An accelerated rate calorimeter in combination with a battery cycler and a precision multimeter was used to measure the heat dissipation from, and heat accumulated in, commercially available lithium‐ion cells during cycling over a range of operating parameters within the limits recommended by the manufacturer. An integral energy balance was used to determine the total heat generated in the test cell during cycling. From the measurements during temperature transients, the heat capacity of the test cell was found to be relatively independent of temperature, ranging from 0.82 to 1.07 J g−1 K−1. This value agrees relatively well with separate measurements using an adiabatic calorimeter which yield slightly higher values. DC current interruption technique was used to determine the time‐dependent area‐specific impedance, 〈ASIt〉, of the cell which was well correlated with steeply increased heat dissipation rate at the end of discharge. The reversible (entropic) heat effect derived from an energy balance was found to be exothermic during discharge and endothermic during charge. Using two different methods, values were obtained for the entropy of reaction (ΔS) during discharge of the cell. The resulting values obtained with method II, depending on the discharge rate, varied from −41.19 to −80.98 J K−1 per g mole of Li and showed a weak dependence on temperature, in the 35 to 55°C range. The rate dependence of the ΔS values needs further examination in a future study. By extrapolating to zero rate, the reversible entropy of the faradaic reaction for this cell was found to be per g mole of Li within the temperature range. The entropic heat effect and the heat effect associated with nonfaradaic reactions are appreciable and should be included in thermal modeling.