1. A method of measuring a battery residual capacity of an electric vehicle battery during a charging process comprising the repeated steps of: measuring a charge current of the battery; estimating a discharge voltage corresponding to a current battery residual capacity; computing a power of the battery as a product of the measured charge current of the battery and the estimated discharge voltage; and accumulatively adding the computed power to the current battery residual capacity to provide a new current battery residual capacity for display, the method of measuring a battery residual capacity during a discharging process comprising the repeated steps of measuring a discharge voltage and a discharge current of the battery; computing a consumed power as a product of the measured discharge voltage and the measured discharge current; and subtracting the consumed power from a most recent battery residual capacity to produce a new most recent battery residual capacity for display.