Patent Publication Number: US-2012032504-A1

Title: Power source device

Description:
TECHNICAL FIELD 
     The present invention relates to a power supply device that stores electric power in an electric storage element, and prompts the electric storage element to discharge the stored electric power when necessary. 
     BACKGROUND ART 
     In recent years, a fuel saving vehicle has been proposed. This vehicle makes effective use of regenerated electric power produced at an application of the brakes and stored in an electric storage element, thereby saving fuel. An electric power controller used in this vehicle is disclosed in, e.g. Patent Literature 1.  FIG. 5  is a block diagram of this electric power controller. 
     Main power supply  101  is a lead battery, and its positive electrode is connected with load  105  via ignition switch  103 . The positive electrode of main power supply  101  is also connected with vehicle electrical generator  107 , which is mechanically connected to engine  109 . When engine  109  works, electrical generator  107  is driven. Engine  109  is mechanically coupled to tires  111 . When engine  109  works, driving force is generated and then tires  111  rotate for running the vehicle. When the brake is applied for speed reduction, tires  111  still rotates due to inertia of the vehicle, so that engine  109  keeps rotating. This rotational energy drives electrical generator  107 , thereby producing the regenerated electric power. 
     In the foregoing vehicle, electric storage element  115  is connected to electrical generator  107  via DC/DC converter  113  in order to efficiently recover the regenerated electric power. Since electric storage element  115  is formed of an electric double-layered capacitor having a great amount of capacitance, storage element  115  can efficiently recover a large amount of electric power produced within a short period at a sudden speed reduction. DC/DC converter  113  is hooked up with computing device  117  for controlling the work of converter  113 , and computing device  117  includes signal-receiving terminal  119  that receives a variety of signals from the vehicle. Computing device  117  thus receives, through terminal  119 , information about the traveling status of the vehicle, operating state of engine  109 , and a voltage state of main power supply  101 , and then computing device  117  controls DC/DC converter  113  in response to those states, thereby controlling the charge or discharge of storage element  115 . 
     The electric power controller discussed above allows tentatively charging the storage element  115  with the regenerated electric power produced at the speed reduction and including a large amount of electric power produced within a short period. The electric power controller also allows supplying charged regenerated electric power, at the times other than the speed reduction, to main power supply  101  or load  105 . As a result, the energy consumed in applying the brakes can be recovered efficiently. 
     However, DC/DC converter  113  has an efficiency of approx. 80% when an electric current of 100 A flows, so the remaining 20% is wasted as heat loss. However, when electric storage element  115  in this circuitry is charged or discharged, it needs electric current running through DC/DC converter  113 . The heat loss thus happens twice, namely, once during charging and again during discharging. The efficiency in total can be thus lowered. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Unexamined Japanese Patent Application Publication No. H06-296332 
     SUMMARY OF THE INVENTION 
     The present invention aims to provide a power supply device to be mounted in a mobile body, and this power supply device can increase total efficiency in charging and discharging an electric storage section thereof by using a constant voltage control. 
     The power supply device of the present invention has a main power supply that can be charged and discharged, an electrical generator, a DC/DC converter, an electric storage section, an output voltage detecting circuit, a control circuit, and an adjusting voltage control section. The electrical generator includes an output terminal and a grounding terminal, and is connected to the main power supply at the output terminal. The electrical generator produces regenerated electric power when the mobile body undergoes a speed reduction. The DC/DC converter includes a first end and a second end, and is electrically connected to the output terminal of the electrical generator at the first end. The electric storage section is electrically connected to the second end of the DC/DC converter. The output voltage detecting circuit is electrically connected directly between the output terminal and the grounding terminal of the electrical generator for detecting output voltage (Vd) at the output terminal. The control circuit is electrically connected to the DC/DC converter and the output voltage detecting circuit. The adjusting voltage control section supplies an adjusting voltage (Va) to the electrical generator for adjusting the output voltage (Vd), and sets the adjusting voltage (Va) at a first predetermined output voltage (Vdc 1 ) at a timing when the electrical generator produces the regenerated electric power. The control circuit then controls the DC/DC converter such that the output voltage (Vd) become a lower control voltage (Vdk) that is lower than the first predetermined output voltage (Vdc 1 ). In addition, the adjusting voltage control section sets the adjusting voltage (Va) at a second predetermined output voltage (Vdc 2 ) that is lower than the first predetermined output voltage (Vdc 1 ) at a timing when the electrical generator ends the production of the regenerated electric power. The control circuit then controls the DC/DC converter such that the output voltage (Vd) become an upper control voltage (Vdu) that is higher than the second predetermined output voltage (Vdc 2 ). The lower control voltage (Vdk) is set at a value with which the regenerated electric power can be charged maximally into the main power supply. 
     Alternatively, an adjustment terminal electrically connected to the electrical generator is disposed to an objective circuit portion for the electrical generator to control a voltage, and the output voltage detecting circuit is directly connected to the adjustment terminal instead of to the output terminal of the electrical generator such that the detecting circuit can detect the output voltage (Vd) that is supposed to be adjusted relative to the adjustment terminal. 
     The present invention allows controlling the output voltage (Vd) to be the lower control voltage (Vdk) that has been set at a value with which the regenerated electric power can be charged maximally into the main power supply. As a result, a charging amount to the main power supply can be increased when the regenerated electric power is charged into both the electric storage section and the main power supply. This increment allows reducing the heat loss incurred in the DC/DC converter or caused by resistances in wirings when the electric storage section is charged. The present invention thus can increase the efficiency of the power supply device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a power supply device in accordance with a first embodiment of the present invention. 
         FIG. 2A  shows a speed variation with time of a vehicle to which the power supply device is mounted in accordance with the first embodiment is mounted. 
         FIG. 2B  shows a variation with time of adjusting voltage Va for an electrical generator of the power supply device in accordance with the first embodiment. 
         FIG. 2C  shows a variation with time of output voltage Vd from the electrical generator of the power supply device in accordance with the first embodiment. 
         FIG. 2D  shows a variation with time of reference voltage Vst of a DC/DC converter of the power supply device in accordance with the first embodiment. 
         FIG. 2E  shows a variation with time of voltage Vc of an electric storage section of the power supply device in accordance with the first embodiment. 
         FIG. 3A  shows a speed variation with time of a vehicle to which a power supply device is mounted in accordance with a second embodiment of the present invention. 
         FIG. 3B  shows a variation with time of adjusting voltage Va for an electrical generator of the power supply device in accordance with the second embodiment. 
