In this study, the heat generation and dissipation terms are classified for both irreversible and reversible heat. The irreversible heat is formulated according to the Joule-heating model with internal resistance in order to circumvent the calculation of electrochemical reactions that require significant computational loads. The internal resistance in the Joule-heating model is determined via electrochemical impedance spectroscopy (EIS). The EIS tests are conducted with state-of-charge (SOC) increments of 5%. To capture rapid changes in the internal resistance, the EIS tests are conducted with smaller intervals in the 0–10% and 90–100% SOC regions. Additionally, an analytical formulation of the resistance is derived from the electrochemical kinetic equations to explain the rapid changes. Analysis reveals that the rapid changes are due to the Li intercalation phenomenon in the electrodes. Thermal analyses are conducted according to the proposed model for Li-ion single cells, and the simulation results exhibits good agreement with the experimental data. Finally, a thermal analysis for a full-scale battery tray comprising 280 cylindrical cells is performed, and the applicability of the proposed model is confirmed by comparing the simulation results with the experimental findings.