A thermal model to predict the heat generation during the charge and discharge of a battery pack is an essential tool to manage the thermal behavior, performance and life of the batteries. In this work, a battery cell (LiFePO4) is modeled on COMSOL Multiphysics with the Batteries and Fuel Cells module. Once its heat generation known, the corresponding heat source term is used to study the temperature reached in a battery pack made of multiple cells. The thickness of the electrodes, the weight, the area, and the size of the particles were measured and implemented in the model. A 5C discharge (Ref. 2.) was then simulated. The temperature elevation (about 14°C for a single battery cell) was compared to an experiment to validate the result (Ref. 2.). The heat generation was then implemented for the simulation of the battery pack and the temperature elevation increased to 19.6°C. The models were set up with the general parameters to end up with an error of about 9% on the temperature elevation in the pack. This is a good approximation to do preliminary design in engineering.