the life of a lead acid storage battery is extended by changing an over - frequency equalized charge interval performed on a lead acid storage battery , in accordance with a transition situation of a state of charge of the lead acid storage battery . the lead acid storage battery is also made to be advantageous in terms of cost by reducing the equalized charge with a low degree of urgency to reduce the power and cost for equalized charge , and by reducing the number of stops of a natural energy storage system . a lead acid storage battery and a lead acid storage battery system whose operational management can be easily performed are achieved by a method in which the future timing when the equalized charge is performed can be grasped by people operating the lead acid storage battery .