WO2015156648A1 - Active cell balancing circuit using cell voltage measurement circuit of capacitor charging method - Google Patents

Abstract

Disclosed is a battery pack comprising: a plurality of battery cells; and a temporary power storage unit configured to be selectively connected to the plurality of battery cells. A third process is configured to be performed, the third process comprising: a storage step for storing, in the temporary power storage unit, part of the power stored in a first battery cell among the plurality of battery cells; and a providing step for providing the power stored in the temporary power storage unit to a second battery cell among the plurality of battery cells. In addition, the third process is configured to be repeated until the difference between a first voltage of the first battery cell and a second voltage of the second battery cell becomes less than or equal to a predetermined value.

Description

Active Cell Balancing Circuit Using Capacitor Charge Cell Voltage Measuring Circuit

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active cell balancing circuit using a circuit for measuring individual cell voltages through a capacitor charging method.

The cell voltage measurement circuit in the battery management system applied to various types of electric vehicles (xEVs), energy storage systems, and uninterruptible power supplies that are currently used is a photomos relay for the purpose of insulating the energy storage unit and the system unit. Cap Charging Method using is mainly used.

In addition, although it is not a problem in a single cell application, cell balancing caused by capacity imbalance is an important problem in an application of multiple cells in which several cells are connected in series. This capacity imbalance in the cell is seen as an unbalance in the cell voltage in terms of the battery management system. In addition, since charging or discharging is prohibited by certain cells in the overcharging and overdischarging areas during the charging and discharging process, normal cell capacity cannot be used. Repeated use through this process acts as a factor in reducing the efficiency of the battery pack and shortening the life of a particular cell.

As a method mainly used for cell balancing, passive cell balancing, which discharges a high voltage cell through a resistor and maintains the same voltage as other cells, is widely used. Passive type has the advantage of low price. In addition, in addition to this method, active cell balancing through the application of a DC / DC converter (Converter), etc. are under study. However, such an active cell balancing method is expensive and expensive, and the system is complicated and thus cannot be expanded.

As a related prior art, the related art is disclosed in Korean Patent Registration No. 10-1187766.

In order to solve the above problems, the present invention is to provide a circuit that can perform cell balancing while measuring the cell voltage.

According to an aspect of the present invention, a cell balancing circuit includes: a battery unit including a plurality of battery cells 10 and a balancing resistor R_Bal connected to each of output terminals of the plurality of battery cells 10; A first relay unit P-R6 connected to an output terminal of the battery unit; And a charging unit C2 connected to an output terminal of the first relay unit.

In this case, the first current limited by the first balancing resistor connected to the first battery cell B1 having the highest voltage among the plurality of battery cells may be charged to the charging unit through the first relay unit.

The energy charged in the charging unit may be configured to charge the second battery cell B4 having the lowest voltage among the plurality of batteries through the first relay unit.

In this case, the cell balancing circuit may further include a converter 80 connected between the battery unit and the first relay unit. In this case, the converter may be configured to control the operation state through the control unit 90 included in the converter.

In this case, when the voltage of the battery unit is high, the battery unit operates as a step-down converter to charge the charging unit with the voltage of the battery unit, and when the voltage of the battery unit is low, the battery unit operates as a boost converter. May be arranged to discharge.

The cell balancing circuit comprises: a capacitor; A second relay; And an AD converter. In this case, the capacitor may be connected to an output terminal of the battery unit, the second relay may be connected to one end of the capacitor, and the AD converter may be connected to an output terminal of the second relay.

In this case, the voltage of any one of a plurality of batteries included in the battery unit is charged in the capacitor, and when the second relay is turned on, the voltage charged in the capacitor is supplied to the AD converter. It may be intended to provide.

In this case, the first relay unit may be implemented using a photomoss relay, and the charging unit may be implemented using an ultracapacitor or a small capacity lithium ion battery. The first converter may be implemented using a bidirectional DC / DC converter.

