JP2009071936A - Voltage equalization system for battery pack - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve optimum voltage equalization even if a plurality of cells constituting a battery pack have variations in capacity. <P>SOLUTION: Based on the capacity SOC of the battery pack calculated in an SOC operation unit 21, a voltage equalization implementation threshold value operation unit 25 operates a voltage equalization implementation threshold value VDd, a threshold value in implementing the voltage equalization. An equalization implementation determination unit 26 compares the voltage equalization implementation threshold value VDd with a voltage difference VDr between the maximum voltage and the minimum voltage among cells to determine whether the voltage equalization is implemented or not. When an expression of VDd<VDr is satisfied, a voltage equalization circuit 10 is operated. When an expression of VDd≥VDr is satisfied, the voltage equalization circuit 10 is halted. This ensures the balance of the cells fully charged so as to maximize the capacity usable as the battery pack even if there are variations in capacity among cells. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an assembled battery voltage equalization system for equalizing voltages of a plurality of cells constituting an assembled battery.

  Generally, in an assembled battery configured by connecting a plurality of cells in series, a voltage equalization circuit that equalizes the voltage of each cell is used for the purpose of utilizing the maximum capacity of the battery and preventing the occurrence of battery variation due to deterioration. Used together. As this voltage equalization apparatus, as disclosed in, for example, Patent Document 1 and Patent Document 2, devices of various circuit systems are known.

  The equalization circuit disclosed in Patent Document 1 sets a bypass circuit including a bypass switch for each cell, and in a cell that is in a fully charged state, by turning on the bypass switch and flowing a charging current to the bypass circuit, This avoids overcharging of the cell.

In addition, the equalization circuit disclosed in Patent Document 2 uses a transformer having a coil with the same winding ratio for each cell, and switches a switch provided between each cell and the coil at a predetermined frequency. Energy transfer through the coil is repeated to equalize the voltage of each cell.
JP 2003-289629 A JP 2006-16615 A

  In the conventional techniques as disclosed in Patent Documents 1 and 2, it is assumed that the initial characteristics (capacity and internal resistance) of each battery cell are uniform, and the actual cell capacity variation is taken into consideration. Therefore, always operate the voltage equalization circuit so that no voltage difference occurs between the cells, or operate the voltage equalization circuit when a predetermined voltage difference occurs between the cells. Yes.

  However, the maximum usable capacity of the battery is when the cell is balanced when fully charged, such as a lithium ion battery, nickel metal hydride battery, lead acid battery, etc. If the voltage drops, the voltage will drop relatively rapidly.In areas where the battery capacity is low, there will be a voltage difference between cells due to variations in the initial characteristics. There arises a problem that a cell voltage difference is generated and the usable capacity as a battery is reduced.

  The present invention has been made in view of the above circumstances, and provides an assembled battery voltage equalization system capable of performing optimum voltage equalization even when there are capacity variations in a plurality of cells constituting the assembled battery. The purpose is that.

  In order to achieve the above object, a voltage equalization system for an assembled battery according to the present invention is a voltage equalization system having a voltage equalization circuit for equalizing the voltages of a plurality of cells constituting the assembled battery, And a control unit that calculates a threshold value for determining whether to operate the voltage equalization circuit based on the capacity of the voltage, and controls the operation of the voltage equalization circuit based on the threshold value. To do.

  According to the present invention, optimal voltage equalization can be performed even when there are capacity variations in a plurality of cells constituting an assembled battery.

  Embodiments of the present invention will be described below with reference to the drawings. 1 to 6 relate to an embodiment of the present invention, FIG. 1 is an overall configuration diagram of a voltage equalization system, FIG. 2 is a functional block diagram of a controller, and FIG. 3 shows a relationship between battery capacity and open voltage. FIG. 4 is an explanatory diagram showing the relationship between the battery capacity and the voltage equalization threshold, FIG. 5 is an explanatory diagram showing the charge / discharge voltage variation of the cell, and FIG. 6 is an explanatory diagram showing the voltage equalization result.

