With the increasing emphasis on energy conservation and emission reduction policies, the number of new energy vehicles in China has been growing annually. As a core component, the high-voltage power battery, often referred to as the EV power battery, plays a critical role in vehicle performance. However, failures in the China EV battery system, such as insulation degradation, can lead to issues like inability to power on or charge, severely impacting the user experience. More alarmingly, these faults may cause significant safety hazards, including explosions, fires, and electric shocks, posing substantial risks to life and property. Insulation performance is a key indicator for ensuring the safe operation of high-voltage power batteries; a decline in this performance can result in internal short circuits, increased leakage risks, and thermal runaway. Therefore, studying the factors influencing the insulation performance of China EV batteries is of paramount importance.
In our research, we employed insulation resistance testing to evaluate 60 high-voltage power batteries with abnormal insulation, predominantly sourced from northern cities in China. The testing method involved connecting one end of an insulation meter to the fast-charging positive or negative terminal of the EV power battery and the other end to the ground. If the resistance between the fast-charging terminal and the ground was less than 10 MΩ, it was considered an insulation anomaly. All 60 batteries confirmed insulation issues upon testing. Subsequent disassembly and inspection revealed that the primary causes of insulation abnormalities could be categorized into four types: internal coolant leakage, external moisture intrusion, insufficient application of insulating adhesive, and seal failure of the pressure relief valve. This analysis aims to delve into these factors and propose effective countermeasures to enhance the reliability of China EV batteries.
To ensure stable operating temperatures, high-voltage power batteries in China EV systems often use coolant, primarily composed of ethylene glycol, which is conductive. If coolant leaks and comes into contact with high-voltage components such as battery cells or high-voltage lines, it can reduce insulation resistance, leading to leakage or short circuits. In our study, coolant leakage was mainly attributed to poor compatibility between the rubber coolant pipe and the battery’s coolant interface. Measurements showed that the coolant interface height exceeded the standard upper limit by 0.8 mm (the standard upper limit for disassembled batteries was 20.3 mm). During installation, the locking clip of the rubber pipe was secured at the upper edge of the interface groove, resulting in an insecure fit. Over time, this caused the pipe to loosen and leak coolant. To address manufacturing defects leading to coolant leakage, we recommend post-production measurements of the coolant interface to ensure it meets standards. Additionally, conducting air tightness tests after assembly can verify integrity. For batteries already affected, structural modifications, such as adding a secondary automatic locking mechanism, can be implemented. This mechanism features claws that automatically deploy upon installation, accompanied by an audible click, ensuring a secure connection. The relationship between leakage probability and interface tolerance can be expressed using a statistical model: $$ P(L) = \frac{1}{1 + e^{-k(\Delta h – \theta)}} $$ where \( P(L) \) is the probability of leakage, \( \Delta h \) is the height deviation, \( k \) is a constant, and \( \theta \) is the threshold deviation. This formula helps in predicting and mitigating risks in China EV battery designs.
| Factor | Description | Impact on Insulation | Proposed Measure |
|---|---|---|---|
| Interface Height Deviation | Exceeds standard by 0.8 mm | Reduces resistance by up to 50% | Implement secondary locking |
| Material Compatibility | Rubber pipe and metal interface | Increases leakage risk over time | Use enhanced sealants |
| Environmental Conditions | Temperature fluctuations | Accelerates degradation | Regular maintenance checks |
External moisture intrusion is another critical factor affecting the insulation performance of China EV batteries. Pressure relief valves, also known as breathing valves, are essential safety components that balance internal and external pressure differences. While they control moisture ingress, some water vapor can still enter the battery, forming condensation. When condensation adheres to insulating materials, it lowers insulation resistance and may create conductive paths, increasing leakage risks and compromising the reliability of the EV power battery. To counteract condensation-induced insulation faults, we propose the use of humidity control sheets. These sheets consist of microporous materials with absorbent fiber cores. When the water vapor partial pressure exceeds the design threshold of the micropores, moisture is absorbed; when it falls below, moisture is released into the atmosphere, maintaining stable humidity levels inside the battery. The effectiveness of this approach can be modeled using Fick’s law of diffusion: $$ J = -D \frac{\partial C}{\partial x} $$ where \( J \) is the diffusion flux, \( D \) is the diffusion coefficient, and \( \frac{\partial C}{\partial x} \) is the concentration gradient. This ensures that humidity remains within safe limits, preventing condensation. Installation involves removing the protective film and adhering the sheets to specific locations within the battery casing, such as the left and right cold plate brackets. It is crucial to complete this process within two hours of exposure to avoid sheet失效 due to environmental exposure.