According to the present invention, unlike the existing active cell balancing circuit, the power source can be separated and the leakage current can be prevented. In addition, since the existing battery management system is used, a low cost system can be realized.

1 illustrates a passive cell balancing circuit according to one embodiment.

2 illustrates an active cell balancing circuit using a transformer in accordance with one embodiment.

3A shows a cell voltage measuring circuit by a capacitor charging method.

FIG. 3B illustrates a first process of performing voltage measurement of each battery cell in the cell voltage measuring circuit shown in FIG. 3A.

4A illustrates a cell voltage measurement and cell balancing circuit according to an embodiment of the present invention.

4B shows an example of a temporal relationship between several processes performed in the circuit shown in FIG. 4A.

4C shows a timing diagram of a process performed in the circuit shown in FIG. 4A.

5A illustrates a cell voltage measurement and cell balancing circuit according to another embodiment of the present invention.

FIG. 5B shows an example of the bidirectional DC / DC converter of the circuit shown in FIG. 5A.

FIG. 6A shows the internal structure of the bidirectional DC / DC converter shown in FIG. 5A.

FIG. 6B shows a control unit of the bidirectional DC / DC converter shown in FIG. 6A.

FIG. 7A shows a case where the bidirectional DC / DC converter shown in FIG. 5A operates in the step-down mode.

FIG. 7B is a diagram for describing a case where the first transistor Q1 is in an on-state in the step-down DC / DC converter illustrated in FIG. 7A.

FIG. 7C is a diagram for describing a case where the first transistor Q1 is in an off-state in the step-down DC / DC converter shown in FIG. 7A.

FIG. 8A shows a case where the bidirectional DC / DC converter shown in FIG. 5A operates in a boost mode.

FIG. 8B is a diagram for describing a case where the second transistor Q2 is in an on-state in the boost type DC / DC converter illustrated in FIG. 8A.

FIG. 8C is a diagram for describing a case where the second transistor Q2 is in an off-state in the boost type DC / DC converter illustrated in FIG. 8A.

Hereinafter, with reference to the accompanying drawings an embodiment of the present invention will be described. However, the present invention is not limited to the embodiments described herein and may be implemented in various other forms. The terminology used herein is for the purpose of understanding the embodiments and is not intended to limit the scope of the invention. Also, the singular forms used below include the plural forms unless the phrases clearly indicate the opposite meanings.

1 illustrates a passive cell balancing circuit 100 according to one embodiment.

The passive cell balancing circuit 100 shown in FIG. 1 includes a battery pack 110 including a plurality of battery cells 10, a balancing resistor R_Bal, a photo coupler 12, and an MCU I / It is comprised including O (11).

The passive cell balancing circuit 100 calculates an average value of the voltages of the battery cells 10, selects the first battery cell 10 (ex: battery cell B1) higher than the average value by a predetermined level, and selects the I of the MCU. The photocoupler 12 is operated through / O 11 to forcibly discharge the first cell 10 with the balancing resistor R_Bal. Accordingly, the voltage of the first battery cell 10 is balanced with the voltage of the other battery cells 10 (eg, battery cells B2, B3, and B4). In this case, the photocoupler 12 is used for separating power from the power of the battery management system and the battery pack.

The passive cell balancing circuit 100 according to FIG. 1 has an advantage that the circuit configuration is simple and can be implemented at low cost. On the other hand, there is a disadvantage in that heat generation occurs in the balancing resistor R_Bal during discharging and efficiency decreases, and it is difficult to perform balancing above a predetermined current due to the characteristics of the discharge method through the resistor.

2 illustrates an active cell balancing circuit 200 using a transformer in accordance with one embodiment.

The active cell balancing circuit 200 shown in FIG. 2 uses a Buck DC / DC converter for stepping down the voltage of the battery pack 110 to the voltage of the battery cell 10. At this time, the plurality of battery cells (B1, B2, ..., Bn) included in the battery pack 110 in a state in which the primary and secondary sides are insulated by the transformer included in the step-down DC / DC converter (10) Cell balancing (eg, B4) 10 having a low voltage is performed through the step-down type DC / DC converter.

