Quantifying early warning feasibility for battery safety failures: A dimensionless analysis of multi-signal dynamics

Force signals are poised to provide early safety warnings due to their early detectability when monitoring battery expansion behaviors. However, the feasibility of using expansion force as an early warning signal for safety failures across various battery packaging formats under diverse trigger scenarios has remained unquantified. This study qualitatively and quantitatively investigates the multi-dimensional features of pouch and prismatic cell units during thermal runaway (TR) and its propagation in a module, particularly focusing on the feasibility of early warning methods. Qualitative results demonstrate that an expansion force anomaly, characterized by a force rising rate of 5 N/s, serves as a superior early warning signal for battery safety failures. This signal is effective for both prismatic and pouch batteries under overcharging and overheating conditions. Furthermore, a dimensionless analysis method is introduced for the quantitative evaluation of different signals, revealing that the expansion force signal is also effective for early warning at the module level. Specifically, for prismatic battery modules, the force-based warning method is most effective in overheating scenarios, as evidenced by the highest normalized indicators (W) for assessing warnings (W = 4). Conversely, for pouch battery modules, the W values are smallest in overheating scenarios (W < 1). Finally, the application of expansion force signal warning is experimentally verified in a large format module, presenting over 15 min in advance of TR onset. These findings contribute to the development of early warning strategies and the safety design of battery modules in energy storage systems.