Battery ageing is inevitable for chemistry of any kind and still occupies a central place in battery research and development. However, the operando monitoring of battery ageing under real working conditions remains largely unexplored. Herein, we demonstrate the feasibility of using fiber Bragg grating sensors to operando monitor the thermal (temperature and heat) evolutions of commercial 18650 sodium-ion cells during long-term cycling ageing. With the delicate heat deconvolution, the evolutions of entropy and overpotential heat rates upon the cycling ageing are decoded, while the ageing-driven changes in overpotential components are further analysed. Drawing also on thermodynamics, high-resolution enthalpy profiles are computed from operando heat and voltage profiles, enabling to track and unravel redox variations during the cycling ageing. Collectively, this work shows that fiber optic sensors are essential tools for operando deciphering cell ageing and offer tremendous opportunities for academia and industry alike.