A numerical model denoted the “Tri-bred model” is developed to accurately reproduce Li-ion battery nail penetration tests. The model considers the movement of the nail as well as the thermal decomposition reaction. Moreover, the “combustion volume” was defined to quantitatively evaluate the degree of combustion risk in a unique manner. The validity of the model was verified by comparison with experimental results. The model suitably described the experimental phenomena; the behavior of the “combustion volume” indicated a similar tendency to the gas emission degree during the experiment. To investigate the relationship between the experimental conditions and the degree of risk, a parametric study was performed and the results of “combustion degree” were compared. As a result, it was found that the nail speed was more strongly correlated with combustion risk than penetration position.