Abstract:
A charge and discharge circuit that reduces the power consumption at charging and discharging without shortening the life of secondary batteries is described. The charge and discharge circuit includes charge current control circuits connected in parallel to the secondary batteries for performing bypass control of a charge current supplied to the secondary batteries. A potential difference detection circuit detects a voltage difference between the secondary batteries and controls the charge current control circuits in accordance with the voltage difference to selectively bypass the charge current supplied to the secondary batteries.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a charge circuit, and, more particularly, to a charge circuit, a charge and discharge circuit and a battery pack that are suitable for a secondary battery used in portable electronic apparatus, such as notebook-size personal computers. 
     FIG. 1 is a schematic circuit diagram of a prior art charge and discharge circuit  100 , which is conventionally built in a battery pack that is installed in a notebook-size personal computer. The function of the prior art circuit is described in the following to illustrate the limitations of conventional charge circuits. The charge and discharge circuit  100  controls charging and discharging of a battery  1  built in the battery pack. The battery  1  includes a plurality of (for example, three) lithium ion batteries  1   a - 1   c  connected in series. The charge and discharge circuit  100  includes a balanced current setting circuit  4  and a charging and discharging control circuit  7 . 
     The positive electrode of the battery  1  is connected to a positive input/output terminal t 1  via a discharge control switch  2  including a P-channel MOS (PMOS) transistor and a charge control switch  3  including the PMOS transistor. The negative electrode of the battery  1  is connected to a negative input/output terminal t 2 . When charges are charged into the battery  1 , a direct current voltage is applied from a personal computer to the positive and negative input/output terminals t 1 , t 2 . When the charges of the battery  1  are discharged, driving power is supplied from the positive and negative input/output terminals t 1 , t 2  to the personal computer. 
     When charges are charged into the battery  1 , even if the discharge control switch  2  is turned off, a charge current is supplied from the personal computer to the battery  1  via the charge control switch  3  that is turned on and a parasitic diode D 2  of the discharge control switch  2 . When the charges of the battery  1  are discharged, even if the charge control switch  3  is turned off, a discharge current is supplied from the battery  1  to the personal computer via the discharge control switch  2  that is turned on and a parasitic diode D 3  of the charge control switch  3 . 
     The balanced current setting circuit  4  includes three charge current control circuits  4   a - 4   c  connected in parallel to the lithium ion batteries  1   a - 1   c , respectively. The charge current control circuits  4   a - 4   c  include resistors  5   a - 5   c  and N-channel MOS (NMOS) transistors  6   a - 6   c , respectively. When each of the NMOS transistors  6   a - 6   c  is turned on, charges of the corresponding lithium ion batteries  1   a - 1   c  are discharged. 
     The charging and discharging control circuit  7  includes a cell voltage detection circuit  8 , an overcharge detection circuit  9  and an over discharge detection circuit  10 . The cell voltage detection circuit  8  includes three cell voltage amplifiers  8   a - 8   c  that detect inter-terminal voltages (cell voltages) V a -V c  of the lithium ion batteries  1   a - 1   c , respectively. Each of the cell voltage amplifiers is preferably an operational amplifier (op amp) having an amplification factor “ 1 ”. 
     Specifically, a non-inverting input terminal of the first cell voltage amplifier  8   a  is connected to the positive electrode of the first lithium ion battery  1   a  and the inverting input terminal thereof is connected to the negative electrode of the first lithium ion battery  1   a  (positive electrode of the second lithium ion battery  1   b ). The first cell voltage amplifier  8   a  detects the inter-terminal voltage (cell voltage) V a  of the first lithium ion battery  1   a  and supplies the cell voltage V a  to the overcharge detection circuit  9 . 
     A non-inverting input terminal of the second cell voltage amplifier  8   b  is connected to the positive electrode of the second lithium ion battery  1   b  and the inverting input terminal thereof is connected to the negative electrode of the second lithium ion battery  1   b  (positive electrode of the third lithium ion battery  1   c ). The second cell voltage amplifier  8   b  detects the inter-terminal voltage (cell voltage) V b  of the second lithium ion battery  1   b  and supplies the cell voltage V b  to the overcharge detection circuit  9 . 
     A non-inverting input terminal of the third cell voltage amplifier  8   c  is connected to the positive electrode of the third lithium ion battery  1   c  and the inverting input terminal thereof is connected to the negative electrode of the third lithium ion battery  1   c . The third cell voltage amplifier  8   c  detects the inter-terminal voltage (cell voltage) V c  of the third lithium ion battery  1   c  and supplies the cell voltage V c  to the overcharge detection circuit  9 . 
     The overcharge detection circuit  9  includes three comparators  9   a - 9   c  and an OR circuit  9   d . The first cell voltage V a  is supplied to a non-inverting input terminal of the first comparator  9   a  and a second reference voltage VTH is supplied to the non-inverting input terminal thereof. The first comparator  9   a  supplies a low-level detection signal to the OR circuit  9   d  when the first cell voltage V a  is below the second reference voltage VTH. When the first cell voltage V a  is equal to or greater than the second reference voltage VTH, a high-level detection signal is supplied to the OR circuit  9   d . That is, the first comparator  9   a  supplies the high-level detection signal to the OR circuit  9   d  when the inter-terminal voltage V a  of the first lithium ion battery  1   a  reaches the second reference voltage VTH. 
