Battery modules are the beating heart of every electric car. They ensure mobility and security and their reliability determines whether a vehicle passes or fails in production. The BMW Group launched the i3, its first fully electric production vehicle, back in 2013 so the company is well aware of the pitfalls of battery production. These pitfalls grow in complexity with every uptick in capacity. Up to 144 laser welds are required for contacting a fifth-generation battery module.
Thanks to the Ophir BeamWatch Integrated laser measurement system, the automaker can check the laser beam in the laser welding process before manufacturing each new battery module, independent of cycle times. Before, in order to confirm ideal weld depths, spot checks by means of micrographs were carried out on a weekly basis.
Testing the beam
Laser welding in the production of battery cells requires absolute precision. Regularly checking the key parameters of the laser beam before welding contributes significantly to the quality of the finished battery pack. From the very first discussion with the experts from the BMW Group, the focus was on how to do this as completely – and with as little impact on cycle time – as possible. Within the ongoing process, the key metrics to be captured were focus diameter, laser power, focus position and focus shift.
The latter two are particularly crucial for single-mode fiber lasers, but measuring them quickly proved difficult. The only method capable is non-contact measurement of the laser beam based on the principle of Rayleigh scattering – a technology developed by MKS and employed in the Ophir BeamWatch product family. The company designed the BeamWatch Integrated system specifically for automated manufacturing.
This combined beam profiler and power gauge has various interfaces that permit direct implementation in industrial networks. For the experts in technology development at the BMW Group, this instrument came at exactly the right time.
The first BeamWatch Integrated system was put into operation in 2019 as part of the prototyping and basic definition for the battery production line. An intensive test phase in technology development followed. It soon became clear that the measurement device could ascertain all the required parameters.
Moreover, the results quickly revealed that the thermal focus shift in the process had been underestimated. Measurements with the BeamWatch Integrated system showed a thermal focus shift in the millimeter range that the engineering team had not expected. Once the magnitude of this shift was known, adjustments could be made in the manufacturing process.
For the automaker, this was an important step toward attaining a consistent weld depth for all contact welds. In the development phase, micrographs of the contact welds were used to determine the relationship between the focus shift and weld depth. Based on this, and working in close cooperation with the Ophir experts, the team determined the limit values for the focus shift.
Another finding that emerged from the test phase involved the optics of the single-mode fiber lasers used for welding. They are affected by spatter on the protective glass. Spatter made its mark not only in terms of the focus shift but even more directly on the focus diameter. The effects of laser beam defocusing due to a smudged protective glass, for example, could also be analyzed through the measurements taken with the BeamWatch Integrated system.
Because of these deviations in the laser beam parameters, the team also checked the weld penetration depth of the respective weld seams. Although the seams looked completely flawless, the weld depth was clearly too shallow. It was concluded that measurement technology had to be integrated into the series production process – as only then could the quality of the contact welds be guaranteed over the long term.
Production today
Today, the BeamWatch Integrated system is built into all the automated production lines where BMW fifth-generation battery modules are made worldwide. During the loading and unloading process, the laser is briefly operated at full power to determine the focus shift. Only after confirmation (IO signal) that the parameters of the laser beam correspond to specifications does the welding begin.
While production was still in the start-up phase, measurements were taken after each battery module was produced. For series production at full capacity, however, the decision was made to measure the laser beam after every 10th module produced. Should the BeamWatch Integrated detect a deviation in one of the previously defined parameters, it displays a warning message. The operator responsible for the line then checks the protective glass of the laser and cleans or exchanges it, as necessary.
Thus, potential errors due to deviations of the laser beam’s caustic can be avoided from the outset. In addition to this proactive testing, all modules are subjected to an electrical function test after the welding process.
The production of a battery module is a very complex process overall. The individual battery cells are stacked, pressed and placed into a frame. Then, all positive and negative poles must be welded in the cell contacting system – upward of 15,000 laser welds per hour are performed in each system. The engineering team is certain that the quality of the manufactured modules depends in large part on the consistently high quality of the laser beam parameters.
The BeamWatch Integrated system is an essential tool for monitoring the laser beam in the production of the battery modules. And the equipment operators in the production lines are convinced of this, too. Overall, MKS measurement technology has contributed to the optimization of the BMW Group’s laser welding process as a whole and the quality of the battery modules.