         FIG. 3C  shows a variation with time of output voltage Vd from the electrical generator of the power supply device in accordance with the second embodiment. 
         FIG. 3D  shows a variation with time of reference voltage Vst of a DC/DC converter of the power supply device in accordance with the second embodiment. 
         FIG. 3E  shows a variation with time of voltage Vc of an electric storage section of the power supply device in accordance with the second embodiment. 
         FIG. 4  is a block diagram of a power supply device in accordance with a third embodiment of the present invention. 
         FIG. 5  is a block diagram of a conventional electric power controller. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments of the present invention are demonstrated hereinafter with reference to the accompanying drawings. In each embodiment, similar elements to precedent ones have the same reference signs, and detailed descriptions thereof are sometimes omitted. 
     Exemplary Embodiment 1 
       FIG. 1  is a block diagram of a power supply device in accordance with this first embodiment of the present invention.  FIGS. 2A-2E  show variations with time of respective characteristics of the power supply device in accordance with the first embodiment.  FIG. 2A  shows a speed variation with time of a vehicle to which the power supply device is mounted in accordance with the first embodiment.  FIG. 2B  shows a variation with time of adjusting voltage (Va) for an electrical generator of the power supply device in accordance with the first embodiment.  FIG. 2C  shows a variation with time of output voltage (Vd) from the electrical generator of the power supply device in accordance with the first embodiment.  FIG. 2D  shows a variation with time of reference voltage (Vst) of a DC/DC converter of the power supply device in accordance with the first embodiment.  FIG. 2E  shows a variation with time of voltage (Vc) of an electric storage section of the power supply device in accordance with the first embodiment. In  FIG. 1 , the wide lines indicate power-carrying wirings, and the narrow lines indicate signal-carrying wirings. In respective drawings of  FIG. 2 , X-axis represents time. The power supply device is mounted in a mobile body, e.g. a vehicle, and is formed of main power supply  17  that can be charged and discharged, electrical generator  11 , DC/DC converter  23 , electric storage section  29 , output voltage detecting circuit  31 , control circuit  35 , and detecting circuit  33  for detecting a voltage of the electric storage section. 
     Main power supply  17  is formed of, e.g. a lead battery. Electrical generator  11  includes output terminal  13  and grounding terminal  22 , and is connected via the power-carrying wirings to main power supply  17  at output terminal  13 . Electrical generator  11  produces regenerated electric power during a speed reduction of the vehicle. To be more specific, electrical generator  11  is connected to an engine (not shown) mechanically, so that it produces electric power in response to the rotation of the engine. 
     Output voltage Vd from electrical generator  11  at output terminal  13  is set by a signal, i.e. adjusting voltage Va, supplied from vehicle-side control circuit  37 . In this embodiment, adjusting voltage Va is formed of two values, namely, first predetermined output voltage Vdc 1  and second predetermined output voltage Vdc 2 . First predetermined output voltage Vdc 1  is set at 15V, and second predetermined output voltage Vdc 2  is set at 13V. Vehicle-side control circuit  37  thus works as an adjusting voltage control section that supplies adjusting voltage Va to electrical generator  11  for adjusting output voltage Vd at output terminal  13  of electrical generator  11 . Alternatively, control circuit  35  or another control section disposed at a different place receives a signal supplied from vehicle-side control circuit  37 , and then supplies adjusting voltage Va to electrical generator  11 . 
     Further, output terminal  13  is electrically hooked up with load  19  and starter  21  via the power-carrying wirings. Load  19  represents a variety of accessories mounted in the vehicle, and starter  21  represents a motor for starting the engine. An ignition switch (not shown) controls the start or stop of starter  21 . Grounding terminal  22  of electrical generator  11  is electrically connected to the ground at the vehicle side. 
     DC/DC converter  23  includes a first end, i.e. input/output terminal  25 , and a second end, i.e. electric storage section terminal  27 , and is electrically connected via the power-carrying wirings to output terminal  13  of electrical generator  11  at input/output terminal  25 . Electric storage section  29  is electrically connected to electric storage section terminal  27  of DC/DC converter  23 . Electric storage section  29  is formed of four capacitors connected in series. Each one of those capacitors is an electric double layer capacitor and has a rated voltage of 2.5V. In this case, electric storage section  29  has an upper limit voltage Vcu of 10V, while a lower limit voltage Vck is set at 5V because the withstand current of DC/DC converter  23  is taken into consideration. Electric storage section  29  thus changes its voltage Vc within a range from 5V to 10V. 
     Output voltage Vd can be varied by adjusting voltage Va within a range from 13V to 15V, so that a voltage at input/output terminal  25  of DC/DC converter  23  can vary within the same range. A voltage at electric storage section terminal  27  represents voltage Vc of electric storage section  29  and varies within a range from 5V to 10V. DC/DC converter  23  thus lowers the voltage at input/output terminal  25  for charging electric storage section  29 , and also raises voltage Vc of electric storage section  29  for discharging storage section  29  from input/output terminal  25 . Converter  23  thus has a bi-directional converter configuration. 
     Output voltage detecting circuit  31  is electrically connected between output terminal  13  and grounding terminal  22  of electrical generator  11  directly via the signal-carrying wirings, and it detects output voltage Vd at output terminal  13 . This structure allows circuit  31  to accurately detect output voltage Vd, relative to grounding terminal  22 , free from influences of internal resistance values in the respective wirings. 
     Electric-storage section voltage detecting circuit  33  is connected between both the ends of electric storage section  29 , i.e. between terminal  27  and the grounding, via the signal-carrying wirings, and detecting circuit  33  detects voltage Vc of storage section  29 , and then inputs the voltage Vc to control circuit  35 . In other words, detecting circuit  33  is electrically connected to electric storage section  29  and control circuit  35  and detects voltage Vc of storage section  29 . 
     DC/DC converter  23 , output voltage detecting circuit  31 , electric-storage section voltage detecting circuit  33 , and vehicle-side control circuit  37  are electrically connected to control circuit  35  via the signal-carrying wirings. In other words, control circuit  35  is electrically connected to DC/DC converter  23  and output voltage detecting circuit  31 . Control circuit  35  outputs a control signal (cont) for converter  23  based on output voltage Vd detected by circuit  31 , voltage Vc detected by circuit  33 , and regeneration signal (reg) supplied from vehicle-side control circuit  37 . Control circuit  35  is formed of a comparison circuit that compares output voltage Vd and voltage Vc of storage section  29  with their reference signals, a logic circuit, a pulse-wave generating circuit that switches DC/DC converter  23 , and the like. Control circuit  35  can be also formed of a digital circuit including a microprocessor and its peripheral circuit. 
