JP2009038876A - Voltage equalizer for battery pack - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a voltage equalizer for a battery pack, which properly equalizes the voltage by less charge and discharge, the equalizer which equalizes the voltage of the battery pack where a plurality of cells to be charged and discharged are connected in series. <P>SOLUTION: This voltage equalizer detects the voltage Vb(k) of each unit cell (step S100), and charges or discharges a cell where a voltage difference from a voltage (reference voltage Vbst) to become a reference is at or over a threshold Vr among detected voltages thereby equalizing the voltage. For example, it equalizes the voltage by discharging the cell (steps S102-114) whose voltage difference from the lowest voltage Vbmin being the lowest voltage among the detected voltage is at or over the threshold Vr. Here, the threshold Vr is set at a different value (step S104), according to the reference voltage Vbst (the lowest voltage Vbmin) so that it reduces the difference, by the reference voltage Vbst (for example, the lowest voltage Vbmin), in the range of the charge state corresponding to the threshold Vr. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an assembled battery voltage equalizing apparatus, and more particularly to an equalizing apparatus for equalizing voltages of a plurality of unit cells in an assembled battery formed by connecting a plurality of unit cells in series.

  In an assembled battery in which a plurality of chargeable / dischargeable cells (secondary batteries) are connected in series, self-discharge current of individual cells constituting the assembled battery and variations in the degree of deterioration (deterioration) over time, Due to variations in charging efficiency, etc., the voltage may vary between these single cells. Such variations in voltage may cause some of the cells to be overcharged or overdischarged to reduce the life of the cells. For this reason, an equalizing device that eliminates voltage imbalance of the cells constituting the assembled battery has been proposed. For example, in Patent Document 1, by detecting the voltage of each unit cell constituting the assembled battery and discharging a unit cell in which a voltage difference of a certain value or more is generated between the unit cell having the lowest voltage among them. A technique for equalization is described.

JP 2003-282155 A

  Here, when the discharge amount for voltage equalization increases, the effective capacity (energy efficiency) of the assembled battery decreases. Therefore, it is desirable to effectively avoid inconveniences (for example, an event in which some cells are overcharged or overdischarged) with a smaller discharge amount, which may be caused by voltage variations. Therefore, the present invention is an apparatus for equalizing the voltage of an assembled battery in which a plurality of chargeable / dischargeable cells are connected in series, and can be appropriately equalized with a smaller amount of charge / discharge. An object is to provide a voltage equalizing apparatus.

  The inventor has characteristics indicating a relationship between an open circuit voltage (hereinafter also referred to as “OCV”) and a state of charge (hereinafter also referred to as “SOC”) of the unit cell. It was noted that the slope of the curve varies depending on the SOC of the battery, and therefore the SOC width corresponding to a predetermined voltage difference varies depending on the voltage (OCV). Then, it has been found that the above problem can be solved by changing the condition for starting charging or discharging for voltage equalization (that is, the operating condition of the equalization circuit constituting the equalization processing unit) according to the SOC. The present invention has been completed.

  ADVANTAGE OF THE INVENTION According to this invention, the equalization apparatus which equalizes the voltage of this several unit cell in the assembled battery in which the unit cell (secondary battery) which can be charged / discharged was connected in series is provided. The equalization apparatus includes a voltage detection unit that detects the voltage of the unit cell. In addition, the cells having the voltage difference from the reference voltage Vbst, which is a reference voltage among the detected voltages, are charged or discharged to equalize the voltages (that is, the voltage of the cells). When the voltage of the unit cell is higher than the reference voltage Vbst by a threshold Vr or more, the unit cell is discharged so that the voltage of the unit cell is higher than the reference voltage Vbst. Is also provided with an equalization processing unit that charges the unit cell when the threshold value Vr is lower than the threshold value Vr. Here, the threshold value Vr is set to a different value according to the reference voltage Vbst so as to relieve the difference in the width of the charging state corresponding to the threshold value Vr due to the reference voltage Vbst.

