Abstract:
In a battery pack manager that manages series-connected rechargeable unit cells, a cell equalizer equalizes the cell voltages by individually discharging the unit cells according to deviations from reference voltages. An overcharge/overdischarge detector detects an overcharge and an overdischarge state of each unit cell. An inhibit circuit prevents the cell equalizer from discharging the unit cells when the overcharge/overdischarge detector is activated to reduce the cell voltage variability, which would otherwise occur as a result of interference from the overcharge/overdischarge detector, so that the overcharge/overdischarge states of all unit cells can be determined with precision. Connecting lines of the unit cells are monitored to detect a line-cut. The inhibit circuit further inhibits the cell equalizer when the connecting lines are being monitored to reduce the cell voltage variability, which would otherwise occur as a result of interference from the line-cut detection, so that false line-cut detection is avoided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based on Japanese Patent Application No. 2004-335002, filed Nov. 18, 2004, which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to battery pack controllers, and more specifically to a battery pack controller that performs control on the voltages and charge/discharge states of unit cells and cell-modules of a battery pack mounted on a motor vehicle.  
         [0004]     2. Description of the Related Art  
         [0005]     In motor vehicles such as electric cars and hybrid cars, it is the usual practice to assemble a large number of lithium unit cells into a single package, known as battery pack, and bundle their power-feed lines to reduce their wiring cross-section and the loss of their associated switching elements. Since lithium cells are susceptible to over-charging and over-discharging, the voltages developed by the unit cells tend to vary significantly from one cell to another. To eliminate the voltage variability, a cell equalizer is used as an indispensable element to prevent the unit cells from becoming over-charged or over-discharged. One example of such cell equalizer is shown and described in Japanese Patent Publication 2002-325370. Since the number of unit cells in a battery pack is as large as several hundred, they are equally divided into a number of groups, called “modules”, and a plurality of inter-cell cell equalizers are provided, one for each module to equalize the cell voltages of each module. An inter-module cell equalizer may also be used to eliminate a voltage difference that can possibly occur between the modules.  
         [0006]     Another indispensable tool for the management of a battery pack involves the use of an overcharge/overdischarge detector that monitors the charge/discharge state of each unit cell and cuts off its charge/discharge currents or provides a preventive measure by controlling the cell currents based on the monitored state. In a hybrid car where a high-voltage battery pack is used, the number of unit cells is significantly large and the voltage difference between the unit cells is high. Because of this reason, the unit cells of the high-voltage battery pack are equally segmented into multiple modules in like manner to the cell equalizer and each module is controlled by a module controller. In addition to the segmentation, the overcharge/overdischarge detector is provided to supply its output to a battery pack controller that provides an overall management of the battery pack in order to achieve simplification of its wiring and isolation of its input and output lines.  
         [0007]     Also known is a line monitor that monitors the lines connecting the opposite terminals of each module and the opposite terminals of each unit cell to the module controllers for detecting a possible disconnection, as shown and described in Japanese Patent Publications 2002-325370 and 2003-084015.  
         [0008]     However, when the module controllers provide control on the cell equalizer and the overcharge/overdischarge detector in an attempt to reduce cell-to-cell voltage differences while preventing each cell of a module from being overcharged or overdischarged, the operation of the overcharge/overdischarge detector conflicts with the operation of the cell equalizer, causing the voltage of each unit cell of the module to vary significantly. As a result, the charge/discharge state of each unit cell cannot precisely detected. In particular, when the cell equalizer is operating a unit cell in a discharge mode and a disconnection should occur in the line connecting this unit cell to the discharge resister of the cell equalizer, a unit-cell voltage will be impressed in error on a cell charge/discharge monitor that is associated with a unit cell adjacent to the discharge-mode unit cell, resulting in a significant error on the determination of overcharge/overdischarge state of the unit cell.  
         [0009]     Furthermore, if line-cut detection is performed when the cell equalizer is operating to equalize cell voltages, the current that flows through the discharge resistor of the equalizer interferes with the line-cut detection and a false line-cut decision could result.  
       SUMMARY OF THE INVENTION  
       [0010]     It is therefore an object of the present invention to provide a battery pack manager capable of performing overcharge/overdischarge detection, a line-cut detection and cell-equalization without causing mutual interference and without introducing complexity and increasing power dissipation.  
         [0011]     According to a first aspect of the present invention, there is provided a battery pack manager for a plurality of unit cells of secondary battery connected in series, comprising a cell equalizer, associated with the unit cells, that equalizes cell voltages developed by the unit cells, an overcharge/overdischarge detector, associated with the unit cells, that operates to detect an overcharge and an overdischarge of the unit cells, and an inhibit circuit that prevents the cell equalizer from discharging the unit cells when the overcharge/overdischarge detector is operating. Since cell voltage variability which would otherwise occur as a result of interference from overcharge/overdischarge detection is reduced by the inhibit circuit, overcharge/overdischarge states of the unit cells can be determined with precision.  