     Next, the operation of the power supply device discussed above is demonstrated hereinafter with reference to  FIGS. 2A-2E . In the following description, “travel of the vehicle” is defined as the vehicle being in one of the following traveling states: “accelerated travel”, “travel at a constant speed”, or “decelerated travel (inertia travel) without applying the break”. Decelerated travel involving the application of the break by a driver is defined as “braking”. 
     As shown in  FIG. 2A , the vehicle starts traveling at time “t 0 ”, and increases the speed with time. Rotation of the engine prompts electrical generator  11  to generate electric power involving fuel consumption. Vehicle-side control circuit  37  outputs adjusting voltage Va of which value is in response to the travel and the braking of the vehicle. As shown in  FIG. 2B , adjusting voltage Va stands at 13V during the travel. At this time, as shown in  FIG. 2C , output voltage Vd from electrical generator  11  is controlled by adjusting voltage Va such that it can stands at second predetermined output voltage Vdc 2  (13V). 
     From the time “t 0 ” to time “t 1 ” at which the acceleration ends, the vehicle travels, so that, as shown in  FIG. 2B , adjusting voltage Va of 13V is supplied to electrical generator  11 , which is then controlled such that the output voltage Vd become 13V. As a result, as shown in  FIG. 2C , output voltage Vd stays at 13V from time “t 0 ” to time “t 1 ”. 
     At time “t 0 ”, as shown in  FIG. 2E , electric storage section  29  is discharged, and voltage Vc is lower limit voltage Vck (5V). DC/DC converter  23  controls the voltage Vc for electric storage section  29  not to be further discharged. In other words, converter  23  controls the voltage at electric-storage section terminal  27 . The control target, i.e. reference voltage Vst, is the lower limit voltage Vck. 
     At this time, DC/DC converter  23  is supposed to control a voltage (output voltage Vd) at input/output terminal  25  to be second predetermined output voltage Vdc 2  (13V); however, when voltage Vc reaches the upper limit voltage Vcu (10V) or lowers to the lower limit voltage Vck (5V), converter  23  controls voltage Vc to stay at voltage Vcu or voltage Vck in order to prevent electric storage section  29  from being over-charged or over-discharged. If voltage Vc of electric storage section  29  is lowered too much, the efficiency of converter  23  is lowered. Therefore, the lower limit voltage Vck is set to avoid this reduction in efficiency. DC/DC converter  23  thus will not control the voltage (output voltage Vd) at input/output terminal  25 , but output voltage Vd can be determined only by adjusting voltage Va. The wide dotted line drawn from time “t 0 ” to time “t 1 ” shown in  FIG. 2D  indicates the reference voltage Vst relative to the voltage at input/output terminal  25 , where this voltage is supposed to be controlled originally. To be more specific, from time “t 0 ” to time “t 1 ” in  FIG. 2D , output voltage Vd is supposed to be controlled originally at upper control voltage Vdu (=13.1V) which is described later. However, since voltage Vc stands at lower limit voltage Vck, converter  23  controls voltage Vc to stay at lower limit voltage Vck. The wide dotted line drawn in  FIG. 2D  after time “t 1 ” has the same meaning, namely, this wide dotted line indicates reference voltage Vst relative to the voltage at input/output terminal  25  supposed to be controlled originally. 
     Subsequently, the driver starts applying the brakes to the vehicle at time “t 1 ” in  FIG. 2A , and electrical generator  11  produces regenerated electric power and at the same time vehicle-side control circuit  37  supplies regeneration signal “reg” to control circuit  35 . Vehicle-side control circuit  37  turns the regeneration signal ON based on a vehicle speed signal and at least one of a braking signal, fuel injection signal, and accelerator signal. In a case where vehicle-side control circuit  37  produces the regeneration signal based on a vehicle speed signal and the braking signal, the following two conditions are satisfied: the vehicle speed signal indicates that the vehicle is in speed reduction state according to the vehicle speed signal, and the braking signal is turned ON by applying the brake. In a similar way, when vehicle-side control circuit  37  produces the regeneration signal based on the vehicle speed signal and fuel injection signal, the following two conditions are satisfied: the vehicle is in speed reduction state according to the vehicle speed signal, and the accelerator pedal is not stepped on and the fuel injection signal is turned OFF. In a case where control circuit  37  produces the regeneration signal based on the vehicle speed signal and the accelerator signal, the accelerator signal is turned OFF although the vehicle speed signal indicates that the vehicle is in an accelerated state. In other words, when the vehicle travels on a downhill road, circuit  37  produces the regeneration signal. The regeneration signal can be also produced based on any three signals out of the vehicle speed signal, braking signal, fuel injection signal, and accelerator signal, or it can be produced based on all those signals. The braking signal can be an ON/OFF signal of the brake pedal or a brake oil-pressure signal. In this embodiment, those signals are collectively called the braking signal. 
     Vehicle-side control circuit  37  works as an adjusting voltage control section, and as shown in  FIG. 2B , it adjusts adjusting voltage Va of electrical generator  11  to 15V. Electrical generator  11  thus tries to raise its output voltage Vd to 15V; however, DC/DC converter  23  switches the control target to output voltage Vd at the same time, namely, converter  23  controls the voltage at input/output terminal  25 . Reference voltage Vst thus becomes lower control voltage Vdk as shown in  FIG. 2D . 
     The lower control voltage Vdk refers to a voltage lower than first predetermined output voltage Vdc 1  by first predetermined voltage range ΔV 1 . Lower control voltage Vdk is set such that main power supply  17  can be charged maximally with the regenerated electric power. To be more specific, it is preferably determined in the following way: 
     In the first place, first predetermined voltage range ΔV 1  is determined as total error range ΔVer that is a total of the maximum error range of the output voltage from electrical generator  11  and the maximum control error range of the voltage of DC/DC converter  23 . The latter range includes a detection error of output voltage detecting circuit  31 . For instance, the former range is approx. 0.05V, and the latter range is approx. 0.05V, then the total error range ΔVer turns out 0.1V. Lower control voltage Vdk is thus determined lower than first predetermined output voltage Vdc 1  (15V) by first predetermined voltage range ΔV 1  (0.1V), i.e. lower control voltage Vdk is determined 14.9V. 
