Method for determining degradation of secondary battery

Abstract

To provide a method for determining degradation of a secondary battery which can determine an abnormal degradation of a positive electrode.SOLUTION: The method for determining degradation of a secondary battery includes: (a) acquiring a first internal resistance value of a battery when a current value is at least a first current value, subsequent to continuous discharge for a predetermined time; and (b) evaluating the relation between a first internal resistance value acquired in the step (a) and one of a second internal resistance value and the capacity maintenance rate of the battery when the current value is not larger than a second current value.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a method for determining deterioration of a secondary battery. [Background technology]

[0002] Patent Document 1 discloses a battery system that, in a secondary battery in which the battery voltage is more affected by the positive electrode potential than by the negative electrode potential, obtains the relationship between the resistance change rate and current in a secondary battery whose state of charge has been reduced, and, based on the obtained relationship, distinguishes between deterioration due to wear of the secondary battery and deterioration due to salt concentration distribution inside the secondary battery, thereby determining the deterioration state of the secondary battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2013 / 121466 Summary of the Invention [Problem to be solved by the invention]

[0004] It is known that the materials constituting a secondary battery deteriorate (deteriorate over time) depending on, for example, the temperature, SOC (State of Charge), and elapsed time of the secondary battery. In particular, the materials constituting the positive electrode may deteriorate abnormally.

[0005] An object of the present disclosure is to provide a method for determining deterioration of a secondary battery that can determine abnormal deterioration of a positive electrode. [Means for solving the problem]

[0006] The present inventors have found that the above problems can be solved by the following means. <Aspect 1> Methods for determining deterioration of secondary batteries, including: (a) obtaining a first internal resistance value of the battery following continuous discharge for a predetermined period of time at a current value equal to or greater than a first current value; (b) evaluating the relationship between the first internal resistance value obtained in step (a) and a second internal resistance value or a capacity retention rate of the battery when the current value is equal to or lower than a second current value. <Aspect 2> The method according to aspect 1, wherein in the step (b), a relationship between the first internal resistance value obtained in the step (a) and a second internal resistance value when the current value is equal to or less than a second current value is evaluated. <Aspect 3> The method according to aspect 1, wherein in the step (b), a relationship between the first internal resistance value obtained in the step (a) and a capacity retention rate of the battery is evaluated. Effect of the Invention

[0007] According to the present disclosure, it is possible to provide a method for determining deterioration of a secondary battery, which is capable of determining abnormal deterioration of a positive electrode. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a graph showing the relationship between discharge time and internal resistance. [Diagram 2] FIG. 2 is a flowchart showing a process for determining the deterioration state of a secondary battery from the relationship between the current and the resistance ratio. [Diagram 3] FIG. 3 is a graph showing the relationship between current and resistance ratio. [Figure 4] FIG. 4 is a flowchart showing a process for determining the degradation state of a secondary battery from the relationship between the capacity maintenance rate and the resistance change rate. [Diagram 5] FIG. 5 is a graph showing the relationship between the capacity retention rate and the rate of resistance change. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the present disclosure.

[0010] <Method for determining deterioration of secondary batteries> The disclosed method for determining deterioration of a secondary battery includes (a) obtaining a first internal resistance value of the battery when the current value is equal to or greater than a first current value following continuous discharge for a predetermined period of time, and (b) evaluating the relationship between the first internal resistance value obtained in step (a) and a second internal resistance value or a capacity retention rate of the battery when the current value is equal to or less than a second current value.

[0011] The present inventors have discovered that a secondary battery in which the positive electrode has abnormally deteriorated in addition to deterioration over time has a higher internal resistance when discharged at a large current for a long period of time, compared to a secondary battery that has only deteriorated over time, and have thus come up with the present disclosure.

