Patent Publication Number: US-6707271-B2

Title: Power supply apparatus with chargeable battery and charge/discharge method

Description:
This application is a divisional of prior application Ser. No. 09/761,754 filed Jan. 18, 2001, which is a divisional application of Ser. No. 09/139,025 filed on Aug. 24, 1998 now U.S. Pat. No. 6,204,633. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power supply apparatus using chargeable batteries. Portable devices such as a notebook personal computer, etc. can usually be operated by both AC mains and a battery. When the portable devices are operated by the AC mains, an AC adapter is used. The AC adaptor is designed so as to charge the batteries while also supplying a load with power. The present invention relates to a power supply apparatus for such a device installed with a plurality of chargeable batteries. 
     2. Description of the Related Art 
     FIG. 1 shows the configuration of a conventional power supply apparatus with chargeable batteries. The operation of this conventional power supply apparatus is described in detail in the following prior application. 
     Laid-open Patent Publication No. 8-137814 (9-322431) 
     Inventor: Seiya Kitagawa 
     Title of the Invention: Power supply apparatus 
     In FIG. 1, an AC adaptor is connected to the external power source terminal, power supplied from the AC adaptor drives a load  10  through a diode  9 , and also charges a battery  14  through a DC—DC converter  11  for charging. When the AC adaptor is not connected or the voltage drops to an abnormally low level, the potential of the cathode side of the diode  9  falls, this fall of potential is detected by a charge/discharge monitor circuit  16 , and the PWM (Pulse With Modulation) control circuit  25  inside the DC—DC converter  11  is controlled by the amount of potential drop. Thus, a FET  21  is always kept on, and the discharge current of the battery  14  is supplied to the load  10  through the DC—DC converter  11 . 
     The DC—DC converter  11  is mainly used to regulate the voltage between the AC adaptor and the battery  14 . The DC—DC converter  11  controls the charge to the battery  14  by switching on/off the FET  21  according to the control of a voltage error amplifier, a current error amplifier and a PWM comparator inside the PWM control circuit  25 . For the details of these operations, see the above-mentioned prior application. 
     In the conventional example shown in FIG. 1, since only one chargeable battery  14  (a single package) can be used, there was a problem that the operation hours of a device driven by the battery cannot be extended by connecting a plurality of chargeable batteries in parallel. This is because when there is an imbalance in the charge states of batteries connected in parallel, energy flows from charged batteries to less-charged batteries, and such a charging overcurrent which occurs in this situation may damage the batteries. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a power supply apparatus such that the energy of the batteries may be effectively used, even if a plurality of chargeable batteries are connected in parallel, and a charge/discharge method of the batteries. 
     One embodiment of the present invention comprises a charge/discharge monitor unit for judging whether or not the power supply apparatus with a plurality of chargeable batteries connected in parallel between a node of an external power source and a load, and a common ground of the external power source and the load, is in a charge state when the external power source outputs voltage sufficient to drive a load and to charge the batteries, or is in a discharge state where the external power source does not output sufficient voltage and a current is discharged from the chargeable batteries to the load; switches inserted between each of the plurality of batteries, and an ON/OFF control unit for controlling the on/off operation of the switches according to the output of the charge/discharge monitor unit and the charge/discharge state of each of the plurality of batteries. 
     By controlling the switches for controlling the charge or discharge current of each battery, the ON/OFF control unit prevents a current from flowing back from charged batteries to less-charged batteries, if there is an imbalance in the charge states of the batteries. 
     Another embodiment further comprises a voltage equivalence detector unit for detecting the equivalence of battery voltages between chargeable batteries, and a battery current direction judgement unit for judging whether a current in each battery flows in a charge direction or discharge direction. 
     When the charge/discharge monitor unit detects a charge state, for example, out of two batteries, one battery in which current is judged to flow in a charge direction is charged by switching on/off switches inserted in series and corresponding to the other battery in which current is judged to flow in a discharge direction, by the battery current direction judgement unit. When a voltage equivalence is detected between one battery during charging and the other battery by the voltage equivalence detector circuit, the charge of the other battery is then controlled by the ON/OFF control unit. 
     Another embodiment further comprises a discharge completion detector unit for detecting the discharge completion state of each chargeable battery, and a battery current direction detection unit for detecting whether a current in each battery flows in a charge direction or discharge direction. 
     When the charge/discharge monitor unit detects a shift in status from a charge state to a discharge state, for example, out of two batteries, one battery in which a current is judged to flow in a charge direction is charged by switching on/off switches inserted in series and corresponding to the other battery in which a current is judged to flow in a discharge direction, by the battery current direction detection unit. When the discharge time reaches a predetermined value, a control is performed to repeat the processes of the current direction detection and after by the battery current direction detector unit. 
     For the charge method of the plurality of chargeable batteries of the present invention, for example, the following methods are used in a power supply apparatus with a DC—DC (direct current-direct current) converter between the node of an external power source terminal and a load, and the node in parallel with a plurality of batteries for being PWM-controlled when the batteries are charged with a current, and composing a directly-connected discharge route when a current is discharged from a battery to a load. 
     When the DC—DC converter is constant-voltage-controlled so that an output voltage may become constant, for example, out of two batteries, only one battery in which a current flows in a charge direction when charging is started is charged, and when the voltage of the battery during charging and the voltage of the other battery during not charging become equal, a control is performed so that the battery not charged may be charged. 
     When the DC—DC converter is constant-current-controlled so that an output current is constant, for example, out of two batteries, one battery with a lower voltage is first charged, and when the voltage of the battery during charging and the voltage of the other battery during not charging become equal, a control is performed so that the battery not charged may also be charged. 
     For the discharge method of the plurality of chargeable batteries of the present invention, when the status of the power supply apparatus shifts from a battery charging state to a battery discharging state, for example, a control is performed so that out of two batteries, one battery during charging may be first discharged, and after the apparatus detects the completion of the discharging, the other battery is discharged. 
     As described above, according to the present invention, in a power supply apparatus where a plurality of chargeable batteries are connected in parallel, switches are switched on/off so that the charged energy of the batteries may be effectively used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more apparent from the following description, when taken in conjunction with the accompanying drawings, in which; 
     FIG. 1 shows the configuration of a conventional power supply apparatus with a chargeable battery. 
     FIG. 2 shows the principle configuration of the present invention. 
     FIG. 3 shows the basic configuration of the power supply apparatus of the present invention. 
     FIG. 4A shows a configuration example of a current detector circuit. 
     FIG. 4B is a graph showing the relationship between a detected current and an output voltage in a current detector unit. 
     FIG. 5 shows a configuration example of a charge/discharge monitor circuit. 
     FIG. 6 shows a configuration example of a battery current direction detector circuit. 
     FIG. 7 shows a configuration example of a constant current/constant voltage judgement circuit. 
     FIG. 8 shows another configuration example of a constant current/constant voltage judgement circuit. 