     DC/DC converter  23  sets reference voltage Vst at lower control voltage Vdk as discussed above. Converter  23  thus works to lower the output voltage Vd to lower control voltage Vdk (14.9V) while electrical generator  11  tries to raise its output voltage Vd to 15V. As a result, converter  23  charges electric storage section  29 . As shown in  FIG. 2C , this mechanism allows output voltage Vd to stay at lower control voltage Vdk from time “t 1 ” to time “t 2 ” during the braking. Since electric storage section  29  is charged with the regenerated electric power as discussed above, electric-storage section voltage Vc rises with time from time “t 1 ” until time “t 2 ”. 
     On the other hand, output voltage Vd stands at 14.9V as shown in  FIG. 2C , and it is thus higher than open circuit voltage of 13V of main power supply  17 , which is thus charged with the regenerated electric power too. In general, a large amount of electric current of the regenerated electric power is absorbed in a greater amount by electric storage section  29 , which is formed of capacitors, than by main power supply  17  formed of storage batteries. For this reason, lower control voltage Vdk is generally set at approx. 14V; however, in this embodiment, voltage Vdk is set at 14.9V. Since output voltage Vd in regeneration is rather higher as discussed above, a charged amount to main power supply  17  increases, so that main power supply  17  is charged with a greater amount than in a general case, and electric storage section  29  is charged with a smaller amount than in a general case. As a result, a charged amount of the regenerated electric power directly to main power supply  17  increases, and an amount charged from electric storage section  29  to main power supply  17  decreases, so that a smaller amount of electric power flows through DC/DC converter  23  that incurs a large amount of heat loss, and the total efficiency of the vehicle is thus improved. 
     Some electric double layer capacitors employed in electric storage section  29  or some components of DC/DC converter  23  don&#39;t have enough thermal resistance against heat generated in an engine room where electrical generator  11  is disposed. It is thus necessary for electric storage section  29  and converter  23  of the power supply device to be disposed in a place avoiding a high temperature, e.g. the cabin or the trunk of the vehicle. This placement requires a long wiring between electrical generator  11  and converter  23 , and in a case of charging electric storage section  29  with the max. current when the regenerated electric power is produced, the resistance in this wiring incurs heat loss. For instance, when the resistance in the wiring is 0.02 Ω and the max. current is 100 A at the production of the regenerated electric power, the heat loss amounts to as much as 200 W. 
     In the case where electric storage section  29  is charged with a smaller amount of electricity at the production of regenerated electric power as discussed previously, the heat loss can less affect the efficiency of the vehicle together with the smaller heat loss produced in DC/DC converter  23 . As a result, although the long wiring is needed between electrical generator  11  and converter  23 , the power supply device can work efficiently. 
     Since electric storage section  29  is formed of electric double layer capacitors that are excellent in quick charge/discharge characteristics, it can be easily charged with an amount of sudden transition although the transition is steep at an initial time, i.e. time “t 1 ”, of production of the regenerated electric power. A recovery ratio as a total is thus improved. 
     It is preferable that lower control voltage Vdk is as close as possible to first predetermined output voltage Vdc 1  in order to make effective use of the regenerated electric power efficiently with the foregoing control method. However, if voltage Vdk is excessively close to voltage Vdc 1 , output voltage Vd possibly exceeds first predetermined output voltage Vdc 1  due to a variation in output voltage Vd or a control error of converter  23 . In such a case, electric storage section  29  cannot be charged although the regenerated electric power is produced, but also electric storage section  29  discharges electric power which thus flows backward. Because of the foregoing reason, lower control voltage Vdk is set lower than first predetermined output voltage Vdc 1  by first predetermined voltage range ΔV 1 . Here, lower control voltage Vdk is set such that the regenerated electric power can be charged maximally into main power supply  17 . In particular, first predetermined voltage range ΔV 1  is preferably set at total error range ΔVer. This setting allows reducing the backward flow from electric storage section  29 , and charging the regenerated electric power as much as possible into main power supply  17 , so that the efficiency as a total can be improved. First predetermined voltage range ΔV 1  can be determined a greater value than total error range ΔVer, however, it is preferable to set first predetermined voltage range ΔV 1  close to total error range ΔVer in order to charge the regenerated electric power as much as possible into main power supply  17  as well as reduce the heat loss. 
     The voltage control discussed above requires a direct connection of output voltage detecting circuit  31  to both of output terminal  13  and grounding terminal  22 . This connection allows detecting output voltage Vd without the influence of the resistance of power-carrying wirings between electrical generator  11  and converter  23 . This accurately detected output voltage Vd allows converter  23  to be controlled such that first predetermined voltage range ΔV 1  can be accurate enough on the order of 0.1V. This accurate control thus allows main power supply  17  to be charged with the regenerated electric power as much as possible, and the electric current running through converter  23  can be reduced, so that the heat loss can be lowered. As a result, the efficiency of the power supply device can be improved. 
     The description is now returned to time “t 2 ” and onward shown in  FIGS. 2A-2E . At time “t 2 ” during the braking, as shown in  FIG. 2E , voltage Vc of electric storage section  29  reaches upper limit voltage Vcu (10V). DC/DC converter  23  thus controls voltage Vc to stay at upper limit voltage Vcu as shown in  FIG. 2D  in order to prevent electric storage section  29  from being over-charged. In other words, converter  23  controls voltage Vc, detected by electric-storage-section voltage detecting circuit  33 , at electric-storage-section terminal  27 . As a result, output voltage Vd, i.e. the voltage at input/output terminal  25 , is not anymore the control target for converter  23 . Then output voltage Vd at time “t 2 ” as shown in  FIG. 2C  is controlled by adjusting voltage Va to become first predetermined output voltage Vdc 1  (15V), and the regenerated electric power is not only charged into main power supply  17  but also supplied to load  19 . 
     Next, the vehicle is accelerated again to travel at time “t 3 ” as shown in  FIG. 2A , regeneration signal (reg) is then turned OFF. Adjusting voltage Va then returns to second predetermined output voltage Vdc 2  (13V) as shown in  FIG. 2B . At this time, DC/DC converter  23  switches reference voltage Vst to upper control voltage Vdu (13.1V) as shown in  FIG. 2D  in order to discharge the regenerated electric power stored in electric storage section  29 . Converter  23  then controls output voltage Vd to be the same value as upper control voltage Vdu. 
