Analysis of safety failure modes of power battery cells and BMS

1. Analysis of battery cell safety failure modes and technical support measures

1. The positive and negative electrodes in the battery cell are short-circuited, and the production equipment guarantee capability is the key

Direct cause: defects in the production process of the battery cell or deformation of the battery cell due to long-term vibration and external force.

Failure analysis: The elimination of positive and negative short-circuit defects in the battery cell during the production process is guaranteed by the relevant equipment of the production line. If the equipment cannot guarantee this, there will be quality problems in batch products. This is a basic requirement for battery manufacturers. my country has implemented catalog management for power battery manufacturers, which can basically ensure that compliant battery manufacturers will not have quality problems in batch products.

Safety technical measures: Screen out unqualified cells (short circuit between positive and negative poles inside the cell), with the target value being greater than 99.999%.

Danger level: Once a serious internal short circuit occurs, it cannot be prevented and controlled, and the external insurance does not work, smoke or combustion will definitely occur. However, if the battery manufacturer has sufficient protection capabilities, the probability of occurrence is very low. Even if it occurs, the vehicle manufacturer and automatic fire extinguishing device must play a protective role.

2. Battery monomer leakage, the key is to implement process discipline

Direct causes: damage due to external force; damage to the sealing structure caused by collision or improper installation; poor sealing performance caused by welding defects or insufficient amount of sealing glue, etc.

Failure analysis: Compared with soft pack and plastic shell batteries, metal shell cells are more prone to leakage, resulting in insulation failure. After battery leakage, the insulation of the entire battery pack fails. Single-point insulation failure is not a big problem, but if there are two or more insulation failures, an external short circuit will occur.

Safety technical measures: Strictly enforce process discipline and improve the level of production automation.

Danger level: Very dangerous; the probability of a car fire accident caused by battery leakage is several orders of magnitude greater than the probability of a short circuit between the positive and negative poles inside the battery cell. For this kind of external short circuit, external insurance comes into play.

3. The battery cell is swollen and the key is to meet the environmental (air) indicators of the production site

Direct cause: It is mainly because of the side reactions inside the battery that produce gas, the most typical of which is the side reaction with water.

Failure analysis: Production line environment deviates from specified requirements.

Safety technical measures: This can be avoided by strictly controlling the moisture content during the battery cell production process.

Danger level: Once battery bloating occurs, leakage will occur. This is a more serious quality problem in product production quality. Preventive measures can be formulated, and unqualified products are strictly prohibited from leaving the factory.

4.Poor capacity consistency, automated production line is the key

Direct causes: Insufficient battery manufacturing process technology, battery storage time, temperature differences during battery charging and discharging, charging and discharging currents, etc.

Failure analysis: There are specific technical indicators for consistency. Once it is lower than the specified indicator, it indicates that there are serious problems with the quality assurance system of the production line, and improvement measures must be taken. Improving product capacity consistency is a continuous improvement process.

Safety technical measures: Strengthen process management and review measures to continuously improve the capabilities of the enterprise's quality management system.

Danger level: Danger will not occur immediately. The capacity consistency is poor, and the manufacturer should make quality improvements in a timely manner, and it is strictly forbidden to ship unqualified products.

2. Analysis of safety failure modes and technical support measures for battery management system (BMS)

1. Voltage detection line failure and measures

Phenomenon: Battery overcharge leads to fire and explosion; most lithium iron phosphate batteries will smoke when overcharged to more than 5V, and ternary batteries will explode once overcharged.

Cause: BMS voltage (connection, wire pressing process or poor contact) voltage detection line failure, resulting in battery overcharging or over-discharging, caused by overcharging or over-discharging.

a) The electrolyte decomposes and releases gas, which causes the battery to swell, and in severe cases, it may even smoke and catch fire;

b) Over-discharging of the battery will cause damage to the molecular structure of the positive electrode material of the battery, resulting in failure to charge;

c) The battery voltage is too low, causing the electrolyte to decompose, dry up and deposit lithium, causing a short circuit in the battery.

Measures: Technically, high-reliability voltage collection lines are selected to prevent the collection lines from not being in place; in terms of management, a charging measures system is formulated and responsibilities are assigned to individuals.

2. Current detection failure and measures

Phenomenon: BMS cannot collect current, SOC cannot be calculated, and the deviation is large.

Cause: The Hall sensor fails, the charging current is large, and the heat inside the battery cell is high, which will cause the diaphragm solidification capacity to decay.

Measures: Select high-quality Hall sensors to ensure qualified installation quality.

3. Temperature detection failure and measures

Phenomenon: The battery operating temperature is too high, which may cause swelling, leakage and explosion.

Cause: Temperature detection failure.

Measures: Strictly control the operating temperature of the battery between 20-45 degrees Celsius to avoid short circuits caused by lithium deposition during low-temperature charging and thermal runaway during high temperatures.

4. Insulation monitoring failure and measures

Phenomenon: Electric shock may occur.

Cause: Insulation failure will occur when the power battery is deformed or leaks.

Measures: For monitoring sensors, high-quality and highly reliable products must be selected.

5. Other failures and measures

(1) SOC estimation has large deviations. The current inspection standard requires that the deviation be within 5%, which is actually quite difficult. We need to continue to improve accuracy.

(2) The electromagnetic compatibility is not good enough to resist electromagnetic interference, which leads to the failure of BMS communication. It is still difficult to ensure that it is in place. The product must also pass the mandatory third-party inspection.