Abstract:
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.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority from the following Japanese Patent Applications: Hei 11-47000, filed Feb. 24, 1999; Hei 11-59309, filed Mar. 5, 1999; Hei 11-356921, filed Dec. 16, 1999; and Hei 11-367565, filed Dec. 24, 1999; the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of controlling condition of an assembled battery mounted in a self-generation electric vehicle such as a hybrid vehicle. 
     2. Description of the Related Art 
     An assembled battery mounted in an electric vehicle is discharged mostly when the vehicle accelerates and charged when the vehicle runs at a constant speed or decelerates. Therefore, it is necessary to maintain the condition of such a battery at a intermediate level between fully-charged level and fully discharged level. 
     In order to maintain the battery condition at the intermediate level, it is necessary to frequently detect the remaining capacity. In the past, the remaining capacity was estimated by summing up battery charging currents and battery discharging currents. 
     JP-A-10-51906 discloses such a method of controlling, as the battery condition, an SOC (State of Charge) ratio which represents the remaining capacity. Data related to terminal voltage and current of an assembled battery at a maximum normal SOC ratio and a minimum normal SOC ratio are stored in a map and are compared with an estimated SOC ratio calculated from currently detected terminal voltage and charge or discharge current of the battery. 
     If the estimated SOC ratio approaches the maximum normal SOC ratio, the battery is discharged. On the other hand, the battery is charged if the estimated SOC ratio approaches the minimum normal SOC ratio. This control is repeated frequently. 
     The more errors are accumulated in the estimated SOC ratio as more frequently the above steps are repeated. This may cause improper control of the battery condition. 
     In the disclosed method, the battery is always controlled so that the SOC ratio stays either at the maximum normal SOC ratio or at the minimum normal SOC ratio. If the battery is controlled at the minimum normal SOC ratio, it is difficult to supply driving energy from the battery to the wheels. On the other hand, if the battery is controlled at the maximum normal SOC ratio, it is difficult to charge the battery with the electric power regenerated from the driving energy of the wheels. 
     Further, because the assembled battery is comprised of a plurality of battery cells of different remaining capacities (or different SOC ratios) and deterioration speeds, it is always possible that any one of the battery cells is over-discharged or deteriorating. 
     SUMMARY OF THE INVENTION 
     A main object of the invention is to provide an improved method of controlling condition of an assembled battery of an electric vehicle which needs not a map. 
     According to a feature of the invention, a method of controlling condition of a battery of a self-generation electric vehicle comprising the following steps: detecting battery voltage and battery current repeatedly; estimating a constant-power voltage on the basis of the battery voltage and battery current, the constant-power voltage being a function of a remaining capacity of the battery when a preset constant power is charged to or discharged from the battery; 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. 
     The following steps can be added to the above steps: setting a maximum-normal voltage and a minimum-normal voltage between a voltage level of the battery that is fully charged and a voltage level of the battery that is completely discharged; and stopping the battery current if the constant-power voltage level becomes out of a range between the maximum-normal voltage and the minimum-normal voltage. 
     Therefore, this method ensures accurate control of the battery condition. 
     The target voltage is preferably set at the middle of the maximum-normal voltage and the minimum-normal voltage. 
     Therefore, battery can be charged or discharged easily. This step also prevents any one of series-connected battery cells of an assembled battery from being over-discharged. 
     The above step may also have the following steps: estimating the remaining capacity of the battery by summing up the battery current; and correcting the estimated remaining capacity to the target remaining capacity when the constant-power voltage approaches the target voltage. 
     Therefore, errors are not accumulated in the estimated SOC ratio even if the step of estimating the remaining capacity is repeated, so that improper control of the battery condition can be prevented effectively. 