The design and control of the DC / DC converter for the active cell balancing circuit 200 is not easy, and there is a disadvantage in that a redesign is required according to the pack configuration. In addition, the use of a DC / DC converter adversely affects EMI (electromagnetic interference) and EMC (electromagnetic compatibility) characteristics, and the cost increases.

Hereinafter, a cell voltage measuring circuit according to an exemplary embodiment will be described with reference to FIGS. 3A and 3B.

3A illustrates a cell voltage measuring circuit 300 using a capacitor charging method.

FIG. 3B illustrates a first process (Process 1) for performing voltage measurement of each battery cell 10 in the cell voltage measurement circuit 300 shown in FIG. 3A.

The cell voltage measuring circuit 300 illustrated in FIG. 3A includes a battery pack 110 including a plurality of battery cells 10, and a positive terminal (+) and a negative terminal (−) of each battery cell 10. A resistor R_Cap having one terminal connected thereto, a relay unit 20 connected to the other terminal of the resistor R_Cap, a voltage measuring capacitor C1 30, and a voltage measuring relay unit P-R5. 40), and the AD converter (50). In this case, the resistor R_Cap is a resistor for limiting a current charged in the voltage measuring capacitor 30.

In this case, the relay unit 20 may include a plurality of relays (P-Rn, where n is a natural number). For example, the first relay unit 21 may include two relays P-R1, and the fourth relay unit 24 may include two relays P-R4. In this case, the relay P-Rn may be implemented using a photomos relay.

Meanwhile, as illustrated in FIG. 3B, the first process of measuring the voltage of the battery cell 10 may be represented by a timing diagram of each of the relay units 20 and 40 over time.

As shown in FIG. 3B, when the relay unit P-R1 21 is turned on, the voltage measuring capacitor 30 is charged with the voltage of the first battery cell B1. After a predetermined time has elapsed, when the first relay 21 is in an OFF state and the voltage measuring relay unit P-R5 40 is in an on state, the first capacitor 21 is charged to the voltage measuring capacitor 30. The read voltage is read into the AD converter 50. When the second relay unit P-R2 22 is in an on-state, the voltage of the second battery cell B2 10 is charged in the voltage measuring capacitor 30. After a certain time has elapsed, when the second relay unit 22 is in the off-state and the voltage measuring relay unit (P-R5) 40 is in the on-state, the voltage charged in the voltage measuring capacitor 30 is changed. It is read by the AD converter 50. By repeating the above process, the voltage of the battery cell 10 is read in an insulated state so that the cell voltage can be measured.

Hereinafter, a cell voltage measurement and cell balancing circuit according to an embodiment of the present invention will be described with reference to FIGS. 4A to 8.

In this embodiment, it is assumed that the battery pack 110 includes four battery cells B1, B2, B3, and B4 10. Among the four battery cells 10, the voltage of the first battery B1 has the highest value, the voltage of the fourth battery B4 has the lowest value, and the second battery B2 and the third battery ( Assume that the voltage in B3) has an average value.

 4A illustrates a cell voltage measurement and cell balancing circuit 400 in accordance with one embodiment of the present invention. 4B illustrates an example of a temporal relationship between several processes performed in the circuit 400 shown in FIG. 4A. 4C shows a timing diagram of the process performed in the circuit 400 shown in FIG. 4A.

4A is compared with FIG. 3A, the cell voltage measurement and cell balancing circuit 400 shown in FIG. 4A includes a balancing resistor R_Bal and a charging relay in the cell voltage measurement circuit 300 shown in FIG. 3A. The unit 60 may further include a charging capacitor C2 70. Accordingly, there is an advantage that it can be implemented at low cost.

In this case, the balancing resistor R_Bal is connected between the output terminal of each battery cell 10 and the first side of each relay unit 20.