     Upper control voltage Vdu indicates a voltage higher than second predetermined output voltage Vdc 2  by second predetermined voltage range ΔV 2 , and is close to the voltage of main power supply  17 . Voltage Vdu is set at a value which allows electric storage section  29  to discharge. Upper control voltage Vdu is preferably determined in the following way: 
     In the first place, second predetermined voltage range ΔV 2  is found as total error range Ver (0.1V). Based on the value, upper control voltage Vdu is determined higher than second predetermined output voltage Vdc 2  (13V) by second predetermined voltage range ΔV 2  (0.1V), namely, upper control voltage Vdu is determined to be 13.1V. 
     Reference voltage Vst is thus set at upper control voltage Vdu as discussed above, then DC/DC converter  23  tries to set output voltage Vd, which is about to be lowered to 13V by electrical generator  11 , at upper control voltage Vdu (13.1V). As a result, converter  23  discharges the regenerated electric power stored in electric storage section  29 . As a result, as shown in  FIG. 2C , output voltage Vd can be maintained at upper control voltage Vdu during the travel from time “t 3 ” to time “t 4 ”. Since electric storage section  29  is discharged as discussed above, voltage Vc of storage section  29  lowers with time from time “t 3 ” to time “t 4 ” as shown in  FIG. 2E . 
     In general, upper control voltage Vdu is set at approx. 14V, i.e. an intermediate between first predetermined output voltage Vdc 1  and second predetermined output voltage Vdc 2  in order to give a priority to the discharge from electric storage section  29 . In this embodiment, however, upper control voltage Vdu is set at 13.1V that is lower than the intermediate one. This setting of upper control voltage Vdu as low as at 13.1V from time “t 3 ” to time “t 4 ” allows reducing an amount of the regenerated electric power, tentatively stored in electric storage section  29 , to be charged into main power supply  17 , so that the regenerated electric power is supplied mainly to load  19  instead. The loss due to internal resistance in main power supply  17  can be thus minimized when electric storage section  29  charges main power supply  17  with the regenerated electric power. The foregoing voltage control method employed in discharging electric storage section  29  assists the power supply device in working more efficiently in addition to the increase in directly charged amount of the regenerated electric power into main power supply  17 . 
     It is preferable to set upper control voltage Vdu at a value as close as possible to second predetermined output voltage Vdc 2  in order to implement the foregoing voltage control for the better efficiency. However, setting at a value excessively close to voltage Vdc 2  possibly causes output voltage Vd to be lower than second predetermined output voltage Vdc 2  because of the variation in output voltage Vd or control error of DC/DC converter  23 . In such a case, electric storage section  29  is charged with the electric power generated by electrical generator  11  while it discharges the regenerated electric power. At this time, since the vehicle is not applied with the brake, it consumes fuel for generating electric power, thereby lowering the fuel efficiency. Because of the reasons discussed above, upper control voltage Vdu is set higher than second predetermined output voltage Vdc 2  by second predetermined voltage range ΔV 2 , which is set as narrow as possible. The foregoing voltage setting method allows setting upper control voltage Vdu at a value enabling electric storage section  29  to discharge and also as close as the voltage of main power supply  17 . To be more specific, for instance, second predetermined voltage range ΔV 2  is set at total error range ΔVer, whereby the possibility of charging electric storage section  29  can be decreased while the regenerated electric power stored in storage section  29  is supplied to load  19  as much as possible instead of being used for charging main power supply  17 . As a result, the total efficiency can be improved. 
     Second predetermined output voltage range ΔV 2  can be determined greater than total error range ΔVer; however, in order to reduce as much as possible the charging from electric storage section  29  to main power supply  17  as discussed above, second predetermined voltage range ΔV 2  is preferably set closer to total error range ΔVer. 
     In the previous discussion, first predetermined voltage range ΔV 1  is set equal to second predetermined voltage range ΔV 2 ; however, the present invention is not limited to this instance. For instance, in a case where an accuracy of output voltage detecting circuit  34  changes in response to an absolute value of the voltage, or in a case where a control accuracy by converter  23  over electric storage section  29  while it is charged differs from a control accuracy over storage section  29  while it is discharged, first predetermined voltage range ΔV 1  and second predetermined voltage range ΔV 2  can be set independently and appropriately in response to the respective cases. 
     Then as shown in  FIG. 2E , when voltage Vc reaches lower limit voltage Vc at time “t 4 ”, reference voltage Vst switches to lower limit voltage Vck as shown in  FIG. 2D . As a result, DC/DC converter  23  keeps voltage Vc at lower limit voltage Vck after time “t 4 ”. At the same time, converter  23  is not anymore involved in controlling output voltage Vd, so that output voltage Vd becomes second predetermined output voltage Vdc 2  (13V) as shown in  FIG. 2C . This state is identical to that at time “t 0 ” and onward. The operation discussed above is repeated for the regenerated electric power to be charged and discharged, thereby improving the efficiency in total. 
     As discussed above, the power supply device in accordance with the first embodiment allows vehicle-side control circuit  37 , which works as an adjusting voltage control section, to set adjusting voltage Va at first predetermined output voltage Vdc 1  at the timing when electrical generator  11  starts producing the regenerated electric power, i.e. at the time when the brakes is applied. Control circuit  35  controls DC/DC converter  23  such that output voltage Vd becomes lower control voltage Vdk that is lower than first predetermined output voltage Vdc 1  by first predetermined voltage range ΔV 1 . 
     Since lower control voltage Vdk is set at a value which allows main power supply  17  to be charged maximally with the regenerated electric power, the amount of the regenerated electric power directly charged to main power supply  17  increases, while an amount thereof charged to electric storage section  29  decreases. As a result, an amount of the electric power traveling through DC/DC converter  23  decreases. The electric power traveling through DC/DC converter  23  incurs a great amount of heat loss. The efficiency of the vehicle in total can be thus improved. 
     On the other hand, vehicle-side control circuit  37  sets adjusting voltage Va at second predetermined output voltage Vdc 2 , which is lower than first predetermined output voltage Vdc 1 , at the timing when electrical generator  11  ends the production of the regenerated electric power, i.e. at the end of braking. Control circuit  35  controls converter  23  such that output voltage Vd becomes upper control voltage Vdu that is higher than second predetermined output voltage Vdc 2  by second predetermined voltage range ΔV 2 . 
     In this mechanism, upper control voltage Vdu is preferably set at a voltage which enables electric storage section  29  to discharge and also as close as the voltage of main power supply  17 . This setting allows reducing the possibility of charging electric storage section  29 , while the regenerated electric power stored in storage section  29  can be supplied as much as possible to load  19  instead of being used for charging main power supply  17 . As a result, the efficiency in total can be improved. 
