It is known that temperature variations inside a battery may greatly affect its performance, life, and reliability. In an effort to gain a better understanding of the heat generation in Lithium ion batteries, a simple heat generation models were constructed in order to predict the thermal behaviour of a battery pack. The Lithium ion battery presents in this paper is Lithium Iron Phosphate (LiFePO 4 ). The results show that the model can be viewed as an acceptable approximation for the variation of the battery pack temperature at a continuous discharge current from data provided by the manufacturer and literature.