     FIG. 9 shows a configuration example of a battery voltage comparator circuit. 
     FIG. 10 shows a configuration example of a battery voltage equivalence detector circuit. 
     FIG. 11 explains the operation of the equivalence detector circuit shown in FIG.  10 . 
     FIG. 12A shows a configuration example of a battery charge completion detector circuit. 
     FIG. 12B shows an example of setting a threshold voltage Vth in the battery charge completion detector circuit in FIG.  12 A. 
     FIG. 13 shows a configuration example of a battery discharge completion circuit. 
     FIG. 14 shows the first embodiment of the ON/OFF control circuit of the present invention. 
     FIG. 15 is a time chart showing the operation of the first embodiment. 
     FIG. 16 shows the second embodiment of the ON/OFF control circuit of the present invention. 
     FIGS. 17A and 17B are time charts showing the operation of the second embodiment. 
     FIG. 18 shows the third embodiment of the ON/OFF control circuit of the present invention. 
     FIGS. 19A and 19B are time charts showing the operation of the third embodiment. 
     FIG. 20 shows the fourth embodiment of the ON/OFF control circuit of the present invention. 
     FIG. 21 is a time chart showing the operation of the fourth embodiment. 
     FIG. 22 shows the fifth embodiment of the ON/OFF control circuit of the present invention. 
     FIGS. 23A and 23B are time charts showing the operation of the fifth embodiment. 
     FIG. 24 shows the sixth embodiment of the ON/OFF control circuit of the present invention. 
     FIG. 25 is a time chart showing the operation of the sixth embodiment. 
     FIG. 26 shows the seventh embodiment of the ON/OFF control circuit of the present invention. 
     FIG. 27 is a time chart showing the operation of the seventh embodiment. 
     FIG. 28 shows the eighth embodiment of the ON/OFF control circuit of the present invention. 
     FIG. 29 is a time chart showing the operation of the eighth embodiment. 
     FIG. 30 shows the ninth embodiment of the ON/OFF control circuit of the present invention. 
     FIG. 31 is a time chart showing the operation of the ninth embodiment. 
     FIG. 32 shows a configuration example of a power supply apparatus in the case where an ON/OFF control circuit is composed by a microprocessor. 
     FIG. 33 is a flowchart showing a process corresponding to the ON/OFF control circuit of the first embodiment. 
     FIG. 34 is a flowchart showing a process corresponding to the ON/OFF control circuit of the second embodiment. 
     FIG. 35 is a flowchart showing a process corresponding to the ON/OFF control circuit of the third embodiment. 
     FIG. 36 is a flowchart showing a process corresponding to the ON/OFF control circuit of the fourth embodiment. 
     FIG. 37 is a flowchart showing a process corresponding to the ON/OFF control circuit of the fifth embodiment. 
     FIG. 38 is a flowchart showing a process corresponding to the ON/OFF control circuit of the sixth embodiment. 
     FIG. 39 is a flowchart showing a process corresponding to the ON/OFF control circuit of the seventh embodiment. 
     FIG. 40 is a flowchart showing a process corresponding to the ON/OFF control circuit of the eighth embodiment. 
     FIG. 41 is a flowchart showing a process corresponding to the ON/OFF control circuit of the ninth embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 shows the principle configuration of the present invention. The diagram is the principle configuration of a power supply apparatus in the case where a plurality of chargeable batteries  5  are connected in parallel between the node of an external power source, for example, an AC adaptor and a load  10 , and a common ground. 
     In FIG. 2, a charge/discharge monitor unit  1  judges whether this power supply apparatus is in a charging state where an external power source drives a load and outputs a voltage sufficient to charge batteries, or in a discharging state where the external power source does not output a voltage sufficient to charge, and a current should be discharged from the chargeable batteries. 
     The current detector unit  2  detects the current flowing in each of a plurality of batteries  5 . Although the current detector unit  2  is not needed in all the embodiments of the present inventions described later, the current detector unit  2  is shown in FIG. 2, since the current detector unit  2  is needed in many of the embodiments. 
     Switches  3  are inserted in series in each of the plurality of batteries  5 . When one of the switches  3  is switched on, a current route is established, and a charging or discharging current flows through a corresponding battery  5  connected to the switch  3 . Meanwhile, when the switch  3  is switched off, the current route is disconnected, and the charging or discharging current is stopped. 
     The ON/OFF control unit  4  controls the on/off state of the switches  3  according to the charge/discharge state of each of the plurality of batteries  5 . 
     FIG. 3 shows the basic configuration of the power supply apparatus of the present invention. The power supply apparatus shown in FIG. 3 comprises a DC—DC converter  11  for charging, connected between a diode  9  and a load  10 , two switches  12   x  and  12   y  connected in parallel with the output side of the DC—DC converter  11 , two resistors for current detection  13   x  and  13   y  connected in series between a corresponding switch and ground, two chargeable batteries  14   x  and  14   y , two current detector circuits  15   x  and  15   y  for detecting the charge/discharge current of each battery, a charge/discharge monitor circuit  16 , and an ON/OFF control circuit  17  for performing a characteristic operation in the present invention. 
     The DC—DC converter for charging comprises a capacitor  20  for eliminating the ripple on a voltage supplied from a power source connected to an external power source terminal, for example, an AC adaptor through a diode  9 , and a FET  21  switched on/off when two batteries  14   x  and  14   y  are to be charged to control charging current, and is always on when these batteries are discharged, a smoothing reactance  22 , a smoothing capacitor  23 , a fly-wheel diode  24 , and a PWM control circuit  25  for controlling the on/off of the FET  21  when the batteries  14   x  and  14   y  are to be charged. In FIG. 3, although all connections needed in the embodiments described later are shown, all the connections are not necessarily needed in all the embodiments. 
     In the present invention, although the on/off operation of both switches  12   x  and  12   y  is controlled according to a charge/discharge state of the batteries  14   x  and  14   y , the control is performed by the ON/OFF control circuit  17 . Therefore, the embodiment of the ON/OFF control circuit  17  is the main content of the present invention. Prior to the description of the embodiment of the ON/OFF control circuit  17 , partial circuits needed to describe the embodiment of the ON/OFF control circuit  17  are described below with reference to FIGS. 4 through 13. 
     FIG. 4 explains a configuration example of the two current detector circuits  15  shown in FIG.  3 . In FIG. 4A, the current detector circuit  15  comprises three operation amplifiers  27  through  29  and seven resistors r 1  through r 7 . This circuit is a so-called instrumentation amplifier, in which r 2 =r 3 , r 4 =r 5  and r 6 =r 7 . 
     In FIG. 4A, when a current flowing in a resistor R for current detection is assumed to be I, the output voltage of the instrumentation amplifier V out  can be given by the following expression; 
     
       
           V   out =( r   7   /r   4 ) {(1+2 r   2   /r   1 )  RI}+V   ref    
       
     
     where r 2 =r 3 , r 4 =r 5  and r 6 =r 7 . 
