacturing and safety control. Then, to improve the safety awareness of batteries, we must first recognize which factors affect battery safety. "To pay attention to safety, it is necessary to find out all kinds of factors that affect safety as much as possible, so as to make sure that there are many factors without leakage." Gao Junkui, executive vice president of Tianjin Lishen, said.
Battery thermal runaway is the main cause of electric vehicle fires
Gao Junkui emphasized that the internal short circuit of the battery cell, the heat dissipation of the single cell, the temperature rise caused by performance degradation, the mechanical reliability and the internal short circuit, heat dissipation, electrical connection reliability, and mechanical reliability of the battery module are important factors related to battery safety. The thermal management capability of the battery system, short circuit in the system, insulation problems, sealing and waterproof performance, BMS (battery management system) management capability and reliability are the factors that determine the safety of the battery pack. He further pointed out that battery safety involves various aspects such as safety system, safety design and manufacturing process.
Among the many potential safety hazards, Gao Junkui emphasized that battery thermal runaway is the main cause of new energy vehicle fire accidents, and this kind of overheating is most likely to occur during battery charging and discharging. As the name suggests, thermal runaway is the accumulation of heat inside the battery, which eventually becomes uncontrollable and leads to serious accidents. At present, there are no effective measures to prevent lithium battery fires, and electric vehicles can only be allowed to burn until the end of the thermal runaway process. So, what causes the battery thermal runaway? At present, the industry generally believes that the main causes of battery thermal runaway are thermal radiation, internal short circuit of the battery, and abuse in harsh environments. How to manage thermal runaway? The research of the expert group of the "Guidelines" has made it clear that when thermal runaway causes fire and explosion, the safety protection device on the battery cell should be activated. To some extent, the temperature and composition of the ejected gas should also be studied and analyzed to prevent the occurrence of secondary short circuit disasters.Also read:Lithium Battery Manufacturers
In addition, the charging speed of lithium-ion batteries is closely related to the service life. For power battery packs that do not have fast charging and thermal performance, if conditions permit, the use of fast charging should be reduced and charging at a low rate should be selected as much as possible. Although fast charging has become a major marketing gimmick for manufacturers and an important reference for consumers to buy vehicles, from a safety point of view, fast charging should not be advocated. This is also a problem that manufacturers and consumers inside and outside the industry need to face up to.
■Using temperature affects the safety of lithium batteries
In addition to the battery itself, the environment of use also affects the safety of the battery.
Why do electric vehicles charge poorly in winter? This may be a problem that plagues most consumers. In the eyes of industry experts, lithium-ion batteries have the best operating temperature range, and difficulty in charging at low temperatures is an inevitable problem under current technical conditions. Not only that, lithium-ion batteries may even cause safety problems when they exceed the optimal operating temperature range. Use at higher temperatures may easily cause thermal runaway safety problems, and lithium-ion deposition may occur at the negative electrode under low-temperature charging. Then, how to ensure the safety of the battery in the non-optimal operating temperature range of more than 45°C and below 0°C? Relevant industry experts pointed out that the charging and discharging strategy should be controlled, such as reducing the rate, to ensure that the battery works within a safe range. The "Guide" also emphasizes that the charging method (including charging temperature, charging rate and charging voltage) should be controlled during the use of lithium-ion batteries to ensure safety.
At present, different systems and designs of single batteries have different charging methods. In order to ensure safety, the "Guidelines" recommends that battery single manufacturers should provide temperature-rate-charging voltage relationship when providing single battery products. Figure, design the system charging strategy according to the battery cell specifications.
Temperature not only affects battery usage, but is also an important factor affecting battery life and storage and transportation safety. Wang Zidong pointed out that the long-term storage performance of lithium-ion batteries at high temperatures will seriously attenuate, and the batteries stored for a long time should not be directly charged by fast charging when they are used again. The "Guide" also clarifies that the packaging of batteries and modules should be waterproof and moisture-proof, avoid extrusion and damage, and should be isolated and fixed with the smallest unit to ensure a safe distance. During transportation, the temperature should also be monitored to avoid sunlight, rain and moisture.
