Impact of Thermal Gradients on Calorimetry Testing of Battery Cells

Detailed modeling of battery thermal runaway and propagation often requires a source term that represents the chemical heat release of the cell as a function of temperature. The shape of this heat release trend typically comes from cell testing data. Accelerating rate calorimetry (ARC) tests provide concise information on cell self-heating, since the cell is kept nearly isothermal and adiabatic. Also, compared to differential scanning calorimetry (DSC) tests of battery component materials, it contains all interactions between components. Converting the temperature rise rate data to heat release rate is theoretically very simple, only requiring the heat capacity of the cell. Practically, however, careful analysis is required to avoid artifacts arising from limitations of the test setup. This study uses a simple one-dimensional transient heat transfer model to illustrate the runaway process inside a cell and describe two error sources present in many ARC tests. As the cell temperature increases, temperature gradients tend to get amplified, which transitions into a reaction front. At that point, the isothermal assumption has broken down. When the reaction front reaches the thermocouple measuring the cell temperature, it is registered as a sudden jump in temperature rise rate that is observed in many tests. In addition, any fixturing, especially compression plates often used for pouch cells, is shown to act as a heat sink, which interferes with the adiabatic assumption and introduces temperature gradients. The impact of these errors on the predicted heat release rate is described, as well as potential countermeasures.