     In FIG. 4A, the positions of the battery side and switch side are the reverse of the positions shown in FIG. 3, and the positive direction of a current I is the discharge direction of the battery current. 
     According to the above expression, as shown in FIG. 4B, it is found that if the output voltage V out  of the instrumentation amplifier, that is, the current detector circuit, is greater than a reference voltage V ref , the current I flowing in the resistor R for current detection is a discharging current, and if the output voltage V out  of the current detector circuit is less than a reference voltage V ref , the current I flowing in the resistor R for current detection is a charging current. 
     FIG. 5 shows the configuration of a charge/discharge monitor circuit  16  shown in FIG.  3 . In the diagram the charge/discharge monitor circuit  16  comprises a comparator  31  for outputting an H (high level) when an input voltage to the DC—DC converter for charging  11  shown in FIG. 3, that is, the voltage on the cathode side of the diode  9 , and the voltage V 0  of a reference voltage source  32 , are inputted, and the voltage on the cathode side of the diode  9  is greater than the voltage V 0 . According to the present invention the two batteries  14   x  and  14   y  are judged to be in a charging state, if an AC adaptor as an external power source is connected to the anode side of the diode  9  in FIG.  3  and power is supplied from the AC adaptor to a load  10 , while both batteries  14   x  and  14   y  are judged to be in a discharging state, if an AC adaptor is disconnected and power is supplied from both batteries  14   x  and  14   y  to the load  10 . 
     Since if the AC adaptor is disconnected, the voltage on the cathode side of the diode  9  becomes lower compared with when the AC adaptor is connected, an L (low level) for indicating a discharging state is outputted from the comparator  31 , if the voltage on the cathode side of the diode  9  becomes lower than the voltage V 0  of the reference voltage source  32  in FIG.  5 . The V 0  of this reference voltage source  32  is set, for example, to a level lower than the voltage on the cathode side of the diode  9  when the AC adaptor is connected, and to a level higher than the voltages of the batteries  14   x  and  14   y.    
     FIG. 6 shows a configuration example of a battery current direction detector circuit used in some embodiments described later. In the diagram the current detector circuit  15  shown in FIG. 4A is used, and in addition to the current detector circuit  15  is provided a comparator  35  to which the output of the current detector circuit  15  and the reference voltage V ref  shown in FIG. 4A are inputted. The comparator  35  provides an output for indicating the discharging direction of a current as an H and the charging direction of the current as an L when the output V out  of the current detector circuit  15  is greater than the reference voltage V ref , and when the output V out  of the current detector circuit  15  is less than the reference voltage V ref , respectively. It is clear from FIG. 4B that the H and L mean discharging and charging, respectively. 
     As explained in the above-mentioned prior application, when charging the batteries, such a constant voltage control that the output voltage of the DC—DC converter  11  is constant is performed over a small current range, and such a constant current control that the output current of the converter is constant is performed if the current reaches a certain level. In some embodiments described later the on/off control of a switch is performed corresponding to either a constant voltage control (mode) or a constant current control (mode). For this reason, it becomes necessary to judge whether the charging of the batteries  14   x  and  14   y  is performed in a constant voltage mode or a constant current mode. 
     FIG. 7 shows a configuration example of a constant current/constant voltage judgement circuit for judging whether the charging of the batteries  14   x  and  14   y  is performed in a constant voltage mode or a constant current mode. The circuit shown in FIG. 7 comprises a voltage error amplifier  38  and a current error amplifier  39  which compose the PWM control circuit  25  shown in FIG. 3, and a comparator  42  for comparing the output of the voltage error amplifier  38  and the output of a current error amplifier  39 . As described above in the prior application, the PWM comparator  40  outputs a gate control signal for switching on the FET transistor  21  shown in FIG. 3 when a higher input voltage value out of the two inputs to the two inversion input terminals is lower than the voltage of a triangular wave outputted by an oscillator  41 . If the output of the voltage error amplifier  38  is higher than the output of the current error amplifier  39 , charging is performed in a constant voltage mode, and the gate signal of the FET  21  is controlled by the PWM control circuit  25  so that the voltage error may be reduced. On the other hand, if the output of the current error amplifier  39  is higher than the output of the voltage error amplifier  38 , charging is performed in a constant current mode, and the gate signal of the FET  21  is controlled by the PWM control circuit  25  so that the current error may be reduced and the output current is constant. Accordingly, it is judged that charging is performed in a constant voltage mode and a constant current mode, if the comparator  42  outputs an H and an L, respectively. 
     FIG. 8 shows another configuration example of a constant current/constant voltage judgement circuit. The judgement circuit of FIG. 8 comprises a comparator  48  and a reference voltage source  49 . Since when charging is performed in a constant voltage mode, the charging voltage of the batteries, for example, the output voltage of the converter  11  in FIG. 3, becomes higher than the output in a constant current mode, the comparator  48  outputs an H if the charge voltage becomes higher than the voltage Vs of the reference power source  49  corresponding to the value at the time of constant current charging, and it is judged from the output that the charging is performed in a constant voltage mode. 
     In some embodiments described later, when charging two batteries in a constant current mode, a control is performed so that a less-charged battery may be charged first. A battery voltage comparator shown in FIG. 9 is used to compare the charge states. That is, it is judged that a battery with a higher voltage has a greater charge. Although the circuit shown in FIG. 9 is substantially the same as the battery current direction detector circuit shown in FIG. 6, the circuit shown in FIG. 9 differs in that the inputs are not the voltages across the resistor for current detection, but are the voltages Vx and Vy of both batteries. Then, the comparator  51  outputs an H when Vx&gt;Vy. 
     In some embodiments described later the on/off control of the switches is controlled, if the voltages of both batteries are almost the same. FIG. 10 shows a configuration example of a battery voltage equivalence detector circuit for judging whether or not the voltages of both batteries are almost the same in order to control the on/off operation. The circuit in FIG. 10 comprises two comparators  53  and  54 , two offset voltage sources  55  and  56 , and an AND circuit  57 . 
     FIG. 11 explains the operation of the equivalence detector circuit shown in FIG.  10 . In FIG. 11 the output A of the comparator  53  and the output B of the comparator  54  corresponding to the voltages Vx and Vy, respectively, and the output C of the AND circuit  57 , are shown. As shown in FIG. 11, the output C of the AND circuit  57  becomes H while the sum of Vx and an offset voltage is greater than Vy, and the sum Vy and the offset voltage is greater than Vx. That is, if the absolute value of the difference between Vx and Vy is less than the offset voltage, an H for indicating that both battery voltages are equal is outputted. 
