A general energy balance has been developed for insertion battery systems by using enthalpy potentials. This leads to a new calculation method for the heat-generation rater The same result is also derived from an alternative model based on local heat generation in an electrochemical cell. A new concept, the effective open-circuit potential of an insertion battery, was proposed to characterize the open-circuit state of the battery during galvanostatic discharge. Simulation results for are presented heat generation in a lithium cell under galvanostatic discharge. The analysis of these results focuses on effects of the shape of the open-circuit potential and ohmic losses in the electrolyte in the porous cathode. It is shown that a single reaction may look like two reactions due to the presence of two plateaus in the open-circuit potential.