The anti-shunt tab design is there to enable a better resistance micro spot weld.
The busbar for a cylindrical or prismatic cell has to be welded blind. Meaning you can only weld from the topside of the busbar as the cell itself is a sealed system.
The anti-shunt tab design shown on the left welded to the top of a cylindrical cell [2].
The aim of the anti-shunt design is to increase the resistance between the two spot weld probes through the busbar.
The slot in the busbars increases the path length and hence the resistance through the tab, hence increasing the current flow through the joints to the cell. There is also an electrical path through the Cell Tab, this is the path that would dominate in the closing of the circuit for the resistance weld.
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Having tested both designs with a small resistance spot welder the anti-shunt tab design generates a weld with more mechanical strength. However, it should be noted that this will be down to a number of factors:
- busbar tab
- tab thickness
- tab width
- tab material
- cell
- cell can material (note that +ve tab is often a different material to -ve on can)
- cell can thickness
- resistance welder
- internal resistance
- weld current
- pulse duration
- weld probe design and material
Mikno and Kowieski [1] describe a model of the welding and then look at different parameters, including the length of the anti-shunt slot.
“In the analyzed case of a shunt made of copper with a thickness of 0.1 mm, an unexpected effect was found in the form of an increase in the weld diameter when the shunt length (indentation) was reduced. The obtained results can be explained by greater heating and thus plasticization of the shunt material, which is confirmed by the larger indentation of the electrodes in the welded material.”
The electrical connections to the cells are critical to the overall performance and life of the battery pack. Hence, these joints need to be understood when first made and at end of life. Variation in the joints is also a critical parameter and hence quality control measures are required in production.
References
- Zygmunt Mikno, Szymon Kowieski, Micro-resistance spot welding of cylindrical battery packets in FEM calculations, Welding Technology Review 2019
- Clad Metals for Use as Connectors for Lithium Ion Batteries, Engineering Materials Solutions
Single Side 21700 Busbars
In the case of cylindrical cells it is possible to connect to both the positive and negative terminals of the cell on the top surface.
Thus leaving the bottom of the cell free for cooling.