Thermal resistance is a critical parameter in the design of supercapacitor and battery systems. It determines how effectively heat moves from inside a cell to its surroundings and therefore influences operating temperature, performance, reliability, lifetime, and cooling system requirements.
However, thermal resistance is not a single, universal value. The result depends heavily on how the cell is tested, how it is connected, and how it is cooled. This means the more useful question is not necessarily, “What is the correct thermal resistance?” but rather, “Which thermal resistance is useful for the system being designed?” It is key to understand the customer system requirements, the demands of the application, and the operating environmental conditions.
What determines a cell’s thermal resistance?
Heat generated inside a cell must travel through several materials and interfaces before it reaches the surrounding air, cooling fluid, or heat sink. Two mechanisms rule this process:
- Conduction, where heat passes through the cell’s internal materials and structure.
- Convection, where heat is transferred from the cell surface to a surrounding fluid such as air.
Applying both mechanisms, the overall thermal resistance of a cell can be divided into two main components:
Internal cell thermal resistance describes the path from the hottest point inside the cell to its outer surface. This is primarily determined by the cell’s materials, geometry, and construction.
External cooling thermal resistance describes the path from the cell surface into the surrounding air, liquid, or solid cooling structure. This depends mainly on the module and cooling-system design.
The distinction matters because the cell manufacturer can influence the first component, while the system or module designer largely controls the second. In Skeleton's case, having control over both the cell and the module/system design offers significant advantages.
Why direct thermal measurements can be misleading
Thermal resistance is commonly determined by cycling a cell at high current inside a climate chamber and measuring the resulting temperature rise. In principle, the calculation is straightforward: thermal resistance is the temperature difference divided by the heat generated inside the cell. In practice, however, the test setup affects the result.
High-current testing requires large power cables. These cables do not simply carry current, but they also conduct heat away from the cell. Measurements show that cable cooling can remove approximately 30–50% of the total power loss during testing.
The climate chamber may also create forced airflow around the cell, providing considerably more cooling than the cell would experience under natural convection or inside a real module. This means a directly measured value characterizes the complete test setup, not just the cell.
Combining physical testing with thermal simulation
To separate the cell’s behaviour from the influence of the test equipment, physical measurements can be combined with a detailed three-dimensional thermal-flow simulation.
The model includes:
- The internal cell design
- Power cables and electrical connections
- Ambient temperature
- Airflow and cooling conditions
- Heat generation inside the cell
The simulation is calibrated against measurements at different load conditions until it reproduces the observed temperature behaviour.
Once validated, the model can be modified to remove the cables, replace forced airflow with natural convection, and calculate temperatures that are difficult to measure directly, particularly the maximum temperature inside the cell.
Three ways to report thermal resistance
The analysis produces three different thermal-resistance values.
1. Directly measured thermal resistance
This value comes directly from the physical test. It includes the cooling effect of the cables and the specific airflow inside the climate chamber.
It is a valid measurement, but it is difficult to apply to another system because the operating environment is unlikely to match the laboratory setup.
2. Corrected thermal resistance
The corrected value removes heat transfer through the cables and replaces the climate chamber’s forced convection with a defined natural-convection condition.
This provides a more representative indication of how the cell behaves under standardized assumptions. However, it still includes an external cooling environment that may differ from the final product.
3. Pure cell thermal resistance
The pure cell value considers only the temperature difference between the cell’s internal hot spot and its outer surface, and therefore isolates the thermal resistance caused by the cell itself, independently of the external cooling system.
For system designers, this is generally the most transferable value. It can be combined with the thermal resistance of the intended cooling solution to estimate the complete heat path:
Total system thermal resistance = cell thermal resistance + cooling-system thermal resistance
From datasheet value to better system design
No single thermal-resistance value can fully describe cell behaviour in every possible application.
A corrected value is useful for comparing thermal performance under defined conditions. A pure cell value is more useful when designing a real module, because it allows engineers to add the thermal characteristics of their own cooling concept.
Providing both values gives customers a clearer picture: one describes expected cell behaviour under specified assumptions, while the other can be used directly in application-level thermal modelling.
The result is better cooling-system design, more accurate temperature predictions, and a more reliable foundation for optimizing performance, lifetime, and total cost of ownership.