     The first comparator  9   a  has hysteresis and maintains the output of a high-level detection signal until the first cell voltage V a  drops to a predetermined voltage VTL smaller than the second reference voltage VTH after the first cell voltage V a  reaches a voltage equal to or greater than the second reference voltage VTH at one time. 
     The second cell voltage V b  is supplied to a non-inverting input terminal of the second comparator  9   b  and the second reference voltage VTH is supplied to the non-inverting input terminal thereof. The second comparator  9   b  supplies a low-level detection signal to the OR circuit  9   d  when the second cell voltage V b  is below the second reference voltage VTH. When the second cell voltage V b  is equal to or greater than the second reference voltage VTH, a high-level detection signal is supplied to the OR circuit  9   d . That is, the second comparator  9   b  supplies the high-level detection signal when the inter-terminal voltage V b  of the second lithium ion battery  1   b  reaches the second reference voltage VTH. 
     The second comparator  9   b  has hysteresis and maintains the output of a high-level detection signal until the second cell voltage V b  drops to the predetermined voltage VTL smaller than the second reference voltage VTH after the second cell voltage V b  reaches a voltage equal to or greater than the second reference voltage VTH at one time. 
     The third cell voltage V c  is supplied to a non-inverting input terminal of the third comparator  9   c  and the second reference voltage VTH is supplied to the non-inverting input terminal thereof. The third comparator  9   c  supplies a low-level detection signal to the OR circuit  9   d  when the third cell voltage V c  is below the second reference voltage VTH. When the third cell voltage V c  is equal to or greater than the second reference voltage VTH, a high level detection signal is supplied to the OR circuit  9   d . That is, the third comparator  9   c  supplies the high-level detection signal to the OR circuit  9   d  when the inter-terminal voltage V c  of the third lithium ion battery  1   c  reaches the second reference voltage VTH. 
     The third comparator  9   a  has hysteresis and maintains the output of a high-level detection signal until the third cell voltage V c  drops to the predetermined voltage VTL smaller than the second reference voltage VTH after the third cell voltage V c  reaches a voltage equal to or greater than the second reference voltage VTH at one time. 
     The output terminal of the OR circuit  9   d  is connected to the gate terminal of the charge control switch  3  (PMOS transistor). The OR circuit  9   d  turns off the charge control switch  3  when at least one of the detection signals of the comparators  9   a - 9   c  is at the high level. That is, when at least one inter-terminal voltage of the lithium ion batteries  1   a - 1   c  reaches the second reference voltage VTH, the charge control switch  3  is turned-off and charging is stopped. 
     The detection signals of the comparators  9   a - 9   c  are also supplied to gate terminals of NMOS transistors  6   a - 6   c , respectively. For example, when the detection signal of the comparator  9   a  is at the high level, the NMOS transistor  6   a  is turned on and the charges of the first lithium ion battery  1   a  is discharged via the NMOS transistor  6   a . Thus the inter-terminal voltage V a  of the first lithium ion battery  1   a  drops to the predetermined voltage VTL. When the inter-terminal voltages V a -V c  of the lithium ion batteries  1   a - 1   c  drop to the voltage VTL, charging is restarted. 
     When the overcharge detection circuit  10  receives the inter-terminal voltages V a -V c  of each of the lithium batteries  1   a - 1   c  from the cell voltage amplifiers  8   a - 8   c  and at least one of the inter-terminal voltages V a -V c  drops below a predetermined third reference voltage, the charge control switch  2  is turned off and the discharging of the battery  1  is stopped. That is, when at least one of the inter-terminal voltages V a -V c  of the lithium ion batteries  1   a - 1   c  drops to a voltage equal to or smaller than the predetermined third reference voltage, the supply of driving power from the battery  1  to the personal computer is stopped. 
     Since there is dispersion in the lithium ion batteries  1   a - 1   c , a difference arises in the inter-terminal voltages V a -V c  when charging starts or a difference arises at the time when the inter-terminal voltage reaches the second reference voltage VTH. In this case, the inter-terminal voltages V a -V c  of the lithium ion batteries  1   a - 1   c  will not reach the second reference voltage VTH at the same time. As a result, for example, when the first lithium ion battery  1   a  reaches the second reference voltage VTH, charging is temporarily stopped. After the inter-terminal voltage V a  is dropped to the voltage VTL by discharging the charging charges of the first lithium ion battery  1   a , the charging is restarted. The lithium ion batteries  1   a - 1   c  are evenly charged by repeating charging and discharging in this manner. 