[0012] The disclosed method for determining deterioration of a secondary battery includes (a) acquiring a first internal resistance value Rp when the current value is equal to or greater than a first current value Ip following continuous discharge for a predetermined time, particularly acquiring a first internal resistance value Rp when the current value is equal to or greater than a first current value Ip following continuous discharge at a predetermined current value for a predetermined time. Here, the "predetermined current value" may be the same as the first current value Ip, and may be, for example, 150A or more, 200A or more, or 250A or more, and may be 500A or less, 400A or less, or 300A or less.

[0013] The discharge time T can be measured, for example, by a timer. The "predetermined time" can be, for example, a time in which the length of the discharge time T is equal to or longer than a reference time Tp that can distinguish the deterioration state of the secondary battery verified in advance by a test or the like. For example, as shown in FIG. 1, under a predetermined condition, when the discharge time T is 10 seconds, the internal resistance of a battery whose positive electrode has abnormally deteriorated becomes significantly larger than that of a secondary battery in an initial state and a secondary battery that has deteriorated over time. In this case, the reference time Tp can be set to 10 seconds. Here, the initial state is a state that serves as a reference when evaluating the deterioration of a secondary battery, and can be, for example, a state immediately after the secondary battery is manufactured. Note that no deterioration of the secondary battery occurs in the state immediately after the secondary battery is manufactured.

[0014] The first current value Ip can be any value that can determine the deterioration state of the battery. The first current value Ip may be, for example, 150 A or more, 200 A or more, or 250 A or more, and may be 500 A or less, 400 A or less, or 300 A or less.

[0015] In the method of the present disclosure relating to the first embodiment, step (b) includes evaluating the relationship between the first internal resistance value Rp obtained in step (a) and the second internal resistance value Rd when the current value is equal to or less than the second current value Id.

[0016] That is, the deterioration of a secondary battery can be evaluated using the resistance ratio, which is a value obtained by dividing a first internal resistance value Rp when the current value is equal to or greater than a first current value Ip by a second internal resistance value Rd when the current value is equal to or less than a second current value Id.

[0017] Here, the second current value Id may be the current value of the secondary battery in an initial state.

[0018] The system for determining the deterioration of a secondary battery includes an ECU, which typically includes at least a ROM (Read Only Memory) that stores a program for performing such control, a CPU (Central Processing Unit) capable of executing the program, a RAM (Random Access Memory) that temporarily stores data, and an input / output port.

[0019] Various signals from a voltage sensor, a current sensor, a temperature sensor, etc. are input to the ECU via an input port. In addition, drive signals to loads (power consumers and / or power suppliers) etc. are output from the ECU via an output port.

[0020] The ECU constitutes a discharge time determination means, an internal resistance acquisition means, and a discrimination means.

[0021] The discharge time determination means is configured to determine whether the length of the discharge time T is equal to or longer than a reference time Tp that can determine the deterioration state of the secondary battery. The method for determining the length of the discharge time T is not particularly limited, and may be, for example, a method of comparing whether the length of the discharge time T measured by a timer is equal to or longer than a reference time Tp that is previously verified by a test or the like. The current value during discharge may be a first current value Ip.

[0022] The internal resistance acquisition means is configured to acquire (estimate) the internal resistance of the secondary battery. The method of acquiring the internal resistance is not particularly limited, and any conventionally known method commonly used in general battery systems may be adopted. For example, a method of estimating the internal resistance by dividing the voltage change during charging and discharging by the current change at that time based on various data detected by a voltage sensor and a current sensor (for example, a method of linearly approximating a parameter of the current change amount and a parameter based on the voltage change amount and impedance change amount, and calculating the slope of the approximated straight line as the impedance of the battery) is exemplified.

[0023] The internal resistance acquiring means can acquire a first internal resistance value Rp when the current value is equal to or greater than a first current value Ip, and a second internal resistance value Rd when the current value is equal to or less than a second current value Id.

[0024] The determining means is configured to determine the deterioration state of the secondary battery based on information relating to the first internal resistance value Rp.

