A microcomputer monitors, on the basis of outputs from a current sensor and voltage sensor, a voltage drop due to an internal resistance when the drop in the terminal voltage due to polarization occurring during the discharge of a battery is saturated, and a dischargeable capacity of the battery corresponding to the value resulting when the saturated voltage drop of the terminal voltage is subtracted from the chargeable capacity of the battery. Such a configuration provide a battery status monitoring device capable of knowing the battery status correctly, a saturation polarization detecting method which is useful to know the charging state of the battery and a dischargeable capacity detecting method.