# 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 internal battery temperature increased exponentially at the onset of TR, calculated to be 259 \({}^{\circ}\)C up to 750 \({}^{\circ}\)C, at which point the temperature increase slowed before reaching a maximum internal temperature of 853.5 \({}^{\circ}\)C.  
[PERSON] [65] completed a study on TR and the fire behavior of an NMC cathode and a natural graphite anode 18650 LIBs at various SOC using a modified cone calorimeter. The purpose of the study was to investigate TR, fire behavior, temperature and voltage variations, gas generation, and the HRR. A radiative heat source was used to heat the batteries and trigger TR. Traditionally, the radiative heat source is located a predetermined distance away from the battery and exposes one side of the battery directly to the heat source and one side indirectly. However, this study utilized a modified cone calorimeter whereby the 18650 battery was inserted into a cylindrical heater with the intention to provide uniform heating across the entire battery. The application of the conventional cone calorimeter method in determining the exact onset temperature to TR is questionable as the internal temperature alters across the battery due to the direct and indirect exposure to the \begin{table}
\begin{tabular}{c|c c} \hline \hline \multicolumn{1}{c}{**State of Charge**} & \multicolumn{1}{c}{**Onset Time (s)**} & \multicolumn{1}{c}{**End Time (s)**} \\ \hline \multicolumn{3}{c}{Safety Venting} \\ \hline
0\% & 1160 \(\pm\) 27 & 3103 \(\pm\) 58 \\
25\% & 1166 \(\pm\) 36 & 1685 \(\pm\) 16 \\
50\% & 1105 \(\pm\) 39 & 1697 \(\pm\) 31 \\
100\% & 899 \(\pm\) 39 & 1366 \(\pm\) 35 \\ \hline \multicolumn{3}{c}{Thermal Runaway} \\ \hline
0\% & _NP_ & _NP_ \\
25\% & 1685 \(\pm\) 16 & 2221 \(\pm\) 27 \\
50\% & 1697 \(\pm\) 31 & 2019 \(\pm\) 36 \\
100\% & 1366 \(\pm\) 35 & 1556 \(\pm\) 46 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Timing of safety venting for NMC LIB [80]. radiative heat source. The 18650-type batteries used in this study [65] were NMC cathode and a natural graphite anode. The electrolyte was a solution of LiPF6 and a mixture of EC, EMC, and DMC. The batteries were equipped with safety vent ports at the positive end of the battery. Tests were conducted under nine test conditions with at least three repetition times, eight of which utilized a 150 W heat source at 0, 25, 50, 70, 75, 80, 90, and 100% SOC. The ninth test condition used a 200 W heat source at 100% SOC. The test results of onset temperature to TR and HRR are shown in Table 5. Considering other studies reviewed [66, 80, 84, 85, 86, 87, 88], it can be concluded that there is a direct correlation between the onset temperature to TR, HRR, and the SOC of the LIBs. [PERSON] [84] conducted abusive testing on LIBs to determine the influence of SOC on thermal reactions. Four commercially available cylindrical cells based on LFP with rated capacities of 2.68, 3, and 4 Ah, and lithium nickel cobalt aluminum oxide (NCA) cathode materials were investigated. The cells were cycled to three states of charge: 0, 50, and 100% and thermally analyzed using an accelerating rate calorimeter (ARC). As part of the ARC testing, the cylindrical cells were heated to 50 \({}^{\circ}\)C and then followed by a 15 min rest period with the intent to stabilize the cell temperature. During this period, the calorimeter measured the self-heating process and if no exothermic reaction took place within the cell greater than the sensitivity threshold of 0.02 \({}^{\circ}\)C min\({}^{-1}\), then the sequence was repeated, this time with an increase of 5 \({}^{\circ}\)C. Nail penetration and crush tests were performed on the cylindrical cells at 100% SOC. Onset temperature, maximum temperature, and exothermic time (103-3221 s) for three SOC's of four types of cells are presented in Table 6.  
The results show that cells at 100% SOC have the lowest onset to TR temperatures.