Simulations of the battery oven test, as one of the safety regulations, have been conducted for large-scale lithium-ion batteries for EV applications. A three-dimensional model is developed and compared with a lumped model. The lumped model simulates the battery temperature well under normal discharge conditions, but becomes inaccurate when the thermal runaway occurs. The effect of battery scale on thermal runaway is investigated. It is found that the smaller batteries can dissipate heat more efficiently than the larger batteries, and the possibility of experiencing thermal runaway is lower. The radiative heat transfer is found to be a significant means of battery heat dissipation. The internal temperature distribution during the thermal runaway is plotted and investigated. Propagation of local hot spot and strong internal temperature gradient are observed. It is concluded that the large-scale batteries are vulnerable to the oven test, and other means of heat dissipation, such as heat sinks may be needed to pass the oven test.