1. An apparatus for estimating an OCV (Open Circuit Voltage) of a battery, comprising: a voltage sensing unit for measuring an output voltage of a battery; a temperature sensing unit for measuring a temperature of the battery; a data storing unit for periodically receiving the battery output voltage and the temperature data from the voltage sensing unit and the temperature sensing unit and then storing the battery output voltage and the temperature data in a memory; an OCV variation estimating unit for calculating an OCV variation from a varying pattern of battery output voltages measured in the past and at the present, stored in the memory, by applying a mathematical model that defines a correlation between the varying pattern of battery output voltages and the OCV variation, and then estimating an OCV variation at a current stage by reflecting a correction factor corresponding to the battery temperature on the calculated OCV variation; and an OCV estimating unit for estimating an OCV of the battery at the current stage by reflecting the estimated OCV variation on the OCV estimated at a last stage.