         [0012]     According to a second aspect of the present invention, there is provided a battery pack manager for a plurality of unit cells of secondary battery connected in series, comprising a cell equalizer, associated with the unit cells, that equalizes cell voltages developed by the unit cells, a line monitor, associated with the unit cells, that monitors connecting lines that connect the unit cells to the cell equalizer, an overcharge/overdischarge detector, associated with the unit cells, that detects an overcharge and an overdischarge of the unit cells, and an inhibit circuit that prevents the cell equalizer from discharging the unit cells when the line monitor is monitoring the connecting lines. Since cell voltage variability which would otherwise occur as a result of interference from line-cut detection is reduced by the inhibit circuit, detection of false line-cut can be avoided.  
         [0013]     In a preferred embodiment, the cell equalizer is activated to perform cell voltage equalization when a main switch of the battery pack is turned off. This allows the cell equalizer to use open-circuit cell voltages as reference voltages, false detection of overcharge/overdischarge states of the unit cells and false detection of line-cut on the connecting lines can be easily avoided using a simple circuit.  
         [0014]     In a further preferred embodiment, the overcharge/overdischarge circuit is inhibited when the cell equalizer is activated. This allows the cell equalizer to be inhibited with a simple circuit when overcharge/overdischarge detection or line-cut detection is performed.  
         [0015]     According to a third aspect of the present invention, there is provided a battery pack manager for a plurality of unit cells of secondary battery connected in series, comprising a cell equalizer, connected to the unit cells via a plurality of connecting lines, that equalizes cell voltages developed by the unit cells, an overcharge/overdischarge detector, connected to the unit cells via the plurality of connecting lines, that makes a decision on whether the unit cells are overcharged or overdischarged, a line-cut detector that detects a line-cut on the connecting lines, and an inhibit circuit that inhibits decision output of the overcharge/overdischarge detector when a line-cut is detected by the line-cut detector on one of the connecting lines. Since the output signal of the overcharge/overdischarge detector that arises at the instant a line-cut is detected is not used, false battery-pack management can be avoided.  
         [0016]     According to a fourth aspect of the present invention, there is provided a battery pack manager for a plurality of unit cells of secondary battery connected in series, comprising a cell equalizer, connected to the unit cells via a plurality of connecting lines, that equalizes cell voltages developed by the unit cells, an overcharge/overdischarge detector, connected to the unit cells via the connecting lines, that makes a decision on whether the unit cells are overcharged or overdischarged, a line-cut detector that detects a line-cut on the connecting lines, and an inhibit circuit that inhibits discharge operation of the cell equalizer when a line-cut is detected by the line-cut detector on one of the connecting lines. Since the cell equalizer is inhibited immediately following the detection of a line-cut, cell equalization, which would otherwise be performed using differing cell voltages caused by the line-cut, is inhibited and hence useless discharging of the unit cells can be avoided. In addition, since the line voltage on the line where the line-cut is detected tends to vary significantly due to cell equalization on that failed line and useless cell equalization ensues, the present invention prevents the unit cells from being discharged by such useless cell equalization.  
         [0017]     According to a fifth aspect, the present invention provides a battery pack manager for a plurality of rechargeable unit cells connected in series, wherein the unit cells are divided into a plurality of battery-pack modules. The battery-pack manager comprises a battery-pack controller, connected to the battery-pack modules via a pair of battery-pack connecting lines and a plurality of module-connecting lines, for successively detecting a voltage value of each of the battery-pack modules. The controller then determines that a line-cut has occurred on one of the battery-pack connecting lines if a single zero-voltage value is detected and determines that a line-cut has occurred on one of the module-connecting lines if two zero-voltage values are detected in succession.  
         [0018]     According to a further aspect, the present invention provides a method of detecting a line-cut on a plurality of connecting lines connecting a plurality of series-connected rechargeable unit cells to control circuitry, wherein the unit cells are divided into a plurality of battery-pack modules, comprising the steps of (a) successively detecting a voltage value of each of the battery-pack modules, and (b) determining that a line-cut has occurred on one of the battery-pack connecting lines if a single zero-voltage value is detected or that a line-cut has occurred on one of the module-connecting lines if two zero-voltage values are detected in succession. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The present invention will be described in detail with reference to the following drawings, in which:  
         [0020]      FIG. 1  is a block diagram of a battery pack manager of the present invention;  
         [0021]      FIG. 2  is a block diagram of a module controller of the present invention;  
         [0022]      FIG. 3  is a circuit diagram of an overcharge/overdischarge detecting comparator of the module controller;  
         [0023]      FIG. 4  is a circuit diagram of the anomaly detector of  FIG. 3 ;  
         [0024]      FIG. 5  is a flowchart of the operation of the battery pack controller inhibits the cell equalizer when it reads output data of an overcharge/overdischarge detector;  
         [0025]      FIG. 6  is a flowchart of the operation of the battery pack controller when it performs a line-cut detection routine;  
         [0026]      FIG. 7  is a flowchart of the operation of the battery pack controller when it performs a modified line-cut detection routine using successively detected module voltages;  
         [0027]      FIG. 8  is a flowchart of the operation of the battery pack controller when it performs a modified line-cut detection routine using successively detected module voltages;  
         [0028]      FIG. 9  is a block diagram showing portions of adjacent module controllers for describing the operation of the battery pack controller when it performs a line-cut detection subroutine on the cell-connecting lines;  
         [0029]      FIG. 10  is a block diagram of an overcharge/overdischarge detector connected to the battery pack controller to perform a line-cut detection on the cell-connecting lines according to a modified embodiment of the present invention;  
         [0030]      FIG. 11  is a block diagram of the overcharge/overdischarge detector according to a further modification of the present invention for detecting a line-cut on the cell-connecting lines; and  
         [0031]      FIGS. 12A and 12B  are flowcharts of a time-shared modes of operation when the main switch of the battery pack is turned off. 