     This embodiment refers to the case where electric storage section  29  is fully charged during the braking. If a period of the braking is too short for storage section  29  to be fully charged, the operation moves immediately to the one at time “t 3 ” although voltage Vc does not yet reach upper limit voltage Vcu, and discharges electric storage section  29 . This mechanism assists in supplying the regenerated electric power even in some small amount to load  19 , so that the efficiency can be improved. 
     This embodiment also refers to the case where voltage Vc reaches lower limit voltage Vck during the travel of vehicle from time “t 3  to time “t 4  as shown in  FIG. 2E . However, if the traveling period is too short, the brake is applied before storage section  29  discharges down to lower limit voltage Vck. In this case, the operation at time “t 1 ” is carried out immediately for charging both of main power supply  17  and storage section  29  with the regenerated electric power. This mechanism allows recovering the regenerated electric power even in some small amount until voltage Vc reaches upper limit voltage Vcu, thereby improving the efficiency. 
     Exemplary Embodiment 2 
       FIGS. 3A-3E  show variations with time of respective characteristics of the power supply device in accordance with the second embodiment of the present invention.  FIG. 3A  shows a vehicle speed variation with time,  FIG. 3B  shows a variation with time of adjusting voltage Va of the electrical generator,  FIG. 3C  shows a variation with time of output voltage Vd from the electrical generator,  FIG. 3D  shows a variation with time of reference voltage Vst of the DC/DC converter, and  FIG. 3E  shows a variation with time of electric-storage section voltage Vc of an electric storage section. 
     In the present embodiment, the power supply device is mounted in an idling-stop vehicle, and its featuring operation is demonstrated hereinafter. The power supply device in accordance with the present embodiment has the same structure as the power supply device in accordance with the first embodiment shown in  FIG. 1 , so that detailed description of the structure is omitted here. In  FIG. 3 , the operation from time “t 0 ” to time “t 3 ” stays the same as that of the first embodiment. 
     As shown in  FIG. 3A , when the vehicle halts at time “t 3 ”, the engine also halts and is thrown into an idling-stop state. In this case, control circuit  35  works in the same manner as that at time “t 3 ” shown in  FIG. 2 . This control allows the regenerated electric power stored in electric storage section  29  to be discharged mainly to load  19 , so that an amount of the charge to main power supply device  17  can be reduced as much as possible. Load  19  can be thus kept being driven by the electric power supplied from storage section  29  even in the idling-stop state, and the efficiency can be improved. Since the engine is kept halted during the period from time “t 3 ” to time “t 5 ”, electrical generator  11  does not produce electric power. However, the vehicle is not in the braking state during this period, vehicle-side control circuit  37  switches adjusting voltage Va to second predetermined output voltage Vdc 2  as shown in  FIG. 3B . 
     Next, the driver of the vehicle moves (switches) his foot from the brake pedal to the accelerator pedal (both are not shown) and makes the vehicle travel. Vehicle-side control circuit  37  detects this switch of the pedals, and then immediately controls an engine start-up circuit (not shown) so as to drive starter  21 . At this time, DC/DC converter  23  controls output voltage Vd to be upper control voltage Vdu (13.1V) as shown in  FIG. 3C . This voltage is higher than the open circuit voltage (13V) of main power supply  17 , so that electric storage section  29  supplies the electric power to starter  21  via converter  23 . Starter thus receives a large amount of current, temporarily, e.g. several-hundred amperes, and voltage Vc sharply lowers from time “t 5 ” to time “t 6 ” as shown in  FIG. 3E . However, voltage Vc does not yet reach lower limit voltage Vck (5V), so that output voltage Vd is kept at upper control voltage Vdu (13.1V). 
     At time “t 6 ”, the drive of starter  21  ends, and the engine restarts, then the vehicle starts traveling as shown in  FIG. 3A . At this time, voltage Vc does not yet reach lower limit voltage Vck, so that voltage Vc lowers with time at the same slant as that between time “t 3 ” and time “t 5 ” provided that load  19  keeps consuming the same amount of electric power. During this period, the operation stays the same as that between time “t 3 ” and time “t 4 ” shown in  FIG. 2 , so that the electric storage section  29  is controlled not to charge main power supply  17  as much as possible. As a result, the efficiency can be improved as the first embodiment proves. 
     Next, when voltage Vc reaches lower limit voltage Vck at time “t 7 ”, converter  23  changes its control target from output voltage Vd to voltage Vc, and controls voltage Vc to be kept at lower limit voltage Vck. This operation is the same as that at time “t 4 ” shown in  FIG. 2 , and it allows reducing the possibility of over-discharge from electric storage section  29 . This operation also allows electrical generator  11  to control the power generation with second predetermined output voltage Vdc 2  (13V), so that the power generation by electrical generator  11  can be halted between time “t 6 ” and time “t 7 ” for saving fuel. 
     At time “t 7 ” and onward, voltage Vc is kept at lower limit voltage Vck until regeneration signal “reg” is turned on. The operation at time “t 7 ” and onward is the same as that at time “t 0 ”, so that similar operations to what are discussed previously are repeated. 
     Even when the power supply device is mounted in the idling-stop vehicle, the structure and operation discussed previously prove that this power supply device can improve the efficiency in total. 
     As discussed in the present embodiment, use of the power supply device in the idling-stop vehicle allows increasing the possibility of discharging the regenerated electric power stored in electric storage section  29  down to lower limit voltage Vck with the aid of starter  21 . The regenerated electric power thus can be more effectively used, and a greater possible amount of the regenerated electric power can be charged at the next braking. The recovery efficiency of the regenerated electric power thus can be further increased. The idling-stop vehicle stops the engine while the vehicle halts, so that the fuel is not consumed during this period. The idling-stop vehicle equipped with the power supply device in accordance with this embodiment allows increasing the efficiency of the vehicle in total and saving the fuel. 
     Exemplary Embodiment 3 
       FIG. 4  is a block diagram of the power supply device in accordance with the third embodiment of the present invention. In  FIG. 4 , a wide line indicates a power-carrying wiring, and a narrow line indicates a signal-carrying wiring. In  FIG. 4 , similar elements to those in  FIG. 1  have the same reference signs and the detailed descriptions thereof are omitted here. The third embodiment differs from the first one in the following two points: 
     (1) adjustment terminal  41  is disposed at a circuit portion which is electrically connected to electrical generator  11  and of which voltage is a control target for electrical generator  11 . In the present embodiment, a voltage control target of a circuit portion for electrical generator  11  is a positive electrode terminal of main power supply  17 . Electrical generator  11  thus controls the voltage at adjustment terminal  41 , i.e. the voltage at the positive electrode of main power supply  17 , to be adjusting voltage Va. Therefore, output voltage Vd discussed in the first and second embodiments corresponds to the voltage at adjustment terminal  41 . Adjusting voltage Va discussed in the first and second embodiments works for adjusting the voltage at output terminal  13 ; however, in the present embodiment, adjusting voltage Va works for adjusting the voltage at adjustment terminal  41  instead. 
