Patent Publication Number: US-8120319-B2

Title: Set battery control method and set battery control circuit as well as charging circuit and battery pack having the set battery control circuit

Description:
RELATED APPLICATIONS 
     This application is the U.S. National Phase under 35 U.S.C. §371 of International Application No. PCT/JP2007/073915, filed on Dec. 12, 2007, which in turn claims the benefit of Japanese Application No. 2006-337182, filed on Dec. 14, 2006, the disclosures of which Applications are incorporated by reference herein. 
     TECHNICAL FIELD 
     The present invention relates to a set battery control method and a set battery control circuit configured to reduce an imbalance in a set battery formed by connecting a plurality of secondary batteries in series as well as a charging circuit and a battery pack having the set battery control circuit. 
     BACKGROUND ART 
       FIG. 6  shows a case where a set battery formed by connecting a plurality of secondary batteries in series is charged and discharged by a charge and discharge method for a secondary battery in the background art, and it is a graph showing, for example, changes of terminal voltages α 11 , α 12 , and α 13  across three secondary batteries  111 ,  112 , and  113 , respectively. 
     Initially, at timing T 101 , the terminal voltages α 11 , α 12 , and α 13  across the secondary batteries  111 ,  112 , and  113 , respectively, are all equal at a cut-off voltage of discharge, Vt (for example, 3.0 V) and the secondary batteries  111 ,  112 , and  113  are in a balanced state. When a charge current is flown to charge the set battery, the terminal voltages α 11 , α 12 , and α 13  gradually increase. 
     A secondary battery deteriorates when it is charged to the extent that the terminal voltage exceeds a predetermined cut-off voltage of charge, Vf. Accordingly, it is set in such a manner that in a case where the set battery as described above is charged, the voltage across the set battery takes a value found by: cut-off voltage of charge, Vf× the number of secondary batteries in series. The cut-off voltage of charge, Vf, is typically 4.2 V in a case where the secondary batteries are, for example, lithium-ion batteries. Hence, in the case of a set battery formed by connecting three secondary batteries  111 ,  112 , and  113  in series, the set battery is charged until the voltage across the set battery reaches 4.2 V×3=12.6 V. 
     Incidentally, the internal resistance of a secondary battery increases when it deteriorates. Accordingly, when a charge voltage is applied across a series circuit formed by connecting a plurality of secondary batteries  111 ,  112 , and  113  in series, the terminal voltage across a secondary battery having larger internal resistance, that is, a deteriorated secondary battery, becomes larger than those of the other intact batteries. When this happens, a charge voltage is no longer distributed equally to the respective secondary batteries. For example, assume that the secondary batteries  111 ,  112 , and  113  are deteriorated more in this order, the terminal voltage α 11  across the most deteriorated secondary battery  111  becomes the highest and the terminal voltage α 13  across the least deteriorated secondary battery  113  becomes the lowest. 
     Then, at timing T 102  in  FIG. 6  when the charge ends, the terminal voltages α 11 , α 12 , and α 13  across the secondary batteries  111 ,  112 , and  113 , respectively, are all different voltages, which gives rise to an imbalance among the secondary batteries  111 ,  112 , and  113 . By discharging the set battery that has been charged in the occurrence of an imbalance as described above by connecting a load to the set battery, then the terminal voltages across the secondary batteries drop faster in ascending order of deterioration. 
     In a case where a secondary battery is discharged, the secondary battery deteriorates when it is overdischarged. Accordingly, a voltage about at which the secondary battery will not be deteriorated is set as the cut-off voltage of discharge, Vt, which is the voltage at which discharge is to end. For example, in the case of a lithium-ion battery, the cut-off voltage of discharge, Vt, is typically set to a voltage of about 3.0 V. It is configured in such a manner that the discharge ends when the lowest voltage among the terminal voltages α 11 , α 12 , and α 13  has dropped to the cut-off voltage of discharge, Vt=3.0 V (timing T 103 ). 
     Consequently, at the timing T 103 , the terminal voltage α 11  across the most deteriorated secondary battery  111  becomes the lowest and the terminal voltage α 13  across the least deteriorated secondary battery  113  becomes the highest. An imbalance among the terminal voltages α 11 , α 12 , and α 13  is therefore increased. 
     In addition, the terminal voltage α 11  across the most deteriorated secondary battery  111  will have dropped to the cut-off voltage of discharge, Vt, before the terminal voltages α 12  and α 13  across the less deteriorated secondary batteries  112  and  113 , respectively, drop to the cut-off voltage of discharge, Vt. The discharge is therefore ended and feeding of a current to the load is stopped. This causes an inconvenience that the capacities of the less deteriorated secondary batteries  112  and  113  are not utilized effectively and the capacity of the overall set battery is reduced. 
     While charge and discharge operations as above are repeated, as are indicated at timings T 104  and T 105 , the secondary battery  111  more deteriorated than the other secondary batteries deteriorates further. This causes an inconvenience that a difference among the terminal voltages α 11 , α 12 , and α 13  becomes larger. 
     Under these circumstances, there is a technique of reducing an imbalance among the secondary batteries forming the set battery by lessening a difference among the terminal voltages across a plurality of secondary batteries by forcedly discharging a secondary battery having the terminal voltage exceeding the cut-off voltage of charge, Vf, when the charge ends so as to lower the terminal voltage (for example, Patent Document 1). 
       FIG. 7  is a graph showing changes of the terminal voltages α 11 , α 12 , and α 13  in a case where a difference among the terminal voltages across a plurality of the secondary batteries is lessened by forcedly discharging a secondary battery having the terminal voltage exceeding the cut-off voltage of charge, Vf, when the charge ends so as to lower the terminal voltage. 
     As is shown in  FIG. 7 , when the charge ends (timing T 202 ), the secondary battery  111  having the terminal voltage exceeding the cut-off voltage of charge, Vf, is forcedly discharged for balance adjustment so as to lower the terminal voltage α 11 , a difference among the terminal voltages α 11 , α 12 , and α 13  across the secondary batteries  111 ,  112 , and  113 , respectively, is lessened (timing T 203 ). 
     According to the technique described in Patent Document 1, however, the most deteriorated secondary battery  111  is forcedly discharged further and the less deteriorated secondary batteries are not discharged. Hence, the chances of being discharged are increased more for the most deteriorated secondary battery  111  than for the less deteriorated secondary batteries, and the most deteriorated secondary battery is further deteriorated. This causes an inconvenience that deterioration varies considerably among the secondary batteries  111 ,  112 , and  113  and an imbalance is increased. 