     
    
     DETAILED DESCRIPTION  
       [0032]     Battery Pack  1   
         [0033]     Referring to  FIG. 1 , a battery pack  1  and a battery pack manager  2  are illustrated. All unit cells of the battery pack  1  are lithium secondary cells whose voltages vary significantly from one cell to another. Because of the difficulty to individually control the cell voltages, the unit cells are segmented into multiple modules M 1 ˜M 8 . For the purpose of disclosure, each battery pack module is made up of four unit cells C 1  through C 4 . All unit cells of the battery pack  1  are connected in series between a high-voltage power line LH and a low-voltage power line LL and these power lines are connected to a generator or an electrical load such as regenerative driving motor, not shown.  
         [0034]     Battery Pack Manager  2   
         [0035]     Battery pack manager  2  comprises a battery pack controller  4 , a current sensor  5 , and a plurality of module controllers  6 - 1  through  6 - 8  of identical configuration, respectively associated with the modules M 1  through M 8  to individually control their unit cells. Current sensor  5  monitors the low-voltage power line LL and informs the battery pack controller  4  of the amount of main current it detects from the power line LL. By integrating the detected main current, the battery pack controller  4  determines the state of charge (SOC) of the battery pack  1 .  
         [0036]     Battery Pack Controller  4   
         [0037]     Battery pack controller  4  is connected to the modules M 1 ˜M 8  through connecting lines L 1 , L 5 , L 9 , . . . , L 29  and L 33 , to the module controllers  6 - 1 ˜ 6 - 8  and further to an external controller, not shown, that controls the generator and electrical load. Battery pack controller  4  exchanges control information with the external controller to jointly provide an overall control on the battery pack  1  so that its operating state is maintained in an optimum range. If the capacity of the battery pack lowers, the battery pack controller  4  communicates this fact to the external controller to request it to recharge the battery pack  1 . When the battery pack reaches its capacity the battery pack controller  4  requests the external controller to stop charging or discharge the battery pack.  
         [0038]     For each of the modules M 1 ˜M 8 , the battery pack controller  4  includes a module discharge circuit  10  formed with a resistor Rm and a switch SWm connected in series between the module connecting lines, which are L 1  and L 5  in the case of module M 1 . CPU  11  is provided in the battery pack controller to control the module controllers M 1 ˜M 8  in response to input signals therefrom as well as from the external controller and the current sensor  5 .  
         [0039]     CPU  11  is provided in the battery pack controller. As will be described later with reference to  FIG. 4 , the CPU  11  controls the switch SWm to discharge the module terminal voltage, and detects abnormal state of the voltages and temperatures of the battery pack  1  based on module terminal voltages from the battery pack  1 , output signals from the module controllers  6  and current sensor  5  to perform one of a set of predetermined recovery actions on the unit cells of one or more modules by way of their associated module controllers  6 .  
         [0040]     Connecting Lines L 1 ˜L 33   
         [0041]     Connecting lines L 1 ˜L 5  supply the terminal voltages of the unit cells C 1 ˜C 4  of module M 1  to the module controller  6 - 1 . Similarly, the connecting lines L 5 ˜L 9  supply the terminal voltages of the unit cells of module M 2  to the module controller  6 - 2 , and so on. Of all the connecting lines, the lines L 1 , L 5 , L 9 , L 13 , . . . , L 29  and L 33  connect the opposite terminals of modules M 1 ˜M 8 , as clearly seen in  FIG. 4 , for detecting their terminal voltages. As will be described, the battery pack controller  4  is provided with a function that monitors these nine module-connecting lines L 1 , L 5 , . . . , L 33  for detecting a line-cut.  
         [0042]     Module Controllers  6 - 1 ˜ 6 - 8   
         [0043]     Since the module controllers  6 - 1 ˜ 6 - 8  are of identical configuration, it would be suffice to illustrate the detail of only one module controller, the module controller  6 - 1 , for example, in  FIG. 2 . As illustrated, the module controller  6 - 1  generally comprises a cell equalizer  8  and an overcharge/overdischarge detector  9 . Cell equalizer  8  and the overcharge/overdischarge detector  9  are connected to the terminals of unit cells C 1 ˜C 4  through the connecting lines L 1 ˜L 5 .  