| Parameter | Value | Effect on Insulation |
|---|---|---|
| Water Vapor Partial Pressure | > 1.5 kPa | High risk of condensation |
| Humidity Control Efficiency | Up to 95% | Maintains resistance above 10 MΩ |
| Exposure Time Limit | 2 hours | Prevents sheet degradation |
Insufficient application of insulating adhesive is a common manufacturing defect that jeopardizes the insulation performance of China EV batteries. High-voltage power batteries comprise multiple battery modules, with voltage differences between positive and negative terminals and other internal components. Proper application of insulating adhesive provides essential protection against short circuits and electrical faults. If the adhesive is inadequately applied, air pockets or bubbles can form, reducing the overall insulation strength. Additionally, moisture and dust may infiltrate, further degrading insulation and accelerating material aging, which increases the likelihood of insulation failures over time. In our disassembly analysis, one EV power battery exhibited clearly insufficient adhesive coverage, particularly at the corners of the first and second-layer battery modules. To address this, we recommend adjusting the dispensing system’s output and using sensor-guided trajectories to ensure 100% coverage of the module surfaces. This not only resolves the issue of insufficient adhesive but also seals the modules against environmental ingress, thereby enhancing the safety and longevity of China EV batteries. The insulation resistance \( R_{ins} \) can be related to the adhesive coverage area \( A \) and thickness \( d \) by: $$ R_{ins} = \rho \frac{d}{A} $$ where \( \rho \) is the resistivity of the insulating material. Maximizing \( A \) and ensuring uniform \( d \) are key to maintaining high \( R_{ins} \).
Seal failure in pressure relief valves is another significant contributor to insulation abnormalities in EV power batteries. These valves are designed to release internal pressure during events like overcharging, overheating, or short circuits, preventing explosions and mitigating thermal runaway. Composed of a valve body, burst disc, and sealing ring, they rely on precise sealing to function effectively. However, in our tests, IPX7 water immersion tests revealed moisture ingress in several batteries, as indicated by color changes in water-sensitive paper. Further inspection showed that the sealing rings of the pressure relief valves were not properly installed, with visible water traces at the inner and outer seal points. To prevent such failures, strict adherence to installation guidelines is essential. Post-installation visual inspections should confirm the absence of distortions or misalignments, and water resistance tests must be conducted to identify defective units. The pressure threshold for valve activation can be described by: $$ P_c = \frac{F}{A} $$ where \( P_c \) is the critical pressure, \( F \) is the force required to rupture the burst disc, and \( A \) is the effective area. Ensuring proper sealing maintains this threshold and safeguards the China EV battery against environmental hazards.
| Fault Type | Frequency (%) | Primary Cause | Mitigation Strategy |
|---|---|---|---|
| Coolant Leakage | 35 | Interface misalignment | Enhanced locking mechanisms |
| Moisture Intrusion | 28 | Condensation formation | Humidity control sheets |
| Insufficient Adhesive | 22 | Manufacturing errors | Automated dispensing systems |
| Seal Failure | 15 | Improper installation | Rigorous testing protocols |
In summary, the issue of insulation abnormalities in China EV power batteries remains an area of ongoing research and refinement. Various factors, such as internal coolant leakage, external moisture intrusion, insufficient insulating adhesive, and seal failures, can compromise the safety and performance of these batteries. Through comprehensive analysis and targeted measures, including structural improvements, humidity control, and automated manufacturing processes, we can enhance the reliability of EV power batteries. As the adoption of new energy vehicles accelerates, a deep understanding of fault diagnosis and prevention technologies is crucial for advancing the widespread use of China EV batteries. Future work should focus on integrating real-time monitoring systems and predictive models to further mitigate risks. The overall insulation performance \( I \) can be optimized by considering multiple variables: $$ I = \sum_{i=1}^{n} w_i f_i(x_i) $$ where \( w_i \) represents weight factors for each influence factor, and \( f_i(x_i) \) denotes the contribution functions. This holistic approach ensures the continued evolution of safe and efficient China EV battery systems.