     In the charging method of the aforementioned lithium ion batteries  1   a - 1   c , however, a stress applies to each lithium ion battery and its life is shortened. Otherwise, since there is dispersion in the lithium ion battery, the charging and discharging are collectively repeated against a specific lithium ion battery and that lithium ion battery deteriorates faster than another lithium ion battery. 
     Japanese Patent Laid-Open Publication No. 7-87673A discloses a charging control system that charges a battery without repeating charging and discharging. The charging control system comprises a plurality of detectors that detect the inter-terminal voltages of a plurality of lithium ion batteries and a plurality of bypass circuits connected in parallel to the plurality of lithium ion batteries, respectively. When the inter-terminal voltages of the lithium ion batteries detected by the detectors reach a predetermined voltage, the associated bypass circuit is conducted and charging is terminated. 
     In the aforementioned charging control system, however, regardless of the dispersion of the inter-terminal voltage of each lithium ion battery, the bypass circuit associated with the battery that reached the predetermined voltage is conducted. Therefore, if the dispersion of the inter-terminal voltage is large, the bypass circuit conducted first is kept in the conduction state for a long time. As a result, the durability of the bypass circuit decreases and the power consumption increases. 
     Japanese Patent Laid Open Publication No. 7-87673A discloses a charging control system that performs charging in the state where the inter-terminal voltages of two lithium ion batteries are always equal. The charging control system includes a resistance voltage dividing circuit that generates half the voltage of the total voltages of two batteries and an amplifier that amplifies the half voltage at an amplification factor “1”. The output of the amplifier is connected to the node of the two lithium ion batteries. When the inter-terminal voltage of the lithium ion battery on the positive electrode side is greater than the inter-terminal voltage of the lithium ion battery on the negative electrode side, the lithium ion battery on the negative electrode side is charged by the amplifier. When the inter-terminal voltage of the lithium ion battery on the positive electrode side is smaller than the inter-terminal voltage of the lithium ion battery On the negative electrode side, the lithium ion battery on the positive side is charged. 
     In the aforementioned control system, however, even when the two lithium ion batteries are discharged, the amplifier and the voltage dividing circuit are activated by the power supply voltages of the batteries, so that the power consumption increases. As a result, the service time of a battery pack is shortened. Moreover, applying the aforementioned control system to three lithium ion batteries or more makes the circuit configuration very complicate. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a charge and discharge circuit that reduces the power consumption at charging and discharging without shortening the life of a secondary battery. 
     In a first aspect of the present invention, a charge circuit that charges a plurality of secondary batteries is described. The charge circuit includes a plurality of charge current control circuits connected in parallel to the plurality of secondary batteries for performing bypass control of a charge current supplied to the plurality of secondary batteries. A potential difference detection circuit is connected to the plurality of charge current control circuits to detect a voltage difference between the plurality of secondary batteries and control the plurality of charge current control circuits in accordance with the voltage difference to selectively bypass the charge current supplied to the plurality of secondary batteries. 
     In a second aspect of the present invention, a charge circuit that charges a plurality of secondary batteries is provided. The charge circuit includes a plurality of charge current control circuits, connected in parallel to the plurality of secondary batteries, respectively, for performing bypass control of a charge current supplied to the plurality of secondary batteries. A plurality of potential difference detection circuits are connected to the plurality of charge current control circuits, respectively. Each potential difference detection circuit detects a voltage difference between a voltage of the associated secondary battery and a voltage of another secondary battery and controls the associated charge current control circuit in accordance with the voltage difference to selectively bypass the charge current supplied to the associated secondary battery. A charge detection circuit is connected to the plurality of potential difference detection circuits to detect whether charging is performed into the plurality of secondary batteries and activate the plurality of potential difference detection circuits when the charging is performed. 
     In a third aspect of the present invention, a charge and discharge circuit that performs the charging and discharging of a plurality of secondary batteries is provided. The charge and discharge circuit includes a plurality of charge current control circuits, connected in parallel to the plurality of secondary batteries, for performing bypass control of a charge current supplied to the plurality of secondary batteries. A potential difference detection circuit is connected to the plurality of charge current control circuits to detect a voltage difference of the plurality of secondary batteries and control the plurality of charge current control circuits in accordance with the voltage difference to selectively bypass the charge current supplied to the plurality of secondary batteries. A charge detection circuit is connected to the potential detection circuit to detect whether charging is performed into the plurality of secondary batteries and activate the potential difference detection circuit when the charging is performed. An overcharge detection circuit detects the voltages of the plurality of secondary batteries and stops the charging into the plurality of secondary batteries when and at least one of the detected voltages is equal to or greater than a predetermined first reference voltage. An over discharge detection circuit detects the voltages of the plurality of secondary batteries and stops discharging of the plurality batteries when at least one of the detected voltages drops to a voltage equal to or smaller than a predetermined second reference voltage. 