[0025] In the first embodiment, the determination means specifies the relationship between the current and the resistance ratio of the secondary battery. Specifically, the determination means calculates the resistance ratio for each current while changing the current of the secondary battery. The obtained current and resistance ratio are plotted on a coordinate system to obtain a map showing the relationship between the current and the resistance ratio. Then, the deterioration state of the secondary battery is determined from the relationship between the current and the resistance ratio by referring to this map. This map can be created in advance by performing a preliminary experiment or the like.

[0026] Next, the process for determining the degradation state of the secondary battery in the first embodiment will be described with reference to the flowchart shown in Fig. 2. The process shown in Fig. 2 is executed by the ECU.

[0027] In step S101, the ECU starts discharging the secondary battery.

[0028] In step S102, the ECU determines whether the length of the discharge time T is equal to or longer than the reference time Tp. If the length of the discharge time T is equal to or longer than the reference time Tp, the ECU executes the process of step S103. If the length of the discharge time T is equal to or shorter than the reference time Tp, the ECU executes the process of step S101 again. The current value during discharge may be a first current value Ip.

[0029] In step S103, the ECU acquires a first internal resistance value Rp for the secondary battery. The first internal resistance value Rp can be calculated from the voltage of the secondary battery and a current value equal to or greater than a first current value Ip. The ECU can acquire the voltage of the secondary battery based on the output of a voltage sensor. Also, the ECU can acquire the current of the secondary battery based on the output of a current sensor.

[0030] In step S104, the ECU acquires the resistance ratio using the first internal resistance value Rp calculated in step S103. Here, the second internal resistance value Rd can be obtained in advance by an experiment or the like and stored in the ROM. In this case, the ECU can calculate the resistance ratio by performing a process of dividing the first internal resistance value Rp calculated in step S103 by the second internal resistance value Rd read from the ROM.

[0031] Since the internal resistance value of a secondary battery may change depending on the temperature of the secondary battery, if information indicating the relationship between the internal resistance value and the temperature is obtained in advance, the internal resistance value corresponding to this temperature can be specified by acquiring the temperature of the secondary battery. The temperature of the secondary battery can be acquired using a temperature sensor. The information indicating the relationship between the internal resistance value and the temperature can be stored in a ROM. The information indicating the relationship between the internal resistance value and the temperature can be expressed as a map or a function.

[0032] In step S105, the ECU acquires the relationship between the current of the secondary battery and the resistance ratio. Specifically, the ECU calculates the resistance ratio for each current while changing the current of the secondary battery. That is, the ECU calculates a first internal resistance value Rp for each current, and calculates the resistance ratio based on the first internal resistance value Rp and the second internal resistance value Rd. The relationship between the current and the resistance ratio is obtained by plotting the acquired current and resistance ratio on the coordinate system shown in FIG. 3.

[0033] In step S106, the ECU determines whether the resistance ratio increases when a large current flows. For example, the ECU determines whether the resistance ratio when a large current flows is greater than a threshold value. At this time, if the resistance ratio when a large current flows is greater than the threshold value, it can be determined that the positive electrode of the secondary battery is abnormally degraded (S107). On the other hand, if the resistance ratio when a large current flows is smaller than the threshold value, it can be determined that the positive electrode of the secondary battery is not abnormally degraded.

[0034] Here, the threshold value can be any resistance ratio value at a current value at which the deterioration state of the secondary battery cannot be determined. For example, as shown in FIG. 3, the resistance ratio when the current value is 60 A can be set as 1, and this value can be used as the threshold value. In this case, if the resistance ratio significantly exceeds the threshold value, it can be determined that the positive electrode of the secondary battery is abnormally deteriorated. In FIG. 3, this corresponds to the case where a current of 250 A flows. In contrast, such an increase in the resistance ratio does not occur in a battery whose positive electrode is not abnormally deteriorated.

