The lead–acid batteries used in hybrid solar-wind systems operate under very specific conditions, and it is often very difficult to predict when the energy will be extracted from or supplied to the battery owing to the highly variable meteorological conditions. To calculate the battery state-of-charge (SOC) variations form the field measured data (battery current, battery voltage etc.), one simple battery simulation model has been given, which has taken into account several factors that affect the battery behaviors. Good agreements were found between the simulation results and the experimental data. Then one year time-series battery state-of-charge (SOC) calculated from the field data of a pilot project have been statistically analyzed, the monthly and hourly variations as well as the probability distributions of the battery SOC have been investigated to show the battery working states in the real hybrid solar-wind system.