     In a fourth aspect of the present invention, a charge and discharge circuit that performs the charging and discharging of a plurality of secondary batteries is provided. The charge and discharge circuit includes a plurality of charge current control circuits, connected in parallel to the plurality of secondary batteries, respectively, for performing bypass control of a charge current supplied to the plurality of secondary batteries. A plurality of potential detection circuits are connected to the plurality of charge current control circuits, respectively. Each potential detection circuit detects a voltage difference between the voltage of associated secondary battery and the voltage of another secondary battery and controls the associated charge current control circuit in accordance with the voltage difference to selectively bypass the charge current supplied to the associated secondary battery. A charge detection circuit is connected to the plurality of potential difference detection circuit to detect whether charging is performed into the plurality of secondary batteries and activate the plurality of potential difference detection circuits. An overcharge detection circuit detects the voltages of the plurality of secondary batteries and stops the discharging of the plurality of secondary batteries when at least one of the detected voltages is equal to or greater than a predetermined first reference voltage. An over discharge detection circuit detects the voltages of the plurality of secondary batteries and stops the discharging of the plurality of secondary batteries when and at least one of the detected voltages drops to a voltage equal to or smaller than a predetermined second reference voltage. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a schematic circuit diagram of a prior art charge and discharge circuit; 
     FIG. 2 is a schematic circuit diagram of a charge and discharge circuit according to a first embodiment of the present invention; and 
     FIG. 3 is a schematic circuit diagram of a charge and discharge circuit according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawings, like numerals are used for like elements throughout. 
     As shown in FIG. 2, a charge and discharge circuit  200  according to a first embodiment of the present invention includes a balanced current setting circuit  4 , a charging and discharging control circuit  7 , and a charge detect resistor RS connected between the positive electrode of a battery  1  and the discharge control switch  2 . The charging and discharging control circuit  7  includes a cell voltage detection circuit  8 , an overcharge detection circuit  9 , an over discharge detection circuit  10 , a charge detection circuit  21 , a gate circuit  22 , and a balanced current control circuit  23  (a potential difference detection circuit). The charge and discharge circuit  200  is preferably built in a battery pack. 
     Each of the three comparators  9   a - 9   c  of the overcharge detection circuit  9  has no hysteresis, generates a high-level detection signal when a cell voltage is equal to or greater than the second reference voltage VTH, and generates a low-level detection signal when the cell voltage dropped below the second reference voltage VTH. 
     The charge detection circuit  21  includes a comparator  21   a  that has a non-inverting input terminal connected to a node between the charge detection resistor RS and the discharge control switch  2  and an inverting input terminal connected to a node between the charge detection resistor RS and the battery  1  via a second reference offset voltage VTH 2 . The second reference offset voltage VTH 2  is smaller than the inter-terminal voltage of the charging detection resistance RS based on the current that flows in the charge detection resistor RS at charging. At charging, the input voltage of the non-inverting input terminal of the comparator  21   a  is greater than the input voltage of the inverting input terminal and the comparator  21   a  generates a high-level output signal. At discharging, the input voltage of the non-inverting input terminal is smaller than the input voltage of the inverting input terminal and the comparator  21   a  generates a low level output signal. That is, the charge detection circuit  21  generates the high-level output signal at charging, and the low-level output signal at discharging. 
     The gate circuit  22  includes an AND circuit  22   a  and an inverter circuit  22   b . The AND circuit  22   a  has a first input terminal that receives an output signal from the charge detection circuit  21  via the inverter circuit  22   b  and a second input terminal that receives an output signal from the overcharge detection circuit  10 . The output terminal of the AND circuit  22   a  is connected to the discharge control switch  2  (the gate terminal of the PMOS transistor). 
     In response to a low-level output signal from the over discharge detection circuit  10  and a high-level output signal from the charge detection circuit  21 , the AND circuit  22   a  supplies a low-level output signal to the discharge control switch  2  and the discharge control switch  2  is turned on. At discharging, when at least one of inter-terminal voltages V a -V c  of lithium ion batteries  1   a - 1   c  drops below a predetermined reference voltage and a low-level output signal is output from the charge detection circuit  21 , the discharge control switch  2  is turned off. 
     The balanced current control circuit  23  includes three potential difference comparison comparators  24   a - 24   c  and three AND circuits  25   a - 25   c.    
     Each of the potential difference comparison comparators  24   a - 24   c  includes a non-inverting input terminal and two inverting input terminals. Each of the comparators  24   a - 24   c  compares the input voltage supplied to the non-inverting input terminal and the smaller input voltage of the input voltages supplied to the two inverting input terminals. Each of the comparators  24   a - 24   c  generates a high-level output signal when the input voltage of the non-inverting input terminal is equal to or greater than the smaller input voltage of either of the inverting input terminals. Each of the comparators  24   a - 24   c  generates a low-level output signal when the input voltage of the non-inverting input terminal drops below the smaller input voltage of either of inverting input terminals. Therefore, each of the comparators  24   a - 24   c  functions as a potential difference circuit. 
