The optimal energy and power performance of lithium ion batteries can be attained if a suitable thermal battery management system is used. Furthermore, to ensure the safe operation, a well functioning temperature control method, is needed. To achieve these goals, the simulation software COMSOL Multiphysics used to construct a three-dimensional electrochemical/thermal model of a monomer lithium ion battery. The simulation makes it possible to study the thermal characteristics at different ambient temperatures and different discharge rates. The obtained main outcomes are 1) The temperature of the lithium ion battery increases with increasing discharge ratio, and a sudden temperature increase is observed for higher discharge ratios, 2) For constant discharge rates, the temperature increase of the battery occurs mainly in the positive and negative electrode region, while lower temperatures are observed in the center and lower-edge region. A comparison between simulation and obtained date, indicates that the three-dimensional electrochemical/thermal model of the monomer lithium ion battery described the lithium ion battery well in terms of both heat generation and heat transfer.