Patent Publication Number: US-2003232237-A1

Title: Voltage control apparatus for battery pack

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the invention  
       [0002] The present invention relates to battery pack voltage control apparatus.  
       [0003] 2. Related Art  
       [0004] There is a technology in the known art which is adopted to drive an electric load by using a battery pack constituted of a plurality of chargeable cells as a power source. The battery pack repeatedly executes a discharge operation to drive the load and a charge operation to recharge the cells. Voltage control is implemented in such a battery pack in order to minimize the inconsistency among terminal voltages at the cells constituting the battery pack by detecting the terminal voltages at the individual cells. For instance, Japanese Laid-Open Patent Publication No. 2001-190030 discloses a technology for achieving uniformity among the terminal voltages at the individual cells by connecting in parallel a balance circuit to each of the cells and partially bypassing the charge current with these balance circuits. As the terminal voltage at a given cell reaches a predetermined value, the corresponding balance circuit bypasses the charge current at the cell. Since the charge current supplied to the cell is lowered if the charge current is bypassed and thus slows down the process of the cell reaching a fully charged state, the extent of the inconsistency between the terminal voltage at this cell and the terminal voltages at the other cells which do not reach a fully charged stage as quickly is reduced.  
       SUMMARY OF THE INVENTION  
       [0005] In a system such as a hybrid electric vehicle, a battery is normally used in a state in which it is charged to approximately 50% and the voltage of the battery mounted in such a system fluctuates in correspondence to its state of charge. For this reason, if control is implemented to minimize the inconsistency among the voltages at the individual cells based upon a voltage level corresponding to a nearly fully charged state, the charge currents are not bypassed as often as they should be during the actual operation and, as a result, the voltages at the cells cannot be controlled to achieve uniformity with ease.  
       [0006] The present invention is to provide an apparatus for controlling a battery pack voltage so as to minimize inconsistency among the voltages at cells in conformance to the state of charges of the cells.  
       [0007] A voltage control apparatus that controls a voltage of a battery pack constituted of a plurality of cells according to the present invention, comprises a plurality of voltage detection devices each detecting a voltage at each of the cells; a plurality of first voltage adjustment devices each adjusting the voltage at each cell so as to achieve a first target value based upon the each voltage value detected by each of the voltage detection devices; and a plurality of second voltage adjustment devices each adjusting the voltage at each cell so as to achieve a second target value based upon the each voltage value detected by each of the voltage detection devices.  
       [0008] A voltage control apparatus that controls a voltage of a battery pack constituted of a plurality of cells according to another aspect of the present invention, comprises a plurality of first circuits each constituted of a first Zener diode having a first breakdown voltage and a first resister connected to the first Zener diode; and a plurality of second circuits each constituted of a second Zener diode having a second breakdown voltage smaller than the first breakdown voltage and a second resister connected to the second Zener diode. The individual first and second series circuits are connected in parallel to each cell and connected in parallel to each other. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 is an overall structure of a vehicle mounted with a battery pack voltage control apparatus achieved in the first embodiment of the present invention  
     [0010]FIG. 2 is a circuit block diagram of a current bypass circuit  
     [0011]FIG. 3 is the relationship between the cell SOC and the cell terminal voltage  
     [0012]FIG. 4 is a circuit block diagram of a current bypass circuit achieved in the second embodiment  
     [0013]FIG. 5 is a circuit block diagram of a current bypass circuit achieved in the third embodiment 
    
    
     DESCRIPTION OF THE PREFEREED EMBODIMENTS  
     [0014] The following is an explanation of preferred embodiments of the present invention, given in reference to the drawings.  
     First Embodiment  
     [0015]FIG. 1 is an overall block diagram of a vehicle mounted with a battery pack voltage control apparatus achieved in the first embodiment of the present invention. In the embodiment, a battery pack is utilized as a power source in a hybrid electric vehicle. In FIG. 1, a battery pack  1  is constituted by connecting in series n cells  11 ˜ 1   n . A voltage control apparatus includes current bypass circuits  21 ˜ 2   n  and voltage detection circuits  31 ˜ 3   n.    