     In addition, as a secondary battery deteriorates further, the capacity decreases and the terminal voltage drops faster at the time of discharge. Then, according to the technique of Patent Document 1, because the secondary battery  111  that is considerably deteriorated and thereby has a reduced capacity is forcedly discharged further after the charge ends, an amount of electricity charged in the secondary battery  111  is reduced further. Hence, when the terminal voltage α 11  across the most deteriorated secondary battery  111  has dropped to the cut-off voltage of discharge, Vt, due to discharge by use (timing T 204 ), the terminal voltages α 12  and α 13  across the other secondary batteries  112  and  113 , respectively, rather become higher than in a case where discharge is not forcedly performed for balance adjustment. This causes an inconvenience that there is a possibility that a difference among the terminal voltages α 11 , α 12 , and α 13  across the secondary batteries  111 ,  112 , and  113 , respectively, is increased and so is an imbalance.
     Patent Document 1: JP-A-2005-176520   

     DISCLOSURE OF THE INVENTION 
     An object of the invention is to provide a set battery control method and a set battery control circuit capable of lessening a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that the most deteriorated secondary battery is deteriorated further as well as a charging circuit and a battery pack having the set battery control circuit. 
     A set battery control circuit according to one aspect of the invention includes: a voltage detection portion that detects a terminal voltage across each secondary battery in a set battery formed by connecting a plurality of secondary batteries in series; a discharge portion that discharges the plurality of secondary batteries; and an imbalance reduction processing portion that performs imbalance reduction processing at set timing preset as timing at which terminal voltages across the plurality of secondary batteries become voltages at or below a cut-off voltage of discharge preset as a voltage at which discharge is to end in such a manner that a secondary battery having a terminal voltage higher than a lowest voltage, which is a lowest terminal voltage among the terminal voltages across the plurality of secondary batteries detected by the voltage detection portion, is discharged by means of the discharge portion so that a difference among the terminal voltages across the respective secondary batteries is lessened. 
     Also, a set battery control method according to still another aspect of the invention includes: a voltage detecting step of detecting each of terminal voltages across a plurality of secondary batteries in a set battery formed by connecting the plurality of secondary batteries in series; and an imbalance reduction processing step of discharging a secondary battery having a terminal voltage higher than a lowest voltage, which is a lowest terminal voltage among the terminal voltages across the plurality of secondary batteries detected in the voltage detecting step, until the terminal voltage across the secondary battery drops to the lowest voltage at set timing preset as timing at which terminal voltages across the plurality of secondary batteries become voltages at or below a cut-off voltage of discharge preset as a voltage at which discharge is to end. 
     According to the set battery control circuit and the set battery control method configured as above, when a set battery formed by connecting a plurality of secondary batteries in series is discharged to a state where at least one of the terminal voltages across the plurality of secondary batteries becomes a voltage at or below the cut-off voltage of discharge preset as the voltage at which the discharge is to end, the terminal voltage in the set battery becomes lower for the secondary battery that is deteriorated more. Accordingly, at the set timing at which the terminal voltage becomes a voltage at or below the cut-off voltage of discharge, it is thought that the secondary battery having the lowest terminal voltage is deteriorated most. 
     Hence, by discharging a secondary battery having the terminal voltage higher than the lowest voltage, that is, a secondary battery thought to be less deteriorated than the secondary battery having the lowest terminal voltage, so as to lessen a difference among the terminal voltages across the respective secondary batteries, it becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that the most deteriorated secondary battery is discharged more and deteriorated further. 
     A charging circuit according to another aspect of the invention includes: the set battery control circuit described above; a charge portion that charges the set battery; and a detection portion that detects timing at which a user makes an operation to charge the set battery using the charge portion as the set timing. The imbalance reduction processing portion performs the imbalance reduction processing when the detection portion detects the set timing. The charge portion charges the set battery after the imbalance reduction processing is performed by the imbalance reduction processing portion. 
     According to the charging circuit configured as above, when the user makes an operation to charge the set battery using the charge portion, that is, when it is highly likely that at least one of the terminal voltages across the plurality of secondary batteries has dropped to or below the cut-off voltage of discharge, the set battery is charged after the imbalance reduction processing is performed. It thus becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that the most deteriorated secondary battery is discharged more and deteriorated further. In addition, by charging the set battery in a state where a difference among the terminal voltages across the respective secondary batteries is lessened, it becomes possible to reduce a cumulative increase of an imbalance among the secondary batteries. 
     Also, a battery pack according to still another aspect of the invention includes the set battery control circuit described above and the set battery. 
     According to the battery pack configured as above, because the imbalance reduction processing can be performed in the battery pack, it becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that the most deteriorated secondary battery is discharged more and deteriorated further in the battery pack. 
     Also, a battery pack according to still another aspect of the invention is a battery pack connected to a charge device that outputs a charge current to a secondary battery according to a command from an outside and includes: the set battery control circuit described above; the set battery; and a detection portion that detects timing at which a user makes an operation to charge the set battery using the charge portion as the set timing. The imbalance reduction processing portion outputs the command to output the charge current to the charge device after the imbalance reduction processing is performed upon detection of the set timing by the detection portion. 
     According to the battery pack configured as above, when the user makes an operation to charge the set battery using the charge portion, that is, when the terminal voltage across the most deteriorated secondary battery is thought to be the lowest voltage, the set battery can be charged by the charge device after a difference among the terminal voltages across the respective batteries is lessened by performing the imbalance reduction processing. It thus becomes possible to reduce a cumulative increase of an imbalance among the secondary batteries by charging the set battery using the charge device in a state where a difference among the terminal voltages across the respective secondary batteries is lessened while reducing the possibility that the most deteriorated secondary battery is discharged more and deteriorated further. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example of the configuration of a charge system using a battery pack having a set battery control circuit according to one embodiment of the invention. 
         FIG. 2  is a circuit diagram showing an example of the detailed configuration of a set battery and a discharge portion shown in  FIG. 1 . 
         FIG. 3  is a view used to describe an example of an operation of a charge system according to a first embodiment. 
         FIG. 4  is a view used to describe an example of an operation of the charge system according to the first embodiment. 
         FIG. 5  is a view used to describe an example of an operation of a charge system according to a second embodiment. 
         FIG. 6  is a view used to describe a charge method according to the background art. 
         FIG. 7  is a view used to describe a charge method according to the background art. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the invention will be described with reference to the drawings. Components labeled with the same reference numerals in the respective drawings are the same components and descriptions thereof are omitted. 