     In some embodiments described later the control operation is switched, if the charging of one of the batteries is completed when charging both batteries. FIG. 12A shows a configuration example of a battery charge completion detector circuit. The detector circuit in FIG. 12A comprises a current detector circuit  15 , a comparator  58  and a threshold voltage source  59  for providing the comparator  58  with a threshold voltage. Since the charging current of the battery decreases as the charge increases, the charging is judged to be completed when the charging current of the battery becomes less than a certain value. As described earlier, the battery current becomes 0 when the output of the current detector circuit reaches the reference voltage V ref . Accordingly, by setting the threshold voltage a little bit lower than this reference voltage V ref , as shown in FIG. 14B, the comparator  58  outputs an H indicating the completion of the charging when the output of the current detector circuit  15  exceeds the threshold voltage Vth. 
     Furthermore, in some embodiments of the present invention, the control operation is switched when the discharging of one battery is completed. FIG. 13 shows a configuration example of a battery discharge completion circuit for switching the control operation. The circuit shown in FIG. 13 is for detecting the completion of the discharging of, for example, the battery  14   x . The comparator  61  outputs an H indicating the completion of the discharging when the voltage of the battery  14   x  becomes lower than the threshold voltage Vth of the threshold voltage source  62 . 
     Next, the embodiment of the ON/OFF control circuit  17  in FIG. 3 of the present invention is described below using a variety of the partial circuits described above. 
     FIG. 14 shows the first embodiment of the ON/OFF control circuit  17  of the present invention. The ON/OFF control circuit  17  in FIG. 14 comprises a battery X charge completion detector circuit  66   x , a battery Y charge completion detector circuit  66   y , two AND circuits  67  and  68 , and two inverters  69  and  70 . 
     Although in FIG. 14 a NOR circuit  64  and an inverter  65  are added to the configuration of FIG. 3, the NOR circuit  64  and inverter  65  are for controlling the on/off operation of the FET  21  using the outputs of the charge/discharge monitor circuit  16  and the PWM control circuit  25 . That is, the output of the charge/discharge monitor circuit  16  becomes H when charging the batteries, and the output of the inverter  65  becomes L. Thus, the output of the PWM control circuit  25  is inverted by the NOR circuit  64 , and is provided to the gate of the FET  21 . Since the FET  21  is, for example, a P channel device, the FET  21  is on when the output of the PWM control circuit  25  is H, that is, the output of the NOR circuit  64  is L, and the on/off operation of the FET  21  is controlled by the output of the PWM control circuit  25 . On the other hand, when discharging the batteries, since the output of the charge/discharge monitor circuit  16  becomes L, the output of the inverter  65  becomes H, and the output of the NOR circuit  64  becomes L regardless of the output of the PWM control circuit  25 . Accordingly, when discharging the batteries, the FET  21  is always kept on. 
     In the first embodiment in FIG. 14, a control operation is performed when discharging the batteries. For example, first, a switch  12   x  is switched on, and the battery  14   x  is charged. Then, after the completion of the charging, switches  12   x  and  12   y  are switched off and on, respectively, and the battery  14   y  is charged. 
     FIG. 15 is a time chart showing the operation of the first embodiment shown in FIG.  14 . When in FIG. 14, an AC adaptor is connected and the power supply apparatus enters the charging state, the output A of the charge/discharge monitor circuit  16  becomes H. At this moment, the outputs B and C of the two charge/discharge completion detector circuits  66   x  and  66   y  are both L, and thereby the output of the AND circuit  67  (AND 1 ) becomes H. Accordingly, the switch  12   x  is switched on, and the charging of the battery  14   x  is started. 
     When the charging is completed, the signal B becomes H. Accordingly, the output of the AND circuit  67  becomes L, and the switch  12   x  is switched off. At this moment, since the signal C is L, all the three inputs to the AND circuit  68  are H, the output becomes H, the switch  12   y  is switched on, and the battery  14   y  is charged. When the charging of the battery  14   y  is completed, the output C of the charge/discharge completion circuit  66   y  becomes H, the output of the AND circuit  68  becomes L, and the switch  12   y  is switched off. 
     FIG. 16 shows the second embodiment of the ON/OFF control circuit of the present invention. The circuit of FIG. 16 comprises a battery voltage comparator circuit  71  for comparing the voltages of the two batteries  14   x  and  14   y  when charging the batteries, and two charge completion detector circuits  66   x  and  66   y  corresponding to each battery, similar to the charge completion detector circuits shown in FIG.  14 . The ON/OFF control circuit compares the battery voltages when charging is started, switches on first a switch corresponding to the battery with a lower voltage, and when the charging of this battery is completed, charges the other battery. 
     FIGS. 17A and 17B are time charts showing the operation of the second embodiment shown in FIG.  16 . First, when the charge/discharge monitor circuit  16  in FIG. 16 detects the charging state of the power supply apparatus, the output becomes H, corresponding to which the output E of a mono-stable multi-vibrator (mono-multi)  72  becomes H. Then, the result is inputted to an AND circuit  74  (AND 1 ). On the other hand, the battery voltage comparator circuit  71  compares the voltages of both batteries  14   x  and  14   y , and when the voltage of the battery  14   x  is greater than the voltage of the battery  14   y , the output D becomes H as shown in FIG.  17 A. In this case, the output of the AND circuit  74  becomes H, and the output F of a flip-flop  75  also becomes H. Since the outputs B and C of the charge completion detector circuits  66   x  and  66   y  are both L at this moment, as a result the output of an OR circuit  78  (OR 1 ) and the output of an OR circuit  80  (OR 2 ) become L and H, respectively. 
     When the output E of the mono-multi  72  becomes L after a certain time elapses, the output AND 1  of the AND circuit  74  becomes L, all inputs to an AND circuit  81  become H, and the output becomes H. Then, the switch  12   y  is switched on, and the battery  14   y  is charged. At this time too, since the outputs B and C of both charge completion detector circuits  66   x  and  66   y  are both L, the output of an inverter  83  is H, and this output is inputted to the AND circuit  81 . 
     When the charging of the battery  14   y  is completed, the output C of the charge completion detector circuit  66   y  becomes H. As a result, the output AND 3  of the AND circuit  81  becomes L, and the switch  12   y  is switched off. Simultaneously, the output of the OR circuit  78  (OR 1 ) becomes H, as a result the output of the AND circuit  79  (AND 2 ) becomes H. Then, the switch  12   x  is switched on, and the charging of the battery  14   x  is started. 
     When the charging of the battery  14   x  is completed, the output B of the charge completion detector circuit  66   x  becomes H, as a result the output AND 2  of the AND circuit  79  becomes L, and the switch  12   x  is switched off. 
     FIG. 17B is a timechart showing the operation in the case where out of the two batteries, the voltage of the battery  14   y  is higher than the voltage of the battery  14   x  in an initial state. In this case, since the output D of the battery voltage comparator circuit  71  is L even if the output A of the charge/discharge monitor circuit  16  becomes H, only the output E of the mono-multi  72  becomes H. Since the outputs of two inverters  77  and  82  are both H, the output AND 2  of the AND circuit  79  becomes H and the switch  12   x  is switched on when the output E becomes L. 
