The air tightness of the battery pack is a crucial indicator in electric vehicles and energy storage systems. The air tightness test of the battery pack is mainly carried out on the battery pack shell, interface, connector, cooling assembly, etc. to ensure that the inside of the battery pack is not contaminated or invaded by impurities such as dust and moisture from the external environment, and that the cooling assembly does not leak, so as to ensure that the battery pack maintains normal performance and life, and does not cause safety accidents such as short circuit or explosion.

1-Battery pack protection level and airtightness testing standard formulation

International Protection Making (IEC60529), also known as foreign body protection level or IP Code. The IP (Ingress Protection) protection level system is a standard established by the International Electrotechnical Commission (IEC) to classify the protection level of electrical equipment housings against foreign body intrusion and water intrusion. The airtightness level of the battery pack case is usually required to reach IP67 or IP68, which means that the battery pack case needs to be completely protected from dust ingress (dustproof level 6) and can be immersed in water at a certain pressure for a period of time without water ingress to a harmful level (waterproof level 7). More stringent requirements are that the battery pack can be immersed in 1m deep water for 60 minutes without water ingress (waterproof level 8). The IP protection level usually consists of two digits. The larger the number, the higher the protection level, as shown in Figure 1:

Figure 1: IP protection level description

In order to ensure the battery pack meets the IP67 and IP68 requirements, the battery pack needs to be submerged in water. This method is time-consuming, destructive to the power battery pack, and has certain safety risks. It is not suitable as an offline test for power batteries. Therefore, it has become a common practice in the industry to use airtight testing to ensure that the battery pack meets the IP67 and IP68 requirements. The formulation of airtight testing standards needs to consider the relationship between pressure drop value and leakage rate, as well as the relationship between aperture and water leakage. The formulation of airtight testing standards involves a series of steps from theoretical extremes to experimental verification to achieve the conversion from IP level to airtight testing standards. For example, taking IP68 as an example:

Figure 2: Steps for formulating airtight testing standards

2- Selection of airtightness testing methods and analysis of testing difficulties

The design and manufacturing quality of the battery pack are key factors affecting air tightness, including the toughness and strength of the battery box cover, the sealing of the battery pack shell, interfaces and connectors, explosion-proof vents, and the sealing of the electrical connector itself. In addition, there will be some problems that affect air tightness during use, such as thermal expansion and contraction problems, material aging, and vibration and impact. In the production and manufacturing of battery pack shells, we pay more attention to poor air tightness caused by problems such as welding points and joint quality, such as uneven welding points, weak or cracked welds, air gaps, and poor sealing of joint connections.

The air tightness test of the battery pack is mainly divided into the air tightness test of the upper shell, lower shell, and assembly parts. The air tightness test of the upper and lower shells must meet the air tightness leakage requirements after assembly. When selecting the air tightness test method for the battery pack, the characteristics of the battery pack, test accuracy requirements, production efficiency, and cost are generally considered comprehensively.

Battery pack shell testing in engineering is generally divided into process airtightness testing and shipping airtightness testing. In addition, the airtightness testing of the upper and lower shells must meet the airtightness leakage requirements after assembly, which puts forward more stringent requirements on the testing standards. To ensure that the airtightness meets the requirements, the following difficulties must be overcome in actual operation:

l Product structure stability: the quality of welds, including plug welds, faucet welds, beam welds, frame bottom plate welds, frame front and rear cover plate welds, etc. Weld leakage problems are mainly concentrated in the arc starting and arc ending points and defects caused by burn-through; cracking caused by welding deformation stress, such as bottom plate cavity side wall welding, bottom plate cavity material stratification, and inability to withstand welding deformation stress.

l Adaptability and Stability of Airtight Fixtures: The design of the fixtures should closely match the shape and dimensions of the tested components, ensuring that the components can be securely fixed to the fixtures during the testing process, thereby reducing testing errors caused by positional shifts or vibrations. However, in practice, the size and shape of battery packs vary significantly, necessitating the design and manufacture of multiple different testing fixtures, which increases costs and operational complexity. Designing a universal fixture would further complicate the design process.

l Repeatability of airtightness test results: Factors such as air pressure, temperature, and dryness of the test workpiece/fixture will affect the airtightness test results.

l For workpieces with many non-penetrating tiny cracks, due to the influence of factors such as the accuracy of the detection equipment and the detection parameters, the leakage source may not be discovered, resulting in missed detection.

Figure 3: Airtightness testing tooling

3-Combination of battery pack air tightness detection solutions commonly used in engineering

The air tightness test of the battery pack shell process generally includes air tightness test and water immersion test. In the air tightness test, the upper cover of the battery box is sealed, leaving only a connector port as the air inlet. The air tightness of the battery pack is judged by controlling the air pressure and observing whether there is air leakage. The water immersion test is to completely immerse the entire battery box in water and judge its air tightness by checking whether there is water in the box.

Helium leak detection is a technology that uses helium as a tracer gas to detect leaks by detecting the helium concentration at the leak point. When helium enters the inside or outside of the device being tested where there may be a leak, if there is a leak, the helium will quickly enter or escape the system through the leak and be detected by the mass spectrometer. The helium leak detection method has a high detection efficiency, especially in detecting small leaks.

Figure 4: Comparison of leak detection methods

In actual production, multiple detection methods are usually combined to improve detection efficiency and accuracy. For example, the helium leak detection method is suitable for high-precision and small leak detection, while the differential pressure method has the characteristics of high precision and fast response. In addition, although the traditional water detection method has low detection accuracy, it is intuitive and low-cost, and is a convenient way to locate leaks.

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