A crucial safety and quality testing step in the fields of electric vehicles, energy storage systems, and consumer electronics is the “battery pack immersion test.” It primarily verifies the integrity of the battery pack’s seal, ensuring it can prevent moisture intrusion and thus avoid serious problems such as short circuits, corrosion, leakage, and even thermal runaway.
The following is a comprehensive analysis of battery pack water immersion testing:
Why is immersion testing so important?
I. Safety First: Moisture is the enemy of batteries. Once it enters the battery pack, it may cause:
- Electrical short circuit: Causes a large instantaneous current, which may lead to overheating, fire, or explosion.
- Corrosion: Damages battery cells, connectors, and PCB boards, leading to performance degradation and malfunctions.
- Insulation failure: Reduces the insulation resistance between high-voltage components and the casing, posing a risk of electric shock.
2. Guarantee performance and lifespan: A well-sealed battery pack ensures a stable internal environment and extends the lifespan of components such as cells and BMS.
3. Meet standard requirements: Domestic and international regulations and standards (such as GB/T, IEC, UL , etc.) have clear requirements for the protection level of battery packs, and water immersion testing is a key means to verify whether they meet the standards.
II. Key Protection Level Standards: IP Code
The dust and water resistance of a battery pack is usually indicated by an IP ( Ingress Protection ) rating, with the second digit being the most relevant.
IP67 : Often referred to as “waterproof”:
- Completely dustproof.
- Under specified conditions, even when immersed in water (usually 0.15-1 meter deep , soaking for 30 minutes ), no harmful water should enter.
IP68 : Protects against continuous immersion in water.
- Under conditions agreed upon by the manufacturer and user (usually more stringent than IP67 , such as water depth exceeding 1 meter and for longer periods), there should be no harmful water ingress even when continuously submerged in water.
IP66/IP69K , etc.: These usually refer to protection against strong water spray or high pressure washing, which is different from “immersion” scenarios.
The core purpose of immersion testing is to verify whether the battery pack truly meets its claimed IP67 or IP68 rating.
III. Main immersion test methods
Water immersion testing is divided into two main categories: direct methods and indirect methods.
A. Direct method (qualitative testing)
This method involves directly immersing the battery pack or sample in water and determining whether there is leakage through observation or internal inspection. Note: Direct immersion of finished battery packs, especially those containing electrolyte, is risky and is typically used for structural verification or destructive testing during the research and development phase.
1. Bubble Detection Method
- Principle: Seal an empty battery pack casing or an unfilled battery pack, fill it with compressed air at a certain pressure (e.g., 0.2-0.5 bar ) through the vent, and then immerse it in a water tank.
- Determination: Observe whether there are continuous bubbles emerging. The location where bubbles appear is the leak point.
Advantages: Simple, intuitive, low cost, and can quickly locate leak points.
Disadvantages: It is a qualitative test and cannot quantify the leakage rate; it is not suitable for finished packages containing electrical components.
2. Direct immersion test
- Principle: According to IP standards (such as IP67 ), the complete battery pack is immersed in water at a specified depth for a specified period of time.
- After testing: Remove the battery pack and open it to check for water stains or droplets inside. Alternatively, you can determine the cause by observing the change in insulation resistance before and after the test.
Advantages: It simulates the most realistic usage environment.
Disadvantages: Contains a risk of damage; if it leaks, the battery pack may be rendered unusable; cannot be used for 100% product testing online.
B. Indirect method (quantitative detection)
This is currently the mainstream, non-destructive online inspection method on production lines.
1. Pressure drop method
- Principle: The pressure relief valve or test port of the battery pack is sealed and connected to a high-precision differential pressure sensor. The instrument will fill the battery pack with clean, dry air at a certain pressure (e.g., 0.5-1.0 bar ), then shut off the air supply and enter the “pressure holding” phase.
- Judgment: During the pressure holding phase, the instrument monitors the decrease in internal pressure. If the pressure drop exceeds a set threshold, it is considered a leak.
Advantages:
- Non-destructive: Will not damage the battery pack.
- Fast: The entire testing cycle is usually completed within tens of seconds, making it suitable for 100% full inspection on the production line.
- Quantitative: It can accurately measure the leakage rate, rather than just a “yes / no” judgment.
- Automation: Easy to integrate into automated production lines.
Shortcomings:
- The leak point cannot be located directly.
- Requirements for the temperature and stability of the testing environment (temperature changes will affect air pressure).
2. Helium mass spectrometry leak detection method
- Principle: This is the most sensitive detection method. The battery pack is evacuated or filled with a helium-gas mixture, and then a “sniffing” probe is used on the outside to detect whether helium is escaping; alternatively, the battery pack is placed in a sealed cavity, evacuated, and then the cavity is checked for helium from inside the battery pack.
- Judgment: The leakage rate is determined by the helium concentration detected by a mass spectrometer.
Advantages:
- Extremely high precision: It can detect extremely small leaks (several orders of magnitude more sensitive than the pressure drop method).
- Locability: The “sniffing” mode can accurately locate the leak point.
Shortcomings:
- The equipment is expensive, and the testing costs are high.
- The testing speed is relatively slow.
- Typically used for spot checks, research and development, or in situations where extremely high sealing requirements are needed.
IV. Typical Industrial Production Process
In modern battery pack production lines, the sealing test process typically follows:
1. Preparation: Complete the final assembly and fastening of the battery pack.
2. Connection: The robot or operator connects the battery pack’s test port to the voltage drop detection equipment.
3. Inflation and Pressure Holding: The equipment automatically inflates, stabilizes, and holds pressure, and monitors pressure changes in real time.
4. Judgment: The system automatically determines “qualified” or “unqualified” based on the preset leakage rate threshold.
5. Data Recording: Test results (including pressure curves, leakage rate values, timestamps, product serial numbers, etc.) are automatically uploaded to the MES system to achieve full lifecycle quality traceability.
6. Handling of non-conforming products: Non-conforming products are removed from the production line for rework or further analysis (such as using the bubble method to locate leak points).
Conclusion
For battery pack immersion testing, the pressure drop method is currently the most widely used, cost-effective, and efficient online full-inspection solution in industrial production. Direct immersion testing and helium testing serve as important supplementary methods for product certification, quality sampling, and in-depth problem analysis. Ensuring that battery packs pass rigorous immersion testing is a key line of defense for guaranteeing their safe and reliable operation throughout their entire lifecycle.
Contact Semco Infratech to discuss your EV & BESS manufacturing requirements and discover how automatic assembly solutions can enhance your production efficiency, ensure product quality, and accelerate your path to market competitiveness.