Thermodynamics experiment and study were carried out for the lithium-ion (Li-ion) batteries that are expected as the power sources for electric and hybrid vehicles. It is confirmed that the heat coming in and going out depend on charging and discharging, respectively. And the thermal generation factors will be decomposed to three elements: reaction heat value Q(r), polarization heat value Q(p), and Joule heat value Q(J). Furthermore, the contribution degree of each factor was able to be expressed quantitatively by dividing these thermal generation factors for charging and discharging. The accuracy of thermodynamics logic was verified since the thermodynamics calculation coincided with the experimental data of thermal generation for the practical Li-ion batteries for electric vehicles (EV). It is possible to utilize the calculation and simulation for the development of Li-ion battery and the establishment of thermal control technology not only for EV but also for hybrid electric vehicles (HEV).