Single side busbars give you some advantages in designing cylindrical cells, but we have always known there is a resistance increase.
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.
The single side busbar uses the cell cap as the positive (+ve) connection and the cell shoulder as the negative connection. This brings the cell to cell connections all one end of the cell, thus freeing up the mechanical and thermal interfaces. However, the negative electrical path is now through the wall of the can.
The illustration shows the Cap to Shoulder (Single Side) connection negative electrical path in blue. This electrical path is not ideal as the cell case is a steel based material and only typically 0.3mm thick.
Let’s make some simple estimations for the resistance over this path:
The cell can is typically ~0.3mm thick steel. Let’s assume Tata Steel Hilumin [2] as that is often the top selection and this has a resistivity of ~0.097 µΩm. The resistance can then be calculated for any geometry of conductor using:
where ρ = resistivity = 0.097 µΩm, L = length ~ 70mm, A = cross-sectional area ~ π x 21mm x 0.3mm
This gives us a resistance, R = 0.34mΩ. This assumes the current passes through the cross-section of the whole can circumference. If we assume the cross-section is a 5mm wide strip and 0.3mm thick the resistance, R = 4.5mΩ.
Interestingly V4Smart [1] quote the Internal resistance DCIR, 30%SOC, 10s, 10A @23°C; Cap-to-Bottom and Cap-to-Shoulder values.
This means the electrical path through the cell can adds 0.66 to 2.06 mΩ.
This post has been built based on the support of and sponsorship from: illumion, Custom Power, Quarto Technical Services, Eatron Technologies and About:Energy
Now we can see we are in the ballpark. A full CAE geometry based calculation including the crimp would be very interesting, but maybe beyond this simple estimation. Plus, we have to remember that the Cap-to-Bottom also brings challenges in terms of the busbars and additions to join both ends of the cells.
References
- V4Smart EDS_V21700 UHP-26
- Hilumin, Tata Steel