     When the charging of the battery  14   x  is completed, the output B of the detector circuit  66   x  becomes H, the output AND 2  of the AND circuit  79  becomes L, and the switch  12   x  is switched off. Simultaneously, the output of the OR circuit  80  becomes H, the output AND 3  of the AND circuit  81  becomes H since the output of the inverter  83  is H, and the switch  12   y  is switched on. Then, when the charging of the battery  14   y  is completed, the output C of the detector circuit  66   y  becomes H, the output AND  3  of the AND circuit  81  becomes L, and the switch  12   y  is switched off. 
     FIG. 18 shows the third embodiment of the ON/OFF control circuit of the present invention. The ON/OFF control circuit of the third embodiment comprises battery current direction detector circuits  86   x  and  86   y  for detecting whether the current of each battery is in a charging direction or a discharging direction, in addition to a battery voltage equivalence detector circuit  85 . The ON/OFF control circuit switches off a switch on the side of the battery in which the current flows in a discharging direction when starting charging, charges the other battery in which the battery current flows in a charging direction, and when the voltage of the battery being charged and the voltage of the battery connected to the switch which is switched off become equal, the switched-off switch is also switched on. This is to prevent an abnormally increased charging current from damaging the batteries, because when the charges in the batteries are imbalanced, the less-charged battery is charged by the charged battery when starting charging. 
     FIGS. 19A and 19B are time charts showing the operation of the third embodiment in FIG.  18 . FIG. 19A is a timechart in the case where it is judged that the charge of the battery  14   x  is greater than the charge of the battery  14   y  and the current flows in a discharging direction, when starting charging. When the power supply apparatus enters a charging state and the output A of the charge/discharge monitor circuit  16  in FIG. 18 becomes H, the outputs of two inverters  89  and  90  are both H, since the outputs of two battery current direction detector circuits  86   x  and  86   y  are both L. As a result, the outputs of two AND circuits  91  and  92  (AND 1  and AND 2 ) become both H, two switches  12   x  and  12   y  are both switched on, and the battery current direction is detected. 
     When, as mentioned above, it is judged that the charge of the battery  14   x  is greater than the charge of the battery  14   y  and the battery current flows in a discharging direction, the output B of the battery current direction detector circuit  86   x  becomes H, as a result the output E of a flip-flop  87  becomes H, and the output AND 1  of the AND circuit  91  becomes L. Thus, the switch  12   x  is switched off and only the battery  14   y  is charged. 
     When the voltage of the battery  14   x  rises and it is judged by the battery voltage equivalence detector  85  that the voltage of the battery  14   x  becomes equal to the voltage of the battery  14   y , the output D of the battery voltage equivalence detector  85  becomes H, and the output E of the flip-flop  87  is reset and becomes L. As a result, the output of the AND circuit  91  becomes H, the switch  12   x  is switched on, and both batteries are charged. 
     FIG. 19B is a timechart in the case where the charge of the battery  14   y  is greater than the charge of the battery  14   x . In this case, although there are some differences from the case shown in FIG. 19A in that the output of the battery current direction detector circuit  86   y  becomes H, the basic operations are the same as those in FIG.  19 A. Therefore, the detailed description is omitted here. 
     FIG. 20 shows the fourth embodiment of the ON/OFF control circuit of the present invention. Although in this fourth embodiment, when starting charging, the direction of each battery current is detected in the same way as in FIG. 18, a switch on the side of the battery in which the current flows in a discharging direction is switched off, and the other battery is charged, the embodiment differs from that in FIG. 18 in that after a certain time elapses, the two switches are both switched on, and the operations of the detection of the direction of the battery current and after are repeated. For this reason, in FIG. 20 a mono-multi  95  for specifying this certain time, an OR circuit  94  at the input stage of the mono-multi  95 , a flip-flop  96  at the output stage of the mono-multi  95 , and an inverter  97  for resetting the flip-flop  96  are added to the configuration shown in FIG.  18 . 
     FIG. 21 is a time chart showing the operation of the fourth embodiment shown in FIG.  20 . When the apparatus enters a charging state and the output A of the charge/discharge monitor circuit  16  becomes H, the outputs of the two AND circuits  91  and  92  become H as shown in FIG. 18, the switches  12   x  and  12   y  are both switched on, and the directions of the battery currents are detected by the two current direction detector circuits  86   x  and  86   y . As shown in FIG. 19A, when the current of the battery  14   x  is judged to be in a discharging direction, the output B of the detector circuit  86   x  becomes H. Accordingly, in FIG. 21, the output D of the flip-flop  87 , the output of the OR circuit  94  (OR 1 ) and the output F of the mono-multi  95  all become H, the output AND 1  of the AND circuit  91  becomes L, and the switch  12   x  is switched off. 
     After a certain time corresponding to the output pulse width of the mono-multi  95  elapses, the output F of the mono-multi  95  becomes L, and as a result the output G of the flip-flop  96  operating on the falling edge becomes H. Then, the flip-flop  87  is reset, the output D of the flip-flop  87  becomes L, and the output AND 1  of the AND circuit  91  becomes H. Since the output of the OR circuit  94  becomes L, the output of the inverter  97  becomes H, and as a result, the flip-flop  96  is reset, the period where the output G of the flip-flop  96  remains H becomes very short. 
     When the switch  12   x  is also switched on, the direction of the battery current is again detected and it is judged that the current direction of the battery is in a discharging direction, the output B of the detector circuit  86   x  becomes H again, as a result the output D of the flip-flop  87 , the output OR 1  of the OR circuit  94  and the output F of the mono-multi  95  all become H. Then, the output AND 1  of the AND circuit  91  becomes L, the switch  12   x  is switched off and the charging of the battery  14   y  is continued. 
     When the output F of the mono-multi  95  becomes L again, the output G of the flip-flop  96  becomes H, and the output D of the flip-flop  87  and the output OR 1  of the OR circuit  94  both become L. As a result, the output AND 1  of the AND circuit  91  becomes H, and the switch  12   x  is switched on. If the results of the current direction detections for the two batteries are both charging directions, after that both batteries  14   x  and  14   y  are charged. 
     FIG. 22 shows the fifth embodiment of the ON/OFF control circuit of the present invention. In the fifth embodiment, when charging is started the batteries, it is judged whether a charging is performed in a constant current mode or in a constant voltage mode. In the case of a constant voltage mode charging is controlled in the same way as in the third embodiment shown in FIG. 18, while in the case of a constant current mode the voltages of two batteries are compared, the battery with a lower voltage is charged first, and when it is judged that the voltages of both batteries become equal, both batteries are charged. 
