# A Review of Experimental and Numerical Studies of Lithium Ion Battery Fires
## 4 Thermal Runaway
### Thermal Runaway in Multiple Cells  
Lithium-ion cells are available in button, cylindrical, pouch and prismatic shapes. In order to deliver the required power and energy from a battery system, the cells are configured in parallel and/or series to increase the capacity and/or the voltage, respectively. A battery system usually consists of a number of battery packs, which are made of multiple battery modules, each containing a number of cells with series and/or parallel configuration. For a battery system, in addition to the thermal hazards that are typically observed in a single battery including high-temperature, safety valve burst, venting, combustion, explosion, and toxic gases during TR, the heat propagation to surrounding batteries can be \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**LIB Chemistry and** **Format** & **Rated Capacity** **(Ah)** & **SOC (\%)** & **Onset Temperature Maximum Temperature Exothermic Time** **(\({}^{\circ}\)C)** & **(\({}^{\circ}\)C)** & **(\({}^{\circ}\)C)** & **(\({}^{\circ}\)C)** \\ \hline \multirow{3}{*}{NCA (1)–18650} & \multirow{3}{*}{3.00} & 100 & 51 & & 838 & 996 & \\  & & 50 & 75.6 & & 704 & 1028 & \\  & & 0 & 75.5 & \({}^{\circ}\) & 320 & 2093 & \\ \hline \multirow{3}{*}{NCA (2)–18650} & \multirow{3}{*}{2.68} & 100 & 100 & & 704 & 103 & \\  & & 50 & 100 & & 739 & 1311 & \\  & & 0 & 120.7 & \({}^{\blacktriangledown}\) & 350 & 1810 & \\ \hline \multirow{3}{*}{NCA (3)–20700} & \multirow{3}{*}{2.68} & 100 & 70 & & 679 & 1297 & \\  & & 50 & 70.5 & & 794 & 1508 & \\  & & 0 & 125.3 & \({}^{\blacktriangledown}\) & 350 & 1629 & \\ \hline \multirow{3}{*}{LFP–3 Ah 26650} & \multirow{3}{*}{4.00} & 100 & 100.2 & & 455 & 1926 & \\  & & 50 & 106 & & 349 & 2859 & \\ \cline{1-1}  & & 0 & 115 & \({}^{\blacktriangledown}\) & 349 & 3221 & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: State of charge and its effect on thermal runaway characteristics revealed using nail penetration and crush tests [84]. also exhibited. Within a battery pack, the propagation is a serious thermal hazard which may result into catastrophic consequences; thus countermeasures should be taken into account to cease the failure propagation [10; 63; 69; 90; 91; 92; 93; 94; 95; 96; 97; 98]. In a study conducted by [PERSON] et al. [69], the fire behaviors of cylindrical 18650 batteries and battery packs under discharge conditions were investigated. Four battery packs (NMC cathode and graphite anode) were used: a single cell, two cells, three cells, and four cells, all of which were positioned and connected electrically in parallel. In total, eight tests were conducted at 100% SOC, four with the discharging treatment and four without. The experiments were performed in a cone chamber with a 2 kW electric heater as the heat source. It was reported that the burning process of the LIBs exhibited four distinct stages: the heating stage, the cracking of the safety vent, battery ignition, and TR. The data of time and temperature to ignition and TR are presented in Table 7. The results indicate that the LIBs under discharge have earlier ignition, earlier ejection of material, and earlier stable combustion than the LIBs not subjected to discharge, and the ejection and combustion is noticeably more intense than the latter. Furthermore, it was found that with increased cell numbers these fire behaviors were amplified. However, temperature to TR did not change significantly. [PERSON] et al. [90] also conducted a comprehensive set of TR propagation tests in cylindrical 18650 packs with different sizes, 2 \(\times\) 2, 3 \(\times\) 3, and 4 \(\times\) 4. It was reported that the TR, initially induced by external heating of a corner battery, propagated with a domino effect within the entire battery pack. The effects of other parameters such as SOC, first TR location, spacing, and the number of heaters on TR propagation were also investigated.