A simplified one-dimensional thermal mathematical model with lumped parameters was used to simulate temperature profiles inside lithium-ion cells. The model makes use of heat-generation parameters established experimentally for the Sony (US18650) cell. The simulation results showed good agreement with temperature measurements at C/2, C/3, and C/6 discharge rates, while some deviation was noticed for the C/1 discharge rate. The model was used to simulate temperature profiles under different operating conditions and cooling rates for scaled-up cylindrical lithium-ion cells of 10 and 100 A h capacity. Results showed a strong effect of the cooling rate on cell temperature for all discharge rates. A significant temperature gradient inside the cell was found only at higher cooling rates, where the Blot number is expected to be > 0.1. At lower cooling rates, the cell behaves as a lumped system with uniform temperature. To establish the limit＇s of temperature allowable in scale-up by the simplified model, commercial lithium-ion cells at different open circuit potentials were tested inside an accelerated rate calorimeter (ARC) to determine the onset-of-thermal-runaway (OTR) temperatures. Sony (US18650) cells at 4.06, 3.0, and 2.8 V open circuit voltage (OCV) were tested and their measured OTR temperatures were found to be 104, 109, and 144 degrees C, respectively. A sharp drop in the OCV, indicating internal short circuit, was noticed at temperatures close to the melting point of the separator material for all open circuit voltages.