     The potential difference comparison comparators  24   a - 24   c  are activated with a high-level output signal output from the charge detection circuit  21 . Each of the comparators  24   a - 24   c  is a differential amplification type comparator and includes a transistor (a constant current source) that is turned on and off by the output signal of the discharge detection circuit  21 . When the output signal of the charge detection circuit  21  is at the high level, the transistor is tuned on and each of the comparators  24   a - 24   c  is activated. When the output signal of the charge detection circuit  21  is at the low level, the transistor is turned off and each of the comparators  24   a - 24   c  is deactivated. Therefore, at discharging, each of the comparators  24   a - 24   c  is deactivated and power is not consumed. 
     The non-inverting input terminal of the first potential difference comparison comparator  24   a  is connected to the output terminal of a first cell voltage amplifier  8   a  of the cell voltage detection circuit  8  via a first reference offset voltage VD (a predetermined first reference voltage). The two inverting input terminals of the first potential difference comparison comparator  24   a  are connected to the output terminal of a second cell voltage amplifier  8   b  and the output terminal of a third cell voltage amplifier  8   c , respectively. 
     The first potential difference comparison comparator  24   a  supplies a high-level output signal to a first AND circuit  25   a  when the output voltage of the first cell voltage amplifier  8   a  (inter-terminal voltage V a  of the first lithium ion battery  1   a ) is equal to or greater than the inter-terminal voltage V b  of the lithium ion battery  1   b  or the inter-terminal voltage V c  of the lithium ion battery  1   c  by the first reference offset voltage VD. 
     The non-inverting input terminal of the second potential difference comparison comparator  24   b  is connected to the output terminal of the second cell voltage amplifier  8   b  via the first reference offset voltage VD. The two inverting input terminals of the second potential difference comparison comparator  24   b  are connected to the output terminal of the first cell voltage amplifier  8   a  and the output terminal of the third cell voltage amplifier  8   c , respectively. 
     The second potential difference comparison comparator  24   b  supplies a high-level output signal to a second AND circuit  25   b  when the output voltage of the second cell voltage amplifier  8   b  (inter-terminal voltage V b  of the second lithium ion battery  1   b ) is equal to or greater than the inter-terminal voltage V a  of the lithium ion battery  1   a  or the inter-terminal voltage V c  of the lithium ion battery  1   c  by the first reference offset voltage VD. 
     The non-inverting input terminal of the third potential difference comparison comparator  24   c  is connected to the output terminal of the third cell voltage amplifier  8   c  via the first reference offset voltage VD. The two inverting input terminals of the third potential difference comparison comparator  24   c  are connected to the output terminal of the first cell voltage amplifier  8   a  and the output terminal of the second cell voltage amplifier  8   b , respectively. 
     The third potential difference comparison comparator  24   c  supplies a high-level output signal to a third AND circuit  25   c  when the output voltage of the third cell voltage amplifier  8   c  (inter-terminal voltage V c  of the third lithium ion battery  1   c ) is equal to or greater than the inter-voltage V a  of the lithium ion battery  1   a  or the inter-terminal voltage V b  of the lithium ion battery  1   b  by the first reference offset voltage VD. 
     The AND circuits  25   a - 25   c  receive output signals from the potential difference comparison comparators  24   a - 24   c , respectively and receive an output signal from the charge detection circuit  21 . When the output signal of the charge detection circuit  21  is at the high level, the AND circuits  25   a - 25   c  supply the output signals of the potential difference comparison comparators  24   a - 24   c  to the gate terminals of NMOS transistors  6   a - 6   c  of charge current control circuits  4   a - 4   c , respectively. 
     Specifically, the output terminal of the first AND circuit  25   a  is connected to the gate terminal of the NMOS transistor  6   a  of the first charge current control circuit  4   a . When the output voltage of the cell voltage amplifier  8   a  is equal to or greater than the inter-terminal voltage V b  of the lithium ion battery  1   b  or the inter-terminal voltage V c  of the lithium ion battery  1   c  by the first reference offset voltage VD, the NMOS transistor  6   a  is turned on. 
     The output terminal of the second AND circuit  25   b  is connected to the gate terminal of the NMOS transistor  6   b  of the second charge current control circuit  4   b . When-the output voltage of the second cell voltage amplifier  8   b  is equal to or greater than the inter-terminal voltage V a  of the lithium ion  1   a  or the inter-terminal voltage V c  of the lithium ion battery  1   c  by the first reference offset voltage VD, the NMOS transistor  6   b  is turned on. 
     The output terminal of the third AND circuit  25   c  is connected to the gate terminal of the NMOS transistor  6   c  of the third charge current control circuit  4   c . When the output voltage of the third cell voltage amplifier  8   c  is equal to or greater than the inter-terminal voltage V a  of the lithium ion battery  1   a  or the inter-terminal voltage V b  of the lithium ion battery  1   b  by the first reference offset voltage VD, the NMOS transistor  6   c  is turned on. 
     Next, the operation of the charge and discharge circuit  200  is described. 
     Hereupon, the second reference voltage VTH supplied to the comparators  9   a - 9   c  may be 4.2 volts and the first reference offset voltage VD may be 0.2 volts. 