         [0044]     Cell Equalizer  8   
         [0045]     Cell equalizer  8  includes a voltage divider  20  formed with a plurality of resistors r of equal value connected in series between the connecting lines L 1  and L 5  to establish a plurality of reference voltages V 2 ′, V 3 ′ and V 4 ′. A plurality of comparators  21 ,  22  and  23  are provided for making comparisons between line voltages V 2 , V 3 , V 4  arranged in descending order from line L 2  to line L 4 , and the reference voltages V 2 ′, V 3 ′ and V 4 ′ likewise arranged in descending order from line L 2  to line L 4 .  
         [0046]     In the cell equalizer  8 , logic circuits  24  and  25  are connected to the comparators  21 ,  22  and  23  to determine which of the unit cells to be discharged based on comparison results supplied from the comparator output terminals. A plurality of discharge circuits  26 ˜ 29  are respectively connected across the terminals of unit cells C 1 ˜C 4  to selectively discharge the unit cells in response to the outputs of comparators  21  and  23  and the outputs of logic circuits  24  and  25 . Each of the discharge circuits  26 ˜ 29  consists of a resistor R and a switch connected in series across the terminals of the associated unit cell.  
         [0047]     When the cell voltages of module M 1  are equal to one another, line voltages V 2 , V 3  and V 4  are equal to the reference voltages V 2 ′, V 3 ′ and V 4 ′, respectively. Since the line voltages are measured with respect to the lowest-voltage line L 5 , if the cell voltage of cell C 1  is higher than the ¼ of the module voltage across lines L 1  and L 5 , the line voltage V 2  is lower than the reference voltage V 2 ′, and if the cell voltage of cell C 4  is higher than the ¼ of the module voltage, the line voltage V 4  is higher than the reference voltage V 4 ′. Therefore, the outputs of comparators  21  and  23  can be used to uniquely determine whether the unit cells C 1  and C 4  must be discharged or not, whereas the logic circuits  24  and  25  are required to determine whether or not the intermediate cells C 2  and C 3  are to be discharged.  
         [0048]     Therefore, when the line voltage V 2  is lower than the reference voltage V 2 ′, the comparator  21  produces an output that turns on the discharge circuit  26  to cause the unit cell C 1  to discharge. When the line voltage V 4  is higher than the reference voltage V 4 ′, the comparator  23  produces an output that turns on the discharge circuit  29  to cause the unit cell C 4  to discharge. The comparison result of comparator  21  and its logical reverse are supplied to the logic circuit  24 , and the comparison result of comparator  23  and its logical reverse are supplied to the logic circuit  25 . The comparison result of comparator  22  and its logical reverse are supplied to both logic circuits  24  and  25 .  
         [0049]     Based on input logical values, the logic circuits  24  and  25  determine whether or not the unit cells C 2  and C 3  are to be discharged and supplies their decision results to the discharge circuits  27  and  28 , respectively. More specifically, the logic circuit  24  produces an output that causes the unit cell C 2  to discharge only when line voltage V 3  is lower than reference voltage V 3 ′ and the unit cell C 1  is not being discharged. Logic circuit  25  produces an output that causes the unit cell C 3  to discharge only when line voltage V 4  is higher than reference voltage V 4 ′ and the unit cell C 4  is not being discharged. (For detailed description of the logic circuits  24  and  25 , reference is made to the aforesaid Japanese Patent Publication 2002-325370).  
         [0050]     In this way, the discharge circuits  26 ˜ 29  are individually turned on to discharge their associated unit cells C 1 ˜C 4  so that the inter-cell voltages V 2 ˜V 4  approach the reference voltages V 2 ′˜V 4 ′.  
         [0051]     A discharge inhibiter  30  is provided in the cell equalizer  8 . Discharge inhibiter  30  comprises a switch  31  and a plurality of low-impedance clamping circuits  32 . Each clamping circuit  32  includes a diode D and a resistor rx connected in series to the switch  31 . Switch  31  is preferably implemented with a photocoupler whose emitter electrode is connected to the connecting line L 5  where the line voltage is the lowest of module M 1  and whose collector electrode is connected through resistor rx to the cathode of diode D whose anode is connected to the control input of the associated discharge circuit. Diode D is thus backward biased to prevent the flow of reverse current to the control terminal of the associated discharge circuit. The photodiode of photocoupler  31  is responsive to a discharge inhibit signal supplied from the battery pack controller  4  by turning on its photo-transistor to forcibly set the switches of all discharge circuits  26 ˜ 29  at the same time into OFF state.  
         [0052]     Overcharge/Overdischarge Detector  9   
         [0053]     Overcharge/overdischarge detector  9  comprises a plurality of comparators  33 ˜ 36  of identical configuration, respectively connected across the opposite terminals of unit cells C 1 ˜C 4  through connecting lines L 1 ˜L 5 . Each of the comparators  33 ˜ 36  produces, at one of their output terminals, a cell overcharge signal Sc or a cell overdischarge signal Sd by comparing the cell voltage of the associated cell with low and high threshold voltages. The output terminals of each of comparators  33 ˜ 36  are connected to an overcharged cell detector  37  and an overdischarged cell detector  38 , respectively. Each of the detectors  37  and  38  is a logic circuit. Based on the logical values of its input terminals, the overcharged cell detector  37  determines whether there is at least one overcharged unit cell in the associated module and supplies the decision output to the CPU  11  of battery pack controller  4 . Likewise, the overdischarged cell detector  38  determines from the logical levels of its input terminals whether there is at least one overdischarged unit cell in the associated module and supplies the decision output to the battery pack controller  4 .  
