The Electric Vehicles Clinic just released details of the Tesla Model 3 cell busbar failures. This is such an important topic that we thought we should cover these findings for the battery design community.
In the Tesla Model 3 with the 21700 cell based battery packs the cell connections are single side and the connections are made using wire bonds.
Single Side 21700 Busbars
If we look at the section through the cylindrical cell we see that the main can of the cell is the negative electrode.
This can gets crimped onto the top-cap with a gasket that isolates, seals and forms the +ve end of the cell.
This means if we that both the positive and negative connections to the cell can be made at one end of the cell.
One of the key design approaches of the Model 3 is that the cells are orientated upside down with the busbars and connections all at the bottom of the pack.
This means that any condensation on the cell or cooling channels is likely to run down the cans and drip from the edge of the cell.
As shown by Electric Vehicles Clinic most of the cells are encased in potting designed to bond the cells to the cooling system and to protect the adjacent cells during thermal runaway.
However, as they show and have found from vehicle repair experience this coverage can be hit and miss.
The result is corrosion on the edge of the cell and around where the negative electrical connections are made to the cell.
The electrical wire is aluminium and the cell can is nickel plated steel.
As stated in the article [1] the vehicles that are frequently charged at DC fast chargers have shown greater resistance to cell corrosion and other moisture-related issues.
References
- Tesla Model 3 / Y – Why It’s Crucial to Address Battery Issues Immediately and not wait!, Electric Vehicles Clinic