This invention provides a method of controlling condition of an assembled battery mounted in a self-generation electric vehicle. The method is comprised of the following steps: detecting battery voltage and battery current repeatedly; estimating a constant-power voltage on the basis of the battery voltage and the battery current; setting a target voltage; comparing the constant-power-discharge voltage with the target voltage to provide a difference therebetween; and charging or discharging the battery to reduce the difference when the battery is not operated. The constant-power voltage is set as a function of a remaining capacity of the battery when a preset constant power is charged to or discharged from the battery.