Battery System

Abstract

To provide a battery system in which deformation of a restraining member can be suppressed and thermal chain of a battery cell can be suppressed.SOLUTION: The charging control of an assembled battery is performed using a set upper limit SOC (set upper limit SOC determined in step ST7 or step ST8) which is the lower upper limit SOC from among a first upper limit SOC (first upper limit SOC calculated in step ST4) that prevents the restraining load of the restraining member due to cell expansion that changes depending on the degree of deterioration of the battery cell from exceeding a predetermined threshold, and a second upper limit SOC that can suppress the occurrence of heat chain (second upper limit SOC calculated in step ST5).SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a battery system, and more particularly to an improvement for suppressing deformation of a restraining member and suppressing thermal chain reaction of battery cells. [Background technology]

[0002] Conventionally, battery modules (batteries) installed in vehicles such as electric vehicles and hybrid vehicles are configured by stacking multiple battery cells, with the stack of battery cells being constrained in the stacking direction by a constraining member. It is known that the higher the state of charge of a battery cell, the greater the expansion (cell expansion), and that even at the same state of charge, the greater the degree of deterioration (the greater the degree of deterioration due to long-term use), the greater the cell expansion.

[0003] Patent Document 1 discloses that the upper limit SOC (State of Charge) is set lower as the deterioration level of the battery cells increases, in order to suppress deformation of the restraining member due to this cell expansion. Patent Document 1 also discloses that the restraining load is estimated from the deterioration level, temperature, and SOC of the battery cells, and fatigue failure of the restraining member is determined based on the cumulative damage amount calculated from the history of the restraining load. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2020-119820 Summary of the Invention [Problem to be solved by the invention]

[0005] Meanwhile, inter-cell separators (made of elastic or insulating material) are interposed between battery cells. If a battery cell (trigger cell) experiences thermal runaway, the inter-cell separator will be exposed to high temperatures, and the loss of the inter-cell separator or its loss of elasticity will reduce the thermal resistance between the battery cells. Furthermore, if the inter-cell separator is compressed due to an increase in the restraining load caused by cell expansion, the thermal resistance between the battery cells will also decrease. In such a situation, there is a concern that heat transfer to adjacent cells could trigger a thermal chain reaction, causing each battery cell to experience thermal runaway in turn.

[0006] In Patent Document 1, by setting the upper limit SOC low to suppress deformation of the restraining member, it is expected that the energy of the battery cell will be reduced and the amount of heat generated during thermal runaway will be reduced, but it is difficult to say that the function of suppressing thermal chain reaction is guaranteed.

[0007] In addition, while it is possible to use high-performance inter-cell separators (for example, those that can maintain high thermal insulation performance) to suppress thermal chain reaction, this would be impractical in terms of securing the mounting space for the battery cells and cost. For this reason, in the prior art, a highly practical technology has not been established to suppress deformation of the restraining member and suppress thermal chain reaction of the battery cells.

[0008] The present invention has been made in consideration of the above points, and its purpose is to provide a battery system that can suppress deformation of restraining members and suppress thermal chain reaction of battery cells. [Means for solving the problem]

[0009] The solution of the present invention to achieve the above object is based on a battery system including an assembled battery formed by stacking a plurality of battery cells and constraining them in the stacking direction by constraint members, and is characterized by comprising: a first upper limit SOC calculation unit that calculates a first upper limit SOC so that the constraint load of the constraint members, which is caused by cell expansion that changes depending on the deterioration level of the battery cells, does not exceed a predetermined threshold; a second upper limit SOC calculation unit that calculates a second upper limit SOC that can suppress the occurrence of a thermal chain reaction based on the likelihood of the occurrence of a thermal chain reaction that changes depending on the deterioration level of the battery cells; and a charge control unit that controls charging of the assembled battery using the lower upper limit SOC of the first upper limit SOC and the second upper limit SOC.

[0010] With this specification, charging of the battery pack is controlled using the lower upper limit SOC of the first upper limit SOC (the upper limit SOC that prevents the restraining load of the restraining member from exceeding a predetermined threshold) and the second upper limit SOC (the upper limit SOC that can suppress the occurrence of thermal chain reaction).Therefore, by keeping the restraining load low, deformation of the restraining member can be suppressed and thermal chain reaction of the battery cells can be suppressed. [Effects of the Invention]

[0011] In the present invention, charging of the battery pack is controlled using the lower of the first upper limit SOC calculated so that the restraining load of the restraining members does not exceed a predetermined threshold and the second upper limit SOC calculated so that the occurrence of thermal chain reaction can be suppressed, thereby suppressing deformation of the restraining members and thermal chain reaction of the battery cells. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a block diagram showing a schematic configuration of a battery system according to an embodiment; [Figure 2] FIG. 2(a) is a diagram showing an example of the relationship between SOC and restraint load depending on the degree of deterioration of the battery cell, FIG. 2(b) is a diagram showing an example of a first upper limit SOC calculation map, and FIG. 2(c) is a diagram showing the result of a thermal chain reaction test, showing the relationship between the degree of deterioration and SOC that enables the suppression of thermal chain reaction. [Figure 3] FIG. 4 is a flowchart illustrating the procedure for determining a set upper limit SOC in the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this embodiment, the present invention will be described as being applied to a battery system including a battery pack (battery module) mounted on an electric vehicle. The vehicle to which the battery system according to the present invention is applied is not limited to an electric vehicle, but may also be a hybrid vehicle, a plug-in hybrid vehicle, or the like.