     First Embodiment 
       FIG. 1  is a block diagram showing an example of the configuration of a charge system  1  using a battery pack  2  having a set battery control circuit according to one embodiment of the invention. The charge system  1  includes the battery pack  2  and a charge device  3  that charges the battery pack  2 . However, the battery pack  2  and the charge device  3  are not necessarily separated from each other and it may be configured in such a manner that, as is shown in  FIG. 1 , the battery pack  2  and the charge device  3  are formed integrally into one circuit. Alternatively, it may be formed as an electric equipment system by further including a load circuit  4  to which power is fed from the battery pack  2 . The battery pack  2  is charged by the charge device  3  in  FIG. 1 . However, the battery pack  2  may be attached to the load circuit  4 , so that it is charged via the load circuit  4 . The battery pack  2  and the charge device  3  are connected to each other by terminals T 11  and T 21  on the DC (Direct Current) high side for power feeding, terminals T 12  and T 22  for a communication signal, and GND terminals T 13  and T 23  for power feeding and a communication signal. Similar terminals are provided also in a case where the battery pack  2  is attached to the load circuit  4 . 
     The terminals T 21  and T 23  of the charge device  3  are connected to the load circuit  4  and a current fed from a set battery  14  is fed to the load circuit  4  via a switching element  12 , a charge path  11 , and the terminals T 11  and T 21 . The load circuit  4  is a load circuit in an electric device driven on power fed from the battery pack  2 . For example, when the power supply switch of an unillustrated electric device is turned ON, a drive current of the load circuit  4  is fed to the load circuit  4  from the battery pack  2 . 
     The battery pack  2  includes the switching element  12 , the set battery  14 , a current detection resistor  16 , a temperature sensor  17 , a control IC (Integrated Circuit)  18 , a voltage detection circuit  20  (voltage detection portion), a discharge portion  23 , and the terminals T 11 , T 12 , and T 13 . As the switching element  12 , a semiconductor switching element, such as an FET (Field Effect Transistor), and a switching element, such as a relay switch, are available. The control IC  18  includes an A/D (analog-to-digital) converter  19 , a control portion  21 , and a communication portion  22 . 
     The switching element  12  is provided inside the battery pack  2  in the charge path  11  on the DC high side extending from the terminal T 11 . The other end of the charge path  11  is connected to the high-side terminal of the set battery  14 . The low-side terminal of the set battery  14  is connected to the GND terminal T 13  via a charge path  15  on the DC low side. The current detection resistor  16  that converts a charge current and a discharge current to a voltage value is provided in the charge path  15 . 
     The set battery  14  includes a plurality of secondary batteries  141 ,  142 , and  143  connected in series. The temperature of each secondary battery is detected by the temperature sensor  17  and inputted into the A/D converter  19  in the control IC  18 . As the secondary batteries  141 ,  142 , and  143 , for example, lithium-ion secondary batteries are available. It should be appreciated, however, that the secondary batteries  141 ,  142 , and  143  are not necessarily separate units of secondary batteries and they may be, for example, a plurality of secondary batteries connected in parallel. 
     The terminal voltages Vb 1 , Vb 2 , and Vb 3  across a plurality of the secondary batteries  141 ,  142 , and  143 , respectively, are detected by the voltage detection circuit  20  and inputted into the A/D converter  19  in the control IC  18 . Further, the current value detected by the current detection resistor  16  is also inputted into the A/D converter  19  in the control circuit IC  18 . The A/D converter  19  converts the respective values inputted therein to digital values, which are outputted to the control portion  21 . 
     In addition, the discharge portion  23  that selectively discharges the respective secondary batteries  141 ,  142 , and  143  is connected to the set battery  14 . 
       FIG. 2  is a circuit diagram showing an example of the detailed configuration of the set battery  14  and the discharge portion  23 . The discharge portion  23  shown in  FIG. 2  includes switching elements SW 1 , SW 2 , and SW 3  and resistors R 1 , R 2 , and R 3 . A series circuit of the switching element SW 1  and the resistor R 1  is connected to the secondary battery  141  in parallel. A series circuit of the switching element SW 2  and the resistor R 2  is connected to the secondary battery  142  in parallel. A series circuit of the switching element SW 3  and the resistor R 3  is connected to the secondary battery  143  in parallel. As the switching elements SW 1 , SW 2 , and SW 3 , semiconductor switching elements, such as an FET, and various switching elements, such as a relay switch, are available. 
     The switching elements SW 1 , SW 2 , and SW 3  come ON or go OFF according to a control signal from the control portion  21 . When the switching element SW 1  comes ON, the secondary battery  141  is discharged via the resistor R 1 . When the switching element SW 2  comes ON, the secondary battery  142  is discharged via the resistor R 2 . When the switching element SW 3  comes ON, the secondary battery  143  is discharged via the resistor R 3 . 
     The control portion  21  includes a CPU (Central Processing Unit) that performs, for example, predetermined arithmetic processing, a ROM (Read Only Memory) in which a predetermined control program is pre-stored, a RAM (Random Access Memory) in which data is stored temporarily, and peripheral circuits. The control portion  21  functions as a charge and discharge control portion  211  and an imbalance reduction processing portion  212  by running the control program pre-stored in the ROM. 
     In this case, the voltage detection circuit  20 , the discharge circuit  23 , and the imbalance reduction processing portion  212  correspond to an example of the set battery control circuit in appended claims. 
     The imbalance reduction processing portion  212  performs imbalance reduction processing when at least one of the terminal voltages across the secondary batteries  141 ,  142 , and  143  has dropped to or below the cut-off voltage of discharge, Vt, as a current is fed from the set battery  14  to the load circuit  4 , so that a secondary battery having the terminal voltage higher than the lowest voltage among the terminal voltages Vb 1 , Vb 2 , and Vb 3  across the secondary batteries  141 ,  142 , and  143 , respectively, detected by the voltage detection circuit  20  is discharged by the discharge portion  23  until the terminal voltage across this secondary battery drops to the lowest voltage. 
     The charge and discharge control portion  211  calculates a voltage value and a current value of a charge current that specify the power to the charge device  3  in response to various values inputted therein from the A/D converter  19  and transmits the computation result to the charge device  3  from the communication portion  22  via the terminals T 12  and T 22  and the terminals T 21  and T 23 . The set battery  14  is thus charged by the charge device  3 . 
     The charge and discharge control portion  211  also performs a protection operation to turn OFF the switching element  12  in response to an abnormality on the outside of the battery pack  2 , such as a short circuit between the terminals T 11  and T 13  and an abnormal current from the charge device  3 , and an abnormal temperature rise of the set battery  14  on the basis of the various values inputted therein from the A/D converter  19 . 
     The charge device  3  receives a command from the charge and discharge control portion  211  at the communication portion  32  serving as communication means in the control IC  30  and feeds a charge current at the voltage value and the current value described above and a pulse width by controlling a charge current feed circuit  33  (charge portion) by means of the charge control portion  31 . 
