As a typical heat source in the operation of lithium-ion (Li-ion) battery, reversible heat (RH) makes a considerable contribution to the temperature variation of the battery, and the determination of this contribution is of great significance, especially for mission-critical applications. In previous research works, the determination of including or excluding the RH was typically made based on the rate of discharge. This study, however, finds that the importance of RH could be more meaningfully judged based on its contribution to the overall thermal response of Li-ion battery at various conditions. An improved electrochemical–thermal coupling model is developed to incorporate the RH. This model is applied to simulate the operation of batteries with different physical properties (e.g., electrode thickness and active material particle size) under different discharge rates. The simulation results are compared in terms of RH generation rates, battery temperature, as well as several other associated characteristics. It is found that the significance of RH to the overall heat generation varies when different intrinsic and extrinsic parameters are considered. This significance becomes more appreciable for batteries with thinner electrodes and/or finer active material particles. A correction diagram is then proposed to modify the results for thermal analyses without including RH.