Both energy density and safety need to be considered
Blindly increasing energy density is considered to be the root cause of the current impact on the safety of electric vehicles. Dong Yang, executive vice president of the China Association of Automobile Manufacturers, once pointed out that the energy density and safety of power batteries are a pair of contradictions. How to ensure the safety of power batteries while improving the energy density of power batteries requires in-depth and meticulous scientific research and experiments to grasp the accuracy. The "degree".
How to grasp this "degree"? Gao Junkui said that under the premise of ensuring safety, the specific energy of the battery should be increased and the charging time should be shortened. "There is a lot of controversy when we discuss the capacity of the battery and the thickness of the battery diaphragm." Wang Zidong said that from the perspective of the safety of the power battery, the increase in the single capacity of the power battery is directly proportional to the instability of the battery system after thermal runaway ; The thickness reduction of the battery separator is directly proportional to the instability of the battery system after thermal runaway. Therefore, it is proposed that the capacity of the power battery should not exceed 100Ah (ternary), 200Ah (lithium iron phosphate), and the thickness of the diaphragm should not be less than 0.012mm. But in this regard, some companies believe that the instability of the system after thermal runaway is related to the level of production control and grouping technology, and it is not appropriate to limit the size of the capacity at present. "After full discussion, it is a long-term and systematic work to determine the capacity of power batteries in the "Guidelines" to continuously increase the specific energy of battery cells. The capacity and specific energy of the monomer. The thickness of the diaphragm is determined as the thickness of the diaphragm is strongly related to the safety of the battery cell. The selection of the thickness of the diaphragm of the power battery is recommended to fully consider the safety risks caused by reducing the thickness of the diaphragm. "
The favor of energy density in new energy vehicle subsidies is considered to be one of the reasons why manufacturers pursue high energy density too much. At present, the subsidy policy for 2019 has not yet been released. Some people in the industry have suggested that the subsidy should adjust the requirements for energy density, so that manufacturers can promote the progress of battery technology from the perspective of safety.
The failure of BMS and battery system should be taken seriously
The statistical results show that BMS failure and battery system failure are the two major factors that cause new energy vehicle safety accidents. Guo Xiaodong, senior director of Neusoft Reach, believes that at present, the industry lacks a unified understanding of the failure modes, possible risks and countermeasures of BMS and battery systems, and lacks basic requirements and practical guidance for the design and development of BMS and battery system safety.Also read:LiFePO4 Battery 12V
It is understood that a serious failure of the BMS may cause the vehicle to catch fire, explode, or cause power interruption or failure of monitoring and protection functions; while a failure of the battery system may cause fire/explosion, power interruption or electric shock, both of which should attract sufficient attention . For this reason, Guo Xiaodong pointed out that for BMS, attention should be paid to standardizing charge and discharge control strategies to avoid safety accidents caused by improper charge and discharge strategies. At the same time, it is necessary to standardize the BMS fault handling mechanism and protection strategy to ensure that when a safety accident occurs, protection measures are triggered reasonably to avoid personal injury. In the development process, it is also necessary to clarify the BMS functional safety development requirements, reduce the product failure rate, and improve the quality and safety of BMS products. For the battery system, the development and design of the power battery system should be optimized to improve the safety of the battery system and reduce safety risks from the design level; optimize battery system testing and production design to control product quality; standardize and optimize battery system after-sales and maintenance requirements to ensure product quality Long service life and reduce safety risks.
In addition, relevant experts pointed out that in the past few years, especially in the initial stage of the promotion and application of electric vehicles, many manufacturers have instilled in consumers the concept that electric vehicles do not need maintenance. In fact, this is a big misunderstanding. Electric vehicles, especially batteries It also requires regular maintenance. The "Guide" also recommends that electric vehicles should be checked regularly at the after-sales service center, and the recommended maintenance cycle is every 5,000 kilometers or half a year. Regular maintenance should include equalization charging, air tightness testing, insulation performance testing, visual inspection, etc. Wang Zidong told reporters: "For safety and reliability, electric vehicles need regular inspection and maintenance. However, the previous power battery system rarely considered the maintainability of the system, making it difficult for users to maintain the power battery system. So , the battery system needs to design a maintenance interface.”