     The charge current feed circuit  33  is formed of an AC-to-DC converter or a DC-to-DC converter. The charge current feed circuit  33  converts an input voltage to a voltage value, a current value, and a pulse width specified by the charge control portion  31  and supplies the conversion result to the charge paths  11  and  15  via the terminals T 21  and T 11  and the terminals T 23  and T 13 , respectively. 
     It should be appreciated that the invention is not limited to a case where the control portion  21  is provided to the battery pack  2 , and the control portion  21  may be provided to the charge device  3 . Alternatively, the control portion  21  may be allocated to both the battery pack  2  and the charge device  3 . 
     An operation of the charge system  1  using the battery pack  2  configured as above will now be described.  FIG. 3  is a view used to describe one example of an operation of the charge system  1  shown in  FIG. 1 . Initially, for example, in a case where the battery pack  2  is connected to the charge device  3 , when the switching element  12  is turned ON by the charge and discharge control portion  211 , a command to start charge at predetermined current and voltage is transmitted from the charge and discharge control portion  211  to the charge control portion  31  via the communication portion  22 , the terminals T 12  and T 22 , and the communication portion  32 . Charge is thus started (timing T 1 ). 
     Accordingly, in response to a control signal from the charge control portion  31 , a charge current is fed from the charge current feed circuit  33  to the set battery  14  via the terminals T 21  and T 11 , the charge path  11 , and the switching element  12  at the current and voltage according to the command from charge and discharge control portion  211 . The terminal voltages Vb 1 , Vb 2 , and Vb 3  increase while the secondary batteries  141 ,  142 , and  143  are charged. 
     At the timing T 1 , the terminal voltages Vb 1 , Vb 2 , and Vb 3  are all equal at the cut-off voltage of discharge, Vt (for example, 3.0 V). The secondary batteries  141 ,  142 , and  143  are therefore in a balanced state. 
     When the secondary batteries  141 ,  142 , and  143  are charged, a difference is generated among the terminal voltages Vb 1 , Vb 2 , and Vb 3  according to the degree of deterioration in the secondary batteries  141 ,  142 , and  143 . To be more concrete, the internal resistance becomes larger and the terminal voltage increases significantly for the secondary battery that is deteriorated more. For example, assume that secondary batteries  141 ,  142 , and  143  are deteriorated more in this order, then the terminal voltage Vb 1  across the most deteriorated secondary battery  141  becomes the highest voltage and the terminal voltage Vb 3  across the least deteriorated secondary battery  143  becomes the lowest voltage. 
       FIG. 3  shows changes of the terminal voltages Vb 1 , Vb 2 , and Vb 3  in the case of CC (Constant Current) charge to feed a constant current to the set battery  14  by way of example. The invention, however, is not limited to a particular charge method and it is possible to adopt various charge methods, such as CV (Constant Voltage) charge to perform charge at a constant voltage, CCCV (Constant Charge Constant Voltage) charge to switch from CC (Constant Current) charge to CV (Constant Voltage) charge, pulse charge to feed a pulse-wise charge current, and trickle charge to perform charge using a minute current. Alternatively, it may be configured in such a manner that the set battery  14  is charged while a load current is fed to the load circuit  4 . 
     For example, when the terminal voltage Vb across the set battery  14  detected by the voltage detection circuit  20  and obtained by the A/D converter  19  reaches a voltage found by multiplying the cut-off voltage of charge, Vf, by the number of secondary batteries in series, for example, 4.2 V×3=12.6 V, a command to stop feeding the charge current is transmitted by the charge and discharge control portion  211  to the charge control portion  31  via the communication portion  22 , the terminals T 12  and T 22 , and the communication portion  32 . An output current of the charge current feed circuit  33  is thus reduced to 0 by the charge control circuit  31 . The charge is thus ended (timing T 2 ). 
     It should be appreciated that the invention is not limited to a case where the charge is ended when the terminal voltage Vb across the set voltage  14  reaches the voltage found by multiplying the cut-off voltage of charge, Vf, by the number of secondary batteries in series (for example, 4.2 V×3=12.6 V). It may be configured in such a manner that deterioration of the secondary batteries  141 ,  142 , and  143  is reduced by ending the charge when the maximum voltage among the terminal voltages Vb 1 , Vb 2 , and Vb 3  reaches the cut-off voltage of charge, Vf (for example, 4.2 V). 
     At the timing T 2 , the terminal voltages across the secondary batteries  141 ,  142 , and  143  becomes higher in ascending order of deterioration. For example, the voltages become higher in order of the terminal voltages Vb 1 , Vb 2 , and Vb 3 . 
     For example, when the unillustrated power supply switch of the load circuit  4  is turned ON, a current fed from the set battery  14  is fed to the load circuit  4  via the switching element  12 , the charge path  11 , and the terminals T 11  and T 21 . The set battery  14 , that is, the secondary batteries  141 ,  142 , and  143  are thus discharged. The terminal voltages Vb 1 , Vb 2 , and Vb 3  then start to drop gradually in response to the discharge of the secondary batteries  141 ,  142 , and  143 . 
     In the process in which the secondary batteries  141 ,  142 , and  143  are discharged, a voltage drops faster in ascending order of deterioration of the secondary batteries. Hence, at the start of discharge, that is, in a state where the set battery  14  is fully charged, the voltages are higher in order of the terminal voltages Vb 1 , Vb 2 , and Vb 3 . However, when the set battery  14  is discharged to some extent, the order of voltages is inverted and the terminal voltages across the secondary batteries become lower in ascending order of deterioration. The voltages therefore become higher in order of the terminal voltages Vb 3 , Vb 2 , and Vb 1 . 
     When the terminal voltage Vb 1 , which is the lowest voltage among the terminal voltages Vb 3 , Vb 2 , and Vb 1  obtained by the A/D converter  19 , has dropped to the cut-off voltage of discharge, Vt, the switching element  12  is turned OFF by the charge and discharge control portion  211  to stop the discharge in order to prevent overdischarge of the set battery  14  (timing T 3 ). Further, the switching elements SW 2  and SW 3  of the discharge portion  23  are turned ON by the imbalance reduction processing portion  212  to discharge the secondary batteries  142  and  143  having the terminal voltages higher than the terminal voltage Vb 1 . The imbalance reduction processing is thus started. 
     The invention is not limited to a case where the imbalance reduction processing is started when the terminal voltage Vb 1 , which is the lowest voltage among the terminal voltages Vb 3 , Vb 2 , and Vb 1 , has dropped to the cut-off voltage of discharge, Vt. For example, it may be configured in such a manner that the imbalance reduction processing is performed when the terminal voltage Vb across the set battery  14  has dropped to or below a value found by multiplying the cut-off voltage of discharge, Vt, by the number of secondary batteries in series. 