  As shown in FIG. 1, the voltage equalization system 1 includes a voltage equalization circuit 10 that equalizes voltages of a plurality of cells that form an assembled battery, and a controller 20 that serves as a control unit that controls the voltage equalization circuit 10. And is configured. Here, the assembled battery to be subjected to voltage equalization is, for example, a lithium ion battery or a nickel metal hydride battery, and a plurality of cells connected in series, connected in parallel, and combined in series and parallel. Connected ones are subject to voltage equalization.

  In the present embodiment, the voltage equalization circuit 10 balances the voltage of each cell using a transformer for an assembled battery D formed by connecting a plurality of n cells D1, D2,..., Dn in series. This is a circuit that performs energy transfer between cells by charging and discharging using the induced voltage of the coil, and equalizes the voltage of each cell. The voltage equalization circuit 10 is not limited to the one using a coil, and may be a circuit using a resistor or a capacitor.

  Specifically, the voltage equalization circuit 10 includes a transformer 11 having a primary coil L0 and a plurality of secondary coils L1, L2,..., Ln corresponding to the cells D1, D2,. It is comprised using. The battery 12 is connected to the primary coil L0 via the switch S0, and the cells D1, L2, L2,..., Ln are respectively connected to the secondary side coils L1, L2,. D2,..., Dn are connected. The coils L1, L2,..., Ln are set to have the same number of turns.

  Each switch S0, S1, S2,... Is composed of a semiconductor switch element such as an FET, and is driven and controlled by a controller 20 comprising a microcomputer or the like. The controller 20 drives the switches S0, S1, S2,... At a predetermined frequency. When the switch S0 is OFF, the switches S1, S2,... Are ON, and when the switch S0 is ON, the switches S1, S2,. As described above, the switches S1, S2,... Are inverted and synchronized with the switch S0 to be turned on and off to equalize the voltages of the respective cells.

  In the cell voltage equalization control by the voltage equalization circuit 10, the controller 20 operates when the cell voltage is equal to or lower than a predetermined threshold value, even if a voltage difference greater than a predetermined value is generated between the cells. If the cell voltage difference is greater than or equal to a predetermined threshold value, the voltage equalization circuit 10 is operated to realize optimum voltage equalization control even for cells having capacity variations. In this embodiment, the threshold value for performing equalization is changed according to the voltage difference between cells so that the cell capacity can be utilized to the maximum.

  Therefore, as shown in FIG. 2, the controller 20 includes an SOC calculation unit 21, a maximum cell voltage calculation unit 22, a minimum cell voltage calculation unit 23, a voltage difference calculation unit 24, a voltage equalization execution threshold value calculation unit 25, an equalization. An implementation determination unit 26 and a switching drive unit 27 are provided, and the total voltage VP, current I, temperature T, and voltage VC1 for each cell of the assembled battery D (n cells) detected by a sensor (not shown). , VC2,..., VCn are input.

  The SOC calculation unit 21 receives the total voltage VP, current I, and temperature T of the assembled battery D as inputs, and calculates the capacity of the assembled battery D as a state of charge (SOC). The capacity SOC is calculated from the relationship between the open circuit voltage VOC based on the internal impedance corrected by the temperature T and the capacity SOC based on the integration of the current I.

  FIG. 3 shows the relationship between the battery capacity SOC and the no-load voltage (open circuit voltage) OCV. FIG. 3 shows a characteristic example of the lithium ion battery. In general, a lithium ion battery shows 4.2 V in a fully charged state (SOC = 100%) and 2.5 V in a full discharge (SOC = 0%), and the voltage characteristic has a slope that decreases as the capacity SOC decreases. It is known that it is steep and has no dependence on the total capacity of the battery.

