Although ideally we'd be able to use a compression pack design for its time and labor savings, it is possible that the design will not work out. Spot-welded packs are not without their weaknesses, but they are more common. If done correctly, they are intuitively quite reliable.

Things to Consider

Spot-welder labor cost: With ~1300 cells, assuming 4 welds per cell (2 on each side), we have over 5000 welds to complete.
Spot-welder lead time/training: 5000 welds does not include the time it takes to obtain, calibrate, and train people on the spot welder.
Minimizing load on welds: Our modules must be designed such that our welds are not loaded, or else we risk cracking them.
Current capacity: Without specialized equipment, we can only spot-weld to nickel strips. We need to consider the resistance added by the nickel strips.
Module repair: If spot-welding our cells, we can no longer replace individual cells. This may have an impact on how we construct our packs.

Proposal

I believe a Tesla-style pack that uses copper busbars with minimal nickel stubs as the primary current carrier is the safest method of construction. Essentially, the idea is to embed the copper busbars directly into the module housings, then solder/bolt/otherwise affix short lengths of nickel strip to the copper. These nickel stubs would then be spot-welded to the battery cells.

The core points in this design address current capacity and preventing weld failures. By embedding the copper into the module housings or otherwise fixing them into place, we take the load off of the nickel strips that the cells are actually welded to. At the same time, using nickel stubs allows us to spot-weld while maximizing current capacity.

Unfortunately, we cannot just modify the current design, as the busbars would not fit in the current cell holders. This would increase pack width, and we were already quite constrained in width. More planning is required to fit the modules in.

See Tesla module picture for inspiration. This Hackaday project is pretty much what I'm thinking of.

Follow-up

2017-09-30 Battery Module - Meeting notes

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