There will be two types of bus bars in our battery pack:1)
- Bars with nickel stripped attached (soldered) to them, running between two rows of cells and sitting flush on the plastic cell holder. The connection resembles a
...
- fish-bone. Each module has 6 type 1 bars, 3 on each side, to connect the 6 rows of cells into 3 pairs.
...
- Bars running orthogonal to the "backbone" bars in 1) to fully connect the 3 pairs of rows in each module together in parallel. These bars also serve as the connection interface between adjacent modules to connect them in series.
Bars of type 2 do not have major ampacity concerns because the current is flowing normal to the long face of the bar (thus the cross sectional area is huge and at least that of type 1). Bars of type 1 need to be sized appropriately to limit the temperature rise under maximum current draw.
Requirements
Most available data for copper conductors give a spec for a 30°C rise. This is too much of an increase for Li-ion batteries, so we will need a safety factor on the ampacity of type 1 bars. We are considering a maximum continuous current draw of 150 A, which means 50 A per type 1 bar.
Analysis
Based on the second table in this page for Copper No. 110 (>99.90% pure), a 1/16" x 1/2" copper bus bar can pass 103 A of current with a 30°C rise in an ambient temperature of 40°C. The area of this bar is 0.03125 sqin, or 20.16 sqmm. In order to provide suitable a safety factor to account for scaling non-linearities and a smaller temperature rise, we should size bus bars with a generous safety factor. We plot the ampacity data for bus bars of 1/16" and 1/8" thickness at various widths below.
...
We observe for each thickness the ampacity is best modelled modeled using a power equation. We generate the following table of extrapolated points using the more conservative 1/8" model. We expect this to bound bus bars of smaller thickness (such as 1/32"). All areas are rounded up to the nearest tenth. Note that in the 60 A - 90 A region the area relationship is approximately linear, but this does not hold true for the entire dataset.
| Ampacity (A) | Projected Area (sq. mm) |
|---|---|
| 90 | 22.3 |
80 | 19.5 |
| 70 | 16.7 |
| 60 | 13.9 |
Recommendations
We recommend a hard lower bound on bus bar area of 17 sq. mm, corresponding to an ampacity of around 70 A with a 30°C rise. This should give a moderate safety factor to ensure a temperature rise lower than 30°C at a 50 A load. Where practical from an engineering tradeoff trade-off standpoint, larger areas should be selected to provide further confidence and design margin during peak operating conditions.