     (2) Output voltage detecting circuit  31  is electrically and directly connected to adjustment terminal  41  instead of output terminal  13 , and also connected electrically and directly to grounding terminal  22  for detecting the voltage at adjustment terminal  41  as output voltage Vd. 
     Structures other than the foregoing two points stay the same as those in accordance with the first embodiment shown in  FIG. 1 . The power supply device in accordance with the present embodiment operates in the same manner as that in the first embodiment shown in  FIGS. 2A-2E , and as that in the second embodiment shown in  FIGS. 3A-3E , so that detailed description thereof is omitted here. 
     There is wiring resistance between output terminal  13  and adjustment terminal  41 . This wiring resistance can incur a voltage drop, which causes DC/DC converter  23  to make a control error when the following conditions are established: Electrical generator  11  controls the voltage, i.e. output voltage Vd, at adjustment terminal  41  with adjusting voltage Va as shown in  FIG. 4 , and output detecting circuit  31  detects the voltage at output terminal  13  as shown in  FIG. 1 . This control error cannot be neglected in the case where output voltage Vd is controlled as accurately as on the order of 0.1V as shown in  FIGS. 2C and 3C . This control error thus may adversely affect the control aiming at efficient operation. For the better efficiency, output voltage detecting circuit  31  thus preferably detects a voltage of the circuit portion of which voltage is a control target for electrical generator  11 . In other words, output voltage detecting circuit  31  desirably detects the voltage at adjustment terminal  41 . To achieve this detection, output voltage detecting circuit  31  is configured to detect the voltage at adjustment terminal  41  as output voltage Vd. This configuration allows forming an efficient power supply device even if electrical generator  11  is electrically connected to adjustment terminal  41 . The preferable structures and controls discussed in the first and second embodiments can be applicable to this third embodiment. 
       FIG. 4  refers to the structure where adjustment terminal  41  is disposed at the positive electrode terminal of main power supply  17 ; however, the present invention is not limited to this structure. For instance, adjustment terminal  41  can be disposed in another circuit portion at a junction between electrical generator  11  and main power supply  17 . In such a case, output voltage detecting circuit  31  can be connected to adjustment terminal  41  so that an efficient control can also be achieved. The structure and control discussed above allow providing the power supply device that can improve the total efficiency even when electrical generator  11  is electrically connected to adjustment terminal  41  and electrical generator  11  controls the voltage (i.e. output voltage Vd) at adjustment terminal  41  to be adjusting voltage Va. 
     Embodiments 1-3 refer to the case where voltage Vc is always kept higher than lower limit voltage Vck (5V); however, if electric storage section  29  is discharged due to its internal resistance when the vehicle stays in a storage state, voltage Vc may stand lower than lower limit voltage Vck at the next starting time. In such a case, control circuit  35  can control converter  23  such that electric storage section  29  is charged until voltage Vc rises up to lower limit voltage Vck after the start of the engine. 
     In embodiments 1-3 the regeneration signal “reg” is turned on or off in response to stepping-on or releasing the brake pedal of the driver; however, regeneration signal “reg” can be turned on or off in response to a fuel cut signal. In this case, electric storage section  29  can be charged with the regenerated electric power during the inertial traveling of the vehicle, so that the recovery efficiency can be further improved. However, in such a case, the recovery of the regenerated electric power invites increment in mechanical load of electrical generator  11  onto the engine, so that the inertial traveling distance can be shorter. Whether or not the regenerated electric power should be recovered during the inertial traveling, or how much the regenerated electric power should be recovered can be determined depending on which one the user gives the priority to traveling distance or recovery amount. 
     Embodiments 1-3 also refer to control circuit  35  that controls converter  23  to maintain voltage Vc at upper limit voltage Vcu when voltage Vc reaches upper limit voltage Vcu, and controls converter  23  to maintain voltage Vc at lower limit voltage Vck when voltage Vc reaches lower limit voltage Vck. However, there is a case where the control by using upper limit voltage Vcu or lower limit voltage Vck is not always needed. To be more specific, such control is not needed for a vehicle system where electric storage section  29  has a great enough capacity comparing with an amount of regenerated electric power obtained at one braking or an amount of electric power consumed by load  19 , and the amount of regenerated electric power is approx. equal to the amount of consumed electric power when the vehicle is operated. The foregoing vehicle system may not have electric-storage-section voltage detecting circuit  33 . Therefore, electric-storage-section voltage detecting circuit  33  is not necessary. 
     Embodiments 1-3 refer to adjusting voltage Va that is changed in two steps. A signal representing this adjusting voltage Va can be either an analog signal or a digital signal; however, use of the analog signal possibly invites variation in adjusting voltage Va due to noises. It is thus preferable to use the digital signal that meets the communication standard for vehicles, and to input adjusting voltage Va in the form of the digital signal to electrical generator  11 . This structure allows minimizing the adverse affect of the noises and transmitting accurately the adjusting voltage Va. As a result, the regenerated electric power can be positively recovered, and the efficiency can be improved. 
     Embodiments 1-3 refer to the structure where adjusting voltage Va is switched between first predetermined output voltage Vdc 1  and second predetermined output voltage Vdc 2 , and this switching is done instantaneously as shown in  FIGS. 2B and 3B . However, some load  19  may operate unsteadily due to this steep change in voltage. It is thus preferable to switch adjusting voltage Va gradually according to a predetermined time-elapse switching function, which is a time function indicating a variation with time of a predetermined adjusting voltage Va. To be more specific, predetermined changing speed “v” (e.g. 1-2V/sec) is used as the predetermined time-elapse switching function for changing adjusting voltage Va. In this case, predetermined changing speed “v” switches adjusting voltage Va gradually to be first predetermined output voltage Vdc 1  or to be second predetermined output voltage Vdc 2 . As a result, output voltage Vd is changed slowly according to predetermined changing speed “v”, so that the possibility of unstable operation of load  19  can be reduced. 
