The heat generation of a 20Ah lithium iron phosphate pouch battery is characterized in this paper through the conduction of isothermal calorimeter measurements. The influence of temperature and current on battery heat generation is examined by including different operating conditions to the testing matrix, and the influence of the SOC on the battery heat rates is also studied throughout stepped current pulse procedures. Besides, the influence of reversible and irreversible heat contributions is also examined by analyzing the obtained output data. The presented examination was carried out as part of the design process of the battery system for EVs within the context of the JOSPEL project