     In this case, as is shown in  FIG. 4 , the imbalance reduction processing is performed when the average value of the terminal voltages Vb 1 , Vb 2 , and Vb 3  has dropped to the cut-off voltage of discharge, Vt. Then, one of the terminal voltages Vb 1 , Vb 2 , and Vb 3  may possibly drop below the cut-off voltage of discharge, Vt. However, it is possible to decrease deterioration caused by overdischarge by discharging only the secondary batteries having the terminal voltage higher than the cut-off voltage of discharge, Vt, by the discharge portion  23  until the terminal voltage drops to the cut-off voltage of discharge, Vt. 
     When the terminal voltages Vb 2  and Vb 3  across the secondary batteries  142  and  143 , respectively, almost coincide with the terminal voltage Vb 1 , the switching elements SW 2  and SW 3  are turned OFF by the imbalance reduction processing portion  212  so as to stop the discharge by the discharge portion  23 . The imbalance reduction processing is thus ended (timing T 4 ). 
     As has been described, the terminal voltages Vb 1 , Vb 2 , and Vb 3  are made almost equal by the imbalance reduction processing by the imbalance reduction processing portion  212 , and an imbalance among the secondary batteries  141 ,  142 , and  143  is reduced. At the timing T 3 , the terminal voltages become higher for the secondary batteries that are deteriorated less. The imbalance reduction processing portion  212  is therefore able to lessen a difference among the terminal voltages across the respective secondary batteries by discharging the less deteriorated secondary batteries by discharging the secondary batteries having the higher terminal voltages at the timing T 3 . 
     Accordingly, in the battery pack  2  shown in  FIG. 1 , the most deteriorated secondary battery will not be deteriorated further by discharging the most deteriorated secondary battery as is in the case of the background art shown in  FIG. 7 , more specifically, as in a case where a difference among the terminal voltages across a plurality of secondary batteries is lessened by forcedly discharging the secondary battery having the terminal voltage exceeding the cut-off voltage of charge, Vf, when the charge ends so as to lower the terminal voltage. It thus becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that a considerably deteriorated secondary battery is deteriorated further. 
     Also, in the battery pack  2  shown in  FIG. 1 , the imbalance reduction processing is performed after the set battery  14  is discharged. Accordingly, when the set battery  14  is charged again (timing T 5 ), the set battery  14  is in a state where a difference among the terminal voltages Vb 1 , Vb 2 , and Vb 3  is lessened. It is therefore possible to reduce a cumulative increase a difference among the terminal voltages across the respective secondary batteries while the set battery is charged and discharged repetitively as in the background art shown in  FIG. 6 . 
     Also, the set battery control circuit formed using the voltage detection circuit  20 , the discharge portion  23 , and the imbalance reduction processing portion  212  as described above does not require a charging circuit in the set battery  14  and it can be formed on the battery pack  2  side alone. It thus becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries in the set battery by including the set battery control circuit in the battery pack without having to change the charger. 
     The above described a case where the imbalance reduction processing portion  212  drops the terminal voltages Vb 2  and Vb 3  until they almost coincide with the terminal voltage Vb 1 , which is the lowest voltage. However, in a case where the lowest voltage is below the cut-off voltage of discharge, Vt, it may be configured in such a manner that only the secondary battery having the terminal voltage higher than the cut-off voltage of discharge, Vt, is discharged by the discharge portion  23  until the terminal voltage drops to the cut-off voltage of discharge, Vt. In this case, it becomes possible to reduce the possibility that the secondary battery having the terminal voltage below the cut-off voltage of discharge, Vt, from becoming an overdischarge state as it is discharged further by the discharge portion  23 . 
     Second Embodiment 
     A charge system  1   a  using a charging circuit according to a second embodiment of the invention will now be described. As with the charge system  1 , the charge system  1   a  is shown in  FIG. 1 . The charge system  1   a  is different from the charge system  1  in the timing at which the imbalance reduction processing is performed by an imbalance reduction processing portion  212   a  in a battery pack  2   a . The charge device  3  corresponds to an example of the charge portion. The voltage detection circuit  20 , the discharge portion  23 , and the imbalance reduction processing portion  212   a  correspond to an example of the set battery control circuit. The charge and discharge control portion  211 , the voltage detection circuit  20 , the discharge portion  23 , and the imbalance reduction processing portion  212   a  correspond to an example of the charging circuit. 
     As with the charge system  1 , the charge system  1   a  may be formed of the battery pack  2   a  and the charge device  3  that are formed integrally into one circuit. Alternatively, it may be formed as an electric equipment system by further including a load circuit  4  to which power is fed from the battery pack  2   a . Also, the battery pack  2   a  may be attached to the load circuit  4 , so that it is charged via the load circuit  4 . Further the control portion  21   a  may be provided to the charge device  3 . Alternatively, the control portion  21   a  may be allocated to both the battery pack  2  and the charge device  3 . 
     Because the other configurations and operations are the same as those of the charge system  1 , the descriptions thereof are omitted herein and a characteristic point of the charge system  1   a  will be described in the following. 
     When the user makes an operation to charge the battery pack  2   a , the imbalance reduction processing portion  212   a  starts the imbalance reduction processing upon detection of this operation. The set battery  14  is charged by outputting a command to output a charge current to the charge device  3  after the imbalance reduction processing is performed. 
     In this case, the operation by the user to charge the battery pack  2   a  is an operation by which user is to charge the battery pack  2   a , for example, an operation by the user to attach the battery pack  2   a  to the charge device  3  or an operation by the user to insert an unillustrated power supply plug of the charge device  3  in a socket. 
     When such an operation is made, for example, a request to start charge is transmitted from the control IC  30  in the charge device  3  to the control portion  21   a  via the terminals T 22  and T 12  and the communication portion  22 , or an unillustrated detection circuit detects that the charge device  3  and the battery pack  2   a  are connected to each other or it detects a charge voltage outputted from the charge device  3 . Upon such detection, the imbalance reduction processing portion  212   a  detects that the user made an operation to charge the battery pack  2   a  and starts the imbalance reduction processing. In this case, the imbalance reduction processing portion  212   a  corresponds to an example of the detection portion. 
     Alternatively, for example, in a case where the user insets the unillustrated power supply plug of the charge device  3  in the outlet, it may be configured in such a manner that an operation power supply voltage for the control portion  21   a  is fed from an unillustrated power supply circuit so that the CPU in the control portion  21   a  starts to operate and functions as the imbalance reduction processing portion  212   a , based on which an operation by the user to charge the battery pack  2   a  is detected. In this case, the control portion  21   a  corresponds to an example of the detection portion. 
