Thermal Conductivity and Specific Heat Capacity of Battery
Thermal Conductivity of battery:
In a battery, Thermal conductivity (k) is a measure of a battery’s ability to conduct heat. High thermal conductivity allows heat generated inside a battery (during charging, discharging, or thermal runaway) to spread quickly, reducing hot spots.
A lithium-ion battery cell is a layered composite, so its thermal conductivity is anisotropic (different in different directions):
- Higher thermal conductivity in-plane direction (along electrode layers – heat spreads better along the foils/electrodes).
- Much lower conductivity through-plane direction (heat doesn’t flow easily across separator/electrolyte).
Thermal properties for pouch cells are measured using a heat flow meter.
Specific heat capacity for batteries:
The specific heat capacity (Cp) of a material is defined as the amount of energy required to raise a unit mass of material by one unit of temperature at constant pressure. In a battery, it tells us how much heat a battery must absorb for its temperature to rise by 1K. It’s critical for thermal modeling and safety analysis.
To design and implement safe battery packs and thermal management systems, it is critical to know the specific heat capacity of the individual cells within the battery pack. Specific heat capacity information is not usually given by cell manufacturers, and therefore it must be determined experimentally.
Specific heat capacity is a useful parameter because it can be used to predict how much the temperature of the cell will rise when heat is applied to the cell. With this knowledge, steps can be taken to ensure that the cell does not reach its onset temperature. The onset temperature of a cell is the temperature at which the cell begins to self-heat, which can lead to a thermal runaway reaction. If a thermal runaway reaction occurs, the battery pack can ignite, which can lead to serious physical and monetary harm.
Standards / References:
ASTM C518-17: “Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus”.
ASTM C1784-14, “Using a Heat Flow Meter Apparatus for Measuring Thermal Storage Properties of Phase Change Materials and Products”.
Equipment used:
- Holometrix Micromet
Picture 1: Holometrix Micromet
Picture 2: Schematic
Example Test :
A Lithium-ion polymer pouch cell at 100% SOC and room temperature (25C) was tested. The specific heat capacity and thermal conductivity was found to be 1.36 J/g.K and 0.68 W/m.K summarized in table below.
| Cell ID | Cell Voltage | Mean Temperature | Specific Heat Capacity | Thermal Conductivity |
|---|---|---|---|---|
| Typical | V | (°C) | (J/g•K) | (W/m•K) |
| Pouch Cell | 4.2 | 25 | 1.36 | 0.68 |
FAQ:
1. How does the heat flow meter work?
In a heat flow meter (HFM), the test specimen is placed between two heated plates controlled to a user-defined mean sample temperature and temperature drop to measure heat flowing through the specimen. The sample thickness (L) corresponds to the actual sample dimension or to match the desired thickness of a compressible sample. The heat flow (Q) through the sample is measured by two calibrated heat flux transducers covering a large area of both sides of the specimen.
After reaching thermal equilibrium, the test is completed. The heat flux transducer output is calibrated with a standard. To calculate thermal conductivity (λ) and heat capacity (Cp), the average heat flux and the thermal resistance R is used, in accordance with Fourier’s Law. Thermal transmittance, also known as the U-value, is the reciprocal of the total thermal resistance. The lower the U-value, the better the insulating ability.
2. Why is it important to measure thermal conductivity?
- Ensures safe thermal management in battery packs.
- Helps in designing cooling systems for electric vehicles and energy storage systems.
- Provides input for thermal simulations to predict temperature distribution.
3. Why is specific heat capacity important?
- Determines how quickly a battery heats up during operation.
- Critical for thermal simulations and safety analysis.
- Helps in estimating energy storage in thermal management systems.