# 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 onset temperature for NCA (1) is considerably lower than NCA (2) and NCA (3), reported to be due to the difference in the unknown electrolyte used by different manufacturers, which leads to variations in the level of thermal stability of the SEI film. NCA (2), NCA (3), and LFP cells showed only minor or zero temperature variation between onset temperatures for cells at 50 and 100% SOC. NCA (1) cells at 50 and 100% SOC recorded an onset temperature variation of 24.6 \({}^{\circ}\)C. No notable onset temperature difference between cells at 0 and 50% SOC was recorded for NCA (1), whilst considerable temperature variations were recorded for all other cells at 0 and 50% SOC. The exothermic reaction time (first exothermic reaction to the completion of TR) was also recorded. The results indicated that the time was shorter for the fully charged cells. The NCA (2) cell had a very short exothermic time of 103 s compared to 996 s for the NCA (1) cell with both at 100% SOC; no indication of why the disparity in the exothermic time between the 18650 cells was provided. The LFP cell at 0% SOC had the greatest exothermic time of 3221 s. \begin{table}
\begin{tabular}{c|c c|c c} \hline \hline
**Condition (SOC)** & **Onset Temperature** & **Total Heat** & **Normalized HRR\({}_{\text{max}}\)** \\ **(Assumed 150 W Heating Power** & **to TR (\({}^{\circ}\)C)** & **Release (kW)** & **HRR\({}_{\text{max}}\) (kW)** & **(kW m\({}^{-}\)2)** \\ \hline
0\% & 278.0 \(\pm\) 6.0 \(\pm\) 19.2 & NP & NP & NP \\
25\% & 280.0 \(\pm\) 19.2 & NP & NP & NP \\
50\% & 257.6 \(\pm\) 12.6 & NP & NP & NP \\
70\% & NP & 30.29 \(\pm\) 0.14 & 2.17 \(\pm\) 0.35 & 517.9 \(\pm\) 83.9 \\
75\% & 256.7 \(\pm\) 8.5 & NP & NP & NP \\
80\% & NP & 35.32 \(\pm\) 20.94 & 2.47 \(\pm\) 1.58 & 590.1 \(\pm\) 378.7 \\
90\% & NP & 47.86 \(\pm\) 2.29 & 3.14 \(\pm\) 0.36 & 750.8 \(\pm\) 85.6 \\
100\% & 235.6 \(\pm\) 7.3 & 52.60 \(\pm\) 8.75 \(\pm\) 3.75 \(\pm\) 0.86 \(\blacktriangledown\) & 895.9 \(\pm\) 205.1 \(\blacktriangledown\) \\
100\%–200 W & 249.6 \(\pm\) 2.5 & NP & NP & NP \\ \hline \hline \end{tabular}  
* Arrows represent upward or downward trend. Normalized heat release rate (HRR) is the HRR divided by the battery surface area. NP indicates that the data were not provided in the results.  
\end{table} 
Table 5: Thermal runaway (TR) and fire behavior of cylindrical 18650 LIBs at varying state of charge (SOC) [65]. SOC was also shown to affect the degradation of LFP/graphite batteries in accelerated storage testing completed by [PERSON] [89]. The study showed that batteries at 100% SOC and at elevated temperatures (55 \({}^{\circ}\)C) experienced the greatest levels of capacity fade. This decay of the battery capacity is due in part to the decomposition of the electrolyte at higher temperatures and the side reactions involving the graphite anode at a higher SOC. The findings [89] show that the peak HRR for higher SOCs were recorded to be marginally lower than for some of the LIBs at lower SOC, particularly when exposed to higher levels of heat flux. It was hypothesized that this may be attributed to three possible factors:  
* Batteries at higher SOC have larger amounts of stored energy leading to more explosive and faster reactions thereby limiting the amount of oxygen consumption, and subsequently incomplete combustion of the LIB;
* Onset of TR occurs earlier for batteries at a higher SOC, resulting in insufficient time to allow chemical reactions to release thermal energy;
* The oxygen (or part of) that is released from the internal electrochemical reactions is consumed in the combustion process, leading to HRR prediction errors based on the oxygen consumption method. A method to assess the thermal hazard was investigated whereby a ratio of the peak HRR and the time to ignition (TTI) is evaluated. The time to ignition has been defined as the duration of time between sample exposure to the incident heat flux and a visible continuous flame.