     [0016] The battery pack  1  supplies a current to an inverter/converter  6  during a discharge. The inverter/converter  6  controls a power output to a motor  7  in response to a command issued by a charge/discharge control circuit  5 . The motor  7  drives wheels  10 . Through the output control implemented by the inverter/converter  6 , a load current of the battery pack  1  is controlled. The battery pack  1  is charged with a current supplied from the inverter/converter  6  during a charge operation. The inverter/converter  6  controls the charge current supplied to the battery pack  1  in response to a command issued by the charge/discharge control circuit  5 . A dynamo-electric generator  8 , which is driven by a gasoline engine  9 , generates electric power and supplies to the inverter/converter  6 .  
     [0017] The charge/discharge control circuit  5  calculates a charge current value and a discharge current (load current) value for the battery pack  1  by using voltage data of the battery pack  1  detected by the voltage detection circuits  31 ˜ 3   n , and outputs a charge control value and a discharge control value to be referenced to achieve these charge and discharge current values to the inverter/converter  6 . It is to be noted that when the vehicle travels, the actual control values of electric power are calculated by using detection values provided by various sensors (not shown) such as an accelerator operation quantity sensor and a brake operation quantity sensor in addition to the voltage data.  
     [0018] The voltage detection circuits  31 ˜ 3   n  are respectively provided in parallel to the cells  11 ˜ 1   n . The voltage detection circuits  31 ˜ 3   n  may each be constituted of, for instance, a differential amplifier circuit. The voltage detection circuits  31 ˜ 3   n  each detect the terminal voltage of the corresponding cell to which it is connected in parallel while the battery pack  1  is charged or discharged and output a detection signal. The current bypass circuits  21 ˜ 2   n  are provided in parallel to the cells  11 ˜ 1   n  respectively. The voltage detection signals from the voltage detection circuits  31 ˜ 3   n  are input to the current bypass circuits  21 ˜ 2   n  respectively.  
     [0019] If any of the voltage values indicated by the voltage detection signals input from the voltage detection circuits  31 ˜ 3   n  while the battery pack  1  is charged reaches a decision-making threshold value which is lower than a charge end voltage by a predetermined value, the corresponding current bypass circuit  21 ˜ 2   n  bypasses the charge current at the corresponding cell into the bypass circuit. As a result, the charge current flowing into the cell is lowered, slowing down the speed at which the cell reaches a fully charged state, i.e., a state of charge (SOC) equivalent to 100%. The charge end voltage is the terminal voltage corresponding to SOC 100% at the cell.  
     [0020] The present invention is characterized by the current bypass circuits  21 ˜ 2   n  constituting the voltage control apparatus described above.  
     [0021]FIG. 2 is a circuit block diagram illustrating the current bypass circuit  21  achieved in the first embodiment. In FIG. 2, the voltage detection circuit  31  and the current bypass circuit  21  are connected in parallel to the cell  11 . Voltage detection circuits and current bypass circuits identical to those shown in FIG. 2 are connected to the other cells. The current bypass circuit  21  includes a first voltage comparator circuit  211 , a second voltage comparator circuit  212 , a transistor  213  and a resistor  214 .  
     [0022] A first target voltage V 1  is applied by a voltage generator circuit  41  to a reference terminal T 1  of the first voltage comparator circuit  211 . The target voltage V 1  is slightly lower than the terminal voltage at the cell corresponding to a fully charged state (SOC 100%). A second target voltage V 2  is applied by the voltage generator circuit  41  to a reference terminal T 2  of the second voltage comparator circuit  212 . The target voltage V 2  is set at the value of the terminal voltage at the cell corresponding to SOC 50%.  