     When a charger is connected to input/output terminals t 1 , t 2  and charging to the battery  1  is started, the charge detection circuit  21  outputs a high-level output signal and the balanced current control circuit  23  is activated. 
     When each of the inter-terminal voltages V a -V c  of the lithium ion batteries  1   a - 1   c  is 3.8 volts, for example, the potential of 0.2 volts (first reference offset voltage VD) does not exist in the inter-terminal voltages V a -V c  of the lithium ion batteries  1   a - 1   c . Therefore, the potential difference comparison comparators  24   a - 24   c  supply low-level output signals to the AND circuits  25   a - 25   c  and the charge current control circuits  4   a - 4   c  are nonconducted. Thus, charging is evenly performed to the lithium ion batteries  1   a - 1   c  toward 4.2 volts. 
     In the following a case is described where the potential difference equal to or greater than 0.2 volt (first reference offset voltage VD) exists in the inter-terminal voltages V a -V c  of the lithium ion batteries  1   a - 1   c  is described. For example, when the inter-terminal voltage V a  of the first lithium ion battery  1   a  is 3.8 volts and the inter-terminal voltages V b , V c  of the second and third lithium ion batteries  1   b ,  1   c  are 3.5 volts, the first potential difference comparison comparator  24   a  supplies a high-level output signal to the first AND circuit  25   a  and the second and third potential difference comparison comparators  24   b ,  24   c  supply low-level output signals to the second and third AND circuits  25   b ,  25 C, respectively. Thereupon, only the first charge current control circuit  4   a  is conducted and charging is evenly performed to the second and third lithium ion batteries  1   b ,  1   c  toward 3.8 volts. At this time, since the charging is continuously performed, even if the first charge current control circuit  4   a  is conducted, the first lithium ion battery  1   a  will not discharge charges via the first charge current control circuit  4   a.    
     When the potential difference between the inter-terminal voltages V b , V c  of the second and third lithium ion batteries  1   b ,  1   c  and the inter-terminal voltage V a  of the first lithium ion battery  1   a  is reduced to less than 0.2 volt, the first potential difference comparison comparator  24   a  supplies a low-level output signal to the first AND circuit  25   a . As a result, all of the charge current control circuits  4   a - 4   c  are nonconducted and charging is evenly performed to the lithium ion batteries  1   a - 1   c  toward 4.2 volts. 
     Shortly, when charging is evenly performed with the potential difference of less than 0.2 volt and at least one of the lithium ion batteries  1   a - 1   c  reaches substantially 4.2 volts, the charge control switch  3  is turned off and the charging is terminated. At this time, the charge detection circuit  21  outputs a low-level output signal and the balanced current control circuit  23  is deactivated. 
     The charge and discharge circuit  200  of the first embodiment has the following advantages: 
     (1) Until the lithium ion batteries  1   a - 1   c  are evenly charged into 4.2 volts, the battery that was first charged into 4.2 volts will not repeat charging and discharging. Therefore, the deterioration of a lithium ion battery is suppressed and long-life batteries and battery packs are provided. 
     (2) Until the lithium ion batteries  1   a - 1   c  are evenly charged into 4.2 volts, charging operation is continued. Therefore, compared with a conventional method in which charging and discharging are repeated, the charging time is reduced so long as the charging operation will not stop at charging. 
     (3) The charge current control circuit that corresponds to the lithium ion battery charged first into 4.2 volts is maintained in a nonconducted state until another lithium ion battery is charged into 4.2 volts. Therefore, since no current will flow into the charge current control circuit for a long time, the durability of the charge current control circuit does not deteriorate and the increase of power consumption is prevented. 
     (4) The charge detection circuit  21  deactivates the balanced current control circuit  23  at discharging. Therefore, the increase of the power consumption of a lithium ion battery at discharging is prevented. Moreover, the NMOS transistors  6   a - 6   c  of the charge current control circuits  4   a - 4   c  are also turned off at discharging. Therefore, the charges of the respective lithium ion batteries  1   a - 1   c  will not be discharged via each of the charge current control circuits  4   a - 4   c . In other words, at discharging, power is not consumed by the charge current control circuits  4   a - 4   c.    
     (5) When the battery  1  is charged, the charge detection circuit  21  and the gate circuit  22  that turn on the discharge control switch  2  are provided regardless of the detection result (low-level output signal) of the over discharge detection circuit  10  indicating at least one voltage of the lithium ion batteries  1   a - 1   c  is overdischarging. Therefore, when at least one voltage of the lithium ion batteries  1   a - 1   c  is a discharge voltage, no charge current flows via the parasitic diode D 2  of the discharge control switch  2  (PMOS transistor). As a result, the discharge control switch  2  (PMOS transistor) will not be heated due to the charge current that flows into the parasitic diode D 2 . 
