NOVELTY - The apparatus i.e. battery electronic control unit (20), has a state-of-charge estimating unit calculates a present state of charge based on a peak battery voltage drop and a peak discharge current. The voltage drop represents a difference between a battery voltage measured before initiating power supply and a lowest battery voltage measured. The estimating unit sets an initial value based on the estimated present state of the charge and estimates a state of charge of a secondary battery (30) e.g. lithium-ion battery, based on the initial value.    USE - Battery control apparatus i.e. battery electronic control unit, for use in an electric vehicle (claimed) i.e. hybrid electric vehicle for receiving operational information e.g. battery voltage and battery temperature, from a secondary battery, estimating an state-of-charge (SOC) of the secondary battery, and transmitting an estimated SOC together with other battery information to a hybrid electronic control unit. Can also be used for an electric vehicle which includes a motor generator as a driving power source.    ADVANTAGE - The apparatus speedily and accurately estimate a state-of-charge (SOC) of a secondary battery immediately after a charge/discharge operation resumes. The apparatus prevents the state-of-charge estimating unit from estimating the present state of charge and prohibits initiating the power supply. The apparatus prevents the secondary battery from over discharging even if an estimated SOC is inaccurate due to measurement errors in a detection of a discharge current and a battery voltage.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a computer-readable medium having a set of instructions for performing a processing for estimating state of charge of a secondary battery    (2) an electric vehicle comprising a motor generator to generate a driving power.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a hybrid electric vehicle.    Battery electronic control unit (20)    Secondary battery (30)    Temperature sensor (32)    Voltage sensor (34)    Current sensor (36)    Relay (38)    Hybrid electronic control unit (40)    Inverter (50)    Motor generator (52)    Driving power distribution device (56)    Engine (60)