  The highest cell voltage calculation unit 22 and the lowest cell voltage calculation unit 23 receive the cell voltages VC1, VC2,..., VCn of each cell as input, and become the highest cell voltage VCmax and lowest voltage among these voltages. The minimum cell voltage VCmin is calculated. The highest cell voltage VCmax and the lowest cell voltage VCmin are input to the voltage difference calculation unit 24, and the voltage difference calculation unit 24 calculates the voltage difference VDr between the highest cell voltage VCmax and the lowest cell voltage VCmin.

  The voltage equalization execution threshold value calculation unit 25 calculates a threshold value (voltage equalization execution threshold value) VDd for performing voltage equalization based on the capacity SOC of the assembled battery. As shown in FIG. 4, the voltage equalization execution threshold VDd is set as a voltage value corresponding to the voltage difference between cells, and is set so that the value of the voltage equalization execution threshold VDd increases as the capacity SOC decreases. ing. As will be described below, by stopping the voltage equalization circuit 10 in a region where the capacity SOC is small, the cell balance is prevented from being lost at the time of full charge, and the usable capacity as an assembled battery is maximized. This is to ensure that

  The voltage equalization execution threshold VDd may be set as a value of a specific capacity SOC, and by prohibiting (stopping) the operation of the voltage equalization circuit 10 in a region where the capacity SOC is smaller than the specific value, Similarly, it is possible to prevent the cell balance from being lost when fully charged, and to maximize the usable capacity of the assembled battery.

  The equalization execution determination unit 26 compares the highest and lowest voltage difference VDr between cells with the voltage equalization execution threshold value VDd, and determines whether or not to perform voltage equalization. When VDd <VDr, a control command for operating the voltage equalization circuit 10 is output to the switching drive unit 27. When VDd ≧ VDr, a control command for stopping the voltage equalization circuit 10 is output to the switching drive unit 27. To do.

  FIG. 5 shows an example of data when cells having substantially the same capacity are connected in series, full discharge is performed from full charge, and full charge is further performed. Initially, all the cells are 4.2 V. However, since current is drawn uniformly from all the cells, it can be seen that there is a voltage variation between the cells due to a slight variation in cell capacity at the end of discharge. After that, when fully charged, all cells converge to 4.2V.

  In the conventional equalization control, the voltage equalization circuit always operates so as not to generate a voltage difference between the cells, or operates when a predetermined voltage difference occurs between the cells. I try to let them. Therefore, when the voltage equalization circuit is operated in the state of FIG. 5 in which the optimum cell balance is achieved, the voltage difference between cells due to the influence of the variation in the initial characteristics occurs in the region where the capacitance SOC is small, and therefore the capacitance SOC is small. By the way, the cell voltages are equalized, and conversely, a voltage difference is generated between the cells near full charge.

  That is, from the characteristics of the capacity SOC and the open-circuit voltage OCV shown in FIG. 3, considering the case where an assembled battery is configured with cells of uniform capacity (ideal state in which no cell voltage difference occurs in any area of the capacity SOC) as a reference. For example, when there is a 50 mV cell-to-cell voltage difference when the capacity SOC is near 0%, there is a decrease in usable capacity of about 1%, but when the capacity SOC is near 100%, the same voltage difference occurs between 50 mV. In this case, the usable capacity is reduced by about 10%.

  Therefore, in this system, the smaller the capacity SOC is, the larger the threshold for carrying out voltage equalization is, so that the cell balance at the time of full charge is aligned and the usable capacity as an assembled battery is maximized. . In particular, in an electric vehicle or the like, by limiting a charging area under a certain condition such as when charging with a constant current by CC charging as an implementation area of voltage equalization, an area with a small capacity by grasping an accurate capacity of the battery Thus, the voltage equalizing circuit 10 can be stopped reliably, which is effective.

  When the voltage equalization execution threshold value VDd is set as a specific value of the capacity SOC, the equalization execution determination unit 26 replaces the highest and lowest voltage difference VDr between cells with the current capacity SOC of the assembled battery. Is compared with the voltage equalization execution threshold VDd. When VDd <SOC, the voltage equalization circuit 10 is operated, and when VDd ≧ SOC, the operation of the voltage equalization circuit 10 is prohibited (stopped).

