When we consider the battery thermal management system we need to start with the basics and look at the heat sources and paths in a battery pack. We don’t have to minimise all of these sources of heat, but it is important that we carefully consider them all.
It is important to calculate
- total heat generated
- thermal gradients across the pack
- thermal gradients within a cell
1. Cell
The heat generated in a cell is mostly Joule Heating (irreversible) and the Reaction Heat (reversible). The most significant term is the Joule Heating as we size the system heat management around the fast charge and peak cycle power. The reversible element is important in low power cycles where the heating is endothermic and hence can result in an unexpected cooling of the cell.
Heat generation in a cell can be defined quite simple for the case where the cell is operating within it’s normal limits. The first expression gives the heat flow [W].
The first part of this equation is the irreversible Joule heating term, the I2R term.
The second part is the reversible entropy term or Reaction heat terms. The charge and discharge reaction can be exothermic or endothermic under certain conditions.
2. Cell to Cell Busbars
The cell to cell busbars will have an I2R heating term and this will be determined by the material and cross-sectional area.
Building a cell busbar thermal model is a good way of understanding the mechanisms.
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.
Heat generated in these busbars will be conducted back into the cells. Also, heat generated in the cells will be conducted into the busbars.
3. HV Busbars
High Voltage busbars are normally sized based on a current/cross-sectional area. The heat generation and losses are the same as those for the cell to cell busbars. Hence we can extend the simple model. However, these busbars are often wrapped in electrical insulation that also thermally insulates them. Thermal conduction between the busbar and cells or groups of cells should be considered in both directions.
This can be a significant factor in cell to cell temperature gradients.
4. Electrical Joints
However an electrical joint is made (bolted, welded, mechanical) they will have a resistance and hence generate heat.
A failing electrical joint will have a higher resistance and can generate a significant amount of heat. If this conducts into the cells it can be the source of a significant cell to cell temperature gradient and result in premature ageing of those cells.
5. Fuses
A fuse has a small cross-section based on the design to heat up and fail under certain current fault conditions.
This means they can generate heat, can require cooling and the heat can conduct through the busbar to other components.
Sized correctly they are unlikely to be an issue, but should not be completely ignored.
6. Contactors
Contactors are essentially an electrical joint and as such there is an I2R heating term.
Depending on the rating and demands they can require cooling and they can heat the busbar that then can conduct to other components in the battery pack.
7. Cooling to Case
Shown is the Xpeng G6 battery pack and this has the cooling system applied underneath the main pack enclosure. Thus ensuring robustness against coolant leaks and electrical isolation.
However, it also means the whole pack enclosure is cooled/heated.
This means that where the pack is attached to the vehicle structure that is also a thermal path.
This post has been built based on the support of and sponsorship from: Resonate Testing Ltd, Quarto Technical Services, Eatron Technologies and About:Energy
8. Case to Cell
Heat flux into the battery pack from the environment, either on a cold or hot day will be conducted, radiated and convected to between the cell and the case.
Consider the connections, surfaces and all of the scenarios in which the battery is to operate.