Can we build a quite simple cell busbar thermal model? First thoughts are whether we can make some simple approximations and hence create a spreadsheet model.
We have a current [I] flow through the busbar (orange). The busbar has a resistance defined by it’s resistivity, cross-sectional area and length. Hence we can calculate the heating power.
We know the mass and heat capacity of the busbar and hence we can calulate the temperature. The cells will be a thermal mass at a fixed temperature and hence we will get heat conduction into both cells.
We will also have an air temperature and assumption that this is all in a box with no air movement, hence a convection heat loss of 1 to 4W/m2.K.
If we break the busbar into 3 sections then we can then perhaps get a better prediction of the heat conduction and convection.
This post has been built based on the support and sponsorship from: Thermo Fisher Scientific, Eatron Technologies, About:Energy, Quarto Technical Services and Alvatek.
Assumptions:
- cells are a 0D thermal mass and at a fixed temperature
- heat radiation is minimal and average of cell temperatures are used as surrounding body temperature
- air temperature is fixed
- busbars are uncoated
For an initial model we assumed that the cells were at a fixed temperature.
Cells at 25°C air at 25°C and then having a current of 10A for 90s we get the following maximum temperatures for the busbars.
The 4mm wide copper busbar has the lowest electrical resistance and the lowest electrical heating. Based on the fact that this busbar doesn’t get hot the convective and conductive losses are small. In all cases the radiative losses are small. The 4mm wide copper busbar has a much lower maximum temperature that is closer to the cell temperature and hence the net radiative heat loss is small. This would be even smaller if the copper was nickel plated.
Narrowing the copper busbar down to 1mm gives it a similar performance in terms of temperature to the other busbars. For all of the cases the dominant heat loss mechanism is conduction into the cells. Remember that the cell temperatures have been fixed and in reality the cells will heat up too. This will change the heat loss and gain balance for the busbars. Hence the next stage is to look at adding the cell thermal model into this simple calculation.
The Cell Busbar Thermal Model is now part of the Battery Calculations Workbook.