  In the present specification, the “single cell” is a term indicating individual power storage elements constituting an assembled battery, and includes batteries having various compositions and configurations unless otherwise specified. The “secondary battery” refers to a battery that can be repeatedly charged, and includes so-called storage batteries such as lithium ion batteries and nickel metal hydride batteries. The electric storage element constituting the lithium ion battery is a typical example included in the “unit cell” referred to herein, and a lithium ion battery module formed by connecting a plurality of such unit cells in series is disclosed in the “set” This is a typical example of a “battery”.

As the reference voltage Vbst, for example, the lowest voltage Vbmin that is the lowest voltage among the detected voltages can be preferably used. In this case, the equalization processing unit is configured to discharge the cells having a voltage difference from the reference voltage Vbst (here, the minimum voltage Vbmin) equal to or higher than the threshold Vr to equalize the voltage. obtain. That is, according to the present invention, there is provided an equalizing device that equalizes the voltages of a plurality of unit cells in a battery pack in which a plurality of chargeable / dischargeable unit cells (secondary cells) are connected in series. The equalization apparatus includes a voltage detection unit that detects the voltage of the unit cell. Moreover, the equalization process part which discharges the cell whose voltage difference with minimum voltage Vbmin which is the lowest voltage among the detected voltages is more than threshold value Vr, and equalizes a voltage is provided. Here, the threshold value Vr is set to a different value in accordance with the minimum voltage Vbmin so as to alleviate the difference in the state of charge (SOC) corresponding to the threshold value Vr due to the minimum voltage Vbmin.
As the reference voltage Vbst, for example, the highest voltage Vbmax that is the highest voltage among the detected voltages may be employed. In this case, the equalization processing unit may be configured to charge the cells having a voltage difference from the maximum voltage Vbmax) equal to or higher than the threshold value Vr to equalize the voltages.

  As described above, in general, the SOC and OCV of a secondary battery (for example, a lithium ion battery) are not linearly related, and the SOC-OCV characteristic curve varies depending on the SOC (typically, the slope of the tangent of the curve). Can be grasped.) Therefore, compared with the location where the slope of the characteristic curve is relatively small, the influence of the difference in the SOC (the amount of charge stored in the battery) on the voltage difference at the location where the slope is relatively large. Becomes larger. For this reason, if the threshold value Vr is set to a constant value regardless of the SOC as in the prior art, charging or discharging (equalization processing) for equalization is performed more frequently than necessary or equalized when necessary. There may be a disadvantage that processing is not performed. For example, when the single cell (reference voltage single cell) indicating the reference voltage Vbst is in a charged state where the slope of the characteristic curve is relatively small, the variation in the SOC of the single cells constituting the assembled battery is reduced to a desired range (SOC The specific threshold value Vr (fixed value) is applied as it is when the reference voltage cell is in a charged state where the slope of the characteristic curve is relatively large even if the specific threshold value Vr is appropriate Then, although the degree of variation in SOC is still within the desired range, the equalization process is performed.

In the technique disclosed herein, the threshold value Vr is set to a different value in accordance with the reference voltage Vbst so as to alleviate a difference due to the reference voltage Vbst (for example, the lowest voltage Vbmin) in the state of charge corresponding to the threshold value Vr. (Ie, the threshold value Vr is changed according to the reference voltage Vbst). More specifically, at a location where the slope of the SOC-OCV characteristic curve is relatively large, the threshold value Vr is set to a larger value than at a location where the slope of the curve is relatively small. For example, the threshold value Vr is preferably set such that the SOC width (capacity difference) corresponding to the threshold value Vr is substantially the same over the entire SOC-OCV characteristic curve (that is, regardless of the SOC). .
By setting the threshold value Vr as described above, regardless of the value of the reference voltage Vbst (thus, regardless of the state of charge of the single cell indicating the reference voltage Vbst (SOCst)), the variation in the SOC is a certain allowable limit. The equalization process can be effectively performed only when the value exceeds. Therefore, unnecessary charge / discharge (especially, charge / discharge caused by performing the equalization process even though the degree of variation in the SOC is still within the desired range) is suppressed, and the charge / discharge is made less efficient. The equalization process can be performed well. Thereby, the energy required for the equalization process can be saved.
For example, in the aspect using the minimum voltage Vbmin as the reference voltage Vbst, the threshold value Vr is set as described above, so that the state of charge of the unit cell indicating the minimum voltage Vbmin (therefore, the state of charge of the unit cell indicating the minimum voltage Vbmin) ( Regardless of the SOCmin), the equalization process can be effectively performed only when the variation of the SOC exceeds a certain allowable limit. Therefore, wasteful discharge (especially, discharge caused by performing the equalization process even though the degree of variation in SOC is still within the desired range) is suppressed, and equalization is efficiently performed with a smaller amount of discharge. Processing can be performed. Thereby, the energy of the assembled battery can be used more effectively.