The charging relay unit 60 may include a plurality of relays P-R6. In this case, the first side of the charging relay unit 60 may be connected to the second side of each relay unit 20, and the charging capacitor 70 may be connected to the second side of the charging relay unit 60. have. In this case, the charging capacitor 70 may be implemented using an ultra capacitor or a secondary battery. In this case, the secondary battery may be, for example, a small capacity lithium ion battery.

In this case, the resistor R_Cap connected to the output terminal of each battery cell 10 in the cell voltage measuring circuit 300 shown in FIG. 3A is the balancing resistor R_Bal and the voltage measuring capacitor 30 in FIG. 4A. Located between and to limit the inrush current to the voltage measuring capacitor (30).

On the other hand, the cell voltage measurement and cell balancing circuit 400 according to an embodiment of the present invention is able to perform the measurement of the cell voltage and the balancing between cells at the same time. For example, as illustrated in FIG. 4B, the first process P100, the second process P200, and the third process P300 may be sequentially performed over time.

The first process P100 may be the same as the process of measuring the voltages of the plurality of battery cells 10 described above with reference to FIGS. 3A and 3B.

The second process P200 compares the plurality of voltages of the plurality of battery cells 10 measured by the first process P100 with each other, thereby having the lowest voltage and the battery cell 10 having the highest voltage. The battery cell 10 may be configured to be determined. Alternatively, the ranking among the plurality of battery cells 10 may be determined based on the voltage of each battery cell. In this case, the second process P200 may be performed by, for example, a separate microprocessor connected to the output terminal of the AD converter 50 of FIG. 3A. In addition, in one embodiment, the second process P200 may be performed separately after the first process P100 is completed. In another embodiment, the second process P200 may be together while the first process P100 is being performed. May be performed.

The third process P300 may be configured to perform cell balancing through the battery cells 10 determined by the second process P200.

The time periods from start to completion of the processes P100, P200, and P300 shown in FIG. 4B do not overlap each other, but in other embodiments, the time periods may be overlapped with each other.

Meanwhile, the third process P300 may be as shown in FIG. 4C.

When the first relay unit P-R1 and the charging relay unit P-R6 are turned on, a balancing current limited by the balancing resistor R_Bal may be charged in the charging capacitor 70. At this time, the section in which the first relay unit P-R1 and the charging relay unit P-R6 are in an on-state may be, for example, sections T-C1, T-C2, ..., T-Cn, n may be a natural number).

After a predetermined time elapses, the first relay unit P-R1 and the charging relay unit P-R6 are turned off, and the fourth relay unit P-R4 and the charging relay unit P-R6 are turned off. Can be turned on. In this case, the sixth battery having the lowest voltage among the battery capacitors 10 from the charging capacitor C2 70 charged with the voltage of the first battery cell B1 having the highest voltage among the battery cells 10. The charging power may move to the cell B6. That is, the charging capacitor 70 may mean that it is discharged. At this time, the first relay unit P-R1 and the charging relay unit P-R6 are turned off and the fourth relay unit P-R4 and the charging relay unit P-R6 are on-state. The interval to be may be, for example, intervals T-D1, T-D2, ..., T-Dk, where k is a natural number.

While repeating this process, the voltage of the first battery cell B1 may be lowered and the voltage of the sixth battery cell B6 may be increased.

In this case, as the third process P300 is performed, as the time passes, the voltage of the first battery cell B1 gradually decreases, and the voltage of the fourth battery cell B4 gradually increases. The magnitude of the voltage of the battery cell B1 and the magnitude of the voltage of the fourth battery cell B4 may be balanced to a similar level. Accordingly, the amount of energy (C_Em, where m is a natural number) charged in the charging capacitor 70 may decrease gradually over time (that is, C_E1> C_E2>…> C_Em). In addition, the amount of energy D_Ei discharged from the charging capacitor 70 (where i is a natural number) may also decrease gradually over time (that is, D_E1> D_E2>…> D_Ei).

