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In MS1V the busbars connected the cells together in parallel and series arrangement of 24P2S per module. They were laser cut from EMS Sigma 60 material, which is a layer of nickel, the stainless steel, copper, stainless steel, nickel[1]
Copper is the main conductor(EMS sigma 60 is 60% copper), but it is difficult to spot weld, which is why there is nickel and stainless steel[1]
Information about EMS Sigma 60:
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Due to issues welding the EMS Sigma 60 directly to the cells(requires too much weld energy, so you can’t do a lot of welds in a short time), nickel strips need to be welded to the cells and the sigma 60busbar.[3]
In MSXIV they found that thinner nickel strips were better – 3 criteria were considered: Power loss and heat dissipated, Manufacturability, Safety
Power loss was marginally higher with the thinnest strips(0.1mm vs 0.15 and 0.20mm) but since a low weld energy was needed to be safer and allow lots of welds to be done, the 0.1mm nickel was chosen
Weld projections(basically hole-like impressions) were added to the nickel strips – helps spot welding to be more effective)
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Longer slits were added to force the current through the cell instead of just the copper??[2]
The Sigma Clad Busbar was modified to have square cutouts - The square cutouts allow for more area to be spot welded to better placement of the nickel strips.
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[4]
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General Considerations:
design bars that are wide enough to maximize contact(surface area), just long enough to connect the battery terminals( you don’t want extra material swinging around; risks causing a short), and thick enough to support the amount of current that will be flowing through these batteries[5].
brass busbars are sometimes used, but they are much less conductive than copper and the EMS sigma 60 is probably a much better bet
Interesting stuff to read:
https://interplex.com/busbar-guide/
Questions
What is laser welding vs spot welding that we do - lots of websites mentioned laser welding to minimize contact resistance
Sources:
[1] Battery Module Concept Rev 1