     The above step preferably includes a step of resetting battery condition. This step may be comprised of: setting a reference condition range that is defined by a preset voltage range and a preset remaining capacity range; setting an operation condition defined by the constant-power voltage and the estimated remaining capacity; and if the operation condition is found to be out of the reference condition range, controlling the battery current to change the operation condition to be out of a boundary defined by the maximum-normal voltage and the minimum-normal voltage and, subsequently controlling the battery current to return the constant-power voltage to the target voltage while the battery is not operated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
     FIG. 1 is a block diagram illustrating a control system of a parallel hybrid vehicle; 
     FIG. 2 is a block diagram illustrating battery cells and the related circuits thereof; 
     FIG. 3 is a graph showing operation characteristics of a battery cell of a nickel-hydrogen battery mounted in a hybrid vehicle; 
     FIG. 4 is a graph showing charge-discharge characteristics of a battery cell relative to SOC ratios; 
     FIG. 5 is a graph showing charge-discharge characteristics of a battery cell relative to SOC ratios; 
     FIG. 6 is a graph showing charge-discharge characteristics of a battery cell relative to SOC ratios; 
     FIGS. 7A and 7B are graphs showing battery cell discharge characteristics relative to SOC ratios; 
     FIG. 8 is a graph showing terminal voltages and SOC ratios of a battery module including a battery cell of a different remaining capacity; 
     FIG. 9A is a graph showing terminal voltages of a battery module including a battery cell of a different remaining capacity, and FIG. 9B is a graph showing discharging power characteristics of a battery module relative to vehicle running time; 
     FIG. 10 is a timing chart showing running power in kW relative to vehicle running hour; 
     FIG. 11 is a timing chart showing average temperature change of the battery cells; 
     FIG. 12 is a timing chart showing a calculated SOC ratio curve and real SOC ratio curve; 
     FIG. 13 is a timing chart showing differences in SOC ratio between the calculated SOC ratio curve and the real SOC ratio curve; 
     FIG. 14 is a timing chart showing differences between target voltage VM and 21 kW-constant-power voltage; 
     FIG. 15 is a flow diagram of controlling battery condition according to a first embodiment of the invention; 
     FIG. 16 is a flow diagram showing a method of controlling the SOC ratio of the assemble battery according to the first embodiment; 
     FIG. 17 is a flow diagram showing a method of controlling the SOC ratio of the assemble battery according to the first embodiment; 
     FIG. 18 is a flow diagram showing a variation of the method of controlling the SOC ratio of the assemble battery according to the first embodiment; 
     FIG. 19 is a timing chart showing change of SOC ratios relative to the vehicle running time; 
     FIG. 20 is a graph showing a method of controlling the SOC ratio according to a second embodiment of the invention; 
     FIG. 21 is a flow diagram showing the method according to the second embodiment; 
     FIG. 22 is a graph showing a method of controlling the SOC ratio according to a variation of the second embodiment; 
     FIG. 23 is a flow diagram showing the variation of the method according to the second embodiment; 
     FIG. 24 is a flow diagram showing a variation of the flow diagram shown in FIG. 23; 
     FIG. 25 is a graph showing a method of controlling the SOC ratio according to a third embodiment of the invention; 
     FIG. 26 is a flow diagram showing the method according to the third embodiment; 
     FIG. 27 is a flow diagram showing the method according to a fourth embodiment; 
     FIG. 28 is a flow diagram showing a method of controlling the SOC ratio according to a fifth embodiment of the invention; 
     FIG. 29 is a flow diagram of a method of controlling the SOC ratio according to a sixth embodiment of the invention; and 
     FIG. 30 is a variation of a portion of the flow diagram shown in FIG.  29 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A principle of controlling an assembled battery of a hybrid vehicle according to the invention is described with reference to FIG.  1 -FIG.  9 B. 
     In FIG. 1, a control system of a parallel hybrid vehicle includes engine  11 , generator  12 , inverter  13 , assembled battery system  14 , torque distributor  15 , motor  16 , speed reduction gear  17  and wheels  18 . 
     Generator  12  is driven by a portion of driving power of engine  11  to generate electric output power. Inverter  13  supplies battery system  14  and motor  16  with electric power. Engine power is distributed by torque distributer  15  to wheels  18  and to generator  12 . Motor  16  converts the electric power supplied thereto into driving power of wheels  18  and, when the vehicle is decelerated, the driving power of the wheels into electric power to be charged into battery system  14 . 
