Laser beam welding of electrical contacts for the application in stationary energy storage devices

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The energy sector has been changing in the past few years, driven by the transition toward renewable energy. This affects the technologies, as well as the structure of energy production by means of a decentralized and time-dependent energy generation. The resulting effects on the power grid require local storage systems to store the surplus energy and to limit the feed-in power. For these energy storage systems, the use of commercial 26650 LiFePO4 battery cells is highly promising. Since the capacity of these cells is comparatively low, a large quantity of cells is needed to match the storage requirements. For this reason, the interconnection between individual battery cells is the basic prerequisite for the production of energy storage systems. Recent research has shown that laser beam welding is suitable for the welding of small electrical contacts. However, the welding process of 26650 cells with contacts made of nickel plated steel is very complex. The requirements regarding the heat input during the joining process are high due to a low thickness of the case in the range of a few hundredth of a millimeter and a high temperature sensitivity of the battery materials. Within this work, experiments on laser beam welding of nickel-plated DC04 steel on copper, aluminum, and steel were carried out. The electrical and mechanical properties of the connection joints were evaluated within a comparative study. For the investigation of the heat input during the welding process, the change of temperature inside the battery case was measured. The results presented in this paper show that laser beam welding with continuous wave radiation is a suitable joining process for the electrical connection of 26650 battery cells, while avoiding a critical temperature change within the cells. Electrical joints with a low contact resistance and a high mechanical strength can be achieved. Furthermore, a clamping device for battery modules consisting of 24 battery cells is presented and the application of the welding process for a large scale production of energy storage systems is demonstrated.