     [0023]FIG. 3 shows an example of the relationship achieved between the cell SOC and the cell terminal voltage. In FIG. 3, the horizontal axis represents the state of charge at the cell and the vertical axis represents the terminal voltage. As FIG. 3 shows, the relationship manifests sloping characteristics in which the terminal voltage (open circuit voltage) in a fully charged state (SOC 100%) is the highest and falls as the SOC is lowered. In the case of a lithium ion battery having its negative terminal constituted of hard carbon, the terminal voltage is approximately 4.1 V when the SOC is 100% and it falls to approximately 3.6 V when the SOC is 50%. Accordingly, the voltage generator circuit  41  is constituted so as to achieve a first target voltage V 1  of 4.0 V and a second target voltage of 3.6 V.  
     [0024] The voltage detection signal output from the voltage detection circuit  31  is input to an input terminal T 3  of the first voltage comparator circuit  211  and an input terminal T 4  of the second voltage comparator circuit  212 . Output terminals of the voltage comparator circuits  211  and  212  are both connected to a base terminal of the transistor  213 . The voltage comparator circuits  211  and  212  each output an H-level signal when the level of the signal input to the respective input terminal T 3  or T 4  becomes higher than the level of the signal input to the corresponding reference terminal T 1  or T 2 . In addition, the voltage comparator circuits  211  and  212  each output an L-level signal when the level of the signal input to the respective input terminal T 3  or T 4  becomes equal to or lower than the level of the signal input to the corresponding reference terminal T 1  or T 2 .  
     [0025] The current bypass circuit  21  is structured so as to selectively utilize either the first voltage comparator circuit  211  or the second voltage comparator circuit  212 . Namely, when the voltage control is to be implemented on the battery pack  1  over a range close to SOC 100%, a controller  42  issues instructions for the voltage generator circuit  41  to apply 4.0 V to the reference terminal T 1  of the first voltage comparator circuit  211  and 5.0V to the reference terminal T 2  of the second voltage comparator circuit  212 . In this case, the level of the output signal from the first voltage comparator circuit  211  shifts in conformance to the level of the voltage detection signal input to the input terminal T 3 . Since the terminal voltage of a lithium ion battery never reaches 5V, the output signal from the voltage comparator circuit  212  keeps at L level. It is to be noted that the first voltage comparator circuit  211  and the second voltage comparator circuit  212  are structured so that the transistor  213  is driven as one voltage comparator circuit outputs an H-level signal even if the other voltage comparator circuit is currently outputting an L-level signal.  
     [0026] When the voltage control is to be implemented on the battery pack  1  over a range near SOC 50%, the controller  42  issues instructions for the voltage generator circuit  41  to apply 3.6V to the reference terminal T 2  and apply 5.0 V to the reference terminal T 1 . In this case, the level of the output signal from the second voltage comparator circuit  212  shifts in conformance to the level of the voltage detection signal input to the input terminal T 4 . Since the terminal voltage of lithium ion battery never reaches 5V as explained earlier, the output signal from the first voltage comparator circuit  211  remains at L level.  
     [0027] The transistor  213  is turned on when either the first voltage comparator circuit  211  or the second voltage comparator circuit  212  outputs an H-level signal. When the transistor  213  is turned on, a bypass current flows via the resistor  214  and the transistor  213 . The current bypass circuit  21  controls the bypass current of the cell  11  so as to equalize the terminal voltage Vc at the cell  11  to the voltage applied to the reference terminal T 1  (or T 2 ) of the currently selected voltage comparator circuit  211  (or  212 ).  
     [0028] The transistor  213  is turned off when both the first voltage comparator circuit  211  and the second voltage comparator circuit  212  outputs an L-level signal. As the transistor  213  is turned off, the bypass current of the cell  11  becomes cut off. At this time, the terminal voltage Vc at the cell  11  is either equal to or lower than the voltage applied to the reference terminal T 1  (or T 2 ) of the selected voltage comparator circuit  211  (or  212 ).  
     [0029] The first embodiment explained above is now summarized.  
