A two-dimensional mathematical model is proposed to study the thermal behavior of a long lithium-ion battery. Bernardi's electrochemical heat generation theory is adopted and a finite element method is used to discrete the coupled electrochemical thermal equations. The potential and current density distribution in the electrodes of the batteries are presented as function of depth of discharge. Modelling results shows that the battery temperature increases as increase of discharging time. The temperature distribution in the battery electrodes are calculated and compared to experiments. It is found that the simulation has good agreement with the experimental studies.