     A constant current/constant voltage judgement circuit  101  in FIG. 22 outputs a H while charging is performed in a constant voltage mode, as shown in FIGS. 7 and 8. In the constant voltage mode the output of the ON/OFF control circuit  102  in FIG. 18, that is, the outputs of two AND circuits  91  and  92  become valid, and these outputs are used for the on/off control of the two switches  12   x  and  12   y  through AND circuits  115  and  116  (AND 3  and AND 4 ) and OR circuits  112  and  113  (OR 3  and OR 4 ). In the case of the constant current mode the constant current/constant voltage judgement circuit  101  outputs an L, which is provided to the two AND circuits  110  and  111  (AND 5  and AND 6 ) through an inverter  114 . The on/off control by these two AND circuits of both switches becomes available by using these outputs. 
     FIGS. 23A and 23B are time charts showing the operation of the fifth embodiment. FIG. 23A is a timechart in the case where the voltage of the battery  14   x  is higher than the voltage of the battery  14   y . When the output A of the charge/discharge monitor circuit  16  shown in FIG. 22 becomes H, the output G of a mono-multi  104 , the output H of a flip-flop  105  and the output of an OR circuit  108  (OR 1 ) all become H. Then, when the output E of a battery voltage comparator circuit  71  becomes H, which indicates that the voltage of the battery  14   x  is higher than the voltage of the battery  14   y , the output of an OR circuit  107  (OR 5 ) becomes H, the flip-flop  105  is reset, and the output H of the flip-flop  105 , the output OR 1  of the OR circuit  108  both become L. The output I of a flip-flop  106  and the output of an OR circuit  109  (OR 2 ) are both H. After a certain time corresponding to the output pulse width of the mono-multi  104  elapses, the output G of the mono-multi  104  becomes L, the output of an inverter  117  becomes H, as a result the output AND 6  of the AND circuit  111  becomes H, and the switch  12   y  is switched on through the OR circuit  113 . 
     When it is judged by a battery voltage equivalence detector circuit  85  that the voltages of both batteries become equal and the output D of the battery voltage equivalence detector circuit  85  becomes H, the flip-flop  106  is reset, but the output OR 2  of the OR circuit  109  remains H, and the output OR 1  of the OR circuit  108  also becomes H. Thus, since the output AND 5  of the AND circuit  110  becomes H and the output AND 6  of the AND circuit  111  remains H, both switches  12   x  and  12   y  are switched on and both batteries are charged. 
     FIG. 23B is a timechart in the case where the voltage of the battery  14   y  is higher than the voltage of the battery  14   x  when charging is started. In this case, when the output A of the charge/discharge monitor circuit  16  becomes H, the output G of the mono-multi  104 , the output H of the flip-flop  105  and the output of the OR circuit  108  all become H. Then, when the output G of the mono-multi  104  becomes L, the output AND 5  of the AND circuit  111  becomes H, the switch  12   x  is switched on and the charging of the battery  14   x  is started. When the output D of the battery voltage equivalence detector circuit  85  becomes H, the output OR 2  of the OR circuit  109  becomes H. As a result, both outputs of the AND circuit  111  and the OR circuit  113  become H, the switch  12   y  is switched on and both batteries are charged. 
     FIG. 24 shows the sixth embodiment of the ON/OFF control circuit of the present invention. In this embodiment, when the status of the power supply apparatus charges to a battery discharge state, out of the two batteries one battery is first discharged, and when the discharging completion of this battery is detected, the other battery is discharged. 
     FIG. 25 is a time chart showing the operation of the sixth embodiment shown in FIG.  24 . The timechart is described assuming that the battery  14   x  is first discharged in the diagram. First, when the output A of the charge/discharge monitor detector circuit  16  becomes L, which indicates that the battery is in a discharging state, the output of an AND circuit  123  (AND 1 ) becomes H, the switch  12   x  is switched on and the battery  14   x  is discharged, since at this moment the outputs B and C of two discharge completion detector circuits  66   x  and  66   y  are L, which indicates that the discharging of the battery is not completed. 
     When the discharging of the battery  14   x  is completed, the output B of the discharge completion detector circuit  66   x  becomes H, the output of the AND circuit  123  becomes L, and the output of an AND circuit  124  (AND 2 ) becomes H. Thus the switch  12   y  is switched on and the battery  14   y  is discharged. Then, when the discharging of the battery  14   y  is also completed, the output C of the discharge completion detector circuit  66   y  also becomes H, the output of the AND circuit  124  becomes L, and the switch  12   y  is switched off. 
     FIG. 26 shows the seventh embodiment of the ON/OFF control circuit of the present invention. In the seventh embodiment the power supply apparatus moves to a discharging state in a condition where the charge has been so far controlled, that is, the on/off operation of the two switches has been maintained. When out of the two switches, for example, a switch  12   x  is switched on and a battery  14   x  is charged, the state of the switched is maintained so that the battery  14   x  is discharged first. Then, after the discharging of this battery  14   x  is completed, the other battery  14   y  is discharged in the same way as the sixth embodiment shown in FIG.  24 . 
     FIG. 27 is a time chart showing the operation of the seventh embodiment shown in FIG.  26 . In FIG. 26, the status of the batteries charges to a discharging state in a condition where out of the two outputs of a charge ON/OFF control circuit  130 , an output F is H and the switch  12   x  is switched on through an OR circuit  131  (OR 1 ). 
     When the output A of a charge/discharge monitor circuit  16  becomes L, the output of an inverter  133  becomes H. On the other hand, since the output of an OR circuit  132  (OR 2 ) and the output B of a discharge completion detector circuit  66   x  are both L, the output of an AND circuit  137  (AND 1 ) is H. For this reason, the output of an AND circuit  134  (AND 3 ) and the output D of a flip-flop  135  become both H, the output of the OR circuit  131  remains H, even if the output F of the charge ON/OFF control circuit  130  becomes L, and the switch  12   x  is kept on. 
     When the discharge completion detector circuit  66   x  detects the discharging completion of the battery  14   x , the output B of the discharge completion detector circuit  66   x  becomes H, and the output D of the flip-flop  135  is reset. Then, the output OR 1  of the OR circuit  131  becomes L, and the switch  12   x  is switched off. Since the output OR 1  of the OR circuit  131  becomes L, the output of an inverter  138  becomes H. Since at this time the output C of a discharge completion detector circuit  66   y  is L, the outputs of AND circuits  141  (AND 2 ) and  142  (AND 4 ), the output E of a flip-flop  143  and the output OR 2  of the OR circuit  132  all become H, a switch  12   y  is switched on, and the battery  14   y  is discharged. 
     When the discharging of the battery  14   y  is completed, the output C of a discharge completion detector circuit  66   y  becomes H. For this reason, the outputs of the AND circuits  141  and  142  the output E of the flip-flop  143  and the output of the OR circuit  132  all become L, and the switch  12   y  is switched off. For the discharge ON/OFF control circuit  139  of this seventh embodiment any of the ON/OFF control circuits of the first through fifth embodiments can be used. 