         [0054]     Since the comparators  33 ˜ 36  are of identical configuration, only one comparator  33  is shown in detail in  FIG. 3 . Comparator  33  is comprised of an overcharge detector  39  and an overdischarge detector  40 . In the overcharge detector  39 , the cell voltage V 1  is suitably scaled down by a voltage-divider  41  formed with resistors R 1 , R 2  connected across the lines L 1  and L 2  and supplied to the noninverting input of a comparator  44 . A high-threshold voltage V H  is developed by a reference voltage source  42  formed with a resistor R 5  and a Zener diode ZD 1 , connected across the lines L 1  and L 2 , and supplied to the inverting input of the comparator  44 . Comparator  44  compares the scaled-down voltage with the high threshold voltage V H  and supplies a cell overcharge signal Sc to the overcharged cell detector  37  when the voltage V 1  exceeds a predetermined level of charge.  
         [0055]     Similarly, in the overdischarge detector  40 , the cell voltage V 1  is suitably scaled down by a voltage-divider  45  formed with resistors R 3 , R 4  connected across the lines L 1  and L 2  and supplied to the noninverting input of a comparator  48 . A low-threshold voltage V L  is developed by a reference voltage source  46  formed with a resistor R 6  and a Zener diode ZD 2 , connected across the lines L 1  and L 2 , and supplied to the inverting input of the comparator  48 . Comparator  48  compares the scaled-down voltage with the low threshold voltage V L  and supplies a cell overdischarge signal Sd to the overdischarged cell detector  38  when the voltage V 1  falls below a predetermined level of charge.  
         [0056]     Overcharge Cell Detector  37  and Overdischarge Cell Detector  38   
         [0057]     Overcharged cell detector  37  and overdischarged cell detector  38  have the effect of reducing the number of insulated wires the battery pack controller  4  needs to receive decision outputs from threshold comparators  36 ˜ 39 . When one of the unit cells C 1 ˜C 4  is overcharged, the overcharged cell detector  37  supplies a module overcharged signal Smc through a photocoupler, not shown, to the battery pack controller  4 . Likewise, when one of the unit cells C 1 ˜C 4  is overdischarged, the overdischarged cell detector  38  supplies a module overdischarged signal Smd through a photocoupler to the battery pack controller  4 .  
         [0058]     Module Voltage Detection by Battery Pack Controller  
         [0059]     As shown in  FIG. 4 , the battery pack controller  4  includes a module voltage detector  49  that takes its input voltage through a multiplexer  50  selectively in sequence from the modules M 1 ˜M 8 . Voltage detector  49  performs a pumping action on the voltage supplied from the multiplexer  50  by charging it into a flying capacitor  51  and then closing normally-open switches  52  and  53  to impress the capacitor voltage to a differential amplifier  54 , while at the same time the internal switch of the multiplexer  50  is turned off. Differential amplifier  54  thus produces an output voltage representing the voltage of a module selected by the multiplexer  50 . The output of differential amplifier  54  is converted to a digital signal in an analog-digital converter  57  and supplied to the CPU  11 . CPU  11  compares the voltage of each module selected by the multiplexer  50  with the voltages of other modules and determines whether or not the selected module is overcharged or all module voltages of the battery pack  1  differ significantly from each other.  
         [0060]     Before the next module is selected by the multiplexer  50 , a reset circuit is operated, which is formed with a resistor  55  and a normally-open switch  56 . By closing the switch  56 , the flying capacitor  51  is discharged through the resistor  55 .  
         [0061]     Discharge Inhibition in Response to Outputs of OVC/OVD Detector  
         [0062]     CPU  11  of the battery pack controller  4  responds to each of these output signals Smc and Smd from the overcharge/overdischarge detector  9  by sending a discharge input signal to the photocoupler switch  31  to prevent all discharge circuits  26 ˜ 29  from being discharged. The effect of this inhibit operation is to hold the cell voltages at the values which were detected by the overcharge/overdischarge detector  9 .  
         [0063]     Preferably, the CPU  11  performs reading of the signals Smc and Smd from the detector  9  at periodic intervals and operates the discharge inhibiter  30  by sending an inhibit signal to its switch  31  for an interval that runs from the instant somewhat earlier than the start timing of the reading operation to its end timing.  
         [0064]     Alternatively, as shown in the flowchart of  FIG. 5 , the CPU  11  operates the discharge inhibiter  30  (step  101 ) before it reads the output signals Smc and Smd of detector  9  (step  102 ).  
         [0065]     Since all unit cells C 1 ˜C 4  are not simultaneously discharged, some of the unit cells were being discharged while others were not at the instant the detector  9  produced an output signal. Therefore, the voltage drops of the cells being discharged are different from those of the other cells due to different values of their internal impedance. As a result, undesirable variations can occur in the cell voltages and the reliability of the operation of overcharge/overdischarge detector  9  would be lost.  