     [0030] (1) The voltage control apparatus includes the current bypass circuits  21 ˜ 2   n . If the terminal voltage of any of the cells  11 ˜ 1   n  reaches the first target voltage V 1  (4.0V in the example) which is slightly lower than the voltage corresponding to a fully charged state (SOC 100%) while the battery pack  1  is charged, the charge current at the cell is bypassed. As a result, the charge current to the cell which has approached a fully charged state (SOC 100%) sooner than the other cells is decreased to slow down the process of the cell entering the fully charged state, and since this reduces the difference in the SOC between the cell and the other cells that are not charged as quickly, the inconsistency among the voltages at the individual cells is minimized. Thus, the discharge capacity of the battery pack  1  is not reduced and any cell degradation is prevented. The inconsistency in the SOC among the individual cells is caused by difference in characteristics of the individual cells which are brought during the cell manufacturing stage, varying temperatures at which the individual cells operate in the battery pack  1  and the like.  
     [0031] (2) Since a value slightly lower than the voltage value corresponding to the fully charged state (SOC 100%) is selected for the first target voltage V 1 , the bypass current stars to flow before the cell reaches SOC 100% to prevent the cell from becoming over charged. It is to be noted that the current bypass circuit has a function of bypassing the charge current and discharging the cell to lower the terminal voltage if the terminal voltage at the cell exceeds the target voltage.  
     [0032] (3) The second target voltage V 2 , which is different from the first target voltage V 1 , is also provided to allow the current bypass circuit  21  to selectively use either the target voltage V 1  or the target voltage V  2 . The target voltage V 2  corresponds to SOC 50% and thus represents a terminal voltage which is often used in an actual application in a hybrid vehicle or the like. When the second target voltage V 2  is selected and the terminal voltage of any of the cells  11 ˜ 1   n  reaches SOC 50%, the corresponding current bypass circuit  21 ˜ 2   n  bypasses the charge current at the cell. As a result, the charge current at the cell having neared SOC 50% sooner than the other cells is reduced, thereby slowing down the process of the cell reaching the state corresponding to SOC 50%. Therefore, the difference in the SOC between this cell and the other cells that are not charged as quickly is reduced in this manner, so that the inconsistency among the voltages at the individual cells is minimized. Thus, unlike in the related art that reduces the inconsistency among the voltages at the individual cells only over the range around SOC 100%, the inconsistency of the voltages at the individual cells is reduced over the range around SOC 50% as well. Consequently, since the inconsistency of the voltages can be reduced in correspondence to the state of charge of the battery pack  1 , the discharge capacity of the battery pack  1  is not lowered and battery degradation is prevented.  
     [0033] (4) Since the battery pack is constituted of cells such as lithium ion cells that achieve sloping characteristic, as shown in FIG. 3, the SOCs of the individual cells can be uniformly adjusted by adjusting the terminal voltage (open circuit voltage) of each cell detected by the corresponding voltage detection circuit to the target value (the first target voltage V 1  or the second part of voltage V 2 ).  
     [0034] The current bypass circuit  21  should select the first voltage comparator circuit  211  while the system is engaged in operation (while the vehicle is traveling) and select the second voltage comparator circuit  212  while the system is in a non-operating state (while the vehicle is parked), for instance. For example, when the controller  42  receives a signal indicating a vehicle running state, the first target voltage V 1  is set to be 4.0 V and the second target voltage V 2  is set to be 5.0 V, so that the first voltage comparator circuit  211  is operated. On the other hand, when the controller  42  receives a signal indicating a vehicle parking state, the first target voltage V 1  is set to be 5.0 V and the second v target voltage V 2  is set to be 3.6 V, so that the second voltage comparator circuit  212  is operated.  
     Second Embodiment  
     [0035] The current bypass circuits may be constituted by using Zener diodes. FIG. 4 is a circuit block diagram illustrating a current bypass circuit achieved in the second embodiment. In FIG. 4, a bypass circuit  21 A and a bypass circuit  21 B are individually connected in parallel to the cell  11 . Bypass circuits identical to those shown in FIG. 4 are connected to the other cells as well. The bypass circuit  21 A includes a resistor R 1  and a Zener diode ZD 1  connected in series to each other. The bypass circuit  21 B includes a resistor R 2  and a Zener diode ZD 2  connected in series to each other. The second embodiment does not include any voltage detection circuits to engage the bypass circuits in operation.  