     As shown in FIG. 2, battery system  14  includes a battery pack  21  having a plurality of series-connected modules  22 , temperature sensor  23 , voltage detecting circuit  24 , battery temperature detecting circuit  25 , battery current detecting circuit  26 , battery control circuit  27 , and vehicle control unit  28 . Each module  22  is comprised of a plurality of battery cells. Battery control circuit  27  sums up terminal voltages of respective modules  22  to obtain the terminal voltage of battery pack  21 . The capacity of battery pack  21  is calculated on the basis of the respective signals of voltage detecting circuit  24 , battery temperature detecting circuit  25 , and battery current detecting circuit  26 . Battery control circuit  27  also provides data for controlling charging and discharging of battery pack  21 . 
     Voltage detecting circuit  24  and temperature detecting circuit  25  can be connected to each battery cell if cost increase does not cause a big problem. 
     In FIG. 3, curve L indicates terminal voltage of one of 240 series-connected battery cells of battery pack  21  relative to battery charging current when battery pack  21  discharges maximum normal power 21 kW. 
     Dotted line  31  shows current-voltage characteristics of the battery cell when battery pack  21  is fully charged. 
     Point Pmax at the intersection of curve L and line  31  represents voltage Vmax at full capacity of the battery cell when the battery pack  21  discharges the maximum normal power, 21 kW. Point Pmax′ represents voltage Vmax′ at full capacity of the battery cell when the current charged or discharged is 0. 
     Dotted line  32  shows current-voltage characteristics when battery pack  21  is almost completely discharged or at zero capacity. 
     Point Pmin at the intersection of curve L and line  32  represents voltage Vmin at zero capacity of the battery cell when the battery pack  21  discharges the maximum normal power, 21 kW. Point Pmin′ represents voltage Vmin′ at zero capacity of the battery cell when the current charged or discharged is 0. 
     FIG. 4 shows voltage characteristics of a battery cell relative to SOC ratios while battery pack  21  discharges the maximum normal power. The SOC ratio represents battery capacity, and is expressed as follows: 
     
       
         SOC ratio=(remaining capacity)/(rated capacity)×100%.  
       
     
     In FIG. 4, curve  41  represents discharge voltage characteristics of a battery cell of battery pack  21  that is fully charged, and curve  42  represents charge voltage characteristics of the battery cell of the battery pack  21  that is almost completely discharged. Thus, curve  41  and curve  42  form a boundary hysteresis curve. 
     Curve  43  represents a lower boundary of normal voltage characteristics of the battery cell when battery pack  21  discharges the maximum normal power, 21 kW. It extends from maximum-normal point PHi, where the battery cell voltage is maximum-normal voltage VHi and the SOC ratio is 80%, to minimum-normal point PLo, where the battery cell voltage is minimum-normal voltage VLo and the SOC ratio is 40%. Curve  44 , which extends from point PLo to point PHi, represents a upper boundary of normal voltage characteristics of the battery cell when battery pack  21  is charged with 21 kW. Thus, curves  43  and  44  form a small hysteresis curve. Target voltage VM is a voltage of the point approximately at the center of the area surrounded by curves  43  and  44  on the vertical line of SOC 60%. 
     Battery pack  21  is controlled to be charged or discharged so that the SOC ratio can stay between 80% and 40%. In other words, it is controlled so that the voltage of the battery cell can stay between maximum-normal voltage VHi and minimum-normal voltage VLo. Accordingly, if the voltage of the battery cell is around the target voltage VM, the SOC ratio is 60%±5%. 
     The target voltage VM can be determined according to a vehicle required power. No-load voltage VM′ of the battery cell as shown in FIG. 5 or voltage with a certain load (not shown) can be substituted for VM. More accurate SOC ratio could be obtained if the charge-discharge characteristics of the battery cell are detected at various temperatures. 
     If the battery cell is charged and discharged repeatedly, the characteristic curves of the battery cell form many hysteresis curves within the boundary hysteresis curve as shown in FIG.  6 . 