         [0066]     Module Voltage Equalization  
         [0067]     Module voltage equalization of each module is performed by the discharge circuit  10  provided for this module in the battery pack controller  4 . If the CPU  11  determines that the module  1  is overcharged or all module voltages of the battery pack  1  differ significantly from each other, it turns on the switch SWm of discharge circuit  10 . As a result, the module M 1  is discharged until it equals the voltages of the other modules. Since a discharge circuit  10  is provided for each module, all module voltages are equalized.  
         [0068]     Inhibition of Equalization Circuits During Module Voltage Detection  
         [0069]     When the module voltage detector  49  is operating to successively detect module voltages from the battery pack  1 , the battery pack controller  4  preferably sends a control signal from its CPU  11  to the cell discharge inhibiter  30  of cell equalizer  8  and/or the switch SWm to prevent all unit cells from being discharged by the discharge circuits  26 ˜ 29  as well as by the module discharge circuit  10 . This eliminates possible voltage variations that can occur between lines L 1  and L 5  and ensures reliability of operation for the module voltage detector  49 .  
         [0070]     Detection of Line-Cut on Module-Connecting Lines and Battery-Pack-Connecting Lines  
         [0071]     The following is a description of a method of detecting a line-cut which can occur on the module connecting lines L 1 , L 5 , L 9 , L 13 , L 17 , L 21 , L 25 , L 29  and L 33 . It is known that most line-cut events occur on a single module connecting line. This method is performed by the CPU  11  according to the flowchart shown in  FIG. 6 . Initially, the CPU  11  operates the discharge inhibiter  30  of all modules to inhibit the operation of their cell equalizer  8  and makes a search for a line-cut by monitoring the module connecting lines (step  201 ).  
         [0072]     If a line-cut occurs on one of the module connecting lines L 5 , L 9 , L 13 , L 17 , L 21 , L 25 , L 29 , the module voltage detector  49  of the battery pack controller  4  receives a zero voltage when two adjacent modules are successively selected by the multiplexer  50  and no voltage is charged into the flying capacitor  51 . CPU  11  receives zero-voltage values from the differential amplifier  54  at two successive instants of module selection. If a line-cut occurs on the line L 5 , the CPU  11  reads two zero-voltage values in succession from the modules M 1  and M 2  when they are selected by the multiplexer  50 .  
         [0073]     Therefore, if the CPU detects two zero-voltage values in succession from the output of module-voltage detector  49 , it is determined that a line-cut has occurred on one of the module connecting lines (step  202 ). If the decision is affirmative at step  202 , flow proceeds to step  203  to inhibit the reading of Smc and Smd output data from the affected two modules.  
         [0074]     If no zero-voltage values are detected in succession, the decision is negative at step  202 , and flow proceeds to step  204  to check to see if a zero voltage detected on one of the connecting lines L 1  and L 33 .  
         [0075]     If the CPU  11  detects a zero voltage value when the module M 1  is selected by the multiplexer  50 , the CPU  11  recognizes that a line-cut has occurred on the line L 1 . Similarly, if the CPU  11  detects a zero voltage value when the module M 8  is selected, the CPU  11  recognizes that a line-cut has occurred on the line L 33 .  
         [0076]     Therefore, if a single zero-voltage value is detected at step  204 , which is not followed by another zero-voltage value, it is determined that a line-cut has occurred on one of the lines L 1  and L 33 , and flow proceeds to step  205  to inhibit the reading of Smc and Smd output data from one of the affected modules M 1  and M 8 . If the decision is negative at step  204  or steps  203  and  25  are executed, the CPU  11  proceeds to the end of the routine.  
         [0077]     Since the line-cut detection routine is performed with the cell equalizer  8  of all modules being disabled initially at step  201 , zero-voltage values can be precisely detected due to the absence of interference from the turn-on of discharge circuits  26 ˜ 29 .  
         [0078]     A modified embodiment of the line-cut detection routine is shown in  FIG. 7 . CPU  11  starts with step  301  by setting a count value N to 1. Count value N represents the number of times the CPU  11  repeatedly performs the line-cut detection routine. At step  302 , a count value CM is initialized to zero. This variable indicates the number of matches that occur on a given battery-pack module “M” between its previous voltage value and its most recent voltage value.  
         [0079]     At step  303 , a variable M representing each battery-pack module is set to 1. At step  304 , the CPU  11  compares the current voltage value supplied from the module voltage detector  49  with its previous voltage value, and determines if they match or mismatch (step  305 ). If they match, flow proceeds from step  305  to step  306  to increment the count value C M  by 1 and moves to decision step  307 . If they mismatch at step  305 , step  306  is skipped and flow proceeds to step  307 . If the variable M is smaller than the maximum number (=8) of modules provided in the battery pack  1  (step  307 ), the CPU  11  proceeds to step  308  to increment the variable M by 1 and returns to step  304  to repeat the process on the next module. If M=8, flow proceeds from step  307  to step  309  to check to see if the count value N is equal to a predetermined value K. If not, flow proceeds from step  309  to step  310  to increment the count value N by 1 and returns to step  303 .  