     [0036] The Zener diode ZD 1  of the bypass circuit  21 A has a breakdown voltage that corresponds to the first target voltage (4.0V) mentioned earlier. The resistance value of the resistor R 1  is set so that the value of the current flowing to the Zener diode ZD 1  when the SOC of the cell is 100% (when the terminal voltage at the cell is 4.1V) does not exceed the maximum rated current value of the Zener diode ZD 1 . A bypass current IZ 1  achieved by the bypass circuit  21 A adopting this structure is expressed as in formula (1) below. 
       IZ   1 =( Vc−VZ   1 )/ r   1   (1) 
     [0037] In the expression above, Vc represents the terminal voltage at the cell  11 , VZ 1  represents the breakdown voltage of the Zener diode ZD 1  and r 1  represents the resistance value of the resistor R 1 .  
     [0038] The Zener diode ZD 2  of the bypass circuit  21 B has a breakdown voltage that corresponds to the second target voltage (3.6V) mentioned earlier. The resistance value of the resistor R 2  is set so that the value of the current flowing to the Zener diode ZD 2  when the SOC of the cell is 100% (when the terminal voltage at the cell is 4.1V) does not exceed the maximum rated current value of the Zener diode ZD 2 . A bypass current IZ 2  achieved by the bypass circuit  21 B adopting this structure is expressed as in formula (2) below. 
       IZ   2 =( Vc−VZ   2 )/ r   2   (2) 
     [0039] In the expression above, Vc represents the terminal voltage at the cell  11 , VZ 2  represents the breakdown voltage at the Zener diode ZD 2  and r 2  represents the resistance value of the resistor R 2 .  
     [0040] In the current bypass circuit shown in FIG. 4, as the terminal voltage Vc at the cell rises until it exceeds the breakdown voltage VZ 2 , i.e., SOC 50% while the battery pack  1  is charged, the Zener diode ZD 2  of the bypass circuit  21 B enters an on state. As a result, the bypass current IZ 2  flows via the resistor R 2  and the Zener diode ZD 2  and the terminal voltage Vc at the cell  11  becomes equal to the second target voltage V 2 .  
     [0041] As the terminal voltage Vc at the cell keeps rising and exceeds the breakdown voltage VZ 1 , i.e., a voltage level corresponding to an SOC slightly lower than 100%, the Zener diode ZD 1  of the bypass circuit  21 A is turned on. As a result, the bypass current IZ 1  flows via the resistor R 1  and the Zener diode ZD 1  and the terminal voltage Vc at the cell  11  becomes equal to the first target voltage V 1 .  
     [0042] In the second embodiment, the bypass current IZ 2  constantly flows once the terminal voltage Vc at the cell exceeds the voltage (3.6V) corresponding to SOC 50%. While this reduces the extent of inconsistency among the terminal voltages of the individual cells, it also lowers the charge efficiency. Accordingly, the bypass current IZ 2  is set lower than the bypass current IZ 1  by ensuring that the resistance value r 1  and the resistance value r 2  have a relationship expressed as r 1 &lt;r 2 . As a result, the power consumption is reduced.  
     [0043] In the current bypass circuit, the resistance value r 1  should be set as low as possible over the range in which the bypass current IZ 1  does not exceed the maximum rated current value of the Zener diode ZD 1 . Since the bypass current IZ 1  increases as the resistance value r 1  is lowered, the cell can be quickly charged to the SOC 100% while being prevented from becoming over charged.  