     In FIG. 6, if the battery cell is charged at minimum-normal point PLo with the SOC ratio being 40%, the voltage of the battery cell changes along charge characteristic curve  44  and the SOC ratio thereof becomes 80% at point PHi. If the same is discharged, the cell voltage changes from PHi along discharge characteristic curve  43  to point P 1 . If the same is charged again, the cell voltage changes along charge characteristic curve  61  to point P 2  and finally to point PHi. If the same is discharged again at point P 2 , the cell voltage changes along discharge characteristic curve  62  to point P 3  and further to point P 1 . 
     That is, if generator  12  is controlled so that the discharge voltage of the battery cell at a maximum normal power (hereinafter referred to as the constant-power voltage) approaches the target voltage VM as described above, the SOC ratio approaches 60%. In other words, as long as the operation point of the battery cell stays in the normally operable range surrounded by the boundary hysteresis curves  43  and  44 , the SOC ratio returns to around 60%. 
     As shown in FIG. 6, SOC ratio 60% is at the middle of maximum and minimum normal SOC ratios, SOC 1  on discharge characteristic curve  43  at target voltage VM and SOC 2  on charge characteristic curve  44  at target voltage VM. Assuming that a vehicle runs with battery pack  21  with maximum-normal point PHi. If the vehicle is driven in a manner of spending battery power so that the operation point the battery cell further moves along discharge characteristic curve  43  to point P 1 , the battery pack  14  is charged along charge characteristic curve  61  so that the constant-power voltage approaches the target voltage VM. Thus, the SOC ratio further approaches 60%. 
     FIG. 7A is a graph showing discharge characteristics relative to the SOC ratios of one of 240 battery cells of an assembled battery which discharges the maximum-normal power of 21 kW. Curve  7   a  represents initial discharge characteristics, and curve  7   b  represents discharge characteristics when the SOC ratio of the battery decreases by 30%. 
     FIG. 7B is a graph showing discharge characteristics of two modules each of which includes 24 battery cells. Curve  7   c  represents discharge characteristics of a first module which includes all the battery cells that correspond to curve  7   a  of FIG. 7A, and curve  7   d  represents discharge characteristics of a second module which include  23  battery cells that correspond to curve  7   a  of FIG. 7A, and one battery cell that corresponds to curve  7   b  of FIG.  7 A. 
     As the first module of 100%-SOC ratio is discharged, the discharge voltage decreases along curve C and sharply drops when the SOC ratio approaches 20%. The SOC ratio of point Q of curve C is much lower than 40% in SOC ratio. On the other hand, the discharge voltage of the second module of 100% in the SOC ratio decreases along curve D and sharply drops when the SOC ratio approaches 40%. Thus, the more the capacity of the battery cells is different from one another, the less the dischargeable power of the battery becomes. 
     If the second module is further discharged and the discharge voltage becomes minimum-normal voltage Vp (which corresponds to minimum-normal voltage VLo of the battery cell) at point P of curve D, the discharge voltage of the battery cell corresponding to curve B becomes less than 0 volt. 
     If the module discharges the maximum normal power and the module voltage becomes less than minimum-normal voltage Vp, the module is controlled to reduce discharge power from the maximum normal power. Thus, the voltage drop across the internal resistance is controlled so that the discharge voltage can be maintained to be not lower than minimum-normal voltage Vp. 
     A method of controlling the discharge power (discharge control method) is described with reference to FIG.  8  and FIGS. 9A and 9B. 
     FIG. 8 is a graph showing discharge characteristics of a module including 24 battery cells including one battery cell whose capacity is 30% less than the others, where the SOC ratio is not controlled. Curve  8   a  represents average voltage change of the module (in terms of the voltage of one battery cell) relative to the vehicle running time. Curve  8   b  represents voltage changes of the 30%-less-capacity cell, and curve  8   c  represents average SOC ratios of the module. Curve  8   b  indicates that the 30%-less-capacity cell is over-discharged, and curve  8   c  indicates that the SOC ratio of the 30%-less-capacity cell decreases to 10%, which is 20% less than the normal SOC ratio. 
