Cell aging and state-of-health (SOH) estimation is widely acknowledged as a challenge in state-of-the-art battery management systems deployed today. Towards addressing this issue, gas evolution monitoring from side reactions using embedded sensors was investigated as a parameter of interest for SOH. Li-ion battery cells with a Mn-rich chemistry were subjected to overcharge experiments. Two cells were repeatedly overcharged and the evolution of gaseous CO2 was measured using fiber optic colorimetric sensors, which were incorporated and sealed into the side pouch of the battery pouch cells. A ratiometric read-out principle has been employed for the optical measurements. Initial results indicate a non-reversible gas evolution inside the battery cells during overcharge, wherein the onset of gas evolution is delayed in time relative to the overcharge condition. An increase in the sensing signal can be observed over a time span of 40 – 50 minutes during each overcharge cycle. This investigation provides real-time information on the dynamics of gas evolution in Li-ion pouch cells during overcharge experiments and allows for an early detection of potentially hazardous cell states.