Over one billion people around the world lack access to grid electricity. Over the last few years, solar panel prices have dropped dramatically leading to more affordable solar electric systems. A typical solar home system comprises one or more solar panels, a charge controller, inverter and a bank of lead acid batteries. However, the weak point in these systems is the battery bank which requires careful maintenance to last a reasonable length of time. An alternative approach is to consider a system in which the battery is integrated with the solar panel and charge controller into a single unit. This paper describes the design, performance simulation and testing of such a unit. The system consisted of a 12.8 V, 26.4 Ah lithium-iron-phosphate battery integrated with a 50 W solar panel and a charge controller. It was sized for 2.5 days of useful battery storage. The load consisted of only DC appliances that took about 10 hours to discharge the battery to a 60% state-of-charge (SOC). A DC electronic load was used to discharge the battery to 60% SOC, a process that was repeated three times. Each process was considered to represent one cycle of the lifetime of the battery strictly in the context of this study. Experimental IV data were collected on the solar panel under different solar insolation conditions and a single equation used to model this data. RC circuits were used to model the charge and discharge of the battery. Details of the modeling and experimental results are presented in this paper.
