Thermal and Electrical Simulations for Thermal Runaway Propagation in Lithium-Ion Battery Modules
Authors: Uemura Yohei , Zhang Wen , Morimoto Sayaka , Yagi Ryosuke 
Published: December 2024

In recent years, standards for evaluating thermal runaway (TR) propagation in battery systems have been developed following a series of fire accidents, such as those occurred in electric vehicles. Those standards require the safety of battery modules even if one of the component cells goes to TR. To predict module safety, simulation models for TR propagation have been actively researched. However, limited studies have focused on the short-circuit currents that occur during TR propagation between electrically connected parallel cells.
In this study, we investigate the impact of short-circuit currents between parallel-connected cells on TR propagation. For this purpose, we combine an electrochemical cell model and an electric circuit module model to simultaneously calculate the heat generation from chemical reactions and electric currents. The TR reaction in each battery cell is modeled by fitting differential scanning calorimetry (DSC) data obtained from both cathode and anode with theoretical curves. The accuracy of the individual cell model is validated through adiabatic reaction calorimetry (ARC) measurements. The model for 2P12S battery module is constructed by connecting cell models thermally and electrically. The short-circuit current is modeled based on the potential difference between parallel-connected cells, which is caused by the voltage drop in the TR cell. First, we investigate the TR propagation when cells are connected in parallel by busbars, through experiments and simulations. Next, we simulate the TR propagation when busbars are severed and short-circuit currents are absent, to explore how electrical short circuit affects TR propagation. The result revealed that the short-circuit currents between parallel-connected cells accelerate the TR propagation.
Figure (a) illustrates the simulation results when a cell located on the side of a 3x8 cell array module is heated. Each line represents the cell temperature, revealing that many cells go to TR following the first trigger cell. The schematics shows the arrangement of cells with colors corresponding to the cell temperature plots, and numbers indicating the order of TR. The result shows TR propagates across the long side of cells, subsequently spreading in the short side direction. This observation highlights the significant influence of thermal resistance between cells on TR propagation. Importantly, the simulation result corresponds to the test conducted on an actual battery module.
Figure (b) represents short-circuit currents, with their paths schematically depicted in the right-hand figure. Assuming that each cell shorts above 180°C, these short-circuit currents arise due to the voltage difference between parallel-connected cells when one of them reaches 180°C, and stops when both cells exceed 180°C. As the result demonstrates, short-circuit currents are inevitable as long as cells are connected in parallel, leading to the generation of Joule heat.
Figure (c) illustrates the simulation result when short-circuit currents between cells are absent. Notably, the TR does not propagate beyond two cells, emphasizing the critical role of short-circuit currents. In other words, interrupting the short-circuit current can effectively prevent TR propagation. During the session, we will discuss the detailed evaluation focusing on how heat transfer and Joule heat contribute to TR propagation.

Figure 1