It is important for large-scale lithium-ion secondary cells to function in a safe and stable manner. Little information is available on the parameters determining the transitions between non-heating, self-heating, and thermal runaway processes of degraded lithium-ion secondary cells. Thermal characterization of the degraded cells is important to identify the impact of degradation on the safety limits of these cells. Accelerating rate calorimeter (ARC), operated in a heat-wait-search mode, is capable of characterizing the thermal behavior of lithium-ion cells. Here, the self-heating and relative heat generation rates of storage-degraded lithium-ion cells during thermal runaway are investigated. Twenty-five 18650-type LiCoO2-based secondary cells are degraded during storage at 80 °C with various states of charge (SOCs), then the thermal behavior of the cells was analyzed by carrying out ARC measurements. The correlations between the onset temperature of thermal runaway, self-heating rate, and heating rate of each cell are investigated. It was found that the self-heating rate correlates linearly with the onset temperature of thermal runaway, while the relative heat generation rate correlates with it exponentially. The cells charged to 100% SOC presented the lowest onset temperatures of thermal runaway.