     [0044] In the second embodiment described above, the inconsistency among the voltages at the individual cells can be reduced over the range around SOC 100% and over the range around SOC 50% as in the first embodiment. Since the inconsistency of the voltages can be reduced in conformance to the state of charge at the battery pack  1 , the discharge capacity of the battery pack  1  is not lowered and cell degradation is prevented. In addition, since the Zener diode ZD 1  (ZD 2 ) is used and the voltage at which the bypass current starts to flow is set in conformance to the breakdown voltage of the Zener diode corresponding to the target voltage V 1  (V 2 ), the voltage detection circuit  31  for detecting the terminal voltage Vc is not required, thereby achieving a cost reduction over the structure which includes the voltage detection circuit  31 .  
     [0045] The resistance value r 2  of the resistor R 2  may be variable. If the resistance value R 2  is variable, the resistance value r 2  can be set high during the system operation (while the vehicle is traveling) to prevent the system efficiency from becoming poor (to prevent the fuel efficiency from being compromised) by keeping down the bypass current IZ 2 . When the system is in a non-operating state (while the vehicle is parked), on the other hand, the resistance value r 2  can be set low to prevent the inconsistency among terminal voltage Vc at an early stage by flowing the bypass current IZ 2  actively. As a result, the inconsistency of the terminal voltages Vc is minimized at a restart of the vehicle to maximize the performance of the battery pack  1 . It is to be noted that while the system is in a non-operating state, the resistance value r 2  should be set to a value that enables a flow of the bypass current IZ 2  which will eliminate any inconsistency among the terminal voltages Vc at the individual cells within a period of approximately 12 hours.  
     Third Embodiment  
     [0046]FIG. 5 is a circuit block diagram illustrating a current bypass circuit achieved in the third embodiment. It differs from the current bypass circuit in FIG. 4 in that an additional component, i.e., a relay RLY, is connected in series to the bypass circuit  21 B. Bypass circuits identical to those shown in FIG. 5 are connected to the other cells as well. Open/close control is implemented on the relay RLY by using a drive signal S 1  output from the controller  42 .  
     [0047] The relay RLY is controlled so that it remains open while the system is in operation (while the vehicle is traveling). Thus, the bypass current IZ 2  is cut off to prevent the fuel efficiency from becoming poorer. When the system is in a non-operating state (while the vehicle is parked), control is implemented on the relay RLY so as to allow it to remain closed. Since the bypass current IZ 2  flows so as to reduce the inconsistency among the terminal voltages Vc, a state with little inconsistency of the terminal voltages Vc is achieved at a restart of the vehicle to maximize the performance of the battery pack  1 . It is to be noted that the relay RLY should achieve characteristics whereby it closes when the drive signal S 1  is not input (when no signal is input), since this eliminates the need to generate a drive signal for closing the rely RLY while the system is in a non-operating state.  
     [0048] The third embodiment explained above, in which the bypass current IZ 2  is cut off by opening the relay RLY when the system is in operation (when the vehicle is traveling), does not allow the system efficiency to be lowered, i.e., does not allow the fuel efficiency to become poor, in addition to achieving the advantages of the second embodiment.  
     [0049] While an explanation is given above on an example in which the present invention is adopted in a hybrid electric vehicle (HEV), the present invention may also be adopted in a fuel cell vehicle (FCV).  
     [0050] In the explanation given above, the value of the target voltage V 1  is set at the cell terminal voltage corresponding to a state close to the fully charged state (SOC 100%) and the value of the target voltage V 2  is set to the cell terminal voltage corresponding to a state close to SOC 50%. However, the values of the target voltages do not need to be set in correspondence to the SOC values used in the example and may be set to other values as appropriate in correspondence to the normal SOC operating range. The number of target voltages does not need to be 2, and 3 or more target voltages may be set.  
     [0051] In addition, the voltage values (4.0V, 3.6V, etc.) quoted above are voltage values applicable to lithium ion cells and appropriate voltage values should be set in conformance to specific cell characteristics when other types of cells are utilized.  
     [0052] It is to be noted that components other than those used in the structures explained above may be adopted as long as the function characterized in the present invention is not compromised.  
     [0053] The disclosures of the following priority application are herein incorporated by reference: Japanese Patent Application No. 2002-00039 filed Jun. 12, 2002