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If the heat is too intense or is applied for too long during spot welding, the cell can be damaged.
If too much pressure is applied during spot welding, the cell can be deformed, especially if its casing is thin. This can compromise the integrity of the cell.
If a fully charged cell were connected to a fully discharged cell, then the more likely scenario would be that both cells are damaged.
When such a connection is made in parallel, there will be a large current flow from the charged cell to the discharged cell in an attempt to equalize their voltages. This surge can generate excessive heat, potentially damaging both cells.
It is possible that the high current can cause only the discharged cell to overheat, which might lead to swelling or leakage (neither of which were noticed in the dead cells).
Maybe the dead cell was initially degraded and died later on when discharged?
Would be worthwhile to check if cells that are adjacent to the dead cell are performing worse than cells that are further away from the dead cell
The stress from high current flow may degrade the internal chemistry of the fully charged and fully discharged cells, reducing their lifespan.
Discharging the modules without a battery management system (BMS) could have resulted in cell voltage imbalances, with certain cells dying after accidentally being over-discharged [Source].
The BMS prevents imbalances and acts as a safeguard. Removing this safeguard, whether for testing or by accident, could have led to imbalances, with some cells being discharged below safe levels while other cells remain at a safe capacity.
This is the most likely cause of the dead cell, as testing was not consistently performed using a BMS. It would also make sense that only 1 cell was dead in each pack, as discharging was always carefully monitored to ensure that the modules would not become over-discharged. Differing internal resistances for each cell may have resulted in the dead cell, as one cell with a significantly lower internal resistance may have discharged more quickly and died, while the others remained fine.