     FIG. 28 shows the eighth embodiment of the ON/OFF control circuit of the present invention. In this eighth embodiment, when the status of the batteries of the power supply apparatus charges to a discharging state, the detection of the current direction of the batteries is performed, a switch corresponding to the battery in which the battery current flows in a charging direction is switched off, and the other battery is discharged. Then, every time a certain time elapses, the directions of the battery currents are detected, and the control corresponding to the result is repeated in the same way. 
     FIG. 29 is a time chart showing the operation of the eighth embodiment shown in FIG.  28 . The timechart shown in FIG. 29 is described for the case where it is judged that out of the two batteries  14   x  and  14   y , the current of the battery  14   x  is in a charging direction. 
     When the output A of the charge/discharge monitor circuit  16  in FIG. 28 becomes L, which indicates a discharging state, the outputs of two AND circuits  149  and  152  (AND 1  and AND 2 ) become both H, both switches  12   x  and  12   y  are switched on, and current flows in both batteries, since in an initial condition the outputs D and E of two flip-flops  147  and  155 , respectively, are both L. When the output B of the battery current direction detector circuit  86   x  becomes L, which indicates the charge status of the battery, the output D of the flip-flop  147  becomes H, and the output AND 1  of the AND circuit  149  becomes L, and the switch  12   x  is switched off. Simultaneously, the output of an OR circuit  158  (OR 3 ) becomes H, and the output F of a mono-multi  159  becomes H. Then, since the switch  12   x  is switched off, the battery current direction detector circuit  86   x  stops the current direction detection operation, and after a little while the output B of the battery current direction detector circuit  86   x  becomes H. Then, the output OR 3  of the OR circuit  158  becomes L. 
     When the output F of the mono-multi  159  becomes L, the output G of a flip-flop  160  operating on the falling edge becomes H, the flip-flop  147  is reset through an OR circuit (OR 1 ), and the output D of the flip-flop  147  becomes L. Thus, the output of the AND circuit  149  becomes H, the switch  12   x  is switched on, and the direction of the battery current is detected again. At this time the output of an OR circuit (OR 2 ) also becomes H. 
     When the output B of the battery current direction detector circuit  86   x  becomes L again, which indicates a charging direction, the output D of the flip-flop  147  becomes H again, the output AND 1  of the AND circuit  149  becomes L, and the switch  12   x  is switched off. Then, the output OR 3  of the OR circuit  158  becomes H, and the output F of the mono-multi  159  becomes H again. 
     After a time corresponding to the pulse width of the output F of the mono-multi  159  elapses again, the output G of a flip-flop  160  becomes H in the same way as described earlier, the flip-flop  147  is reset, and the output AND 1  of the AND circuit  149  becomes H. For this reason, the switch  12   x  is switched on, and the current direction is detected for a third time. When it is assumed that the two current direction detector circuits  86   x  and  86   y  detect that the currents of both batteries  14   x  and  14   y  are in a discharging direction, the flip-flop  147  is not reset, the output AND 1  of the AND circuit  149  remains H, and the switch  12   x  is kept on. After that both batteries  14   x  and  14   y  are discharged. 
     FIG. 30 shows the ninth embodiment of the ON/OFF control circuit of the present invention. In the ninth embodiment, like the eighth embodiment shown in FIG. 28, when the status of the batteries of the power supply apparatus charges to a discharging state, the direction of the battery current is detected, a switch corresponding to one battery whose current flows in a charging direction is switched off, and the other battery is discharged. Then, when it is judged that the voltages of two batteries become equal, the switch which has so far been off is switched on, and both batteries are discharged. 
     FIG. 31 is a time chart showing the operation of the ninth embodiment shown in FIG.  30 . The timechart shown in FIG. 30 is described for the case where the current direction of the battery  14   x  is in a charging direction when charging is started. 
     The output A of the charge/discharge monitor circuit  16  in FIG. 30 becomes L, and as in FIG. 29 the outputs of two AND circuits  168  and  174  (AND 1  and AND 2 ) both become H, both switches are switched on, and the current directions are detected. At this moment, the outputs of two OR circuits  169  and  172  (OR 1  and OR 2 ) become both L, since the output A of the charge/discharge monitor circuit  16  which was H becomes L. 
     When the output B of the current direction detector circuit  86   x  becomes L, as described earlier, the output E of a flip-flop  166  becomes H, the output AND 1  of the AND circuit  168  becomes L, and the switch  12   x  is switched off. Then, when the output D of the battery voltage equivalence detector circuit  85  becomes H, which indicates that the voltages of both batteries are equal, the outputs of the two OR circuits  169  and  172  both become H, the flip-flop  166  is reset, and the output E of the flip-flop  166  becomes L. For this reason, the output AND 1  of the AND circuit  168  becomes H, the switch  12   x  is switched on, and after that both batteries are discharged. 
     Although the hardware configurations of the variety of embodiments of the ON/OFF control circuit of the present invention have been so far described in detail, this ON/OFF control circuit can also be configured using a microprocessor, and thereby the on/off operation of both switches can also be controlled by way of software. FIG. 32 shows a configuration example of such a power supply apparatus. In FIG. 32 a microprocessor  180  is used instead of the ON/OFF control circuit  17  shown in FIG.  3 . The on/off control of two switches in this power supply apparatus is described below with reference to FIGS. 33 through 41. 
     FIG. 33 is a flowchart showing a process corresponding to the ON/OFF control circuit of the first embodiment shown in FIG.  14 . When the process is started as shown in FIG. 33, first in step S 1  the switches  12   x  and  12   y  are switched on and off, respectively, and the battery  14   x  is charged. In step S 2  it is judged whether or not the charging of the battery  14   x  is completed. If the charge is completed, in step S 3 , contrary to the above, the switches  12   x  and  12   y  are switched off and on, respectively, in steps S 4  it is judged whether or not the charging of the battery  14   y  is completed, and if the charging is completed, the process is terminated. 
     FIG. 34 is a flowchart showing a process corresponding to the ON/OFF control circuit of the second embodiment in FIG.  16 . When the process is started as shown in FIG. 34, first, in step S 6  both switches  14   x  and  14   y  are switched off. In step S 7  the voltages of both batteries are compared, and it is judged whether or not the voltage Vx of the battery  14   x  is higher than the voltage Vy of the battery  14   y . If the voltage Vx is not higher than the voltage Vy, in steps S 8  through S 11 , as shown in FIG. 33, both batteries, first  14   x  and then  14   y  are charged. 
     If in step S 7  the voltage Vx of the battery  14   x  is higher than the voltage Vy of the battery  14   y , in steps S 12  through  15  both batteries, first  14   y  and then  14   x , are charged. 
