Development and Validation of a Degradation Model for Lead-Acid Batteries Used in Off-Grid Solar Systems in Rural Nigeria

This study develops and validates a physics-based degradation model for valve-regulated lead-acid batteries used in off-grid solar home systems deployed in rural Nigeria, filling a critical evidence gap on battery lifetime prediction under the specific duty cycles and climatic conditions of the Nigerian rural context. VRLA batteries represent the largest single cost component in solar home systems, and premature battery failure, which is widespread in Nigerian rural deployments, significantly increases the effective cost of solar access for low-income households. Three sets of VRLA battery test samples representing capacity ratings of 50Ah, 100Ah, and 150Ah commonly used in Nigerian solar home systems were subjected to accelerated cycle life testing under duty cycles representative of three Nigerian deployment contexts: high solar resource daily cycling in Sokoto, variable resource intermittent cycling in Enugu, and deep cycle abuse with incomplete daily charging simulating typical user behaviour. The degradation model is based on the coupled Wöhler fatigue model for cycle life and the Arrhenius thermal ageing model for calendar life, with parameters identified from the accelerated test data. Electrochemical impedance spectroscopy was applied at monthly intervals to track degradation state. Model predictions were validated against independently collected field capacity measurements from 40 deployed solar home system batteries in Jigawa and Anambra States. The validated model achieves a prediction error of less than 9 percent on field battery remaining capacity. The model enables more accurate component replacement scheduling and economic lifetime cost calculation for rural solar programmes. Keywords: VRLA battery, degradation model, off-grid solar, rural Nigeria, lifetime prediction.