# A Review of Experimental and Numerical Studies of Lithium Ion Battery Fires
## 4 Thermal Runaway
### State of Charge and Its Effect on Thermal Runaway  
The SOC is the most crucial state function of a battery system and can simply be explained as the remaining available capacity of a battery (in amp hours, Ah), which is expressed as a percentage of the rated capacity. From a thermodynamics point of view, SOC is the amount of potential chemical energy remaining within the battery cells. The effect of SOC has been studied in almost all research investigating the onset temperature, time to TR, and the energetics associated with thermally induced failure of \begin{table}
\begin{tabular}{c c} \hline \hline
**Abuse Condition** & **Direct Cause** \\ \hline \multirow{4}{*}{Thermal} & Overheat \\  & Extreme cold \\  & Fire \\  & Thermal shock \\ \hline \multirow{4}{*}{Mechanical} & Shock \\  & Drop \\  & Penetration \\  & Immersion \\  & Crush \\  & Vibration \\ \hline \multirow{4}{*}{Electrical} & External short circuit \\  & Internal short circuit \\ \cline{1-1}  & Overcharge \\ \cline{1-1}  & Overdischarge \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Abuse conditions and potential causes. LIBs. [PERSON] and his coworkers [80; 81; 82] established a novel technique known as the copper slug battery calorimeter (CSBC) to measure the heat generated inside the LIB and combined it with oxygen consumption calorimetry to quantify the heat released in the flaming combustion. The slug heats the specimen to initiate the failure process. They reported that most LIB heat released during TR originated from the electric energy of short circuit, chemical reactions, and combustion, where the amount of combustion heat was more than the sum of the other two parts. These studies [80; 81; 82] were extended to determine the timing of the safety venting and TR, and associated temperatures in cylindrical 18650 LIBs containing a carbon anode and three different cathodes: lithium cobalt oxide (LCO), lithium nickel manganese cobalt oxide (NMC), and lithium iron phosphate (LFP) at various states of charge (0, 25, 50, and 100%). The temperature at the time of safety venting and TR data are presented in Table 3.  
\begin{table}
\begin{tabular}{c|c c c c c c|c c  
The results in Table 3 indicate that the onset temperatures to TR vary slightly between LIBs used in the research. The LIB at 100% SOC and containing LCO recorded the lowest onset temperature (221 \({}^{\circ}\)C) to TR with the onset temperature increasing with decreasing SOC up to 308 \({}^{\circ}\)C for the LCO battery at 0%. The NMC battery results were similar with one exception; the LIB at 25% SOC reached the onset of TR 29 \({}^{\circ}\)C before the LIB at 50% with a variation of 105 \({}^{\circ}\)C between 100 and 0%. The LFP batteries produced a variation of only 6 \({}^{\circ}\)C between LIBs at different SOC reaching the onset of TR. The onset temperature to TR is comparable between all LIBs tested and as such there can be no definitive conclusion drawn on the correlation between either SOC or the LIB chemistries to the onset temperature of TR. With the CSBC technique being somewhat new and not widely employed in the testing of LIBs, further studies and data quantification from more industry-accepted methods may be required. The data in relation to the timing of the safety venting for only the LIBs with NMC cathode were measured and are presented in Table 4.  
[PERSON] [83] conducted a study into the TR features of a 25 Ah large-format prismatic battery with an NMC cathode using extended volume-accelerating rate calorimetry (EV-ARC). The results of this study showed that the polyethylene (PE) separator began to breakdown at 123.7 \({}^{\circ}\)C and melted at 140 \({}^{\circ}\)C, though the ceramic coating did not fail until 250 \({}^{\circ}\)C, at which point the electrodes shot circuited. Of the four experiments completed in this study [83], the onset temperature ranged 85-105 \({}^{\circ}\)C and this is the point at which the SEI began decomposition.