Charge transport and chemical reactions during charging and discharging of a battery produce heat that determines temperature behaviors. The elevated temperature causes undesired side reactions that accelerate degradation and potentially result in catastrophic operating conditions like a thermal runaway. The heat generated in an operating battery is generally approximated by the sum of the reversible and irreversible heat. The reversible heat is produced by the change of entropy. The irreversible heat is approximated by either the overpotential heating or Ohmic and reaction heating. Most studies have compared the surface temperature with tuned convection coefficients, but not investigated the heat generation directly. A study conveyed shows that two other heat source terms, enthalpy heating and heat of mixing, should be included to accurately and completely describe the heat generation. The first one is caused by diffusion of lithium ions in the solid phase and the second one by change of the gradient of ion concentrations. An electrochemical thermal model including these additional terms is experimentally validated against calorimetric measurements on a 15.7 Ah LiMn2O4/carbon pouch type power cell using a specially designed calorimeter.