At discharge, a battery releases the stored energy of the chemical reaction partially by passing electric current throughout the external circuit, and partially dissipating it in the form of heat. The amount of heat depends on the discharging current I. Knowing its total amount as well as the heat flow is important for the safety of batteries, especially lithium-ion ones. In battery engineering practice, it is often suggested that the heat flow P follows the Joule-Lenz-Ohm law in the form P = I²R (R is the battery’s internal resistance). Both total heat and heat flow can be determined with relative ease by applying general ideas of thermodynamics when the battery operates under projected use conditions by comparing the dependencies of the open circuit voltage and voltage profile of the discharging battery upon the state of charge or discharge. Although the theoretical background is universal, being applicable to any kind of battery, our paper presents the results of examining three lithium-ion batteries of extensively used chemistries: lithium iron phosphate-graphite, lithium cobalt oxide-graphite, and lithium nickel cobalt aluminum oxide-graphite. Particular attention is given to the effect of the entropy of the reactions on batteries’ thermal behavior.