  In a preferred aspect of the equalization apparatus disclosed herein, a unit cell whose voltage difference from the reference voltage Vbst (for example, the minimum voltage Vbmin) is equal to or greater than a threshold value Vr is set to a value of the reference voltage Vbst (for example, the minimum voltage Vbmin). Regardless of the configuration, the battery is charged or discharged by a certain amount of charge (in the embodiment in which the minimum voltage Vbmin is the reference voltage Vbst, a cell having a voltage difference from the minimum voltage Vbmin equal to or greater than the threshold Vr is discharged). Has been. For example, a unit cell whose voltage difference from the reference voltage Vbst (for example, the minimum voltage Vbmin) is equal to or greater than a threshold value Vr is charged by a constant charge amount equal to or less than the width of the SOC corresponding to the threshold value Vr per one equalization process. Or configured to be discharged (in the embodiment in which the minimum voltage Vbmin is the reference voltage Vbst). According to such an aspect, a simple control mechanism prevents excessive charging / discharging of a unit cell that requires equalization processing (a unit cell having a voltage difference from the reference voltage Vbst equal to or greater than the threshold value Vr). It is possible to appropriately equalize the voltages of the single cells.

  The equalization apparatus disclosed herein, for example, equalizes the voltage of the unit cell in an assembled battery in which the unit cell is a lithium ion battery (in other words, an assembled battery in which a plurality of lithium ion batteries are connected in series). This is suitable as a voltage equalizing device.

  Further, the present invention includes, as another aspect, a voltage equalization apparatus for any assembled battery disclosed herein, and an assembled battery in which a plurality of chargeable / dischargeable cells are connected in series. Provided is a power supply system configured to be able to equalize battery voltage by the voltage equalization device. As still another aspect, the present invention provides a vehicle equipped with the power supply system.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  FIG. 1 shows a schematic configuration of a battery pack voltage equalizing apparatus according to an embodiment of the present invention. The voltage equalizing device 20 is configured as a device that equalizes the voltages of the cells B (1) to B (n) constituting the assembled battery 10. The apparatus 20 includes an electronic control unit (ECU) 30 connected to connection points of the cells B (1) to B (n) through conductive lines L (0) to L (n), Transistors T (1) to T (n) and resistors R (1) to R (n) connected in series between the conductive lines L (0) to L (n), respectively. The voltage equalizing device 20 also constitutes a power supply system 1 including the device 20 and the assembled battery 10.

  The individual cells B (1) to B (n) constituting the assembled battery 10 are, for example, lithium ion batteries. The assembled battery 10 is configured by connecting these single cells B (1) to B (n) in series. The load 12 connected to the assembled battery 10 via the relay 14 may include a power consumer that consumes the power stored in the assembled battery 10. The load 12 may also include a power supply device (charger) that supplies power capable of charging the cells B (1) to B (n).

