Thermal design and management are very important for lithium-ion batteries (LIBs) to prevent thermal runaway under normal and abnormal conditions such as overcharge and short circuit. In this study, in-situ thermal analysis was performed on LiNi0.88Co0.08Mn0.04O2 /Li half-cell under isothermal condition. We have developed a novel in-situ DSC module for thermal analysis that occurs during the charging and discharging of the LIBs, and it has been confirmed to operate very successfully. In addition, the total heat could be separated into the three kinds of ohmic, reversible, and irreversible heat using electro-thermal modeling. The combination of experiments and calculations made it possible to study detail thermal changes depending on charging state of electrode in this work. It was found that the thermal behavior pattern is affected by reversible and irreversible heat source, while the total amount of heat generation highly depends on the applied C-rate. It is suggested that the electro-thermal properties of the active material, such as the thermal variation of the equilibrium potential (dU/dT), would be characterized using the present method. The developed thermal analysis can be used as a very powerful tool for current LIBs research that require thermal stability and can be applied as a guideline for material selection in the future.