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A long-term in-situ measurement method for evolved gases in commercial 18650 cylindrical lithium ion batteries (LIBs) is proposed using Raman spectroscopy. Hydrogen, methane, carbon dioxide and carbon monoxide were the main gases detected from cells at 4.2-4.8 V for 1800 h. Gas evolution rates were determined by the aging time and the staying potential, resulting in a nonlinear partial-pressure-dependence as a function of the aging time. Initially, the evolution of carbon dioxide and carbon monoxide was significant. After potential-dependent onset times, hydrogen and methane generation increased suddenly. At low potential ranges of 4.2-4.4 V, mostly hydrogen gas was generated, whereas at high potential ranges (>4.6 V), methane becames dominant. Even at 4.4 V, importantly, the absolute accumulative H2 gas pressure was >3 atm, raising the requirement to monitor such gas for better safety even under nominal operating conditions. Moreover, cumulative partial pressures of the detected gases exceeded the range 5-10 atm, which was associated with the staying potential. An evolution mechanism through which the gas is converted from hydrogen to methane is proposed and discussed. The electrochemical analysis of the aged LIBs showed that the capacity fade was accelerated by the increase in the staying potential while the resistances remained similar. [ABSTRACT FROM AUTHOR]