The behaviour of lead-acid batteries during dynamic operation on a long-period basis is analyzed in this paper. An algorithm which takes into account all possible battery conditions during real operation has been developed and is used for the battery state of voltage, SOV, predictions. This battery algorithm is general and therefore can be used for other types of accumulators, provided that the model parameters are known. For model validation, a typical battery was incorporated for energy storage in a stand-alone, renewable power supply system and the experimental results were compared with the algorithm predictions. The results show that very good agreement between measured and simulated battery SOVs has been achieved for monthly periods of continuous system operation.