Manufacturing capabilities in battery production are developing rapidly and Tesla just gave promising insights on their strategy towards cheaper and better batteries on their recent battery day. Now a less well-known but not less innovative company revealed interesting insights on laser-welding of cylindrical cells.
The connection between battery cells and the electric conducting sheet (also known as busbar) is a vital factor with positive or negative effects on the overall battery lifetime, performance, safety and stability. In the last years different methods have evolved rapidly with welding becoming a new industry standard, and within this technology laser-welding seems to win the paradigm shift.
Initially starting with bonding – which was introduced and optimized in the electronic industries starting in the 1990’s – bonding has today been replaced by various forms of welding. Due to availability and stability of high-scale production facilities it was self-evident that bonding was the premier choice for transaction batteries in the beginning. However, the technology lacks vibration resistance and quality of connection in resistance and tensile forces. Disadvantages which are of less importance to electronic applications but not to e.g., e-mobility batteries. This is also the reason why welding has become more into the focus of battery manufacturers. Spot, ultrasonic and laser welding are the common system. Whereas laser-welding has initially high investment costs it is seen as the preferred option especially with pouch and prismatic cell formats. However, for a long-time laser-welding was not supposed to work with cylindrical cells. The laser-beam was not precise enough and resulted in violations of the cell-material. This technology barrier was the starting point of the company VOLTLABOR in 2008 in Austria, with a long history in laser-welding.
VOLTLABOR is a pioneer in laser welding of cylindrical cells and had managed in 2008 to laser-weld cylindrical cells on both sides with unrivalled quality at zero-failure rate. At this time the first to do so globally. This led to increasing international interest and brought the interest of top players in the field to the picturesque village in the north of Austria. Since then, the technology has increasingly developed. In recent years, the laser-technology has developed constantly with rather positive effects on laser welding, as Martin Reingruber, CTO at VOLTLABOR states: “In the last 10 years the technology has become faster and more precise.” This allows that laser-welding today outperforms other connecting technologies such as bonding or resistance welding. Together with other positive effects such as high homogeneity and high contact-forces a new standard has been set. The effects have also been scientifically investigated and proven. In various scientific papers of German and Austrian universities, which serve as development partner from VOLTLABOR, a team of engineers compared laser with spot and ultrasonic welding. Across various aspects laser-welding outperformed spot and ultrasonic welding. Contact resistance, max. tensile strength are significantly better, furthermore laser-welding allows a much higher flexibility regarding alloys and materials combinations. The following tables summarizes key insights and differences regarding the different welding methods:
Table 1: Quantitative comparison of welding technologies
| Spot | Ultrasonic | Laser | |
| Min. electric resistance | 0,167mOhm | 0,167mOhm | 0,130mOhm |
| Max. tensile strength | 316,78N | 661,32N | 876,8N |
| Highest temperature on cell-poles | 31,0°C | 110,7°C | 86,86°C |
| Highest temperature on cell cylindrical housing | 25,0°C | 55,2°C | 32,5°C |
As stated in the table, Laser-Welding delivers best results regarding electric resistance and tensile strength. Whereas spot-welding leads to a limited temperature effect on cell pole/housing level ultrasonic welding results in the highest temperatures. This again may affect the overall cell quality and lead to negative effects in the overall production process due to negatively high process temperatures.
Similar results regarding the benefits of laser-welding versus other contacting methods have been made by the university of Uppsala (Sweden). In a thorough analysis joining methods have been compared and evaluated along different criteria. Whereas resistance welding leads to the overall worse results, laser-beam welding outperforms on average. The following table summarizes the results, with factors rated between 1-5 with 1 lowest and 5 highest:
Table 2: Evaluation of joining methods[1]
| Criteria | RSW Resistance Welding | UWB | LBW Laser Beam Welding |
| Joint resistance (similar materials) | 4 | 4 | 5 |
| Joint resistance (dissimilar materials) | 2 | 5 | 3 |
| Heat transfer | 3 | 3 | 4 |
| Potential mechanical damage. | 4 | 4 | 5 |
| Joint current capacity | 3 | 3 | 5 |
| Joint durability | 4 | 4 | 4 |
| Potential vibration damage | 5 | 5 | 5 |
| Cycle time | 4 | 4 | 5 |
| Repeatability | 3 | 4 | 4 |
| Cost per battery connection | 5 | 5 | 4 |
| Investment | 4 | 4 | 2 |
| Easy automation | 4 | 4 | 5 |
| Flexibility | 3 | 3 | 3 |
| Safety | 4 | 4 | 3 |
Another benefit from laser-welding results in the flexibility of the respective material mix that is possible. In contrast to spot or ultrasonic welding laser-welding allows for a myriad of different combinations of nickel/cooper/iron steel across different alloys. Thickness of the busbar has been optimized, leading to lower overall weight of the battery and minimized material costs. Depending on client wishes VOLTLABOR is able to laser-weld a broad spectrum of different materials and also cell-formats. This is especially interesting as there seems to be a clear industry shift towards bigger cell formats. With Teslas new cell-format 4680, better energy density, lower manufacturing costs and increased safety is proposed.
In the last year the company VOLTLABOR has made another big step towards professionalism becoming one of the rising stars in the industry. Through a joint-venture with established Austrian Tier-1 supplier Miba, the company has exclusive access to MIBAs FLEXcooler® Technology – a highly innovative cooling system that avoids gapfillers, reduces CO2 footprint, weight and costs. The FLEXcooler® is a liquid cooling component and enables bottom cooling of cylindric cells. The bottom cooling is the cooling method of the future as shown by Tesla at their battery day. A single cell fuse rounds off the battery concept to a safe and reliable battery pack. This year a highly automated production line with a capacity of approx. 500 MWh per year started its production.
The trend and development definitely supports companies like VOLTLABOR a typically hidden-champion in the industry or as Stefan Gaigg, Managing Director at the Austrian battery producer, states: “The remaining challenge lies in an automated manufacturing process, combined with outstanding product features like an innovative cooling concept. That’s where manufacturer like us still have a competitive edge over other companies.”
Dr. Johannes Kaar | Voltlabor GmbH
[1] Source: Larsson et al: Welding methods for electrical connections in battery systems, Uppsala Universitet, June 2019.
