Fire tests on E-vehicle Battery Cells and Packs

<b>Objective</b> The purpose of this study was to investigate the effects of abuse conditions, including realistic crash scenarios on Li ion battery systems in E-vehicles in order to develop safe practices and priorities when responding to accidents involving E-vehicles. <b>Method</b> External fire tests using a Single Burning Item equipment (SBI) were performed on commercial Li ion battery cells and battery packs for electric vehicle (E-vehicle) application. The two most common battery cell technologies were tested: Lithium Iron Phosphate (LFP) and mixed transition metal oxide (Lithium Nickel Manganese Cobalt Oxide, NMC) cathodes against graphite anodes, respectively. The cell types investigated were “pouch” cells, with similar physical dimensions, but the NMC cells have double the electric capacity of that of the LFP cells due to the higher energy density of the NMC chemistry, 7 Ah and 14 Ah, respectively. Heat Release Rate (HRR) data and concentrations of toxic gases were acquired by oxygen consumption calorimetry and Fourier Transform Infrared Spectroscopy (FTIR), respectively. <b>Results</b> The test results indicate that the state of charge (SOC) affects the heat release rate (HRR) as well as the amount of toxic hydrogen fluoride gas (HF) formed during combustion. A larger number of cells increases the amount of HF formed per cell. There are significant differences in response to the fire exposure between the NMC and the LFP cells in this study. The LFP cells generate a lot more HF per cell, but the overall reactivity of the NMC cells is higher. However, the total energy released by both batteries during combustion was independent of SOC which indicates that the electric energy content of the test object contributes to the activation energy of the thermal and heat release process, whereas the chemical energy stored in the materials is the main source of thermal energy in the batteries. <b>Conclusions</b> The results imply that it is difficult to draw conclusions about higher order of system behavior with respect to HF emissions based on data from tests on single cell or small assemblies of cells. This applies to energy release rates as well, the present data shows that mass and shielding effects between cells in multi-cell assemblies affect the propagation of a thermal event.