The heat management of lithium-ion cells under ceaseless high-current operation plays a pivotal role in preventing an internal short-circuit, which is generally caused by separator meltdown at high temperatures and rapid cell deterioration. However, the simple specification sheets provided by battery manufacturers do not provide explanations nor supporting data on why or how permitted C-rates for charging or discharging at different temperatures can be guaranteed. Thus, the long-term degradation behavior of commercial 18650 cylindrical lithium-ion cells (LiNi0.8Co0.15Al0.05O2/graphite, 2.85 Ah) is systematically and statistically studied to determine the key degradation factors. Specifically, the capacity and power retention as well as the temperature changes of each cylindrical cell are gathered during 1,000 cycles at three C-rates, 0.5C, 1C, and 2C, for both charging and discharging (i.e., nine cases in total). The analysis of variance method shows that the capacity degradation is mainly governed by the charging C-rate, while power degradation and cell temperature behavior are affected by both the charging and discharging C-rates. Also, a thermo-electrochemical model is used to predict how quickly the cells reached the critical temperature under catastrophic conditions, like an adiabatic condition. Thus, this systematic approach can be an indispensable to ensuring the reliability of lithium-ion cells.