     FIG. 9A is a graph showing discharge characteristics of a module including 23 normal battery cells and one battery cell whose capacity is 30% less than the others, where the discharge power is controlled. Curve  9   a  represents average voltage change of the module (in terms of the voltage of one battery cell) relative to the vehicle running time. Curve  9   b  represents the voltage of the 30%-less-capacity cell, and curve  10   c  represents an average SOC ratio of the module. The discharge voltage of the module is controlled according to the voltage of curve  9   a.  Curve  9   c  indicates that the 30%-less-capacity cell is controlled to reduce frequency of being over-discharged, and curve  10   c  indicates that the SOC ratio of the 30%-less-capacity cell only decrease to 20%. 
     FIG. 9B is a graph showing the total discharge power of the battery including 240 battery cells whose discharge power is controlled while a vehicle is running. 
     Curve  9   d  represents dischargeable power levels which do not decrease the discharge voltage of the modules to a level lower than the module&#39;s or the battery-cell&#39;s minimum-normal voltage. 
     Thus, the control of the discharging power is effective to prevent the discharge voltage of the modules from becoming less than the minimum-normal voltage. 
     (First Embodiment) 
     A method of controlling battery condition according to a first embodiment of the invention is described with reference to FIGS. 10-19. 
     At first, voltage of battery pack  21  and battery current charged to or discharged from battery pack  21  are detected to calculate target voltage VM. Then, if the detected constant-power voltage of the battery cell is as high as VHi, battery pack  21  is not further charged. On the other hand if the detected constant-power voltage is as low as VHo, battery pack  21  is not discharged. A provisionally SOC ratio is calculated separately by accumulating the battery currents detected repeatedly. This is corrected thereafter in the following manner. 
     When the provisionally calculated SOC ratio becomes 60%, battery pack  21  is charged or discharged so that the constant-power voltage becomes equal to target voltage VM, which should correspond to the real SOC ratio of 60% as described above. Thus, the charged or discharged current can be considered as an accumulation error, which is corrected at this stage. 
     The operation of a hybrid vehicle in which the above 60%-SOC-ratio-control is carried out is shown in FIGS. 10-14. FIG. 10 shows running power in kW relative to vehicle running hour, FIG. 11 shows average temperature of the battery cells, FIG. 12 shows SOC ratio curve  12   a  calculated in the method according the invention and real SOC ratio curve  12   b  that was measured after the battery remaining capacity had been precisely measured. FIG. 13 shows differences in the SOC ratio between curve  12   a  and curve  12   b.  FIG. 14 shows differences between curve  14   a  of target voltage VM and curve  14   b  of the constant-power voltage (21kW-constant-power discharge-voltage). As shown in FIG. 14, the SOC ratio is controlled within 60%±4%. Control operation of vehicle control unit  28  is described with reference to FIG.  15 . Vehicle control unit  28  controls engine  11  via a engine controller (not shown) according to data related to the vehicle load and the SOC ratio of battery pack  21 . 
     At step S 1000 , the SOC ratio provided by battery control circuit  27  is read. The SOC ratio is then compared with a target SOC ratio to obtain required electric power at step S 1002 . At step S 1004 , a running power value, which is calculated beforehand, is added to the required electric power to provide a total required power. Then, engine  11  is controlled to provide engine power corresponding to the total required power in a well-known manner. Vehicle control unit  28  also controls generator  12  and motor  16  in a well-known manner. 
     The operation of battery control circuit  27  is described with reference to flow diagrams shown in FIGS. 16,  17  and  18 . 
     At step S 901 , battery cell voltage VB, battery cell current IB, and battery cell temperature TB are detected. At step S 902 , the constant-power voltage VBw is calculated by the following equation. Internal resistance Rk is calculated by the least square method.              VBo   =     VB   +     Rk   ×   IB                     VBw   =       {     VBo   +       (       VBo   2     -     4   ×   Rk   ×   α       )     0.5       }     ×   0.5       ,                                
     where VBo is a no-load voltage of the battery cell, and α is a portion of the maximum normal power (21 kW) allocated to each battery cell. 
