An energy storage system includes a lead-acid battery, a battery management unit. The battery management unit defines, based on an open voltage of the lead-acid battery, a first amount of change in a capacity of the lead-acid battery from a reference state, correlating with the open voltage of the lead-acid battery and caused by a first deterioration factor and defines, based on the first amount of change in the capacity and an amount of change in overall internal resistance of the lead-acid battery from the reference state, a second amount of change in a capacity of the lead-acid battery, not correlating with the open voltage and caused by a second deterioration factor. The battery management unit defines, based on the first and second amounts of change in the capacity, at least one of a battery capacity of the lead-acid battery or an amount of change in the battery capacity from the reference state.