Harmonic Ratio Analysis (HRA) is a nonlinear electrochemical diagnostic method for online condition monitoring of electrolyzers and fuel cell systems at cell and stack level. The method evaluates selected harmonic ratios derived from low-frequency multi-frequency modulation of the operating point and provides robust trend-based indicators of asymmetric transport-related and symmetric kinetic nonlinearities. This technical note describes the measurement principle, harmonic ratio definitions, multi-frequency evaluation strategy, quality chain including SNR gating and persistence criteria, and practical implementation aspects for laboratory testing, industrial stack monitoring, and controller integration. The method is designed as a complementary approach to electrochemical impedance spectroscopy (EIS) for field-robust nonlinear diagnostics.