Beam Measurement Systems for Power Battery Welding Quality

The battery module is the heart of an electric vehicle. Electric vehicles are powered by batteries, so the reliability of the battery is critical to the safety of the vehicle. Having launched its first all-electric vehicle, the i3, back in 2013, the BMW Motor Group is well aware of the multiple challenges of battery production. As production capacity continues to increase, so does the complexity of battery production - for example, as many as 144 welds are required in the production of BMW's fifth-generation battery modules.

 

With the Ophir BeamWatch Integrated automated laser beam measurement system, automakers can inspect the laser beam before welding each new battery module without disrupting the production cycle. Prior to Ophir BeamWatch Integrated, the automaker had to perform weekly spot checks with micrographs in order to confirm the ideal weld depth.

Beam quality monitoring without compromising production cycles

Laser welding in battery production requires absolute precision. Regularly checking the key parameters of the laser beam before welding is important to ensure the quality of the battery packs. From the very first discussions with experts from the BMW Motor Group, the Ophir team focused on "how to carry out regular checks of the laser beam while minimizing the impact on the production cycle". The key indicators for laser beam inspection are focus diameter, focus position, laser power, and focus shift.

 

Two of these parameters, laser power, and focus shift, are particularly important for single-mode lasers, but they are very difficult to measure quickly. The only way to do this is by non-contact measurement of the laser beam according to the Rayleigh scattering principle, a technique developed by MKS and used in the Ophir BeamWatch series.

 

The company has designed the Ophir BeamWatch Integrated system specifically for automated manufacturing. This combination of beam profiler and power meter has various interfaces and is directly adapted to industrial production. For the technology development specialists of the BMW Motor Group, this instrument comes at the right time.

Non-contact detection of focus shift

The first Ophir Beam Watch Integrated system was commissioned in 2019 as part of the prototyping and foundation definition of the battery production line. This was followed by an intensive testing phase for the development of the technology, and soon the device was able to determine all the required parameters.

 

However, the test results soon showed that the thermal focus shift generated during the production process had been underestimated. Measurements of the BeamWatch Integrated system showed that the thermal shift of the focus was within millimeters, a result that the engineers had not expected. Fortunately, the engineers knew the magnitude of the focus shift and were able to adjust it during the production process.

 

For automakers, this is an important step in maintaining consistent weld depths for all contact welds. During the development phase, micrographs of the contact welds are used to determine the relationship between the focus offset and the weld depth. On this basis, the automaker works closely with the Ophir team experts to determine the limit values for the focus offset.

Welding spatter causes a reduction in weld depth

In addition, engineers have found that the optics of single-mode fiber lasers used for contact welding are susceptible to spatter on the shielding glass. Spatter not only causes a focus shift but also directly affects the focus diameter.

 

Smudges on the protective glass can defocus the laser beam, which can be analyzed by the BeamWatch Integrated system by means of measurements. Due to the deviation of the laser beam parameters, the engineers checked the depth of each weld seam. Although the welds looked flawless, the depth of the weld was clearly too shallow. The final conclusion was that the measurement technology had to be integrated into the series production process, as only then could the quality of the welds be guaranteed in the long term.

Repeatable contact weld quality

Today, the Ophir BeamWatch Integrated system is integrated into all automated production lines worldwide for BMW's fifth-generation battery modules. During loading and unloading, the laser is operated briefly at full power to determine the focus displacement. Only after confirming (IO signal) that the laser beam parameters are in accordance with the requirements does welding begin. While production is still in the start-up phase, laser beam calibration measurements are performed for every cell module produced; however, after entering the normal high-volume production phase, laser beam calibration measurements can be performed after every ten cell modules are produced.

 

If the BeamWatch Integrated system detects a deviation from one of the previously defined parameters, a warning message is displayed. The operator in charge of the production line then checks the protective glass of the laser and cleans or replaces it if necessary. Potential errors due to deviations in laser beam dispersion can thus be avoided from the outset. In addition to this active inspection, all modules are tested for electrical functionality at the end of the welding process.

Indispensable monitoring tools

Overall, the production of battery modules is a very complex process. Individual battery cells are stacked, pressed, and placed into a frame. Then, the positive and negative electrodes of all the cells must be welded together in battery contact systems - up to 15,000 spot welds per hour per system! The engineers are convinced that the quality of a battery module depends to a large extent on consistently high-quality laser beam parameters, and the equipment operators on the production line are convinced that the Ophir beam watch Integrated system is an essential tool for monitoring the laser beam parameters during the production of the battery module.

 

In short, the Ophir Beam Watch Integrated system, which automatically measures the laser beam quality and detects focus shifts to ensure weld quality without interrupting the production cycle, is a key tool for guaranteeing the quality of welds in the battery production process, contributing to the overall development of the laser welding process at automotive manufacturers and to the optimization of the quality of the battery modules.



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