     FIG. 35 is a flowchart showing a process corresponding to the ON/OFF control circuit of the third embodiment shown in FIG.  18 . When the process is started as shown in FIG. 35, first, in step S 21  both switches  12   x  and  12   y  are switched on. In step S 22 , if as a result of detecting the current directions of both batteries it is judged that the current Ix of the battery  14   x  is in a discharging direction, in step S 23  the switch  12   x  is switched off and the battery  14   y  is charged. When in step S 24  it is judged that the voltages of both batteries are equal, in step S 25  the switch  12   x  is switched on and the battery  14   x  is charged. Then, in step S 26 , when it is judged that the charging of both batteries is completed, the process is terminated. 
     When in step S 22  it is judged that the current direction of the battery  14   x  is not in a discharging direction, in step S 27  it is judged whether or not the current Iy of the battery  14   y  is in a discharging direction. If the current Iy is in a discharging direction, in steps S 28  through  30  the battery  14   x  is charged until the voltages of both batteries become equal. After the voltages of both batteries become equal, both batteries are charged, and the process flow moves to the process in step S 26 . If in step S 27  it is judged that the current of the battery  14   y  is not in a discharging direction, both batteries are charged until in step S 26  it is judged that the charging of both batteries is completed. 
     FIG. 36 is a flowchart showing a process corresponding to the ON/OFF control circuit of the fourth embodiment shown in FIG.  20 . In FIG. 36 the processes similar to those in FIG. 35 are executed. That is, if in step S 22  it is judged that the current flow of the battery  14   x  is in a discharging direction, in step S 23  the switch  12   x  is switched off, and then in step S 31  the state is left as it is for a certain time corresponding to the pulse width of the mono-stable multi-vibrator  95  shown in FIG.  20 . Then, the process flow returns to step S 21 , where both switches are switched on, and the processes of the detection of the battery current direction and after are executed. 
     When in step S 27  it is judged that the current direction of the battery  14   y  is in a discharging direction, in step S 28  the switch  12   y  is switched off, and then the processes in step S 31  and after are executed. If in step S 27  it is judged that the current direction of the battery  14   y  is not in a discharging direction, in step S 26  the judgement process is repeated until the charging of both batteries is completed. 
     FIG. 37 is a flowchart showing a process corresponding to the ON/OFF control circuit of the fifth embodiment shown in FIG.  22 . In FIG. 37, different charge controls are employed depending on whether the charging of the battery is controlled in a constant voltage mode or in a constant current mode. When the process is started, first, in step S 32  it is judged whether or not charging is performed in a constant voltage mode. If the charging is performed in a constant voltage mode, in steps S 33  through S 36 , as shown in steps S 21  through S 24  in FIG. 35, as a result of the current direction detection of the batteries it is judged that the current flow of the battery  14   x  is in a discharging direction, the switch  12   x  is switched off, and only the battery  14   y  is charged until the voltages of both batteries become equal. If it is judged that the voltages of both batteries are equal, in step S 37  the switch  12   x  is also switched on, and if in step S 38  it is judged that the charging of both batteries is completed, the process is terminated. 
     If in step S 34  it is judged that the current direction of the battery  14   x  is not a discharging direction, in steps S 39  through S 41  the same processes as in steps S 27  through S 29  in FIG. 35 are executed, and then the process flow moves to the process of step S 37 . If in step S 39  it is judged that the current direction of the battery  14   y  is not a discharging direction, the flow immediately moves to the process of step S 38 . 
     Next, if in step S 32  it is judged that the charging is not performed in a constant voltage mode, first, in step S 42  both switches  12   x  and  12   y  are switched off, and in step S 43  the voltages of both batteries are compared. If the voltage of the battery  14   x  is higher than the voltage of the battery  14   y , of steps S 44  and S 45  the battery  14   y  is charged until the voltages of both batteries become equal, and then the flow moves to the process of step S 37 . If the voltage of the battery  14   y  is higher than the voltage of the battery  14   x , in steps S 46  and S 47  the battery  14   x  is charged until the voltages of both batteries become equal, and then the flow moves to the process of step S 37 . 
     FIG. 38 is a flowchart showing a process corresponding to the ON/OFF control circuit of the sixth embodiment shown in FIG.  24 . In FIG. 38, first, the battery  14   x  is discharged, and after discharging is completed, the battery  14   y  is discharged. The process is basically the same as that for charging as shown in FIG. 33, except that lastly in step S 52  both switches are switched off. 
     FIG. 39 is a flowchart showing a process corresponding to the ON/OFF control circuit of the seventh embodiment shown in FIG.  26 . In FIG. 39, different control is performed depending on which switch is on in the charging state of a battery. If a battery status charges to a discharging state in a condition where the switch  12   x  has been on in a charging state, in steps S 53  through S 56  the discharging is controlled in the same way as shown in FIG.  38 . 
     On the other hand, if a battery status charges to a discharging state in a condition where the switch  12   y  has been on in a charging state, in steps S 57  through S 59  the batteries, first,  14   y  and then  14   x , are discharged until the discharging of both batteries  14   y  and  14   x  is completed, and then in step S 56  both switches are switched off. 
     FIG. 40 is a flowchart showing a process corresponding to the ON/OFF control circuit of the eighth embodiment shown in FIG.  28 . In FIG. 40, the same on/off control of the switches as in the charge control shown in FIG. 36 is performed, and the discharging is performed. That is, as a result of the detection of the battery current direction, a switch corresponding to the battery in which the current direction is the reverse of a target discharging is switched off, and discharging is performed. Then, the current direction is detected for each a certain time, and the control is maintained. Then, if in step S 67  it is judged that the discharging of both batteries is completed, in step S 68  both switches are switched off and the process is terminated. 
     FIG. 41 is a flowchart showing a process corresponding to the ON/OFF control circuit of the ninth embodiment shown in FIG.  30 . In FIG. 41, the same on/off control of the switches as that for charging as shown in FIG. 35 is performed as a discharging control. That is, the detections of the current directions of both batteries are performed, a switch corresponding to the battery whose current flow is in a charging direction is switched off, and the other battery is discharged. After the voltages of both batteries become equal, the former battery is also discharged. When in step S 76  it is judged that the discharging of both batteries is completed, in step S 77  both switches are switched off, and the process is terminated. 
     Although the ON/OFF control circuits of the embodiments have been so far described roughly classifying into when discharging and when charging, it is natural that the charging embodiments and the discharging embodiments can be used properly combined in an actual power supply apparatus. 
     Furthermore, although the embodiments of the present invention are described above for the case where two chargeable batteries are connected in parallel, the number of the batteries is not limited to two, and the on/off control method of the present invention can be basically applied to the case of three or more batteries. 
     As so far described in detail, according to the present invention, by connecting a plurality of batteries and controlling switches for switching on/off the charging/discharging current flowing in each battery, a current can be prevented from flowing from charged batteries to less-charged batteries, if there is some imbalance between the charging states of the batteries, the charging energy of the batteries can be effectively used, and the performance of a power supply apparatus can be greatly improved.