To provide a capacity estimating device for secondary battery for estimating a total capacity of the secondary battery.

The capacity estimating device for secondary battery includes: a charging control means for supplying a charging current to the secondary battery; a current-superimposing means for detection for superposing a detection current of a prescribed pattern to a charging current supplied to the secondary battery from the charging control means; a detection means for detecting the current and the terminal voltage of the secondary battery, respectively; a charging ratio estimation means for using an adaptive digital filter from measured values of the current and the terminal voltage detected with the detection means for estimating the open-circuit voltage of the secondary battery and estimating the charging ratio of the secondary battery from the estimated open-circuit voltage, on the basis of the relation of a previous open-circuit voltage and the charging ratio; and a capacity estimation means for estimating the total capacity of the secondary battery on the basis of the charging ratio estimated with the charging ratio estimation means. The charging ratio estimation means uses the measured values of the current and the terminal voltage, when the detection current is superposed with the current superimposing means for detection to estimate the charging ratio.

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