In this case, while the third process P300 is performed, the voltage measuring relay unit P-R5 40 is also driven to check the voltages of the battery cells 10 currently being balanced. For example, the relay unit P-R1 is turned on and charged to a voltage in the voltage measuring capacitor C1 30, and then turned off or the relay unit P-R4 is turned on. Whenever the apparatus is turned off after receiving the energy from the voltage measuring capacitor 30, the voltage measuring relay unit P-R5 40 is turned on to turn on the voltage of each battery cell B1 or B4. May be read into AD converter 50. At this time, the section in which the voltage measuring relay unit 40 is in the on-state is, for example, a section T-S1, T-S2, ..., T-Sp, where p is a natural number.

5A illustrates a cell voltage measurement and cell balancing circuit 500 according to another embodiment of the present invention. FIG. 5B illustrates an example of a bidirectional DC / DC converter of the circuit 500 shown in FIG. 5A.

Referring to FIG. 5A in comparison with FIG. 4A, the circuit 500 shown in FIG. 5A is a bidirectional DC located in front of the charging relay unit (P-R6) 60 in the circuit 400 shown in FIG. 4A. It may be configured to further include a / DC converter (Bi-directional DC / DC Converter) (80). Accordingly, since current control is possible, the efficiency of cell balancing can be increased by minimizing or removing the balancing resistor R_Bal.

In this case, when the voltage of the battery cell 10 connected to the bidirectional DC / DC converter 80 is relatively high, the bidirectional DC / DC converter 80 operates as a step-down type DC / DC converter 81 to charge the voltage of the battery cell 10. When the voltage of the battery cell 10 connected to the capacitor 70 is relatively low when the capacitor 70 is charged, it operates as a boost type DC / DC converter 82 to discharge the charge capacitor 70. It is supposed to perform balancing.

In other words, when the battery cell 10 connected to the left side of the DC / DC converter 80 is the battery cell having the highest voltage among the plurality of battery cells, the charge to be supplied with the current from the right side of the DC / DC converter 80 is performed. The DC / DC converter 80 is a step-down type since the voltage of the battery cell having the highest voltage that supplies current from the left side of the DC / DC converter 80 is always higher than or equal to the voltage across the capacitor 70. It is necessary to operate as the DC / DC converter 81.

In comparison, when the battery cell 10 connected to the left side of the DC / DC converter 80 is the battery cell having the lowest voltage among the plurality of battery cells, the current must be supplied from the right side of the DC / DC converter 80. As the charging capacitor 70 supplies current to the battery cell having the lowest voltage, the voltage across the charging capacitor 70 may be lower than the voltage of the battery cell having the lowest voltage. The / DC converter 80 needs to operate as a multiplier type DC / DC converter 82.

In this case, as shown in FIG. 5B, when the bidirectional DC / DC converter 80 operates as the step-down DC / DC converter 81, current flows in the direction 83 from the point S1 to the point S2. And the flow of this current can be controlled. The current cannot flow in the direction 84 from the point S2 to the point S1.

When the bidirectional DC / DC converter 80 operates as a boosted DC / DC converter 82, a current may flow in the direction 85 from the point S2 to the point S1, and the current Flowchart can be controlled. The current cannot flow in the direction 86 from the point S2 to the point S1.

Hereinafter, an internal structure, an operation mode, and a controller of the bidirectional DC / DC converter 80 illustrated in FIG. 5A will be described with reference to FIGS. 6A to 8C.

FIG. 6A shows the internal structure of the bidirectional DC / DC converter 80 shown in FIG. 5A. FIG. 6B shows the control unit 90 of the bidirectional DC / DC converter 80 shown in FIG. 6A.