[0014] Although not shown, the battery pack is configured such that multiple battery cells are stacked together, and the stack of battery cells is constrained in the stacking direction by restraining members. More specifically, inter-cell separators (made of elastic material or heat insulating material) are interposed between the battery cells. End plates are also disposed on both ends of the stack in the stacking direction, and the stack formed by stacking each battery cell, each inter-cell separator, and each end plate is constrained from both sides in the stacking direction by restraining members. The restraining members are configured, for example, by four restraining bands.

[0015] -Battery system configuration overview- Fig. 1 is a block diagram showing a schematic configuration of a battery system 1 according to this embodiment. As shown in Fig. 1, the battery system 1 includes a monitoring unit 2, a battery ECU 3, and a battery pack 4 whose charging is controlled by the battery ECU 3.

[0016] The monitoring unit 2 is equipped with a current sensor 21, a voltage sensor 22, and a temperature sensor 23. The current sensor 21 detects the value of current input to and output from the battery pack 4. The voltage sensor 22 detects the voltage value of the battery pack 4. The current sensor 21 and voltage sensor 22 are disposed, for example, in the electrical circuit of the battery pack 4. The temperature sensor 23 detects the temperature of the battery pack 4. The temperature sensor 23 is disposed, for example, inside the battery pack 4, near a battery cell that is particularly prone to becoming hot. The detected values of the sensors 21, 22, 23 are output to the battery ECU 3.

[0017] The battery ECU 3 includes a control unit 31 and a storage unit 32. The control unit 31 includes a CPU (Central Processing Unit), and the storage unit 32 includes a RAM (Random-Access Memory) and a ROM (Read-Only Memory). The storage unit 32 stores programs and various data for operating the control unit 31. The control unit 31 executes the operation of determining a set upper limit SOC and charge control, which will be described later, in accordance with the programs.

[0018] The control unit 31 includes, as functional units realized by executing the program, an information receiving unit 33, an SOC calculation unit 34, a cell deterioration level calculation unit 35, a first upper limit SOC calculation unit 36, a second upper limit SOC calculation unit 37, a set upper limit SOC determination processing unit 38, and a charge control unit 39. Below, an outline of the functions of each of these units will be described.

[0019] The information receiving section 33 is capable of receiving, from the monitoring unit 2, current value information from the current sensor 21, voltage value information from the voltage sensor 22, and temperature information from the temperature sensor .

[0020] The SOC calculation unit 34 calculates the SOC of the battery pack 4 using at least some of the current value information, voltage value information, and temperature information. The SOC is a value that indicates the current amount of stored electricity relative to the full charge capacity of the battery pack 4 as a percentage. Methods for calculating the SOC are well known, so a description thereof will be omitted here.

[0021] The cell deterioration level calculation unit 35 calculates the deterioration level indicating the degree of deterioration of the battery cells (battery assembly 4). This deterioration level is calculated as an index correlated with the amount of expansion of the battery cells that have expanded due to long-term use of the battery assembly 4 and the amount of gas generated within the battery cells, and the deterioration level increases as the battery assembly 4 is used for a longer period of time.

[0022] In this embodiment, the degree of deterioration is calculated by multiplying a quantity correlated with the amount of expansion and gas generation of the battery cell by a correlation coefficient. As the quantity correlated with the amount of expansion and gas generation of the battery cell, for example, the capacity maintenance rate of the battery pack 4 can be used. The capacity maintenance rate is the ratio of the current full charge capacity to the full charge capacity in the initial state (when new) of the battery pack 4. The correlation coefficient is determined in advance by experiment or the like. Alternatively, the degree of deterioration may be calculated using, for example, an integral value of the discharge current from the initial state of the battery pack 4.

[0023] The first upper limit SOC calculation unit 36 calculates a first upper limit SOC that prevents the restraining load of the restraining member caused by cell expansion from exceeding a predetermined threshold (referred to in the present invention as the first upper limit SOC that prevents the restraining load of the restraining member caused by cell expansion, which changes depending on the degree of deterioration of the battery cell, from exceeding a predetermined threshold) based on the degree of deterioration calculated by the cell deterioration degree calculation unit 35. The first upper limit SOC calculation unit 36 calculates the first upper limit SOC so that the first upper limit SOC decreases as the degree of deterioration increases.

