Prognostics and health management for fuel cells in real-world applications are important for optimization of lifetime and operation. A parameter that has been shown to indicate the state of ageing is the low-frequency intercept with the real axis of the Nyquist diagram in a fuel cell's impedance spectrum. This article presents a method for direct identification of this parameter, without the need to carry out a full electrochemical impedance spectroscopy, and with an algorithm that can be realized within a typical fuel-cell control system. The proposed method is based on relay excitation feedback, a proven solution from controller tuning. (C) 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.