  Next, the operation of the voltage equalization system 1 controlled by the above controller 20 will be described.

  First, the controller 20 obtains the highest cell voltage VCmax and the lowest cell voltage VCmin among the current voltages of the cells constituting the assembled battery D, and calculates the voltage difference Dr. Further, the controller 20 receives the total voltage VP, current I, and temperature T of the assembled battery D as input, obtains the current capacity SOC of the assembled battery D, and sets the voltage equalization execution threshold VDd by referring to the map.

  Then, the voltage equalization execution threshold VDd is compared with the voltage difference VDr which is the maximum value of the voltage difference between cells, and the voltage equalization circuit 10 is activated / stopped according to the comparison result. That is, when VDd <VDr, the switches S1, S2,... Of the voltage equalization circuit 10 are driven at a predetermined frequency, and when VDd ≧, the switches S0, S1, S2,. Stop.

  When the voltage equalization circuit 10 is operated, when the switches S1, S2,..., Sn are turned ON, the voltages V1, V2,..., Vn of the cells D1, D2,. Applied. Next, when the switches S1, S2,..., Sn are turned off, the energy stored in the coils L1, L2,..., Ln passes through the magnetic circuit of the coils L1, L2,. ) And the voltage of each coil is equalized to (V1 + V2 +...) / N.

  Further, when the switches S1, S2,..., Sn are turned on, the voltage difference between the coil winding voltage (V1 + V2 +...) / N and each cell voltage V1, V2,. Energy is exchanged. The primary side switch S1 is turned on and off in synchronization with the secondary side switches S1, S2,..., Sn, and is generated at the time of equalization in the secondary side coils L1, L2,. The energy loss of the switching element is compensated from the battery 12 via the primary side coil L1.

  By repeatedly storing and releasing energy between each cell and the coil as described above, as shown in FIG. 6, finally, the voltage of each cell is equalized and becomes uniform. FIG. 6 shows data obtained by performing voltage equalization on a plurality of cells whose voltages are intentionally varied.

  As described above, the voltage equalization system in the present embodiment can maximize the energy of the assembled battery by optimally controlling the voltage value of each cell of the assembled battery. As a result, for example, when applied to an electric vehicle, it is possible to extend the travel distance with the same battery cell capacity, and the vehicle cost can be reduced by installing a minimum battery capacity in a vehicle that can travel a specified distance. Can be reduced, and the number of times of charging can be reduced, thereby contributing to energy saving.

Overall configuration diagram of voltage equalization system Functional block diagram of controller Explanatory diagram showing the relationship between battery capacity and open circuit voltage Explanatory drawing which shows the relationship between a battery capacity and a voltage equalization implementation threshold value Explanatory drawing showing variation in charge / discharge voltage of cells Explanatory diagram showing voltage equalization results

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Voltage equalization system 10 Voltage equalization circuit 20 Controller D Battery assembly D1, D2, ..., Dn Cell SOC capacity VDd Voltage equalization execution threshold VDr Voltage difference

Claims (4)

A voltage equalization system having a voltage equalization circuit for equalizing the voltages of a plurality of cells constituting an assembled battery,
A controller for calculating a threshold for determining whether or not to operate the voltage equalization circuit based on the capacity of the assembled battery, and for controlling the operation of the voltage equalization circuit based on the threshold; An assembled battery voltage equalizing system.   The threshold value is set as a voltage value to be compared with a voltage difference between the plurality of cells, and the voltage difference for operating the voltage equalization circuit is increased as the capacity of the assembled battery is smaller. The voltage equalization system of the assembled battery according to 1.   2. The assembled battery according to claim 1, wherein the threshold is set as a specific value of the capacity of the assembled battery, and when the capacity of the assembled battery is equal to or less than the specific value, the operation of the voltage equalization circuit is prohibited. Voltage equalization system.   2. The assembled battery voltage equalizing system according to claim 1, wherein the voltage equalizing circuit is operated only when the assembled battery is charged under a certain condition.

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