FIG. 7A shows a case where the bidirectional DC / DC converter 80 shown in FIG. 5A operates in the step-down mode 81. FIG. 7B is a diagram for explaining a case where the first transistor Q1 is in an on-state in the step-down DC / DC converter 81 shown in FIG. 7A. FIG. 7C is a diagram for describing a case where the first transistor Q1 is in an off-state in the step-down DC / DC converter 81 shown in FIG. 7A.

FIG. 8A shows a case where the bidirectional DC / DC converter 80 shown in FIG. 5A operates in the boost mode 82. FIG. 8B is a diagram for explaining a case where the second transistor Q2 is in an on-state in the boost type DC / DC converter 82 shown in FIG. 8A. FIG. 8C is a diagram for explaining a case where the second transistor Q2 is in an off-state in the boost type DC / DC converter 82 shown in FIG. 8A.

As shown in FIG. 6A, the internal structure of the bidirectional DC / DC converter 80 includes a controller 90 and a capacitor configured to control the transistors Q1 and Q2 and the transistors Q1 and Q2. C3 and C4, and the inductor (L). In this case, the bidirectional DC / DC converter 80 is configured to allow current to flow in both directions 83 and 85, and to control the current flowing in both directions 83 and 85 according to an operation mode.

In this case, as shown in FIG. 6B, the controller 90 is configured to perform a control operation in an insulated state through the photocoupler. At this time, the control operation of the DC / DC converter 80 is to perform the operation in the Buck Converter Mode (Bouck Converter Mode) or Boost Mode (Boost Converter Mode).

As shown in FIG. 7A, in the converter 81 configured to operate in the step-down mode, the second transistor Q2 is always turned off by the controller 90 so that the converter 81 is included in the second transistor Q2. The first current flow path 71 through the second parasitic diode D2 may be formed.

In this case, the first transistor Q1 may be turned on or off by the controller 90. When the first transistor Q1 is in the on-state, as illustrated in FIG. 7B, a second current flow path 72 through which current may flow in the direction 83 from the point S1 to the point S2. ) Is formed to control the current flowing through the second current flow path 72. On the other hand, when the first transistor Q2 is turned off by the controller 90, as illustrated in FIG. 7C, the first transistor Q1 may include the first parasitic diode D1 included in the first transistor Q1. Since only the first current flow path 71 through the third current flow path 73 and the second parasitic diode D2 is formed, the current cannot flow in the direction 83.

As shown in FIG. 8A, the converter 82 configured to operate in the boosted mode has the first transistor Q1 always turned on by the controller 90 so that the fourth current flow path 74 can be operated. It may be formed. At this time, when the second transistor Q2 is turned on by the controller 90, current flows in the direction 85 from the point S1 to the point S2 as shown in FIG. 8B. It cannot be. On the other hand, when the second transistor Q2 is turned off, the current flowing through the fourth current flow path 74 is controlled.

6A and 7A to 8C, an ultracapacitor or a secondary battery may be connected to the output terminal S2 of the capacitor C4 side of the bidirectional DC / DC converter 80. In this case, the secondary battery may be, for example, a small capacity lithium ion battery.

By using the embodiments of the present invention described above, those belonging to the technical field of the present invention will be able to easily make various changes and modifications without departing from the essential characteristics of the present invention. The content of each claim in the claims may be combined in another claim without citations within the scope of the claims.

Claims (15)

1. A battery pack comprising a plurality of battery cells, and a charging capacitor adapted to be selectively connected to the plurality of battery cells.

   A storage step of storing a portion of power stored in a first battery cell of the plurality of battery cells in the charging capacitor, and providing power stored in the charging capacitor to a second battery cell of the plurality of battery cells. To perform the charging and discharging step comprising a,

   By repeating the charging / discharging step a plurality of times until the difference between the first voltage of the first battery cell and the second voltage of the second battery cell is equal to or less than a predetermined value, the first battery cell and the second battery Configured to perform a third process of completing cell balancing between cells,

   Battery pack.
2. The method of claim 1,

   Capacitor for voltage measurement; A second relay; And an AD converter,

   The voltage measuring capacitor is connected to the output terminal of the battery unit including the plurality of battery cells,

   An input terminal of the second relay is connected to one terminal of the voltage measuring capacitor,

   The AD converter is connected to the output terminal of the second relay,

   The voltage of any one of the plurality of battery cells included in the battery unit is to charge the capacitor for measuring the voltage,

   And when the second relay is turned on, provides the AD converter with a voltage charged in the voltage measuring capacitor.

