I. Why is Airtightness Testing Critical for Energy Storage Batteries?
1. Safety Assurance: Once moisture (especially liquid water rich in conductive ions) enters the battery enclosure, it may directly cause short circuits in high-voltage components such as cells, busbars, and relays, triggering thermal runaway and posing fire risks.
2. Performance and Lifespan Protection: Even slow ingress of trace moisture can trigger side reactions with the electrolyte, producing gases that increase internal pressure, accelerate capacity decay, and raise internal resistance-significantly shortening cycle life.
3. Insulation Assurance: Humid conditions reduce the battery system's overall insulation resistance, potentially triggering insulation fault alarms in the BMS (Battery Management System) and causing system malfunction.
4. Addressing Complex Environments: Energy storage facilities may be deployed in harsh conditions such as coastal high-humidity areas, locations with significant diurnal temperature variations prone to condensation, or regions with frequent wind, sand, rain, and snow. High-standard sealing performance is fundamental to overcoming these challenges.
II. Mainstream Airtightness Testing Methods and Their Principles Currently, the industry primarily employs the following testing methods:
1. Pressure Decay Method (Pressure Decline Method)
· Principle: This is the most common and direct method. Clean compressed air (or an inert gas like nitrogen) at a specific pressure is introduced into the battery pack enclosure or module. After stabilization, the gas supply is cut off, and a high-precision sensor monitors pressure changes over a set period.
· Determination: If leakage occurs in the enclosure, the pressure will decrease. By calculating the pressure decay value and converting it to an equivalent leakage rate (e.g., cc/min or Pa/s) using the ideal gas equation, product qualification can be determined.
· Advantages: Fast testing speed, relatively low cost, intuitive results, and easy automation integration into production lines.
· Disadvantages: Demands high temperature and stability in the testing environment, as temperature fluctuations cause pressure changes that interfere with accuracy.
2. Differential Pressure Method (Reference Method)
· Principle: An advanced version of the pressure drop method that effectively mitigates environmental temperature fluctuations. Identical pressure is simultaneously applied to the test piece and a sealed "reference piece." After stabilization, the gas supply is shut off for both. A high-precision differential pressure sensor directly measures the pressure difference change between the test piece and the reference piece.
· Determination: Since both components are in the same environment, temperature changes affect them synchronously. Therefore, variations in the differential pressure value solely originate from leaks in the workpiece. This method achieves significantly higher accuracy than the direct pressure drop method.
· Advantages: Ultra-high precision, strong resistance to interference, highly suitable for micro-leak detection and temperature-sensitive large enclosures.
· Disadvantages: Equipment cost is slightly higher than the direct pressure drop method.
3. Flow Method
· Principle: While pressurizing, directly measure the gas flow required to maintain the target pressure inside the workpiece. This flow value represents the real-time leakage rate.
· Determination: Directly read the leakage flow value and compare it with the standard value.
· Advantages: Extremely fast testing speed, suitable for large leak detection or rapid online screening of known leakage paths.
· Disadvantages: Less accurate than the differential pressure method for detecting extremely small leaks.
4. Water Test Method (Bubble Method)
· Principle: Fill the workpiece with pressurized air, submerge it in water, and observe for continuous bubble formation. · Determination: Leak location and severity assessed by observing bubble quantity and frequency. · Advantages: Extremely low cost; provides visual leak localization. · Disadvantages: Low efficiency; highly subjective; cannot quantify leakage rate; requires drying workpieces, posing potential risks to electronics. Has transitioned to an auxiliary localization method on production lines rather than a primary mass production testing approach.
III. Testing Standards and Key Considerations
Testing Standards:
The industry commonly references IP protection ratings (e.g., IP67, IP68) to define sealing performance. IP67 requires products to withstand temporary immersion in 1 meter of water for 30 minutes without water ingress. In airtightness testing, this is typically converted to a maximum allowable leakage rate (e.g., ≤5 cc/min).
Key Considerations:
· Test Pressure: Typically set slightly above the maximum hydrostatic or pneumatic pressure the product may encounter in actual use, commonly ranging from 50 Pa to 20 kPa depending on design requirements.
· Pressurization Time, Stabilization Time, Test Duration: These parameters are critical and must be adequately set to ensure internal pressure equilibrium and stable test results.
· Temperature Compensation: Advanced testing equipment integrates temperature sensors and employs algorithms to compensate for pressure variations due to temperature, enhancing test accuracy.
· Test Specimen Volume: The internal volume of the test specimen must be accurately input prior to testing for the equipment to precisely calculate the leakage rate.
IV. Recommended Test Procedure (Using Differential Pressure Method as Example)
1. Fixtures: Use specialized fixtures to seal all battery pack interfaces (e.g., pressure relief valves, electrical connectors), leaving only one channel connected to the leak tester.
2. Parameter Settings: Configure test pressure, inflation time, stabilization time, test duration, and leakage rate threshold on the testing equipment.
3. Initiate Test: The equipment automatically inflates, stabilizes pressure, performs testing, and calculates results.
4. Result Evaluation: The device automatically displays "PASS" (合格) or "FAIL" (不合格) along with the specific leakage rate value.
5. Non-conforming Product Handling: For failed units, methods such as applying soapy water can be used to locate leaks for repair.
Waterproof and airtightness testing for energy storage batteries is not an optional step but a critical quality lifeline ensuring their safe, reliable, and efficient operation throughout their entire lifecycle. Selecting high-precision, high-stability automated airtightness testing solutions and establishing rigorous testing protocols represent a prudent investment for every energy storage battery manufacturer seeking to enhance product competitiveness while mitigating significant recall and brand risks. As technology advances, online fully automated testing stations integrated into production MES systems will become mainstream, providing robust data support for achieving "zero leakage" at the factory gate.