  The electronic control unit 30 includes a central processing unit (CPU) 32, a ROM (read only member) 34 that stores a program executed by the CPU 32, a random access memory (RAM) 36 that temporarily stores data, And an input / output port (not shown). When the transistors T (1) to T (n) are turned on by the on / off signal output from the electronic control unit 30 through the output port, the power supply system 1 shown in FIG. A current flows through a resistor connected in series to each other, and the power of the corresponding unit cell is consumed (discharged). Note that the ROM 34 stores the value of the OCV and the SOC of the OCV based on the shape of the SOC-OCV characteristic curve (typically, the slope of the tangent line of the characteristic curve) of the cells constituting the assembled battery 10. A data table is stored that indicates a relationship with a voltage difference, that is, a threshold value Vr for keeping the variation within a certain range (within an allowable range). In this data table, the threshold value Vr is set so that the SOC width corresponding to the threshold value Vr is substantially the same over the entire SOC-OCV characteristic curve (that is, regardless of the SOC).

  The operation of the voltage equalizing apparatus 20 configured as described above will be described. FIG. 2 is a flowchart illustrating an example of an equalization processing routine executed by the electronic control unit 30 of the voltage equalization apparatus 20 according to the present embodiment. This routine is repeatedly executed every predetermined time (for example, every hour) immediately after the assembled battery 10 is disconnected from the load 12.

  When the equalization processing routine shown in FIG. 2 is executed, the CPU 32 of the electronic control unit 30 first detects the voltages Vb (1) to Vb (n) of the individual cells B (1) to B (n). Processing is executed (step S100). The voltages Vb (1) to Vb (n) of the individual cells B (1) to B (n) can be detected as a voltage difference between the conductive lines L (0) to L (n). Next, the lowest voltage among the detected voltages Vb (1) to Vb (n), that is, the lowest voltage Vbmin is determined (step S102), and the OCV-Vr data table stored in the ROM 34 is referred to. The value of the threshold value Vr corresponding to the lowest voltage Vbmin (OCV) is read (step S104), and the value 0 is set to the cell counter k (step S106).

  Then, the battery counter k is incremented (step S108), and a voltage difference ΔV between the voltage Vb (k) and the lowest voltage Vbmin of the kth cell B (k) is calculated (step S110), and the calculated voltage The value of the difference ΔV is compared with the threshold value Vr read above (step S112). As a result, when voltage difference ΔV is equal to or greater than threshold value Vr (YES in step S112), it is determined that the variation in SOC cannot be permitted, and corresponding transistor T (k) is turned on (step S114). As a result, the electric power of the unit cell B (k) is consumed by the resistor R (k) so that the variation in the SOC is eliminated. On the other hand, when voltage difference ΔV is equal to or smaller than threshold value Vr (NO in step S112), it is determined that the variation in SOC is within an allowable range, and corresponding transistor T (k) is turned off (step S115). . Note that when the transistor T (k) is already in the off state, the state is maintained.

  Then, it is checked whether or not the unit cell counter k is n (step S116). If the unit cell counter k is not n (NO in step S116), the unit cell counter k is incremented in step S108, and the next The same process is performed for the unit cell. If unit cell counter k is n (YES in step S116), the current equalization processing routine is terminated.

  According to the above-described embodiment, by performing the equalization process so as to keep the variation in the SOC within a certain range, the unit cell constituting the assembled battery is overcharged or overdischarged (in other words, the unit cell Can be effectively prevented. Further, the variation in the SOC is constant by detecting the voltage Vb (k) of each cell B (k) and comparing the detected voltage Vb (k) and the voltage difference ΔV between the minimum voltage Vbmin and the threshold value Vr. Therefore, the determination is made with higher accuracy than the aspect of determining the necessity of equalization based on the difference in the SOC of each unit cell. It is possible. Further, the threshold value Vr is set so that the SOC width corresponding to the threshold value Vr becomes a constant value regardless of the minimum voltage Vbmin. Therefore, the threshold value Vr is effective only when the variation of the SOC exceeds a certain allowable level. The equalization process can be performed. Therefore, the equalization process can be performed efficiently with a smaller amount of discharge.

