In this paper we introduce a Simplified Single Particle Model (SSPM), which is obtained from the general mathematical formulation of Li-ion batteries. The model is validated by using different commercial graphite/LiFePO4 cells, and results show agreement with more complicated models and experimental data for low operating currents of less than 1C. A maximum relative error of less than 2% can be observed to estimate cell voltage in the plateau region of the charge/discharge curves. Therefore, the proposed model is suitable in the case of stand-alone renewable energy systems, where battery current is typically lower than C/10. By increasing the current, the SSPM deviates from more accurate models and experimental data. However, it is sufficiently precise and computationally efficient to be used in the simulation and optimization framework of off-grid renewable energy systems. Finally, the proposed model will be applied to two different case studies; one for an installed PV-battery and the other for a Wind-battery system which are located near Zaragoza, Spain, in order to predict battery state over time.