Renewable energies are in constant growth and evolution, being a clean way to provide the energy required for the sustainable development of human society. In this context, energy storage systems are a key factor in the integration of renewable generation, because through them, the flexibility of the power system can be increased. Lead-acid batteries have been extensively used to provide electricity in isolated and rural locations, and could be integrated to the smart grid in order to improve its performance. However, this is a complex element due to its working principle, specifically during charging periods. In this paper, a general purpose model is formulated from a probabilistic point-of-view in order to determine the range of possible values of state-of-charge due to the uncertainty and to estimate the battery efficiency. A case study is analyzed and the results are compared with Monte Carlo Simulation approach in order to evaluate the proposed model.