[0024] FIG. 2(a) is a diagram showing an example of the relationship between the SOC and the restraining load imposed by the restraining member for different degrees of deterioration of the battery cells. Note that FIG. 2(a) shows the relationship when the temperature of the battery pack 4 is a predetermined value. In FIG. 2(a), the horizontal axis represents the SOC of the battery pack 4, and the vertical axis represents the restraining load imposed by the restraining member. The restraining load is the load generated between the stack and the restraining member due to expansion of the battery cells. Lines D1 to D3 show examples of the relationship between the SOC and the restraining load for different degrees of deterioration, with the degree of deterioration increasing in the order of lines D1, D2, and D3. Note that although not shown in FIG. 2(a), the restraining load also changes depending on the temperature of the battery pack 4, and for each of lines D1 to D3, the restraining load for the same SOC increases as the temperature increases.

[0025] The restraint load threshold FU indicates the upper limit of the restraint load, and is set as a value that indicates the possibility of the restraint member being damaged if a restraint load exceeding this limit occurs. The threshold FU is determined appropriately based on the strength of the restraint member.

[0026] The upper limit SOC value k3 indicates the SOC at which the restraint load becomes the threshold value FU when the degree of deterioration is high as indicated by line D3. The upper limit SOC value k2 (k2>k3) indicates the SOC at which the restraint load becomes the threshold value FU when the degree of deterioration is medium as indicated by line D2. The upper limit SOC value k1 (k1>k2) indicates the SOC at which the restraint load becomes the threshold value FU when the degree of deterioration is low as indicated by line D1. In this way, the higher the degree of deterioration, the lower the upper limit SOC. In this way, the relationship between the SOC and the restraint load is evaluated using battery cells with different degrees of deterioration, and this relationship is stored in the ROM of the storage unit 32 as a map or a relational expression.

[0027] 2(b) is a diagram showing an example of the first upper limit SOC calculation map created in this manner. As shown in FIG. 2(b), in this embodiment, when the deterioration level of the battery cell is in the range Da, the first upper limit SOC is calculated as a value exceeding k1. When the deterioration level is in the range Db, the first upper limit SOC is calculated as a value between k1 and k2 (k1 in the diagram). When the deterioration level is in the range Dc, the first upper limit SOC is calculated as a value between k2 and k3 (k2 in the diagram). When the deterioration level is in a range exceeding Dd, the first upper limit SOC is calculated as a value equal to or less than k3 (k3 in the diagram). Note that the value of the first upper limit SOC in each of the deterioration level ranges Da to Dd may be a constant value for each of the deterioration level ranges Da to Dd as shown in FIG. 2(b), or may change as the deterioration level changes for each of the deterioration level ranges Da to Dd (the higher the deterioration level, the lower the first upper limit SOC).

[0028] The second upper limit SOC calculation unit 37 calculates a second upper limit SOC that can suppress the occurrence of a thermal chain reaction in each battery cell based on the degree of deterioration calculated by the cell deterioration level calculation unit 35. (In the present invention, the second upper limit SOC that can suppress the occurrence of a thermal chain reaction is based on the likelihood of the occurrence of a thermal chain reaction that changes depending on the degree of deterioration of the battery cell.) The second upper limit SOC calculation unit 37 calculates the second upper limit SOC so that the second upper limit SOC decreases as the degree of deterioration increases.

[0029] 2(c) is a diagram showing the results of a thermal chain reaction test, illustrating the relationship between the degree of degradation that enables the suppression of thermal chain reaction and the SOC (thermal chain reaction initiation SOC). As shown in Fig. 2(c), a mini-module was used in which multiple battery cells with a certain degree of degradation were assembled under a restraint load (threshold) FU, and a thermal chain reaction test was conducted under varying thermal chain reaction initiation SOC conditions (causing a trigger cell to go into thermal runaway to determine whether adjacent cells would also go into thermal runaway). A map showing the relationship between the degree of degradation that enables the suppression of thermal chain reaction and the SOC was created, and this map was stored in the ROM of the memory unit 32.

[0030] The set upper limit SOC determination processing unit 38 receives information on the first upper limit SOC calculated by the first upper limit SOC calculation unit 36 and information on the second upper limit SOC calculated by the second upper limit SOC calculation unit 37, and compares these upper limit SOC values (more specifically, determines whether the first upper limit SOC is lower than the second upper limit SOC) to select the lower upper limit SOC (hereinafter referred to as the set upper limit SOC).

