Abstract: In the area of the charge level of 20.about.80% where the charge level of a battery does not appear in the terminal voltage of the battery, the battery ECU estimates charge level by integrating the currents of the charge and discharge of the battery. When the engine ECU detects that the ignition key is turned on, the motor generators are driven by the engine, and the generated electric power is accumulated in the battery. When the area where the charge level appears in the terminal voltage, is reached, the calculation of the charge level is performed from the current and voltage at that time. When this charge level has reached 80%, the calibration of said estimated charge level is performed by this value. Furthermore, it is also possible for this calibration to be prohibited when a heavy current is flowing and the error may become large in the calculation of the charge level. By the above mentioned methods, the charge level of a battery can accurately be detected.

Abstract: Variation of the charged amount among battery blocks composing a battery assembly is detected. By subtracting the detected value of the variation from the width between the upper limit value and the lower limit value of the charged amount, the movable range of the charged amount is found. The position of the present charged amount is detected as the state of charge. For example, it is arranged that both ends of the movable range are 0% and 100%, and that the movable range is the full scale. Then, the position of the charged amount on this scale is specified by the ratio %. The detection of the state of charge in which the variation in charged amount and the change of the movable range are considered, is performed, and on the basis of this state of charge, a preferable charge and discharge control is performed.

Abstract: The charge and discharge are performed by a proper electric power corresponding to the service environment of a battery and the state of the battery. When the temperature of the battery is not more than a specified temperature, or not less than another specified temperature, the upper limit value Pin-s of the charging electric power and the upper limit value Pout-s of the discharging electric power are determined so as to be smaller than those at a normal temperature. The temperature of the battery is detected, and the charging electric power and the discharging electric power are determined so that they may be not more than the upper limit values Pin-s, Pout-s of the charging and the discharging electric power at this temperature, and the charge control of the battery is performed.

Abstract: The voltage of each of battery block of a battery is detected by a voltage sensor. Further, the temperature of the battery is detected by a temperature sensor. These detected values are supplied to a detecting section of a battery ECU. A judging section reads out the threshold value of the voltage stored in a memory, on the basis of the detected values of temperature, and compares the threshold value of the voltage with the detected voltage of each of the battery blocks. Then, if at least any one is less than the threshold value of the voltage, a relay is turned off. Furthermore, the threshold value of the voltage is arranged to be the voltage at the moment when the value of the current is suddenly increased by the discharge of a constant current of the battery.