     There are two ways for switching the adjusting voltage Va with predetermined changing speed “v”: The first one is this: Vehicle-side control circuit  37  directly changes the signal of adjusting voltage Va according to predetermined changing speed “v”. In a case where adjusting voltage Va is raised from second predetermined output voltage Vdc 2  to first predetermined output voltage Vdc 1 , reference voltage Vst of converter  23  will not be switched also immediately to lower control voltage Vdk. Converter  23  is controlled such that reference voltage Vst becomes lower than adjusting voltage Va by first predetermined voltage range ΔV 1 . In this case, control circuit  35  controls reference voltage Vst to stay lower than adjusting voltage Va by first predetermined voltage range ΔV 1  when adjusting voltage Va becomes higher than reference voltage Vst by first predetermined voltage range ΔV 1 . The foregoing control allows preventing a backward flow from electric storage section  29  to output terminal  13 . The foregoing control also allows charging main power supply  17  and storage section  29  with the regenerated electric power at the earliest possible timing when the regenerated electric power is produced and while adjusting voltage Va is switched gradually according to predetermined changing speed “v”. The regenerated electric power thus can be recovered efficiently. 
     In a similar way, in a case where the production of the regenerated electric power ends and adjusting voltage Va is lowered from first predetermined output voltage Vdc 1  to second predetermined output voltage Vdc 2 , DC/DC converter  23  is controlled such that reference voltage Vst becomes higher than adjusting voltage Va by second predetermined voltage range ΔV 2 . In this case, control circuit  35  controls reference voltage Vst to stay higher than adjusting voltage Va by second predetermined voltage range ΔV 2  when adjusting voltage Va becomes lower than reference voltage Vst by second predetermined voltage range ΔV 2 . The foregoing control allows preventing a backward charge from output terminal  13  to electric storage section  29 . The foregoing control also allows discharging the regenerated electric power from storage section  29  at the earliest possible timing when the production of the regenerated electric power ends and while adjusting voltage Va is switched gradually according to predetermined changing speed “v”. The regenerated electric power thus can be effectively used and a period of power generation in electrical generator  11  consuming the fuel by the engine can be shortened. As a result, the higher efficiency can be expected. In a case of employing the foregoing control method, it is necessary to use electrical generator  11  that can output a voltage in multi-steps. 
     Here is another method for switching adjusting voltage Va according to predetermined changing speed “v”: In the first place, the following structure should be prepared: a signal of adjusting voltage Va supplied from vehicle-side control circuit  37  is input also to control circuit  35 . Next, in a case where adjusting voltage Va rises from second predetermined output voltage Vdc 2  to first predetermined output voltage Vdc 1 , for instance, control circuit  35  detects this change, and controls reference voltage Vst to be lower control voltage Vdk according to predetermined changing speed “v”. As a result, output voltage Vd of converter  23  also rises to lower control voltage Vdk according to predetermined changing speed “v”. In a similar way, in a case where adjusting voltage Va lowers from first predetermined output voltage Vdc 1  to second predetermined output voltage Vdc 2 , control circuit  35  controls reference voltage Vst to become upper control voltage Vdu according to predetermined changing speed “v”. The foregoing method also allows the adjusting voltage Va to be switched gradually according to predetermined changing speed “v”. 
     The foregoing description refers to the use of predetermined changing speed “v” as the predetermined time-elapse switching function. In this case, there is a premise that adjusting voltage Va varies linearly relative to time; however, if a sharp change in the voltage of load  19  is desirably minimized, the predetermined time-elapse switching function is not to be a linear function relative to time, and it is preferable that adjusting voltage Va is slowly changed during an initial stage of the switching, and then the changing speed is increased. In other words, the predetermined time-elapse switching function can be a quadratic function, a higher-order function, or an exponential function. Here is another case for the predetermined time-elapse switching function, namely, it can have multiple predetermined changing speeds “v”. Use of one of these predetermined time-elapse switching functions allows electrical generator  11  to change the amount of power generation moderately, so that the load to the engine can be reduced. 
     Embodiments 1-3 refer to the use of electric double layer capacitors for electric storage section  29 ; however, storage section  29  can be formed of electro-chemical capacitors, other capacitors, or a secondary battery. 
     Embodiments 1-3 refer to the case where the power supply device is employed in the vehicle; however, the present invention is not limited to this case, for example, the power supply device can be employed in construction equipment such as a crane, or mobile body such as an elevator, as long as they can generate the recovered electric power. 
     Embodiments 1-3 refer to load  19  or starter  21  electrically connected to output terminal  13  as power consuming accessories; however, they are not essential to the power supply device. To be more specific, in the idling-stop vehicle, for instance, electrical generator  11  works as a motor generator having a function of starter  21 , and in a case where the regenerated electric power stored in electric storage section  29  can be consumed only by the motor generator, load  19  or starter  21  is not needed for the power supply device. In a similar way, in the case where the power supply device is employed in the construction equipment or the elevator, and the motor thereof produces the regenerated electric power, which is stored in electric storage section  29  and consumed only by the motor, then load  19  is not needed for this structure. 
     At time “t 3 ” in  FIG. 2B  in accordance with the first embodiment, or at time “t 3 ” in  FIG. 3B  in accordance with the second embodiment, adjusting voltage Va of electrical generator  11  is switched from first predetermined output voltage Vdc 1  (15V) to second predetermined output voltage Vdc 2  (13V). At this time, since main power supply  17  is slow to follow the voltage change, output voltage Vd takes maximally several seconds before it reaches upper control voltage Vdu (13.1V). However, the delay of this several seconds is so small relative to X-axis, i.e. time scale, that  FIGS. 2C and 3C  depict as if output voltage Vd reaches from first predetermined output voltage Vdc 1  instantly to upper control voltage Vdu. 
     INDUSTRIAL APPLICABILITY 
     The power supply device of the present invention allows controlling the charge/discharge to/from the electric storage section for improving the efficiency in total. The power supply device is thus useful particularly as a power supply device that stores the regenerated electric power in the electric storage section while the brake is applied and discharges the regenerated electric power when necessary. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           11  electrical generator 
           13  output terminal 
           17  main power supply 
           19  load 
           21  starter 
           22  grounding terminal 
           23  DC/DC converter 
           25  input/output terminal 
           27  electric-storage section terminal 
           29  electric storage section 
           31  output voltage detecting circuit 
           33  electric-storage section voltage detecting circuit 
           35  control circuit 
           37  vehicle-side control circuit (adjusting voltage control section) 
           41  adjustment terminal