A storage capacity management system includes an upper limit terminal voltage inducing part for inducing an upper limit terminal voltage, a lower limit terminal voltage inducing part for inducing a lower limit terminal voltage, an upper and lower limit voltage width calculating part for calculating an upper and lower limit voltage width, an intermediate voltage difference calculating part for calculating an intermediate voltage difference, an upper and lower limit voltage ratio calculating part for calculating an upper and lower limit voltage difference, an intermediate determination voltage ratio inducing part, a voltage ratio comparing part, and an intermediate storage capacity inducing part for inducing an intermediate storage capacity based on the upper limit storage capacity, the lower limit storage capacity and the intermediate determination voltage ratio when the results of the comparison by the voltage ratio comparing part satisfy a predetermined condition.