Thermal design and management are important for lithium-ion batteries (LIBs) to prevent thermal runaway under normal and abnormal conditions such as overcharge and short circuit. A sound understanding of the heat generation behaviors of LIBs is needed for their thermal design and management. Since battery characteristics such as capacity and power capability degrade with time and the number of cycles, one can infer that the amount of heat generated by LIBs may also be changed by this degradation. Calorimetry is an effective method of studying the heat generation mechanisms of LIBs. In this study, we apply calorimetry to characterize the heat generation behavior of LIBs during charging and discharging after degradation due to long-time storage. At low rates of charging and discharging, such as 0.1C, significant differences dependent on the degree of degradation are not observed. On the contrary, more degraded batteries exhibit greater heat generation related to overvoltage increase at high rates of charging and discharging, such as 1 C. The solution resistance increase is particularly striking in an LIB stored at 50 °C. The chief cause of this increase may be leakage of electrolyte solution, resulting in greater heat generation at high rates of charging and discharging.