         [0080]     In this way, match counting step  306  is repeated K times when the count value N equals K at step  309 . At decision step  311 , the CPU  11  determines whether or not the match count value CM of module “M” is equal to a predetermined number (=K). If a line-cut has occurred on a line connecting a module “M” to the battery-pack controller  4 , the previous and current voltages of this module will continuously match during the K-time period and the count value C M  will be equal to K at step  311 . When this occurs, the CPU  11  proceeds from step  311  to step  312  to inhibit all cell equalizers  8  and inhibit the reading of output data (Smc, Smd) from the module “M”. If the line-cut has occurred on one of the module connecting lines L 5 , L 9 , L 13 , L 17 , L 21 , L 25 , L 29 , there are two adjacent modules “M(i)” and “M(i+1)”, of which the count values C M(i)  and C M(i+1)  are equal to K. If the line-cut has occurred on one of the battery-pack connecting lines L 1  and L 33 , there is only one module “M” whose count value C M  equals K.  
         [0081]     Detection of Line-Cut on Module-Connecting Lines  
         [0082]     The following is a description of another method of detecting a line-cut on the module-connecting lines L 5 , L 9 , L 13 , L 17 , L 21 , L 25  and L 29  as well as a line-cut on the cell-connecting lines that connect the individual unit cells of each module to the associated module controller with reference to the flowchart of  FIG. 8 . In this embodiment, the CPU  11  begins a line-cut detecting routine with step  401  to monitor the overcharge (Smc) and overdischarge (Smd) output terminals of all module controllers for detecting any two adjacent module controllers that alternately produce overcharge signal Smc and overdischarge signal Smd.  
         [0083]     Assume that a line-cut occurs on the module-connecting line L 5  as indicated by a symbol X in  FIG. 9 , and the discharge circuit  29  of module controller  6 - 1  is turned on in response to an output from the comparator  23 , the line L 4  is coupled through the low-impedance discharge circuit  29  to the line L 5  as indicated by the thick solid line. Therefore, the cell-connecting line L 5  is brought to a level virtually equal to the line voltage V 4 . This drives the line voltage V 4  to a level lower then the reference voltage V 4 ′ and drives the line voltage V 6  of the cell-connecting line L 6  to a level lower than the reference voltage V 6 ′. As a result, the comparator  23  of module controller  6 - 1  turns off its discharge circuit  29  and the comparator  21  of module controller  6 - 2  turns on its discharge circuit  26 , thus coupling the cell-connecting line L 6  through the low-impedance discharge circuit  26  to the module-connecting line L 5 , instead of the line voltage V 4 .  
         [0084]     As a result, the module-connecting line L 5  is now brought to a level virtually equal to the line voltage V 6 . Hence the comparator  21  of module controller  6 - 2  turns off its discharge circuit  26 , while the comparator  23  of module controller  6 - 1  turns on its discharge circuit  29 , coupling the voltage V 4  to the module-connecting line L 5  again.  
         [0085]     Thus, the line-cut on the module-connecting line L 5  causes its line voltage to fluctuate in a range twice the cell voltage, and the comparator  36  of module controller  6 - 1  produces an overcharge signal Sc and the comparator  33  of module controller  6 - 2  produces an overdischarge signal Sd, alternately with the comparator  36 . Therefore, adjacent module controllers  6 - 1  and  6 - 2  alternately produce overcharge signal Smc and overdischarge signal Smd.  
         [0086]     Returning to  FIG. 9 , if the CPU  11  determines that two adjacent module controllers are alternately producing overcharge and overdischarge signals Smc and Smd (step  401 ), the CPU recognizes that a line-cut has occurred on one of the module-connecting lines. In this instance, the decision at step  402  is affirmative and flow proceeds to step  405  to inhibit all cell equalizers  8  and inhibit the reading of output data (Smc, Smd) from the module “M”.  
         [0087]     Detection of Line-Cut on Cell-Connecting Lines  
         [0088]     If no line-cut is detected on the module-connecting lines, the decision at step  402  is negative and flow proceeds to step  403  to detect a line-cut on the cell-connecting lines of all module controllers, i.e., L 2 ˜L 4 , L 6 ˜L 8 , . . . , L 30 ˜L 32 .  
         [0089]     To perform this line-cut detection process at step  403 , the output terminals of all comparators  33  to  36  of each module controller are connected to the CPU  11  as shown in  FIG. 10 . CPU  11  monitors the output terminals of comparators  33  to  36  of all module controllers for detecting one or two adjacent comparators alternately producing overcharge signal Sc and overdischarge signal Sd.  
         [0090]     If a line-cut occurs on the cell-connecting line L 2  and the discharge circuit  26  is turned on in response to an output from the comparator  23  ( FIG. 2 ), the line L 2  is coupled through the low-impedance discharge circuit  26  to the cell-connecting line L 1 . Therefore, the line L 2  is driven to a level virtually equal to the line voltage V 1 . This raises the line voltage V 2  to a level higher than the reference voltage V 2 ′, and lowers the line voltage V 3  to a level below the reference voltage V 3 ′. As a result, the comparator  33  recognizes that the unit cell C 1  is overdischarged, producing an overdischarge signal Sd, while the comparator  34  recognizes that the unit cell C 2  is overcharged, producing an overcharge signal Sc.  
