Materials developed for battery cells must ensure safety. This study describes a method for predicting thermal runaway when a battery cell is heated. Also presented is a method for quantitatively evaluating cell material safety in safety testing without having to scale up the cell. With thermal runaway reactions being a complex series of elementary reactions, the differential isoconversional method based on the approach of Friedman was used to calculate thermal runaway. As a battery is a solid-liquid mixture with electrical energy, thermal runaway reaction behavior varies greatly depending on the cell’s heating rate. As such, a method for selecting kinetically consistent test conditions based on differential scanning calorimetry measurement results was added to enable calculation of thermal runaway behavior of cells under adiabatic conditions. The calculation results were verified with accelerating rate calorimetry, which can measure self-heating rates under adiabatic conditions, to confirm that the thermal runaway onset timing matched. Increasing the accuracy of heating rate calculations will require further research into how to treat changes in specific heat during decomposition reactions. Safety diagrams accounting for regulation and standard test conditions were created for a battery based on this calculation method. This method has revealed quantitative metrics for evaluating battery safety.