  In step S114, the transistor T (k) corresponding to the kth unit cell B (k) is turned on and a timer is started, and the timer B causes the unit cell B (k) to have a constant discharge charge amount Q (typically Specifically, the charge amount is set to be equal to or less than the SOC width corresponding to the threshold value Vr.) The discharge timing is detected, and the transistor T (k) is turned off at that timing. May be. According to such a configuration, an event in which the unit cell B (k) is excessively discharged (until the voltage falls below the minimum voltage Vmin) is avoided, so that the equalization process can be performed efficiently with a smaller amount of discharge. The threshold value Vr is set so that the corresponding SOC width becomes a constant value regardless of the minimum voltage Vbmin. Therefore, when the discharge charge amount reaches the constant value Q in this way, the transistor T (k) is turned on. It is suitable for controlling to turn off.

In order to confirm the application effect of the present invention, the following experiment was conducted.
That is, the positive and negative electrode sheets each holding the positive electrode active material and the negative electrode active material on the sheet-like positive electrode current collector and the negative electrode current collector are wound through the separator sheet and accommodated in the case together with the electrolyte. An 18650 type lithium ion battery was prepared. The positive electrode current collector of the lithium ion battery prepared here is an aluminum foil, the negative electrode current collector is a copper foil, the positive electrode active material is lithium nickelate (LiNiO ₂ ), and the negative electrode active material is artificial graphite. The electrolyte of the lithium ion battery has a composition containing LiPF ₆ at a concentration of 1 mol / L in a 3: 7 (volume ratio) mixed solvent of ethylene carbonate and diethyl carbonate. FIG. 3 shows an SOC-OCV characteristic curve of the lithium ion battery used in this experimental example.

<Example 1>
Six lithium ion batteries having the above-described configuration (hereinafter, these may be referred to as Ba1, Ba2, Bb1, Bb2, Bc1, and Bc2), two sets of three (first set: Ba1 and Ba2, second set) : Bb1 and Bb2, and the third set: Bc1 and Bc2).
Among them, the first set of batteries Ba1 and Ba2 was adjusted so that the SOC was 60%, and these lithium ion batteries (unit cells) were connected in series to form the assembled battery 10 having the configuration shown in FIG. This assembled battery 10 is connected to a voltage equalizing apparatus 20 having the configuration shown in FIG. 1 and performing a voltage equalizing process according to the flowchart shown in FIG. The voltage equalizing apparatus 20 is set to read (adopt) 5 mV as the threshold value Vr corresponding to the allowable range of SOC when the minimum voltage Vmin in FIG. 2 is a voltage corresponding to SOC 60%. In other words, the equalization process is set to start when the voltage difference between the single cells Ba1 and Ba2 becomes 5 mV or more.
For the power supply system 1 having such a configuration, the number of times equalization processing was performed for 30 days at 25 ° C., and the total amount of charges discharged for the equalization processing (total discharge amount) were examined. Here, the total discharge amount is represented by the difference (ΔSOC (%)) between the SOC at the start of the experiment (here, 60%) and the SOC at the end of the experiment. As a result, the number of equalization treatments in this example was 2, and the total discharge amount ΔSOC was 3%.

<Example 2>
The second set of batteries Bb1 and Bb2 were adjusted so that the SOC was 20%, and these lithium ion batteries (single cells) were connected in series to obtain the assembled battery 10 having the configuration shown in FIG. This assembled battery 10 is connected to a voltage equalizing apparatus 20 having the configuration shown in FIG. 1 and performing a voltage equalizing process according to the flowchart shown in FIG. Here, in the SOC-OCV characteristic curve shown in FIG. 3, the slope of the curve when the SOC is 20% (the slope of the tangent line) is almost twice the slope when the SOC is 60%. Therefore, when the minimum voltage Vmin in FIG. 2 is a voltage corresponding to SOC 20%, the voltage equalization apparatus 20 of this example reads 10 mV as the threshold value Vr corresponding to the allowable width of SOC (the same width as in Example 1) ( To be adopted).
For the power supply system 1 having such a configuration, the number of equalization processes and the total discharge amount were examined in the same manner as in Example 1. As a result, the number of equalization treatments was 3, and the total discharge amount ΔSOC was 3%.

