Studying the failure modes of power battery systems is of vital importance to improving battery life, the safety and reliability of electric vehicles, and reducing the cost of electric vehicles. This article analyzes the failure modes and consequences of the external performance of the power battery system and proposes corresponding treatment measures. Various failure modes are considered in the design of the power battery system to improve the safety of the power battery.
The power battery system is usually composed of batteries, battery management systems, Pack systems including functional components, wiring harnesses, structural parts and other related components. Power battery system failure modes can be divided into three different levels of failure modes, namely, battery cell failure mode, battery management system failure mode, and Pack system integration failure mode.
First, Battery failure mode
The failure modes of batteries can be further divided into safety failure modes and non-safe failure modes. The main safety failures of the battery are as follows:
1. The positive and negative poles of the battery are short-circuited:
The internal short circuit of the battery is caused by the inside of the battery cell. There are many reasons for the internal short circuit of the battery. It may be caused by defects in the production process of the battery cell or the deformation of the battery cell due to long-term vibration and external force. Once a serious internal short circuit occurs, the control cannot be prevented, and the external insurance does not work, and smoke or burning will definitely occur.
If we encounter this situation, all we can do is to notify the people in the vehicle to escape. Regarding the internal short circuit problem of the battery, so far, the battery manufacturer has not been able to screen out 100% of the cells that are likely to have an internal short circuit at the factory. They can only do a sufficient test in the later stage to reduce the probability of an internal short circuit.
2. Battery cell leakage:
This is very dangerous and a very common failure mode. Many electric vehicle fire accidents are caused by battery leakage. The reasons for battery leakage are: external force damage; collision, improper installation and damage to the sealing structure; manufacturing reasons: welding defects, insufficient sealing glue, and poor sealing performance.
After the battery leaks, the insulation of the entire battery pack fails, and the problem of single-point insulation failure is not big. If there are two or more insulation failures, an external short circuit will occur. From the perspective of practical applications, soft-pack and plastic-case batteries are more prone to leakage and lead to insulation failure than metal-case single cells.
3. Lithium precipitation in battery negative electrode:
Improper use of the battery, overcharging, low-temperature charging, and high-current charging will cause the battery’s negative electrode to degrade lithium. Lithium iron phosphate or ternary batteries produced by most domestic manufacturers will undergo lithium evolution when charging below 0 degrees Celsius. Above 0 degrees Celsius, they can only be charged with low current according to the characteristics of the battery. After the occurrence of lithium precipitation in the negative electrode, the lithium metal cannot be reduced, resulting in irreversible degradation of battery capacity. Lithium evolution reaches a certain degree of severity, forming lithium dendrites, piercing the diaphragm and causing an internal short circuit. Therefore, the power battery should not be charged at low temperature when in use.
4. Bloating and swelling of the battery cell:
There are many reasons for flatulence, mainly due to the side reaction inside the battery to produce gas, the most typical one is the side reaction with water. The problem of flatulence can be avoided by strictly controlling the moisture in the production process of the battery. Once battery bloating occurs, liquid leakage will occur.
The above failure modes are very serious problems and may cause casualties. Even if a battery cell is used for one or two years without problems, it does not mean that the battery cell will have no problems in the future. The longer the battery is used, the greater the risk of battery failure.
The non-safety failure of the battery cell only affects the performance, mainly as follows:
1. Poor capacity consistency:
The inconsistency of the power battery usually refers to the excessive difference in the remaining capacity and voltage of the battery in a group of batteries, which causes the battery life to deteriorate. There are many reasons for the poor consistency between batteries, including the manufacturing process of the battery, the storage time of the battery, the temperature difference during the charging and discharging of the battery pack, and the size of the charging and discharging current.
The current solution is mainly to improve the control level of the battery production and manufacturing process, to ensure the consistency of the battery as much as possible from the production pass, and to use the same batch of batteries for assembly. This method has a certain effect, but it cannot be cured. The problem of poor consistency of the battery pack will appear after a period of use. After the inconsistency of the battery pack occurs, if it cannot be dealt with in time, the problem will become more serious and even dangerous.
2. Excessive self-discharge:
The irreversible reaction caused by the micro short circuit caused by impurities during battery manufacturing is the main reason for the excessive self-discharge of individual batteries. In most battery manufacturers, the self-discharge of the battery can be ignored in micro-hours. Because the battery is charged and discharged for a long time and placed in storage, the chemical reaction occurs with the environmental conditions, which causes the battery to self-discharge, which reduces the battery power and performance Low, unable to meet the needs of use.
3. Reduced low-temperature discharge capacity:
As the temperature decreases, the low temperature performance of the electrolyte is not good, and the reaction is not enough. The conductivity of the electrolyte decreases, which leads to an increase in battery resistance, a decrease in voltage plateau, and a decrease in capacity. At present, the discharge capacity of batteries of various manufacturers at -20 degrees is basically 70% to 75% of the rated capacity. The discharge capacity of the battery is reduced at low temperatures, and the discharge performance is poor, which affects the performance and driving range of electric vehicles.
4. Attenuation of battery capacity:
The attenuation of battery accommodation mainly comes from the loss of active lithium ions and the loss of electrode active materials. The regularity of the layered structure of the positive electrode active material is reduced, and the passivation film is deposited on the negative electrode active material, the degree of graphitization is reduced, and the porosity of the separator is reduced, resulting in an increase in the charge transfer impedance of the battery. The ability to deintercalate lithium is reduced, resulting in a loss of capacity.