     At step S 903 , the detected current values are accumulated to calculate the SOC ratio. That is: 
     
       
         SOC ratio=(remaining capacity)/(rated capacity)×100(%)  
       
     
     At step S 904 , whether the SOC ratio is 60±3% or not is examined. If the result is NO, step S 907  follows. If YES, step S 906  follows, where the constant-power voltage VBw is compared with target voltage VM. If VBw&lt;VM, it is necessary to charge the battery pack. Then, the calculated SOC ratio is corrected to be lower than 60% so that vehicle control unit  28  can control engine  11 , generator  12 , and motor  16  to charge battery pack  21 . On the other hand, if VBw&gt;VM, it is necessary to discharge the battery pack. Then, the SOC ratio is corrected to be higher than 60% so that vehicle control unit  28  can control engine  11 , generator  12 , and motor  16  to discharge battery pack  21 . Thus, vehicle control unit  28  controls engine  11 , generator  12 , and motor  16  step by step so that the actual SOC ratio stays around 60%. The correction of the SOC ratio each time is between 1 and 0.01%. The corrected SOC ratio is supplied to vehicle control unit  28  at step S 907 . 
     Then, whether the SOC ratio is within 60%-80%-SOC-ratio-control-range or not is examined at step S 908 . 
     A sub routine of this step is shown in FIGS. 17 or  18 . In FIG. 17, at step S 1001 , whether the SOC ratio is larger than 80% is examined. If the result is YES, a charge-stop command is sent to vehicle control unit  28  at step S 1002 . Other steps are readily understandable. 
     Instead of examining the SOC ratio, the constant-power voltage VBw or the no-load voltage VBo is compared with VHi or VHi′ that corresponds to 80% SOC ratio at step S 1101  shown in FIG.  18 . If VHi&lt;VBw, the charge stop command is sent to vehicle control unit  28  at step S 1102 . If VBw is found to be smaller than VLo at step S 1103 , the discharge stop command is sent to vehicle control unit  28  at step S 1104 . Instead of the constant-power voltage VBw, the no-load voltage VBo can be used in the steps as described before. 
     If the vehicle stops at step S 909 , control parameters are stored to be used to the next operation at step S 910  before the operation ends. 
     The above described SOC ratio changes as shown in FIG.  19 . When the vehicle starts and runs at beginning until point  19   a,  the real SOC ratio stays within the set range 57%-63%. The detected SOC ratio is corrected at step S 906 . When the vehicle is driven in a manner to spend much battery power, the detected SOC ratio moves from point  19   a  to point  19   b  where the SOC ratio is much lower than 57%. Accordingly, discharge stop command is sent to vehicle control unit  28  at step S 908 , that is, at step S 1004  or step S 1104 . Then, the detected SOC ratio returns to point  19   c  in the set range, and it is corrected again at step S 906 . Thus, detected SOC ratio can be corrected timely. When the vehicle is driven in a manner to charge much power to battery, the detected SOC ratio moves to point  19   d  where the SOC ratio is much higher than 63%. This is also controlled in substantially the same manner as described above. 
     (Second Embodiment) 
     A method of controlling battery condition according to a second embodiment of the invention is described with reference to FIGS.  16  and  20 - 24 . 
     It has been found that the constant-power voltage does not always move along the normal characteristic curve, due to a deviation in memory effect and polarization of the battery cells. As a result, the SOC ratio can not be accurately controlled to approach 60%. The method according to the second embodiment will solve the above problem. 
     The constant-power voltage is controlled in almost the same manner as the first embodiment so that it can approach target voltage VM, and the following subroutine shown in FIG. 21 is added to the main routine shown in FIG.  16 . 
     At step S 1100 , whether or not a present operating point (SOC ratio, constant-power voltage) is on or below curve  43  is examined. If the result is NO, the subroutine ends. 
     On the other hand, if the result is YES, difference x between 60% and the SOC ratio of operating point C, where charging of battery pack  21  starts, is detected at step S 1102 . At  1104 , X is multiplied by a raising coefficient (between 1 and 2) to obtain X′, and the battery pack  21  is charged so that the SOC ratio can increase by X′. In other words, the operation point moves to point D where the SOC ratio is larger than 60%. Thereafter, the battery cells are discharged so that the constant-power voltage becomes equal to target voltage VM at step S 1106 . That is, the operation point moves to point A. 