     As shown in FIG. 3, a charge and discharge circuit  300  according to a second embodiment of the present invention includes a balanced current control circuit  31  that differs from that of the first embodiment. The balanced current control circuit  31  includes three differential voltage amplifiers  32   a - 32   c  and three priority circuits  33   a - 33   c . Each of the differential voltage amplifiers  32   a - 32   c  amplifies a voltage (differential voltage) obtained by subtracting an input voltage supplied to an inverting input terminal from an input voltage supplied to a non-inverting input terminal. Each of the differential voltage amplifiers  32   a - 32   c  outputs the predetermined minimum output voltage when the differential voltage is equal to or smaller than 0 volt. The minimum output voltage is sufficient to turn off the NMOS transistors  6   a - 6   c  of the charge current control circuits  4   a - 4   c.    
     The differential voltage amplifiers  32   a - 32   c  are activated with a high-level output signal supplied from the charge detection circuit  21 . Each of the differential voltage amplifiers  32   a - 32   c  is a differential amplification type amplifier and includes a transistor (a constant current source) that is turned on and off with the output signal of the charge detection circuit  21 . When the output signal of the charge detection circuit  21  is at the high level in charging, the transistor is turned on and each of the differential voltage amplifiers  32   a - 32   c  is activated. When the output of the charge detection circuit  21  is at the low level in discharging, the transistor is turned off and each of the differential voltage amplifiers  32   a - 32   c  is deactivated. Therefore, at discharging, each of the differential voltage amplifiers  32   a - 32   c  does not consume power. 
     The non-inverting input terminal of the first differential voltage amplifier  32   a  is connected to the output terminal of the first cell voltage amplifier  8   a  of the cell voltage detection circuit  8 . The inverting input terminal of the first differential voltage amplifier  32   a  is connected to the output terminal of the first priority circuit  33   a . The first priority circuit  33   a  is connected to the output terminal of the second cell voltage amplifier  8   b  and the output terminal of the third cell voltage amplifier  8   c  and supplies the smaller voltage of the inter-terminal voltages V b , V c  of the lithium ion batteries  1   b ,  1   c  to the inverting input terminal of the first differential voltage amplifier  32   a.    
     Therefore, the first differential voltage amplifier  32   a  amplifies the differential voltage between the output voltage of the first cell voltage amplifier  8   a  (inter-terminal voltage V a  of the first lithium ion battery  1   a ) and the smaller voltage of the inter-terminal voltages V b , V c  of the Elithium ion batteries  1   b ,  1   c  and outputs an amplification voltage that corresponds to the differential voltage. 
     The output voltage of the first differential voltage amplifier  32   a  is supplied to the gate terminal of the NMOS transistor  6   a  of the first charge current control circuit  4   a . Therefore, the on resistance of the NMOS transistor  6   a  is changed in accordance with the output voltage (i.e., the output voltage according to the differential voltage) of the first differential voltage amplifier  32   a . That is, as the output voltage of the first differential voltage amplifier  32   a  becomes great, the on resistance of the NMOS transistor  6   a  becomes small and the current that flows into the first charge current control circuit  4   a  (i.e., a resistor  5   a  and the NMOS transistor  6   a ) increases. 
     The non-inverting input terminal of the second differential voltage amplifier  32   b  is connected to the output terminal of the second priority circuit  8   b  of the cell voltage detection circuit  8 . The inverting input terminal of the second voltage detection circuit  32   b  is connected to the output terminal of the second priority circuit  33   b . The second priority circuit  33   b  is connected to the output terminal of the first cell voltage amplifier  8   a  and the output terminal of the third cell voltage amplifier  8   c  and supplies the smaller voltage of the inter-terminal voltages V a , V c  of the lithium ion batteries  1   a ,  1   c  to the inverting input terminal of the second differential voltage amplifier  32   b.    
     Therefore, the second differential voltage amplifier  32   b  amplifies the differential voltage between the output voltage of the second cell voltage amplifier  8   b  (inter-terminal voltage V b  of the second lithium ion battery  1   b ) and the smaller voltage of either of the inter-terminal voltages V a , V c  of the lithium ion batteries  1   a ,  1   c  and outputs an amplification voltage. 
     The output voltage of the second differential voltage amplifier  32   b  is supplied to the gate terminal of the NMOS transistor  6   b  of the second charge current control circuit  4   b . Therefore, the on resistance of the NMOS transistor  6   b  is changed in accordance with the output voltage (i.e., the output voltage according to the differential voltage) of the second differential voltage amplifier  32   b . That is, as the output voltage of the second differential voltage amplifier  32   b  becomes great, the on resistance of the NMOS transistor  6   b  becomes small and the current that flows into the second charge current control circuit  4   b  (i.e., a resistor  5   b  and the NMOS transistor  6   b ) increases. 
     The non-inverting input terminal of the third differential voltage amplifier  32   c  is connected to the output terminal of the third cell voltage amplifier  8   c . The inverting input terminal of the third differential voltage amplifier  32   c  is connected to the output terminal of the third priority circuit  33   c . The third priority circuit  33   c  is connected to the output terminal of the first cell voltage amplifier  8   a  and the output terminal of the second cell voltage amplifier  8   b  and supplies the smaller voltage of the inter-terminal voltages V a , V b  of the lithium ion batteries  1   a ,  1   b  to the inverting input terminal of the third differential voltage amplifier  32   c.    
