A battery system comprising: a battery; a current measurement unit configured to measure a current charged to or discharged from the battery; a voltage measurement unit configured to measure an output voltage of the battery; a function creation unit configured to create a function representing a depth-of-discharge characteristic of the battery on the basis of the measured current and output voltage, and further including a current variation measurement unit configured to calculate a depth of discharge of the battery on the basis of the current charged to or discharged from the battery and the output voltage of the battery; a data accumulation unit configured to accumulate data created by associating the output voltage of the battery and the depth of discharge corresponding to the output voltage of the battery; and a formula creation unit configured to create the function representing the depth-of-discharge characteristic of the battery on the basis of the accumulated data, and further including a function supplementing unit configured to supplement the function representing the depth-of-discharge characteristic in a vicinity of a discharge endpoint where the data is not accumulated, by adding a part of a predetermined depth-of-discharge characteristic function in the vicinity of the discharge end point to the function created from the accumulated data; and a remaining capacity calculation unit configured to calculate a remaining capacity corresponding to the output voltage of the battery using the created function. a battery; a current measurement unit configured to measure a current charged to or discharged from the battery; a voltage measurement unit configured to measure an output voltage of the battery; a function creation unit configured to create a function representing a depth-of-discharge characteristic of the battery on the basis of the measured current and output voltage, and further including a current variation measurement unit configured to calculate a depth of discharge of the battery on the basis of the current charged to or discharged from the battery and the output voltage of the battery; a data accumulation unit configured to accumulate data created by associating the output voltage of the battery and the depth of discharge corresponding to the output voltage of the battery; and a formula creation unit configured to create the function representing the depth-of-discharge characteristic of the battery on the basis of the accumulated data, and further including a function supplementing unit configured to supplement the function representing the depth-of-discharge characteristic in a vicinity of a discharge endpoint where the data is not accumulated, by adding a part of a predetermined depth-of-discharge characteristic function in the vicinity of the discharge end point to the function created from the accumulated data; and a data accumulation unit configured to accumulate data created by associating the output voltage of the battery and the depth of discharge corresponding to the output voltage of the battery; and a formula creation unit configured to create the function representing the depth-of-discharge characteristic of the battery on the basis of the accumulated data, and further including a function supplementing unit configured to supplement the function representing the depth-of-discharge characteristic in a vicinity of a discharge endpoint where the data is not accumulated, by adding a part of a predetermined depth-of-discharge characteristic function in the vicinity of the discharge end point to the function created from the accumulated data; and a remaining capacity calculation unit configured to calculate a remaining capacity corresponding to the output voltage of the battery using the created function.