Safety is the red line and quality is the bottom line. Without security, there is no need to discuss the quality of the good or bad. For good quality products, the safety must be high. However, products with high safety are not necessarily high quality products. Car power battery safety is currently a hot topic, and the summit forum on the development of new energy vehicles must have battery safety issues. However, after the discussion, the overall impression of the layman was that the current electric car in China was unsafe, contrary to the original intention of the discussion. So, there was a message on the Internet that "China's electric car is garbage." Fortunately, the practical significance of these discussions is mainly positive and has not caused panic in the electric vehicle market. The author combed these discussions, visited some power battery manufacturers, and studied the safety technology and safety measures of China's vehicle power battery. I have a certain experience for your reference.
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I. Battery safety failure mode analysis and technical safeguard measures
1. The positive and negative internal short circuit of the battery core, the production equipment support ability is the key
(1) Direct cause: Defects in the production process of the battery core or deformation of the battery core due to long-term external vibration.
(2) Failure analysis: The elimination of the short-circuit defects of the positive and negative electrodes inside the battery during the production process is guaranteed by the relevant equipment of the production line. If the equipment cannot be guaranteed, there will be mass product quality problems. This is a basic requirement for battery manufacturers. China has implemented catalog management for power battery manufacturers, which basically guarantees that the qualified battery manufacturers will not have mass product quality problems.
(3) Safety technical measures: The unqualified (positive and negative internal and negative batteries) cells are screened out, and the target value is greater than 99.999%.
(4) Risk: In the event of a serious internal short circuit, the control cannot be prevented, and the external insurance does not work, and smoke or burning will definitely occur. However, under the sufficient guarantee capacity of the battery manufacturer, the probability of occurrence is very low. Even if it occurs, the manufacturer of the vehicle must have a guarantee function.
2. Battery cell leakage, the implementation of process discipline is the key
(1) Direct cause: external force damage; collision, installation is not standardized, resulting in damage to the sealing structure; welding defects, insufficient sealing glue, resulting in poor sealing performance.
(2) Failure analysis: Compared with the soft case and the molded case, the metal case monomer is more prone to liquid leakage, resulting in insulation failure. After the battery leaks, the insulation of the entire battery pack fails. The single-point insulation failure problem is not serious. If there are two or more insulation failures, an external short circuit will occur.
(3) Safety technical measures: Strict process discipline and improve production automation.
(4) Danger degree: very dangerous; the probability of a car accident causing a car fire accident is several orders of magnitude higher than the probability of a positive and negative electric current inside the cell. This external short circuit, external insurance has to play a role.
3. The battery core is inflated and bulged, and the production environment (air) indicator is the key.
(1) Direct cause: Mainly due to the side reaction inside the battery to generate gas, the most typical is the side reaction with water.
(2) Failure analysis: The production line environment deviates from the specified requirements.
(3) Safety technical measures: The water can be strictly controlled during the production process of the battery core, which can be avoided.
(4) Risk: In the event of battery flatulence, liquid leakage may occur. This is a serious quality problem in the production quality of the product. It is necessary to develop preventive measures, and it is strictly forbidden to leave the unqualified products.
4. Poor capacity consistency, automated production line is the key
(1) Direct cause: insufficient battery manufacturing process technology capacity, battery storage time, battery temperature difference during charge and discharge, charge and discharge current, etc.
(2) Failure analysis: There are specific technical indicators for consistency. Once the specifications are lower than the specifications, it indicates that the quality assurance system of the production line is serious. It is necessary to carry out improvement measures to improve the consistency of product capacity is a continuous improvement process.
(3) Safety technical measures: Strengthen the process management and review measures, and continuously improve the ability of the enterprise quality management system.
(4) Risk: Danger does not occur immediately. The capacity consistency is poor, the manufacturer should timely improve the quality, and at the same time prohibit the unqualified products from leaving the factory.
Second, battery management system (BMS) security failure mode analysis and technical safeguards
1. Voltage detection line failure and measures
(1) Phenomenon: The battery is overcharged, causing fire and explosion; most of the lithium iron phosphate overcharged to 5V will emit smoke, and once the ternary battery is overcharged, it will explode.
(2) Cause: The BMS voltage (connection, crimping process or poor contact) voltage detection line is invalid, resulting in overcharging or overdischarging of the battery, charging or overdischarging.
a) decomposition of the electrolyte releases gas, which causes the battery to swell, and even if it is serious, it may cause smoke;
b) The over-discharge of the battery may cause damage to the molecular structure of the positive electrode material of the battery, resulting in no charge and no power;
c) The battery voltage is too low, causing the electrolyte to decompose, and the dryness of the electrolyte occurs, causing a short circuit inside the battery.
(3) Measures: Technically select high-reliability voltage acquisition lines to prevent contact with the collection lines from being in place; management establishes charging measures system, and responsibility is implemented.
2. Current detection failure and measures
(1) Phenomenon: The BMS cannot collect current, the SOC cannot be calculated, and the deviation is large.
(2) Reason: The Hall sensor fails, the charging current is large, and the internal heat of the battery core is large, which will attenuate the curing capacity of the diaphragm.
(3) Measures: Use high-quality Hall sensors to ensure the quality of the installation.
3. Temperature detection failure and measures
(1) Phenomenon: The operating temperature of the battery is too high, and it is prone to bulging, liquid leakage and explosion.
(2) Cause: Temperature detection is invalid.
(3) Measures: Strictly control the operating temperature of the battery between 20-45 degrees Celsius to avoid short circuit caused by low temperature charging of the battery and high temperature thermal runaway.
4. Insulation monitoring failure and measures
(1) Phenomenon: Personnel may be exposed to electricity.
(2) Cause: Insulation failure occurs when the power battery is deformed or leaked.
(3) Measures: For the sensors to be monitored, high quality and high reliability products must be used.
5. Other failures and measures
(1) The SOC estimation deviation is large, and the current inspection standard requirements are all within 5%, which is actually more difficult. To continuously improve the accuracy.
(2) Insufficient electromagnetic interference resistance of electromagnetic compatibility, resulting in failure of BMS communication, how to ensure that it is in place, there are certain difficulties. The product must also be compulsory by a third party.
Third, the battery pack (Pack) integration failure, resulting in a safety accident hazard analysis and measures
1. Bus connection failure
(1) Phenomenon: A large amount of heat is generated at the joint of the conductor, and in extreme cases, the power battery is on fire.
(2) Reason: The bolt connection is in use, and the bolt is oxidized off or vibrated, causing the bolt to loose.
(3) Measures: The battery core is connected with the battery core or the module and the module are connected by laser welding, or the temperature sensor is added at the connection to avoid the failure of the bus bar by means of detection. New connection technologies are constantly emerging, and manufacturers must actively adopt them.
2. Power battery system main circuit connector failure
(1) Phenomenon: The performance of the connector is unreliable, and a virtual connection occurs under vibration, resulting in a high temperature ablation connector.
(2) Cause: Connection failure occurs when the connector temperature exceeds 90 degrees.
(3) Measures: The connector needs to be added with a high-voltage interlock function, or a temperature sensor is attached to the connector. Connectors must use high quality products.
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