In short, we've been seeing this many times over the years, and we have a theory on what the root cause is of isolation faults in 40kWh packs. It's this:
What you're seeing here is a 40kWh battery module, shell off. All of the cells except for the 4th from the right look fine, and the 4th from the right has a rupture in the seal, with 'gunk' behind it. It's also very slowly leaking electrolyte.
Every single isolation fault we've seen in 40kWh packs except one (water ingress) has been because of a cell doing this. It's mostly in older cars, but we've seen it with cars with as little as 80 000km on the clock (about 50k miles). Since it's practically impossible to replace individual cells, we have to replace half-modules (which solves the issue).
At first we thought this was because of the metal shells around the modules deforming as the packs expand due to aging, and then rubbing or cutting into the cells - but that's not true. It's the cells themselves. There has to be some manufacturing defect or degradation mode that causes cells to develop these blisters and eventually rupturing.
What you're seeing here is a 40kWh battery module, shell off. All of the cells except for the 4th from the right look fine, and the 4th from the right has a rupture in the seal, with 'gunk' behind it. It's also very slowly leaking electrolyte.
Every single isolation fault we've seen in 40kWh packs except one (water ingress) has been because of a cell doing this. It's mostly in older cars, but we've seen it with cars with as little as 80 000km on the clock (about 50k miles). Since it's practically impossible to replace individual cells, we have to replace half-modules (which solves the issue).
At first we thought this was because of the metal shells around the modules deforming as the packs expand due to aging, and then rubbing or cutting into the cells - but that's not true. It's the cells themselves. There has to be some manufacturing defect or degradation mode that causes cells to develop these blisters and eventually rupturing.