[0031] The charging control unit 39 controls charging of the battery pack 4 using the set upper limit SOC determined by the set upper limit SOC determination processing unit 38. For example, when charging the battery pack 4 from an external power source or when regenerative charging is performed while the vehicle is running, the charging control is performed with the set upper limit SOC as the upper limit SOC (the SOC at which charging is stopped).

[0032] -Determination of the set upper limit SOC- Next, the procedure for determining the set upper limit SOC will be described with reference to the flowchart in Fig. 3. First, in step ST1, the information receiving unit 33 receives current value information from the current sensor 21, voltage value information from the voltage sensor 22, and temperature information from the temperature sensor 23 from the monitoring unit 2.

[0033] Next, in step ST2, the SOC calculation unit 34 uses the above-mentioned information to calculate the SOC of the battery pack 4. Then, in step ST3, the cell deterioration level calculation unit 35 calculates the deterioration level of the battery cells.

[0034] After the deterioration level of the battery cell is calculated in this manner, in step ST4, the first upper limit SOC calculation unit 36 calculates a first upper limit SOC that prevents the restraining load of the restraining member caused by cell expansion from exceeding a predetermined threshold. In step ST5, the second upper limit SOC calculation unit 37 calculates a second upper limit SOC that can suppress the occurrence of thermal chain reaction in each battery cell.

[0035] After the first upper limit SOC and the second upper limit SOC are calculated in this manner, in step ST6, the set upper limit SOC determination processing unit 38 determines whether the first upper limit SOC is lower than the second upper limit SOC.

[0036] If the first upper limit SOC is lower than the second upper limit SOC and the result of the judgment in step ST6 is YES, the process proceeds to step ST7, where the set upper limit SOC for charging control by the charging control unit 39 is determined to be the first upper limit SOC (the upper limit SOC that prevents the restraining load of the restraining member from exceeding a predetermined threshold).

[0037] On the other hand, if the first upper limit SOC is equal to or greater than the second upper limit SOC and the result of step ST6 is NO, the process proceeds to step ST8, where the second upper limit SOC (the upper limit SOC that can suppress the occurrence of thermal chain reaction) is determined as the set upper limit SOC for charging control by the charging control unit 39.

[0038] By repeating the above operation, charging control of the battery pack 4 is performed while updating the set upper limit SOC according to the degree of deterioration of the battery cells.

[0039] -Effects of the embodiment- As described above, in this embodiment, charging of the battery pack 4 is controlled using the lower upper limit SOC (set upper limit SOC) between the first upper limit SOC, which is set so that the restraining load of the restraining members does not exceed a predetermined threshold, and the second upper limit SOC, which is set so that the occurrence of thermal chain reaction is suppressed. Therefore, in both cases where the first upper limit SOC is higher than the second upper limit SOC and where the second upper limit SOC is higher than the first upper limit SOC, deformation of the restraining members can be suppressed and thermal chain reaction of the battery cells can be suppressed by keeping the restraining load low.

[0040] Furthermore, in this embodiment, there is no need to use a high-performance inter-cell separator (for example, one that can maintain high thermal insulation performance) to suppress thermal chain reaction as the inter-cell separator interposed between the battery cells. In other words, there is no need to use an inter-cell separator with high thermal insulation properties or an inter-cell separator with a large thickness, so there is no need to improve the structure to ensure mounting space for the battery cells, and costs can be reduced, making it possible to provide a battery system 1 that is highly practical for suppressing deformation of the restraining members and suppressing thermal chain reaction of the battery cells.

[0041] -Other embodiments- The present invention is not limited to the above-described embodiments, and all modifications and applications within the scope of the claims and equivalents thereto are possible.

[0042] For example, the above embodiment has been described with reference to a single battery pack 4 restrained by a restraining member. However, a battery pack actually mounted on a vehicle is configured to include multiple battery packs 4. In this case, it is preferable to determine the set upper limit SOC for each battery pack 4 and perform charging control using the lowest set upper limit SOC among these set upper limit SOCs. [Industrial Applicability]

[0043] The present invention is applicable to a battery system including a battery pack mounted on an electric vehicle or the like. [Explanation of symbols]

[0044] 1 Battery System 36 First upper limit SOC calculation unit 37 Second upper limit SOC calculation unit 39 Charging control unit

Claims

[Claim 1] A battery system including a battery pack formed by stacking a plurality of battery cells and constraining them in the stacking direction by a constraining member, a first upper limit SOC calculation unit that calculates a first upper limit SOC so that a restraining load of the restraining member caused by cell expansion that changes depending on a deterioration level of the battery cell does not exceed a predetermined threshold; a second upper limit SOC calculation unit that calculates a second upper limit SOC that can suppress the occurrence of a thermal chain reaction based on the likelihood of the thermal chain reaction occurring, which varies depending on the degree of deterioration of the battery cell; a charge control unit that controls charging of the battery pack using the lower upper limit SOC of the first upper limit SOC and the second upper limit SOC.

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