Leak Testing for Battery Systems in Electromobility: Challenges and Solutions
In recent years, electromobility has experienced remarkable growth, with the development of safe and reliable battery systems being of paramount importance. This article sheds light on the challenges and best practices for leak testing battery cells and housings in electric vehicles.
Figure 1: Exploded view of a battery pack with its components that typically need to be tested for leak tightness.
Fundamental Challenges in Testing:
Battery Housings:
Battery housings typically need to have a substantial volume to achieve the required energy density as well as the capacity for the demands of electric vehicles. This means that the volumes of battery housings can be considerable, making leak testing more complex. Furthermore, the walls of these housings are often thin to reduce weight, which increases the complexity of the testing.
The elasticity of the housings is another crucial factor. Battery housings can deform under various operating conditions, which leads to changes in volume and further on to a falsified test result in cause of pseudo leakage rates. Also, the large surface of battery housings increases the target for temperature variations, which additionally causes challenging effects for the leak testing.
Cooling:
Efficient cooling is essential for maintaining the optimal operating temperature and prolonging the lifespan of batteries. However, cooling also poses another challenge for leak testing. The batteries are large test specimens, and the required cooling principles often necessitate high testing pressures. Furthermore, the optimized high conductivity of the cooling to its environment poses challenges to the leak testing due to an increases target for temperature variations, which again causes unwanted effects for the leak testing.
Higher testing pressures of several barsare necessary to ensure leak-tightness under real operating conditions. This means that the testing equipment and methods must be capable of withstanding these pressures while delivering accurate and precise results.
Individual battery cells:
Another demanding test specimen are battery cells, which due to their energy density require much stricter limits for leak testing to ensure the safety in subsequent use. Due to this stricter requirement, compared to its housing often requires the use of Helium leak detection. Even though surpassing air leak testing in price, Helium leak detection offers a greatly enhanced precision over technologies based on air.
Fundamental Approach to Contacting:
Selecting appropriate contact methods is crucial for conducting leak testing effectively and accurately.
- Utilizing the Later Electrical Interfaces: A proven approach is to use the existing electrical interfaces of the batteries for testing. This minimizes the effort and increases efficiency.
- Possible Use of Two Connections: In some cases, using two connections on the housing can enhance test coverage and accuracy. This allows for a more comprehensive monitoring of the pressure inside the test specimen, thus allowing faster measurements and therefor shorter cycle times.
Flow-Optimized Adapters: Developing specially designed adapters that optimize the flow of air can improve contact and enable precise and faster measurements. Selecting the right adapters is crucial for conducting leak testing effectively in this field.
Figure 2: Customized ZEDcon[1] adapter designed for automated battery leak testing applications
Lack of Manufacturer Standardization:
Another challenge is the absence of standardized connector types or testing requirements in the electromobility industry. Different manufacturers adopt various approaches, necessitating flexible adaptation of testing systems to meet specific requirements. The ability to adapt and be flexible is critical in this regard.
Testing Methods with Air:
Mass Flow Method:
The mass flow method is based on measuring the volume of flowing air (the mass flow) instead of a pressure drop and is a proven method for leak testing battery trays, covers and complete assemblies.
- Principle of Operation: The mass flow method operates based on a specific principle where the mass flow of air is directly monitored to detect leaks. In principle these mass flow sensors consist out of two temperature sensors and a heater placed in between. Here a temperature difference between two temperature sensors gives access to the air flow over this sensor.
- Optimizing Filling Speed: The speed at which air is introduced into the test specimen is critical for the measurement speed. A PID[2]-optimized filling speed ensures that the test specimen is efficiently and precisely tested. Furthermore, a second connection to the test specimen allows a more direct measurement of the pressure inside and thus also decreases the cycle time.
- Bypass: A bypass system allows for the circumvention of specific parts of the test process, which can be useful in meeting certain requirements.
- Flexible Selection of Test Pressure: The mass flow method allows for the adjustment of test pressures, including positive and negative measurement circuits, to meet various testing requirements.
Figure 3: Pressure curve of a typical mass flow measuring circuit. Pressure curve in blue and monitored leak rate in red.
Differential Pressure Method:
The differential pressure method relies on two sensors, a relative pressure sensor to monitor and control the test pressure and a differential pressure sensor which measures the pressure difference between the test specimen and a reference space, which are pressurized to the same level. This offers distinct advantages in leak testing battery cooling systems, modules, trays and covers.
- Principle of Operation: A diagram is used to illustrate the operation principle of this method.
- Low-Noise Sensors: Low-noise sensors ensure precise measurements regardless of the test pressure. This is crucial for the accuracy of the results.
Figure 4: Pressure curve of a typical differential pressure measuring circuit.
Tracer Gas Testing Methods:
Using helium as a tracer gas provides highly precise results and includes two primary methods:
- Sniffing Test: Helium sniffing tests are highly sensitive and allow for the precise detection of leaks. They are based on using a helium detector to capture the leaked helium gas and therefor pose one of the few testing methods which allow a location of the leak itself and give the opportunity to rework defect parts.
- Vacuum Chamber: In a vacuum chamber, the low helium background in vacuum is used to identify smallest leaks most precisely. This method requires a vacuum chamber in which a vacuum is generated while the test specimen is filled with Helium and the test is conducted. This method uses highly sensitive detectors like mass spectrometers and thus requires reaching the regime of fine vacuum.
Leak testing battery cells and housings requires specialized methods and adaptation to individual requirements. Despite the lack of standardization in the industry, the mentioned approaches and techniques are crucial to ensuring the safety and reliability of electric vehicles.
It’s important to emphasize that these challenges and solutions are subject to continuous research and development. New technologies and methods are being developed to further enhance the efficiency and accuracy of leak testing. This is crucial as electromobility plays an increasingly significant role in sustainable transportation, and the safety of battery systems is paramount.
According to Anthony Nobel Head of Strategic Sales!” At ZELTWANGER, we are driving the E-Mobility revolution by enhancing the safety and efficiency of electric vehicle batteries, thereby increasing acceptance. Through our cutting-edge proprietary testing technology, numerous successfully implemented projects, and close collaborations with renowned OEMs, we offer leak testing solutions that cover all critical battery components, from battery cells to complete battery packs. Our comprehensive consulting and development support spans from feasibility assessments to prototype testing and efficient mass production at any level of automation”.
In summary, leak testing individual components of a battery system, and complete battery assemblies and housings is a critical step in the development of electric vehicles. It contributes to ensuring the reliability and safety of these vehicles, enabling consumers to fully realize the benefits of electromobility.
Dr. rer. nat. Raphael Nold
Application Developer
ZELTWANGER Leaktesting & Automation GmbH
[1] ZEDcon – standardized adapter series from ZELTWANGER for manual and automated use
[2] PID control for filling with air is a feedback system that targets the desired air pressure. It does this by adjusting the air supply in response to three factors: the current error from the setpoint (P), accumulated past errors (I), and the rate of change of the error (D). This combination of factors helps achieve and maintain the target air pressure.