     When the user attaches the battery pack  2   a  to the charge device  3  or when the user inserts the unillustrated power supply plug of the charge device  3  in the outlet, it is most likely that the user wishes to charge the set battery  14  because at least one of the terminal voltages across the secondary batteries  141 ,  142 , and  143  has dropped to or below the cut-off voltage of discharge, Vt, and the set battery  14  is no longer able to feed power to the load circuit  4 . 
     In other words, the timing at which it is detected that the user attaches the battery pack  2   a  to the charge device  3  or the user inserts the unillustrated power supply plug of the charge device  3  in the outlet corresponds to an example of the set timing at which at least one of the terminal voltages across a plurality of the secondary batteries  141 ,  142 , and  143  becomes a voltage at or below the cut-off voltage of discharge, Vt, that is preset as the voltage at which the discharge is to end. 
     It may be configured in such a manner that the imbalance reduction processing portion  212   a  starts the imbalance reduction processing at the set timing which is set to a point when at least one of the terminal voltages Vb 1 , Vb 2 , and Vb 3  has dropped to or below the cut-off voltage of discharge, Vt, on the basis of the terminal voltages Vb 1 , Vb 2 , and Vb 3  obtained by the A/D converter  19 . 
     An operation of the battery pack  1   a  using the battery pack  2   a  configured as described above will now be described.  FIG. 5  is a view used to describe an example of an operation of the charge system  1   a  shown in  FIG. 1 . Initially, the charge and discharge operations same as those at the timing T 1  through T 3  in  FIG. 3  are performed. At the timing T 3 , the terminal voltages become lower for the secondary batteries that are deteriorated more. The voltages therefore become higher in order of the terminal voltages Vb 3 , Vb 2 , and Vb 1 . This state is maintained intact because the feeding of a current from the set battery  14  to the load circuit  4  is stopped at the timing T 3 . 
     Subsequently, for example, when the user attaches the battery pack  2   a  to the charge device  3  or the user inserts the unillustrated power supply plug of the charge device  3  into the outlet, for example, a request to start charge is transmitted from the control IC  30  in the charge device  3  to the control portion  21   a  via the terminals T 22  and T 12  and the communication portion  22 . The imbalance reduction processing portion  212   a  then turns ON the switching elements SW 2  and SW 3  of the discharge device  23  for discharging the secondary batteries  142  and  143  having the terminal voltages higher than the terminal voltage Vb 1  (timing T 10 ). The imbalance reduction processing is thus started. 
     When the terminal voltages Vb 2  and Vb 3  across the secondary batteries  142  and  143 , respectively, almost coincide with the terminal voltage Vb 1 , the imbalance reduction processing portion  212   a  turns OFF the switching elements SW 2  and SW 3  so as to stop the discharge by the discharge portion  23 . The imbalance reduction processing is thus ended. Further, a command to start charge at predetermined current and voltage is transmitted from the imbalance reduction processing portion  212   a  to the charge control portion  31  via the communication portion  22 , the terminals T 12  and T 22 , and the communication portion  32 . The charge is thus started (timing T 11 ). 
     Accordingly, in response to the control signal from the charge control portion  31 , a charge current is fed from the charge current feed circuit  33  to the set battery  14  via the terminals T 21  and T 11 , the charge path  11 , and the switching element  12  at the current and voltage according to the command from the imbalance reduction processing portion  212   a . The set battery  14  is thus charged. 
     As has been described, the terminal voltages Vb 1 , Vb 2 , and Vb 3  are made almost equal by the imbalance reduction processing by the imbalance reduction processing portion  212   a  and an imbalance among the secondary batteries  141 ,  142 , and  143  is reduced. Also, at the timing T 10 , the terminal voltages become higher for the secondary batteries that are deteriorated less. Hence, by discharging the less deteriorated secondary batteries by discharging the secondary batteries having the higher terminal voltages at the timing T 10 , the imbalance reduction processing portion  212   a  becomes able to lessen a difference among the terminal voltages across the respective secondary batteries. 
     Accordingly, in the battery pack  2   a  shown in  FIG. 1 , the most deteriorated secondary battery will not be deteriorated further by discharging the most deteriorated secondary battery as is in the case of the background art shown in  FIG. 7 , more specifically, as in a case where a difference among the terminal voltages across a plurality of secondary batteries is lessened by forcedly discharging the secondary battery having the terminal voltage exceeding the cut-off voltage of charge, Vf, when the charge ends so as to lower the terminal voltage. It thus becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that a considerably deteriorated secondary battery is deteriorated further. 
     Also, in the battery pack  2   a  shown in  FIG. 1 , the imbalance reduction processing is performed before the set battery  14  is charged. Accordingly, when the charge is started (timing T 11 ), the set battery  14  is in a state where a difference among the terminal voltages Vb 1 , Vb 2 , and Vb 3  is lessened. It is therefore possible to reduce a cumulative increase of a difference among the terminal voltages across the respective secondary batteries while the set battery is charged and discharged repetitively as in the background art shown in  FIG. 6 . 
     Also, in a case where the imbalance reduction processing is performed at the timing T 3  at which the terminal voltage Vb 1 , which is the lowest voltage among the terminal voltages Vb 3 , Vb 2 , and Vb 1 , has dropped to the cut-off voltage of discharge, Vt, as in the charge system  1  ( FIG. 3 ), in a case where there is a long time since the imbalance reduction processing ends (timing T 4 ) until the charge is started (timing T 5 ), the secondary batteries  141 ,  142 , and  143  undergo self-discharge before the charge is performed, which gives rise to a difference among the terminal voltages Vb 1 , Vb 2 , and Vb 3 . Accordingly, there is a possibility that a difference among the terminal voltages Vb 1 , Vb 2 , and Vb 3  increases cumulatively in the following charge and discharge operations. 
     However, by performing the imbalance reduction processing immediately before the charge is started (timing T 11 ) as in the charge system  1   a  ( FIG. 5 ), even when there is an imbalance caused by the self-discharge, because a difference among the terminal voltages Vb 1 , Vb 2 , and Vb 3  can be lessened before the charge is started, charge can be performed after an imbalance among the secondary batteries  141 ,  142 , and  143  is reduced. It thus becomes possible to reduce a cumulative increase of a difference among the terminal voltages across the respective secondary batteries while the set battery is charged and discharged repetitively. 
