PURPOSE: To provide a method and its device by which the discharge characteristics of a battery can be obtained with a simple construction with ease regardless of the lapse of time, and remaining capacity can simply be estimated.

CONSTITUTION: Voltage, current and temperature from a temperature sensor 22, a voltage sensor 24, and a current sensor 28 are read by a discharge characteristics computing means 36. Approximate discharge characteristics at that time are obtained from the curved surface function of the discharge characteristics on the axes of the voltage, the temperature, and a discharge time using the formula V=alog(H0-H)+bH+C of a memory 38. A discharge time up to present is obtained based on the approximate discharge characteristics by a discharge time computing means 40, and remaining capacity on the approximate discharge characteristics is obtained by a remaining capacity computing means 42.