Lead–acid batteries used in hybrid solar–wind power generation 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 working conditions, no battery model has achieved a good compromise between the complexity and precision. This paper presents a simple mathematical approach to simulate the lead–acid battery behaviors in stand alone hybrid solar–wind power generation systems. Several factors that affect the battery behaviors have been taken into account, such as the current rate, the charging efficiency, the self-discharge rate, as well as the battery capacity. Good agreements were found between the predicted results and the field measured data of a hybrid solar–wind project. At last, calculated from 1-year field data with the simulation model, the time-series battery state-of-charge (SOC) has been statistically analyzed considering the monthly and hourly variations as well as the probability distributions. The results have shown the battery working states in the real hybrid solar–wind power generation system.