     The operation point control from point C to point D and from point D to point A can be carried out after battery pack  21  are charged or discharged several times. 
     Another method of controlling the SOC ratio when the constant-power voltage does not move along the normal characteristic curve is described with reference to a graph shown in FIG. 22 and a subroutine flow diagram shown in FIG.  23 . 
     At step S 1100  in FIG. 23, whether or not a present condition of the battery cells corresponding to an operating point shown in FIG. 22 is on or below curve  43  shown in FIG. 22 is examined by calculation similar to calculation shown in FIG.  16 . If the result is NO, the subroutine ends. On the other hand, if the result is YES, difference X between 60% and the SOC ratio of operating point C is detected at step S 1102 . At step S 1204 , battery pack  22  is charged so that the SOC ratio becomes 60%. In other words, the operation point moves from point C to point H. Thereafter, target voltage VM is rewritten to no-load target voltage VM′ at step S 1206 . 
     In the above method, it is possible to insert steps of dissolving large memory effect of the battery cell between step S 1100  and S 1102  as shown in FIG.  24 . At step S 1302 , whether the operation point is a certain voltage lower than curve  43  is examined. If the result is NO, the step goes to S 1102 . On the other hand, if the result is YES, a deep discharging treatment is given at step S 1304  before going to step S 1102 . 
     The deep discharging treatment is well known as a method of dissolving the memory effect of batteries. 
     (Third Embodiment) 
     A method of controlling battery condition is described with reference to FIGS. 25 and 26. 
     As shown in FIG. 25, if battery pack  21  is discharged excessively and its operation point moves along discharge characteristic curve  41  down to point P 5 , where the SOC ratio (e.g. 20%) becomes much lower than 40%, battery pack  21  is charged again to move the operation point to around PHi, from where the operation point moves along new discharge characteristic curve  45  through point P 7  down to a point P 8  around point PLo, thus returning to the normal control range. 
     The operation point moves from point P 8  to point P 9  along new charge characteristic curve  48  when battery pack  21  is charged again. This changes the SOC-ratio-control-range at target voltage VM from range O-O′ to range P 7 -P 9 . 
     However, this range shift can be eliminated if battery pack  21  is discharged to lower the constant-power voltage to minimum-normal voltage VLo before controlling the constant-power voltage at target voltage VM. The range shift can be also eliminated if battery pack  21  is fully charged before controlling the constant-power voltage at target voltage VM. 
     This resetting operation is carried out regularly or automatically while the vehicle is running without assist of the battery power. In this resetting operation, battery pack  21  is charged continuously and uniformly with certain amounts of electric power in a uniform charging manner as in a flow diagram shown in FIG.  26 . 
     At step S 2000 , battery pack is charged in the uniform charging manner, and whether the constant-power voltage VBw is fully charged or not is examined at step S 2002 . 
     If the result is YES, battery pack  21  is discharged until the constant-power voltage VBw approaches minimum-normal voltage VLo at step S 2006 , and the routine ends. On the other hand, if the result is NO, the uniform charging is continued. 
     (Fourth Embodiment) 
     A method controlling battery condition according to a fourth embodiment of the invention is described with reference to FIGS. 25 and 27. 
     At first, battery pack  21  is charged to move the operation point from point P 5  along charge characteristic curve  46  to point P 6  beyond PHi by a SOC ratio smaller than the difference between PLo and P 5  in FIG. 25, from where the operation point moves along discharge characteristic curve  48 , thus returning to the normal control range. 
     If the battery pack  21  is further discharged along discharge characteristic curve  48  to a point between PLo and P 5 , the operation control range at target voltage VM becomes closer to original control range O-O′. 
     The more the above resetting operation is repeated, the closer to the original range the control range becomes. 
