An approach to predict the remaining lifetime of lead-acid battery is described. The battery is used to provide back-up power to keep essential devices within wind turbine generators operational under emergency conditions. The approach is developed from a circuit model of the battery. From the results of laboratory tests, changes in the model parametric values due to ambient temperature variations, battery aging effects as well as discharge current level have been quantified. Through a developed computational method, the terminal voltage and state of charge of the battery at any other discharging conditions can then be calculated. The discharge profile allows one to predict the remaining lifetime of the battery. The predicted results appear to agree most satisfactorily with that obtained from laboratory measurements.