Development of a Predictive Physics-Based Degradation Model for Lead-Acid Battery Performance Under Nigerian Tropical Operating Conditions

Background: Lead-acid batteries power the majority of Nigeria's off-grid solar energy storage systems, yet premature degradation in tropical conditions substantially reduces system lifetime and raises the cost of off-grid electrification. Physics-based predictive degradation models tailored to Nigerian conditions are absent. Aim: This study developed and validated a physics-based predictive degradation model for lead-acid battery performance under Nigerian tropical operating conditions. Methods: A coupled electrochemical, thermal, and mechanical degradation model was formulated integrating active material dissolution, grid corrosion kinetics, electrolyte stratification, and plate sulfation mechanisms. Model parameters were identified from galvanostatic experiments conducted at temperatures from 25 to 55 degrees Celsius on 60 commercial VRLA batteries. Long-term validation used 36-month field data from 45 off-grid solar installations in five Nigerian states spanning all geopolitical zones. Results: The model predicted battery capacity at 36 months within 6.8% mean absolute error. Operating temperature was the dominant degradation driver, with each 10 degrees Celsius increase above 25 degrees Celsius reducing cycle life by 31%. Partial state of charge operation contributed 22% of capacity loss in improperly designed charge controllers. Conclusion: The physics-based model provides actionable guidance for extending lead-acid battery life in Nigerian solar systems through temperature management and charging protocol optimisation. Keywords: lead-acid battery, degradation model, tropical conditions, off-grid solar, Nigeria.