   Battery pack.
3. The method of claim 1,

   In a time period other than the time period occupied by the storage step and the providing step, the measurement step of measuring the voltage of the first battery cell and the second battery cell is further performed. Supposed to

   Battery pack.
4. The method of claim 1,

   And before the third process, further perform a first process of measuring all voltages of the plurality of battery cells, and a second process of determining battery cells in which voltage balancing is required among the battery cells.

   Battery pack.
5. The method of claim 1,

   The charging capacitor,

   Is connected to the relay unit for selectively connecting any one of the plurality of battery cells to the charging capacitor,

   Battery pack.
6. The method of claim 5,

   The battery pack includes a bidirectional DC-DC converter, and the bidirectional DC-DC converter is connected between the relay unit and the charging capacitor.

   Battery pack.
7. The method of claim 6,

   When power is input to the charging capacitor, the bidirectional DC-DC converter operates in the step-down mode,

   When power is output from the charging capacitor, the bidirectional DC-DC converter operates in a boost mode,

   Battery pack.
8. The method of claim 6,

   The bidirectional DC-DC converter allows the current to flow only in one of a first direction through which current flows toward the charging capacitor and a second direction through which current flows from the charging capacitor, according to its operation mode. Designed to control,

   Battery pack.
9. According to claim 1, A pair of input terminals of the voltage measuring unit which is selectively connected to any one of the plurality of battery cells to measure the voltage of each battery cell is electrically connected to both terminals of the charging capacitor That, battery pack.
10. A battery unit including a plurality of battery cells;

    A first relay unit connected to an output terminal of the battery unit; And

    A charging capacitor configured to be connected to an output terminal of the first relay unit,

    A storage step of storing a first current output from the first battery cell having the highest voltage among the plurality of battery cells in the charging capacitor through the first relay unit, and storing energy charged in the charging capacitor; It is to perform the charging and discharging step comprising providing to the second battery cell having the lowest voltage of the plurality of batteries through a first relay unit,

    By repeating the charging / discharging step a plurality of times until the difference between the first voltage of the first battery cell and the second voltage of the second battery cell is equal to or less than a predetermined value, the first battery cell and the second battery Configured to perform a third process of completing cell balancing between cells,

    Cell balancing circuit.
11. The method of claim 10,

    Capacitor for voltage measurement; A second relay; And an AD converter,

    The voltage measuring capacitor is connected to the output terminal of the battery unit,

    An input terminal of the second relay is connected to one terminal of the voltage measuring capacitor,

    The AD converter is connected to the output terminal of the second relay,

    The voltage of any one of the plurality of battery cells included in the battery unit is to charge the capacitor for measuring the voltage,

    And when the second relay is turned on, provides the AD converter with a voltage charged in the voltage measuring capacitor.

    Cell balancing circuit.
12. The method of claim 10,

    The cell balancing circuit includes a bidirectional DC-DC converter, and the bidirectional DC-DC converter is connected between the first relay unit and the charging capacitor.

    Cell balancing circuit.
13. The method of claim 12,

    When power is input to the charging capacitor, the bidirectional DC-DC converter operates in the step-down mode,

    When power is output from the charging capacitor, the bidirectional DC-DC converter operates in a boost mode,

    Cell balancing circuit.
14. The cell balancing circuit of claim 10, wherein the first relay unit may be implemented using a photomoss relay.
15. The cell balancing circuit of claim 10, wherein the charging capacitor may be implemented using an ultra capacitor or a small capacity lithium ion battery.

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