     Also, as with the battery pack  2  ( FIG. 3 ), in a case where the imbalance reduction processing is performed at the timing T 3  at which the terminal voltage Vb 1 , which is the lowest voltage among the terminal voltages Vb 3 , Vb 2 , and Vb 1 , has dropped to the cut-off voltage of discharge, Vt, for example, when the user starts to charge the battery pack  2  by removing it from the load device and attaching it to the charge device  3  before the terminal voltage Vb 1 , which is the lowest voltage among the terminal voltages Vb 3 , Vb 2 , and Vb 1 , has dropped to the cut-off voltage of discharge, Vt, the battery pack  2  is charged without the imbalance reduction processing being performed. Consequently, there is a possibility that a difference among the terminal voltages Vb 1 , Vb 2 , and Vb 3  increases cumulatively in the following charge and discharge operations. 
     However, by performing the imbalance reduction processing immediately before the charge is started (timing T 11 ) as in the charge system  1   a  ( FIG. 5 ), even when the user makes an operation to charge the battery pack  2   a  before the terminal voltage Vb 1 , which is the lowest voltage among the terminal voltages Vb 3 , Vb 2 , and Vb 1 , has dropped to the cut-off voltage of discharge, Vt, a difference among the terminal voltages Vb 1 , Vb 2 , and Vb 3  is lessened before the charge is started, so that the charge can be performed after an imbalance among the secondary batteries  141 ,  142 , and  143  is reduced. It thus becomes possible to reduce the possibility that a difference among the terminal voltages across the respective secondary batteries increases cumulatively while the set battery is charged and discharged repetitively. 
     The imbalance reduction processing shown at the timings T 3  and T 4  and the timings T 10  and T 11  described a case where only the secondary battery having the terminal voltage higher than the other secondary batteries is discharged by turning ON the switching element connected in parallel to the secondary battery having the terminal voltage higher than the other secondary batteries. However, it may be configured in such a manner that the resistors R 1 , R 2 , and R 3  in the discharge portion  23  are set at substantially the same resistance value, so that all the switching elements SW 1 , SW 2 , and SW 3  are turned ON in the imbalance reduction processing. In this case, the term, “substantially the same resistance value”, means that a range of resistance variance caused by an accuracy error or a characteristic variance falls within the same range. 
     When all the switches SW 1 , SW 2 , and SW 3  are turned ON, the resistors having substantially the same resistance value are connected in parallel to the respective secondary batteries  141 ,  142 , and  143 . Accordingly, a current is flown through each of the resistors R 1 , R 2 , and R 3  in such a manner that the terminal voltages across the respective secondary batteries become equal. Hence, a discharge current of the secondary battery having a higher terminal voltage becomes larger whereas a discharge current of the secondary battery having a lower terminal voltage becomes smaller. Consequently, a discharge current of the secondary battery that is deteriorated considerably and thereby has a lower terminal voltage is reduced. It thus becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that the considerably deteriorated secondary battery is deteriorated further. An imbalance among the secondary batteries  141 ,  142 , and  143  can be thus reduced. 
     The set battery control circuit, the set battery control method, the charging circuit, and the battery pack configured as above are suitably used as a set battery control circuit, a set battery control method, and a charging circuit used in battery-mounted devices, such as an electronic device represented by a portable personal computer and a digital camera, an electric car, and a hybrid car as well as a battery pack used as the power supply of these battery-mounted devices, and a charge device that charges the battery pack. 
     A set battery control circuit according to one aspect of the invention includes: a voltage detection portion that detects a terminal voltage across each secondary battery in a set battery formed by connecting a plurality of secondary batteries in series; a discharge portion that discharges the plurality of secondary batteries; and an imbalance reduction processing portion that performs imbalance reduction processing at set timing preset as timing at which terminal voltages across the plurality of secondary batteries become voltages at or below a cut-off voltage of discharge preset as a voltage at which discharge is to end in such a manner that a secondary battery having a terminal voltage higher than a lowest voltage, which is a lowest terminal voltage among the terminal voltages across the plurality of secondary batteries detected by the voltage detection portion, is discharged by means of the discharge portion so that a difference among the terminal voltages across the respective secondary batteries is lessened. 
     According to this configuration, a secondary battery having a terminal voltage higher than the lowest voltage, which is the lowest among the terminal voltages across the plurality of secondary batteries detected by the voltage detection portion, is discharged by the imbalance reduction processing portion by means of the discharge portion at the set timing preset as timing at which the terminal voltages across the plurality of secondary batteries become voltages at or below the cut-off voltage of discharge preset as a voltage at which discharge is to end, so that a difference among the terminal voltages across the respective secondary batteries is lessened. 
     By discharging the set battery formed by connecting a plurality of secondary batteries deteriorated at different degrees in series, the terminal voltage drops faster for a secondary battery deteriorated more. Hence, in the set battery discharged to a state where at least one of the terminal voltages across a plurality of secondary batteries becomes a voltage at or below the cut-off voltage of discharge preset as the voltage at which the discharge is to end, the terminal voltages become lower for the secondary batteries that are deteriorated more. Accordingly, at the set timing at which the terminal voltage becomes a voltage at or below the cut-off voltage of discharge, the secondary battery having the lowest terminal battery is thought to be most deteriorated. 
     Hence, by discharging the secondary battery having the terminal voltage higher than the lowest voltage, that is, the secondary battery thought to be less deteriorated than the secondary battery having the lowest terminal voltage so as to lessen a difference among the terminal voltages across the respective secondary batteries, it becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that the most deteriorated secondary battery is discharged more and deteriorated further. 
     Also, it is preferable that the discharge portion discharges the plurality of secondary batteries selectively, and that the imbalance reduction processing portion selectively discharges a secondary battery having the terminal voltage higher than the lowest voltage by means of the discharge portion in the imbalance reduction processing. 
     According to this configuration, the imbalance reduction processing portion is able to lessen a difference among the terminal voltages across the respective secondary batteries by selectively discharging only the secondary battery having the terminal voltage higher than the lowest voltage by means of the discharge portion. 
     The discharge portion may be configured to discharge the plurality of secondary batteries in such a manner that a discharge current becomes larger for secondary batteries having higher terminal voltages by connecting resistors having substantially a same resistance value in parallel to the plurality of secondary batteries. 
     According to this configuration, when the imbalance reduction processing portion performs a discharge operation by means of the discharge portion, resistors having substantially the same resistance value are connected in parallel to a plurality of secondary batteries by the discharge portion. Accordingly, a current is flown through the resistors connected in parallel to the respective secondary batteries in such a manner that the terminal voltages across the respective secondary batteries become equal. Hence, a discharge current becomes larger for a secondary battery having a higher terminal voltage whereas a discharge current becomes smaller for a secondary battery having a lower terminal voltage. Consequently, because a discharge current of the secondary battery that is considerably deteriorated and thereby has a lower terminal voltage is reduced, it becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that a considerably deteriorated secondary battery is deteriorated further. 
