E-Mobility Engineering | January/February 2024 67 Battery inspection | Focus the part is scanned from all around, algorithms can process the recorded data to produce highly detailed 3D slices through the part. To create these images quickly enough for production line use requires powerful computers, while AI helps rapid interpretation. Thermal imaging uses cameras sensitive to infrared wavelengths to identify temperature variations and potential hotspots within battery modules and packs. Ultrasonic inspection systems use high-frequency sound waves to find defects such as voids in battery components by analysing the echoes returned from interfaces between materials. These are complemented by other techniques such as Electrochemical impedance spectroscopy (EIS), which analyses the electrical behaviour of cells and batteries to assess performance and detect anomalies, while pressure decay and helium leak testing, for example, check for gas leaks at cell, module and pack level. The performance of both established and emerging inspection technologies is benefiting from the application of artificial intelligence (AI) techniques such as machine learning to improve speed and accuracy in processing and interpreting sensor data. Principal challenges Inspecting EV battery systems throughout their manufacture speedily and reliably is particularly challenging, because cycle time is crucial and manufacturers demand quick turnarounds for efficient production. “The need to test 100% of batteries in line adds complexity to the task of maintaining a swift manufacturing pace while maintaining quality control,” stresses an expert from a nondestructive testing (NDT) company employing X-ray and CT technologies including 3D-Inline CT systems. Reliability of inspection and measurement is governed by standards such as the German VDA5, and meeting them adds an additional layer of complexity, the expert adds. “Striking a balance between meeting these standards and maintaining an efficient manufacturing process is a challenge, as rigorous testing may increase cycle times.” Catching defects as early as possible in the manufacturing process is crucial to improving the yield achieved by a facility such as a megafactory. This is because bringing the reject and rework rates to a minimum will bring down battery costs, emphasises an expert from a camerabased inspection system provider. Large facilities also present the problem of round-the-clock monitoring of newly manufactured batteries in storage and the recognition of defects that are not yet visible, a specialist from a developer of infrared cameras and thermographic solutions notes. More broadly, the main challenge with inspection of batteries through the manufacturing process stems from the industry’s relative youth, comments an expert from a provider of inspection defects are electrode overhang, metal contaminants, delamination, deflected anodes, voids/porosity, bad welds, critical structural changes, cycling induced particle cracking or ageing mechanisms such as electrode particle composition degradation, mechanical deformation and assembly glue issues. A broad and expanding range of technologies is applied at all stages of the manufacturing process, with an emphasis on non-destructive methods followed by destructive ones if it is necessary to investigate a detected defect in more depth in a lab environment rather than on the production line. Key inspection technologies Cameras feeding imagery to computer vision systems are employed for visual inspections of surfaces of electrodes and assemblies to look for defects ranging from anomalies in active material to misalignment of components, leakage, and external defects. Techniques such as ultrasound and X-ray technologies including CT scanners are used to examine the internal structure of cells, modules and packs, identifying defects and anomalies without causing damage. Typical, CT scanners consist of an X-ray source and a detector that are positioned on opposite sides of the part being scanned. The source emits a narrow beam of X-rays that passes through the part. Both source and detector are mounted on a circular rotating gantry that surrounds the part and completes a rotation in a matter of seconds. X-rays are partially absorbed by the different materials they encounter as they pass through the part, with dense materials absorbing more X-ray energy and appearing white on a monochrome image, with less dense materials showing up as shades of grey, while air and the least dense materials appear black as most X-rays pass right through. However, ‘false’ colours can be applied to highlight materials of interest with different densities. Because Processed high-magnification image from a ZEISS Xradia 620 Versa X-ray microscope of a 21700 automotive cell shows a thin fracture defect through the cathode layer (Image courtesy of ZEISS)
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