     In FIG. 27 at step S 9060 , whether flag F 1  is 1 or not is examined. If flag is 1, this indicates that battery pack is over-discharged and is charged so that the operation point moves from point P 5  along charge characteristic curve  46  to point P 6  shown in FIG.  25 . If the result is NO, step S 9063  follows. If the result is YES, step S 9061  follows to set the target SOC ratio to 80%+α. Then, set flag F 1  to 1 at the next step S 9062 . At step S 9063 , whether the SOC ratio is 80%+α or not is examined. 
     If the result is NO, the next step is S 907  of the main routine. If the result is YES, the flag F 1  is set to 0 at step S 9064 , because it is considered that the operation point of the battery cell has approached point P 6  along charge curve  46 . Then, whether flag F 2  is 0 or not is examined at step S 9065 . 
     If the result is YES, step S 9066  follows to set the target SOC ratio to 60% because this indicates that battery pack  21  is being discharged so that the operation point moves along discharge curve  45 . Then, flag F 2  is set to 1 at step S 9067 . 
     If the result of step S 9065  is NO, step S 9068  follows to examine whether the SOC ratio has approached 60% or not. If the result is NO, the next step is S 907  of the main routine. If the result is YES, flag F 2  is set to 0 before going to step S 907  of the main routine. 
     (Fifth Embodiment) 
     Another method of controlling battery condition according to a fifth embodiment of the invention is described with reference to FIG.  25  and FIG.  28 . 
     At step S 3000 , battery pack  21  is charged so that the operation point moves from point P 5  along curve  46  to point P 6  further than point PHi by a certain SOC ratio that is smaller than the difference in the SOC ratio between point PLo and point P 5 . Then, battery pack  21  is discharged so that the constant-power voltage becomes VLo at step S 3002 . Thereafter, battery pack  21  is charged again so that the constant-power voltage approaches target voltage VM. 
     (Sixth Embodiment) 
     A method of controlling battery condition according to a sixth embodiment of the invention is described with reference to a flow chart shown in FIGS. 29 and 30. 
     In steps  901 - 907 , the SOC ratio is calculated and running condition data VB, IB, TB in the same manner as described above. At step S 1308 , dischargeable power Wout of the battery cell and no load voltage VBo thereof are obtained by the following equation.              Wout   =       (       (     VBo   -   VP     )     /   Rk     )     ·   VLo                 VBo   =     VB   +     RK   ×   IB                                    
     At step S 1309 , whether or not the constant-power voltage VBw of the battery cell is less than discharge stop voltage 0.9 V is examined. If the result is YES, Wout is set to 0 to stop the battery discharge at step S 1310 . Thereafter, Wout is sent to vehicle control unit  28  at step S 1313 . As long as vehicle control unit  28  controls the dischargeable power within Wout, the constant-power voltage would not become lower than minimum-normal voltage VLo. That is, any battery cell will not become over-discharged. 
     At step S 1311 , whether the SOC ratio is larger than 40% or not is examined. If the result is YES, and the uniform charging is commanded because battery pack  21  may have a battery cell of different remaining capacity. The uniform charging equalizes all the battery-cell capacities. 
     If the vehicle stops at step S 909 , all the parameters used in the steps are stored in a memory for the next control. 
     The SOC ratio at step S 1310  can be stored as the minimum-normal SOC ratio for the next operation. For example, a target SOC ratio is set between the minimum-normal SOC ratio (e.g. SOC 2  in FIG. 6) and the full SOC ratio (i.e. 100%) to equalize the remaining capacities of the battery cells being charged and the remaining capacities of the same being discharged, so that imbalance between the chargeable capacity and dischargeable capacity can be reduced. 
     At step S 1308 , dischargeable power Wout is controlled automatically. However, the constant-power voltage VBw becomes equal to minimum-normal voltage VLo although the constant-power voltage VBw is not examined. 
     As shown in FIG. 30, whether the constant-power voltage VBw becomes lower than minimum-normal voltage VLo is examined beforehand. If the result is NO, step S 1308  is skipped, and Wout is set 21 kW at step S 1313 . On the other hand, if the result is YES, Wout calculated at step S 1308  is sent to vehicle control unit  28  to control the discharge power. 
     In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the broader spirit and scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention in this document is to be regarded in an illustrative, rather than restrictive, sense.