     Therefore, the third differential voltage amplifier  32   c  amplifies the differential voltage between the output voltage of the third cell voltage amplifier  8   c  (inter-terminal voltage V c  of the third lithium ion battery  1   c ) and the smaller voltage of either of the inter-terminal voltages V a , V b  of the lithium ion batteries  1   a ,  1   b  and outputs an amplification voltage. 
     The output voltage of the third differential voltage amplifier  32   c  is supplied to the gate terminal of the NMOS transistor  6   c  of the third charge current control circuit  4   c . Therefore, the on resistance of the NMOS transistor  6   c  is changed in accordance with the output voltage (i.e., output voltage according to the differential voltage) of the third differential voltage amplifier  32   c . That is, as the output voltage of the third differential voltage amplifier  32   c  becomes great, the on resistance of the NMOS transistor  6   c  becomes small and the current that flows in the third charge current control circuit  4   c  (i.e., a resistor  5   c  and the NMOS transistor  6   c ) increases. 
     Next, the operation of the charge and discharge circuit  300  is described. 
     When a charger is connected to the input/output terminals t 1 , t 2  and charging to the battery  1  is started, the charge detection circuit  21  outputs a high-level output signal and the balanced current control circuit  31  is activated. 
     When each of the inter-terminal voltages V a -V c  of the lithium ion batteries  1   a - 1   c  is 3.8 volts, for example, the differential voltage is 0 volt. Therefore, each of the differential voltage amplifiers  32   a - 32   c  outputs the minimum output voltage. The NMOS transistors  6   a - 6   c  are turned off and charging is evenly performed to the lithium ion batteries  1   a - 1   c  toward 4.2 volts. 
     Then, for example, the case where the inter-terminal voltage V a  of the first lithium ion battery  1   a  is 3.8 volts, the inter-terminal voltage V b  of the second lithium ion battery  1   b  is 3.6 volts and the inter-terminal voltage V c  of the third lithium ion battery  1   c  is 3.5 volts is described. The first differential voltage amplifier  32   a  receives the differential voltage of 0.3 volt, the second differential voltage amplifier  32   b  receives the differential voltage of 0.1 volt and the third differential voltage amplifier  32   c  receives the differential voltage of minus 0.1 volt. The first differential voltage amplifier  32   a  outputs the maximum first output voltage, the second differential voltage amplifier  32   b  outputs a second output voltage smaller than the first output voltage and the third differential voltage amplifier  32   c  outputs the minimum output voltage. Thus, the first and second charge current control circuits  4   a ,  4   b  are conducted and the third charge current control circuit  4   c  is nonconducted. Further, the on resistance of the transistor  6   a  of the first charge current control circuit  4   a  is smaller than the on resistance of the transistor  6   b  of the second charge current control circuit  4   b . Therefore, a lot of current flows into the first charge current control circuit  4   a . That is, a charge current increases in descending order of the inter-terminal voltages V a -V c  of the lithium ion batteries  1   a - 1   c . As a result, the inter-terminal voltages V a -V c  of the lithium ion batteries  1   a - 1   c  gradually become equal. 
     When the inter-terminal voltages V a -V c  of the lithium ion batteries  1   a - 1   c  become equal each other, the NMOS transistors  6   a - 6   c  of the charge current control circuits  4   a - 4   c  are turned off and charging is evenly performed to the lithium ion batteries  1   a - 1   c  toward 4.2 volts. 
     The charge and discharge circuit  300  of the second embodiment has the following advantages: 
     In addition to the advantages (1) to (5) of the first embodiment, the balanced current control circuit  23  controls the charge current control circuits  4   a - 4   c  so that the charge current that corresponds to the potential difference of each of the lithium ion batteries  1   a - 1   c  flows into each of the lithium ion batteries  1   a - 1   c . Therefore, the equalization time of the cell voltages V a -V c  of each of the lithium ion batteries  1   a - 1   c  is further shortened. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms. 
     The present invention may apply to not only a charge and discharge circuit of a notebook-size personal computer but also to the charge and discharge circuit of portable electronic apparatus, such as a digital camera, digital video, PDA (personal digital assistant) and cellular phone, for example. 
     The present invention may also apply to a charge and discharge circuit of a battery including two or four lithium ion batteries or more. 
     The present invention may also apply to a charge and discharge circuit of a secondary battery, such as a nickel hydrogen battery, other than a lithium ion battery. 
     If a charger installed in the personal computer or a charger that separately supplies a charge current to a battery pack can surely perform constant current and constant voltage control, the overcharge detection circuit  9  may be omitted. 
     The first reference offset voltage VD is not limited to 0.2 volt. For example, the cell voltages V a -V c  of the lithium ion batteries  1   a - 1   c  are equalized more strictly by setting the first reference offset voltage VD to a smaller voltage than 0.2 volt. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.