Battery capacity degradation is an inevitable problem for batteries. But at present, battery manufacturers should first solve the previous safety failure problems and battery consistency problems, and then consider extending the cycle life of the battery on this basis.
Second, BMS failure mode
The cell failure of the battery is not only related to the battery itself, but also to the failure of the battery management system BMS. BMS failure modes can also cause serious accidents in the following categories:
1. BMS voltage detection failure causes battery overcharge or overdischarge:
The connection, crimping process, or poor contact causes the voltage detection line to fail, the BMS has no voltage information, and it does not stop when it should stop during charging. The battery will catch fire and explode if overcharged. The overcharge of lithium iron phosphate above 5V is mostly just smoke, but once the ternary battery is overcharged, it will explode.
Moreover, overcharging can easily cause the electrolyte in the lithium-ion battery to decompose and release gas, which can cause the battery to swell, or even smoke or catch fire in severe cases; over-discharge of the battery can cause damage to the molecular structure of the battery’s positive electrode material, which will cause the battery to fail to charge; At the same time, the battery voltage is too low to cause the electrolyte to decompose, dry out and release lithium, and return to the battery short-circuit problem. Reliable voltage collection lines should be selected when the system is designed, and strictly controlled during the production process to prevent the failure of the voltage collection lines.
2. BMS current detection fails
The Hall sensor fails, the BMS cannot collect current, the SOC cannot be calculated, and the deviation is large. Failure of current detection may cause excessive charging current. The charging current is large, the internal heating of the battery is large, and the temperature exceeds a certain temperature, which will reduce the curing capacity of the diaphragm and seriously affect the battery life.
3. BMS temperature detection fails
The failure of temperature detection leads to excessively high operating temperature of the battery and irreversible reaction of the battery, which has a great influence on the battery capacity and internal resistance. The calendar life of the battery cell is directly related to the temperature. The number of cycles at 45 degrees is half of that at 25 degrees. In addition, the battery is prone to swelling, leakage, explosion and other problems when the temperature is too high. Therefore, the battery temperature must be strictly controlled during the use of the battery. Between 20-45 degrees Celsius, in addition to effectively improving the service life and reliability of the battery, it can also effectively avoid the short circuit caused by the battery’s low-temperature charging and lithium evolution and the high-temperature thermal runaway.
4. Insulation monitoring failure:
Insulation failure will occur when the power battery system is deformed or leaked. If the BMS is not detected, electric shock may occur. Therefore, the BMS system should have the highest requirements for monitoring sensors, and avoiding the failure of the monitoring system can greatly improve the safety of the power battery.
5. Communication failure for electromagnetic compatibility issues:
For the BMS system, electromagnetic compatibility mainly assesses its ability to resist electromagnetic interference. Electromagnetic interference will cause BMS communication to fail and cause the above problems.
6. Large deviation of SOC estimation:
At present, the common problem of all BMS manufacturers is the difference in deviation size. Basically, the current inspection standard requirements are less than 5%, and most manufacturers’ BMS should be difficult to achieve, because the SOC error will become larger and larger in actual use, because the use environment is more complicated, and there are more conditions that affect accuracy.
Three, Pack system integration failure mode
1. Failure of the busbar:
If it is a bolt connection, in the later use process, the bolt oxidizes and falls off or the vibration causes the bolt to loosen, which will cause a lot of heat at the conductor connection, and in extreme cases, it will cause the power battery to catch fire. Therefore, most power battery system manufacturers use laser welding at the cell and cell connection or module and module connection when designing the pack, or add a temperature sensor at the connection to avoid the failure of the bus bar by means of detection.
2. The main circuit connector of the power battery system fails:
The high-voltage line of the power battery system is connected to the external high-voltage system through a connector. The performance of the connector is unreliable, and virtual connection occurs under vibration, resulting in high temperature ablation of the connector. Generally speaking, the connection failure will occur if the temperature of the connector exceeds 90 degrees. Therefore, in the system design, the connector needs to add a high-voltage interlock function, or add a temperature sensor to the connector, and monitor the temperature of the connector at all times to prevent the connector from failing.
3. Stickiness of high voltage contactor:
The contactor has a certain number of load disconnection, and most of the contactors are ablated when the high current load is closed. In the system design, a dual relay scheme is generally adopted, and the control is closed in sequence to avoid sticking of the high-voltage contactor.
4. The fuse overcurrent protection fails:
The selection and matching of the fuse in the high-voltage system components, the gradient which breaks first and then breaks, needs to be considered comprehensively. Vibration or external impact and extrusion lead to deformation of the power battery, seal failure, and lower IP rating. Therefore, the collision protection of the battery box structure needs to be considered in the system design.
According to the various failure modes of the power battery system mentioned above, researchers and battery manufacturers need to improve the safety of lithium battery cells through continuous improvement of processes and technologies. BMS system manufacturers must fully understand the performance of the battery, based on the safety design principles of power batteries, Designing a safe and reliable battery system, while correct use is the ultimate barrier to ensure battery safety. Users should use the power battery system correctly, prevent mechanical abuse, heat abuse, and electricity abuse, and effectively improve the safety and reliability of electric vehicles.