<Example 3>
The third set of batteries Bc1 and Bc2 was adjusted so that the SOC was 20%, and these lithium ion batteries (unit cells) were connected in series to form the assembled battery 10 having the configuration shown in FIG. This assembled battery 10 was connected to a voltage equalizing apparatus 20 having the configuration shown in FIG. However, the voltage equalization apparatus 20 according to the present example is set to read (adopt) 5 mV as the threshold value Vr regardless of the value of Vmin (that is, regardless of the gradient of the SOC-OCV characteristic curve). .
For the power supply system 1 having such a configuration, the number of equalization processes and the total discharge amount were examined in the same manner as in Example 1. As a result, the number of equalization treatments was 18, and the total discharge amount ΔSOC was 10%.

As shown in these experimental results, in the embodiment of Example 3, the SOC 60 has a relatively small gradient even though the battery is in a state of charge (SOC 20%) where the gradient of the SOC-OCV characteristic curve is relatively large. %, The same threshold value Vr (5 mV) as in Example 1 is adopted as the equalization process start condition. Therefore, even if the degree of variation in the SOC is still within the allowable range of the SOC in Example 1, the equalization process is performed. Started. As a result, in the embodiment of Example 3, the number of equalization treatments was remarkably increased compared to Example 1, and a clearly larger amount of charge was discharged (consumed) for equalization than in Example 1.
On the other hand, in the aspect of Example 2, the threshold value Vr (10 mV), which is the same as that in the case where the allowable SOC range is 60% SOC, is adopted as the equalization processing start condition according to the gradient of the SOC-OCV characteristic curve. Thus, the equalization process is started only when the variation of the SOC becomes larger than the allowable range. As a result, in the aspect of Example 2, it was confirmed that the number of equalization processes was suppressed compared to Example 3, and that equalization was appropriately performed with a smaller discharge amount.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment and Example are only illustrations and what changed and changed the above-mentioned specific example is contained in the invention disclosed here.

It is explanatory drawing which shows typically schematic structure of the voltage equalization apparatus of the assembled battery which concerns on one Embodiment. It is a flowchart which illustrates one operation | movement form of the voltage equalization apparatus of the assembled battery which concerns on one Embodiment. It is a graph which shows the SOC-OCV characteristic curve of the lithium ion battery (unit cell) used in the experiment example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Power supply system 10 Assembly battery 12 Load 14 Relay 20 Voltage equalization apparatus (voltage detection part, equalization process part)
30 Electronic control unit 32 CPU
34 ROM
36 RAM

Claims (4)

In an assembled battery in which a plurality of chargeable / dischargeable cells are connected in series, an equalizing device for equalizing the voltages of the plurality of cells,
A voltage detector for detecting the voltage of the unit cell;
An equalization processing unit that charges or discharges a unit cell having a voltage difference from a reference voltage Vbst, which is a reference voltage among the detected voltages, equal to or higher than a threshold value Vr to equalize the voltage;
With
Here, the threshold voltage Vr is set to a different value according to the reference voltage Vbst so as to alleviate a difference in the state of charge corresponding to the threshold value Vr due to the reference voltage Vbst. Device.   The lowest voltage Vbmin, which is the lowest voltage among the detected voltages, is set as the reference voltage Vbst, and the equalization processing unit discharges a cell whose voltage difference from the reference voltage Vbst is equal to or greater than a threshold Vr to The battery pack voltage equalization apparatus according to claim 1, wherein the voltage equalization apparatus is configured to equalize the battery pack.   A cell having a voltage difference from the reference voltage Vbst equal to or greater than a threshold value Vr is configured to charge or discharge a single charge by a constant charge amount regardless of the value of the reference voltage Vbst, thereby equalizing the voltage. The voltage equalization apparatus for an assembled battery according to claim 1 or 2.   The voltage equalization apparatus for an assembled battery according to any one of claims 1 to 3, wherein the single battery is a lithium ion battery.

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