     Also, it is preferable that in a case where the lowest voltage detected by the voltage detection portion is below the cut-off voltage of discharge, the imbalance reduction processing portion discharges only a secondary battery having a terminal voltage higher than the cut-off voltage of discharge by means of the discharge portion until the terminal voltage across the secondary battery drops to the cut-off voltage of discharge in the imbalance reduction processing. 
     According to this configuration, in a case where the lowest voltage detected by the voltage detection portion is below the cut-off voltage of discharge, the imbalance reduction processing portion discharges only a secondary battery having the terminal voltage higher than the cut-off voltage of discharge by means of the discharge portion until the terminal voltage of this secondary battery drops to the cut-off voltage of discharge. It is therefore possible to reduce the possibility that the secondary battery having the terminal voltage below the cut-off voltage of discharge becomes an overdischarged state as it is discharged further. 
     Also, it is preferable that the imbalance reduction processing portion performs the imbalance reduction processing at the set timing which is set to a point when at least one of terminal voltages across the plurality of secondary batteries has dropped to or below the cut-off voltage of discharge as the set battery feeds a current to a load. 
     According to this configuration, when at least one of terminal voltages across a plurality of secondary batteries has dropped to or below the cut-off voltage of discharge as a current is fed from the set battery to a load, that is, at timing at which the terminal voltage of the most deteriorated secondary battery is thought to be the lowest voltage, the second battery having the terminal voltage higher than the lowest voltage, that is, the secondary battery thought to be less deteriorated than the secondary battery having the lowest terminal voltage, is discharged, so that a difference among the terminal voltages across the respective secondary batteries is lessened. It thus becomes possible to lessen a difference among the terminal voltages of the respective secondary batteries while reducing the possibility that the most deteriorated secondary battery is discharged more and deteriorated further. 
     Also, it may be configured in such a manner that the imbalance reduction processing portion performs the imbalance reduction processing at the set timing which is set to a point when a terminal voltage across the set battery drops to or below a value found by multiplying the cut-off voltage of discharge by the number of the secondary batteries in series as the set battery feeds a current to a load. 
     According to this configuration, the preset timing can be detected by detecting the terminal voltage across the set battery and comparing the detected terminal voltage with a value found by multiplying the cut-off voltage of discharge by the number of secondary batteries in series. Hence, there is no need to detect the terminal voltages across the plurality of secondary batteries and compare each with the cut-off voltage of discharge in order to detect the set timing. The imbalance reduction processing can be thus simplified. 
     A charging circuit according to another aspect of the invention includes: the set battery control circuit described above; a charge portion that charges the set battery; and a detection portion that detects timing at which a user makes an operation to charge the set battery using the charge portion as the set timing. The imbalance reduction processing portion performs the imbalance reduction processing when the detection portion detects the set timing. The charge portion charges the set battery after the imbalance reduction processing is performed by the imbalance reduction processing portion. 
     According to this configuration, when the user makes an operation to charge the set battery using the charge portion, the charge portion charges the set battery after the imbalance reduction processing is performed by the imbalance reduction processing portion. When the user makes an operation to charge the set battery, it is highly likely that the terminal voltage across the set battery has dropped too low to drive the load, that is, it is highly likely that at least one of the terminal voltages across the plurality of secondary batteries has dropped to or below the cut-off voltage of discharge. Hence, as has been described above, it is thought that it is highly likely that the terminal voltage across the most deteriorated secondary battery is the lowest voltage. 
     Hence, by configuring in such a manner that the set battery is charged after the imbalance reduction processing is performed when the user makes an operation to charge the set battery using the charge portion, it becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that the most deteriorated secondary battery is discharged more and deteriorated further. In addition, by charging the set battery while a difference among the terminal voltages across the respective secondary batteries is lessened, it becomes possible to reduce a cumulative increase of an imbalance among the secondary batteries. 
     Also, a battery pack according to still another aspect of the invention includes the set battery control circuit described above and the set battery. 
     According to this configuration, because the imbalance reduction processing can be performed in the battery pack, it becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that the most deteriorated secondary battery is discharged more and deteriorated further in the battery pack. 
     Also, a battery pack according to still another aspect of the invention is a battery pack connected to a charge device that outputs a charge current to a secondary battery according to a command from an outside and includes: the set battery control circuit described above; the set battery; and a detection portion that detects timing at which a user makes an operation to charge the set battery using the charge portion as the set timing. The imbalance reduction processing portion outputs the command to output the charge current to the charge device after the imbalance reduction processing is performed upon detection of the set timing by the detection portion. 
     According to this configuration, as has been described, when the user makes an operation to charge the set battery using the charge portion, that is, when the terminal voltage across the most deteriorated secondary battery is thought to be the lowest voltage, the charge portion charges the set battery after a difference among the terminal voltages across the respective secondary batteries is lessened by performing the imbalance reduction processing. Hence, by charging the set battery using the charge device while a difference among the terminal voltages across the respective voltages is lessened, it becomes possible to reduce a cumulative increase of an imbalance among the secondary batteries while reducing the possibility that the most deteriorated secondary battery is discharged more and deteriorated further. 
     Also, a set battery control method according to still another aspect of the invention includes: a voltage detecting step of detecting each of terminal voltages across a plurality of secondary batteries in a set battery formed by connecting the plurality of secondary batteries in series; and an imbalance reduction processing step of discharging a secondary battery having a terminal voltage higher than a lowest voltage, which is a lowest terminal voltage among the terminal voltages across the plurality of secondary batteries detected in the voltage detecting step, until the terminal voltage across the secondary battery drops to the lowest voltage at set timing preset as timing at which terminal voltages across the plurality of secondary batteries become voltages at or below a cut-off voltage of discharge preset as a voltage at which discharge is to end. 
     According to this configuration, at the set timing preset as the timing at which the terminal voltages across the plurality of secondary batteries become voltages at or below the cut-off voltage of discharge preset as a voltage at which the discharge is to end, a secondary battery having the terminal voltage higher than the lowest voltage, which is the lowest among the terminal voltages across a plurality of the secondary batteries, is discharged until the terminal voltage of this secondary battery drops to the lowest voltage. 
     Accordingly, as has been described, a secondary battery having the terminal voltage higher than the lowest voltage, that is, a secondary battery thought to be less deteriorated than the secondary battery having the lowest terminal voltage, is discharged to lessen a difference among the terminal voltages across the respective secondary batteries. It thus becomes possible to lessen a difference among the terminal voltages across the respective secondary batteries while reducing the possibility that the most deteriorated secondary battery is discharged more and deteriorated further.