Comprehensive Assessment of Energy Balance and Battery Degradation in Autonomous Power Systems with Renewable Sources

Abstract

This study presents a comprehensive experimental and analytical investigation of energy flows in autonomous power supply systems that integrate renewable energy sources and advanced battery storage technologies. The research evaluated the performance and degradation behaviour of lead-acid AGM and lithium iron phosphate (LFP) batteries under varying discharge currents, discharge depths, and temperature conditions. The experiments showed that Lifepo₄ batteries exhibited superior stability, retaining up to 84% of their nominal capacity at 0 °C and maintaining higher efficiency than AGM batteries, which dropped to 65% under the same conditions. Dynamic load simulations revealed significant increases in internal resistance and temperature, particularly in AGM batteries during high-current discharge cycles. The developed mathematical model captured the combined effects of current, temperature, and discharge depth, predicting capacity degradation with an accuracy of ±3.5%. The integration of renewable generation and battery storage enabled daily energy generation of 8.2- 13.8 kWh, with an average conversion efficiency of 85%. These results highlight the advantages of Lifepo₄ batteries for autonomous systems that require reliable performance across diverse operating conditions. We propose and experimentally validate a lightweight correction term that jointly accounts for discharge current, temperature, and depth of discharge within a single degradation model, yielding a prediction error of ±3.5 % across chemistries and operating regimes. We report a rigorously controlled dataset from 12 batteries (6 AGM, 6 LiFePO₄) tested under dynamic duty cycles and sub-zero conditions using high-precision instrumentation. We present an integrated PV–wind–battery testbed with a measured daily energy balance of 8.2–13.8 kWh and a conversion efficiency of up to 91%, providing actionable guidance for sizing and dispatch in autonomous off-grid systems.