This paper proposes an online lifetime estimation methodology for supercapacitors. The online technique uses a Lyapunov-based adaptation law to estimate online the supercapacitor's parameters. Unlike offline time- or frequency-domain characterization techniques that require discontinuation of the system's normal operation, the proposed approach is more suitable for real-time applications, such as electric/hybrid vehicles, as it provides online lifetime estimation. Furthermore, convergence and stability analysis is provided by Lyapunov's stability theory as opposed to many online estimators available in the literature. The effectiveness of the proposed strategy is validated through experiments along with comparison against two different methods.