         [0091]     At the same time, the comparator  21  turns off the discharge circuit  26  and the comparator  24  turns on the discharge circuit  27 , coupling the cell-connecting line L 2  to the line L 3  via the low-impedance discharge circuit  27  so that the line L 2  is driven to a level virtually equal to the line voltage V 3 . This lowers the line voltage V 2  to a level below the reference voltage V 2 ′, and raises the line voltage V 3  to a level above the reference voltage V 3 ′. As a result, the comparator  33  recognizes that the unit cell C 1  is overcharged, producing an overcharge signal Sc, while the comparator  34  recognizes that the unit cell C 2  is overdischarged, producing an overdischarge signal Sd.  
         [0092]     At the same time, the comparator  21  turns on the discharge circuit  26 , and the comparator  24  turns off the discharge circuit  27 . Thus, the line L 2  is driven to a level virtually equal to the line voltage V 1 . Therefore, the line L 2  is returned to the initial state of voltage V 1  that appeared on it immediately following the occurrence of its cut-off. It is seen that the discharge circuits  26  and  27  are alternately turned on and off at short intervals.  
         [0093]     Returning to step  403  of  FIG. 8 , the CPU  11  monitors the outputs of comparators  33  to  36  of all module controller and checks to see if there is at least one comparator that alternately produces overcharge and overdischarge signals at short intervals. If such comparators are detected (step  404 ), flow proceeds to step  405  to perform the inhibition as described above. If no line-cut is detected at step  404 , flow proceeds to the end of the routine.  
         [0094]     Alternatively, the overcharge/overdischarge detector  9  is modified as shown in  FIG. 11  to provide line-cut detection on the cell-connecting lines L 2 , L 3  and L 4  of the module controller  6 - 1 . In this modification, the overcharge detector  37  is divided into an odd-numbered overcharge cell detector  37 A and an even-numbered overcharge cell detector  37 B, and the overdischarge detector  38  is likewise divided into an odd-numbered overdischarge cell detector  38 A and an even-numbered overdischarge cell detector  38 B.  
         [0095]     Odd-numbered overcharge cell detector  37 A receives overcharge signals Sc from the comparators  33  and  35  to produce an overcharge signal Smc(O) and the even-numbered overcharge cell detector  37 B receives overcharge signals Sc from the comparators  34  and  36  to produce an overcharge signal Smc(E). Odd-numbered overdischarge cell detector  38 A receives overdischarge signals Sd from the comparators  33  and  35  to produce an overdischarge signal Smd(O) and the even-numbered overdischarge cell detector  38 B receives overdischarge signals Sd from the comparators  34  and  36  to produce an overdischarge signal Smd(E).  
         [0096]     The outputs of all detectors  37 A,  37 B,  38 A and  38 B are coupled to a transition detector  12  that analyzes the transition states of all input signals. If a line-cut occurs on one of the cell-connecting lines L 2 , L 3  and L 4 , the transition detector  12  locates the failed cell-connecting lines and informs the CPU  11  of the identity of the disconnected line.  
         [0097]     Time-Shared Operation when Ignition Switch is Off  
         [0098]     Another method of time-shared modes of overcharge/overdischarge detection, line-cut detection and cell/module equalization is shown in  FIGS. 12A and 12B .  
         [0099]     If a main switch of the battery pack such as the automobile ignition switch is turned off (step  501 ), the CPU  11  executes the routine of  FIG. 12B  (step  502 ). CPU  11  monitors the main switch (step  501 ) and if it is turned on, the CPU interrupts the routine of  FIG. 12B  (step  503 ).  
         [0100]     CPU  11  executes the routine of  FIG. 12B  by exclusively activating the cell equalizer  8  of all module controllers so that the overcharge/overdischarge detector  9  of all module controllers and the line-cut detection are inhibited (step  504 ). Following step  504 , the CPU  11  exclusively activates all the overcharge/overdischarge detectors  9  so that all cell equalizers  8  and line-cut detection are inhibited (step  505 ). Subsequently, the CPU  11  exclusively performs line-cut detection (step  506 ) so that all cell equalizers and all overcharge/overdischarge detectors are inhibited. At step  508 , the CPU  11  waits a predetermined interval of time before returning to step  504 . Whenever the ignition switch is operated (step  503 ), the CPU  11  interrupts the execution of the routine of  FIG. 12B .  
         [0101]     If the main switch is in a turn-off state for an extended period of time, it is likely that voltage variability develops between the unit cells due to their spontaneous discharge. The activation of the cell equalizers  8  during the turn-on state of the ignition switch can eliminate the variability problem. A further advantage is that, when the cell equalizers are operated the battery pack  1  is in a fully unloaded state, and hence the cell equalizers can utilize the open-circuit voltages of all unit cells to equalize their voltages.