Thermal runaway hazards related to adiabatic runaway reactions in various 18650 Li-ion batteries were studied in an adiabatic calorimeter with vent sizing package 2 (VSP2). We selected two cathode types, LiCoO2 and Li(Ni1/3Co1/3Mn1/3)O2, and tested Li-ion batteries to determine the thermal runaway features. The charged 18650 Li-ion batteries were tested to evaluate the thermal hazard characteristics, such as the initial exothermic temperature (T 0), self-heating rate (dT/dt), pressure rise rate (dP/dt), pressure–temperature profiles, maximum temperature (T max) and pressure (P max), which are measured by VSP2 with a customized stainless steel test can. The thermal reaction behaviors of the Li-ion battery packs were shown to be an important safety concern for energy storage systems for power supply applications. The thermal abuse trials of the adiabatic calorimetry methodology used to classify the self-reactive ratings of the various cathodes for Li-ion batteries provided the safety design considerations.