[0035] Another method for determining the deterioration state of a secondary battery from the relationship between current and resistance ratio is to compare it with a map of a secondary battery that has deteriorated over time. That is, a map showing the relationship between current and resistance ratio of a secondary battery that has deteriorated over time is created in advance, and this map is compared with a map on which calculated values ​​are plotted, so that the deterioration state of the secondary battery can be determined. As shown in FIG. 3, in a secondary battery whose positive electrode is abnormally deteriorated, the resistance ratio when a large current flows is larger than the resistance ratio when a small current flows. In contrast, in a battery whose positive electrode is not abnormally deteriorated, such an increase in the resistance ratio does not occur.

[0036] In this way, when the resistance ratio increases when a large current flows, the ECU determines that the positive electrode of the secondary battery is abnormally degraded (S107). On the other hand, when the resistance ratio does not increase when a large current flows, the ECU determines that the positive electrode of the secondary battery is not abnormally degraded, and ends the process shown in FIG.

[0037] In the method of the present disclosure according to the second embodiment, in step (b), the relationship between the first internal resistance value obtained in step (a) and the capacity retention rate of the battery is evaluated.

[0038] In the second embodiment, the ECU further includes a battery capacity acquisition means.

[0039] The battery capacity acquisition means is configured to acquire (estimate) the chargeable and dischargeable battery capacity of the secondary battery. The method of acquiring the battery capacity is not particularly limited, and a conventionally known method commonly used in a general battery system may be adopted. For example, a method of estimating the battery capacity of the secondary battery according to a battery model formula based on various data detected by a voltage sensor, a current sensor, a temperature sensor, etc. (for example, a method of storing previously obtained open circuit voltage characteristics of the positive and negative electrodes of the secondary battery, extracting the amount of active material, capacity density, and resistance value of the positive and negative electrodes by referring to the stored data and data detected by the voltage sensor, the current sensor, and the temperature sensor, and estimating the battery capacity of the secondary battery using the extracted parameters) is exemplified.

[0040] The determination means in the second embodiment calculates the capacity maintenance rate by subtracting the acquired battery capacity from the initial capacity. The determination means also calculates the resistance change rate by subtracting the initial internal resistance value from the acquired internal resistance value. Then, the deterioration state of the secondary battery is determined by referring to a map showing the relationship between the capacity maintenance rate and the resistance change rate of a secondary battery that has deteriorated over time as shown in FIG. 5. This map can be created in advance by conducting a preliminary experiment or the like. The initial capacity and the initial internal resistance value can also be obtained in advance by conducting a preliminary experiment or the like.

[0041] Next, a process for determining deterioration of a secondary battery in the second embodiment will be described with reference to the flowchart shown in Fig. 4. The process shown in Fig. 4 is executed by the ECU.

[0042] In step S201, the ECU acquires the battery capacity of the secondary battery. For example, the battery capacity can be estimated according to a battery model formula based on data detected by a voltage sensor, a current sensor, and a temperature sensor.

[0043] In step 202, the ECU starts discharging the secondary battery.

[0044] In step S203, the ECU determines whether the length of the discharge time T is equal to or longer than the reference time Tp. If the length of the discharge time T is equal to or longer than the reference time Tp, the ECU executes the process of step S204. If the length of the discharge time T is equal to or shorter than the reference time Tp, the ECU executes the process of step S202 again. The current value during discharge may be the first current value Ip.

[0045] In step 204, the ECU obtains a first internal resistance value Rp when the current value is a first current value Ip. For example, the ECU can estimate the internal resistance value by dividing the voltage change during charging and discharging by the current change at that time, based on various data detected by the voltage sensor and the current sensor.

[0046] In step S205, the ECU calculates a capacity maintenance rate (ΔAh) by subtracting the acquired battery capacity from the initial capacity. The ECU also calculates a resistance change rate (ΔR) by subtracting the initial internal resistance value from the acquired internal resistance value. The ECU obtains the relationship between the capacity maintenance rate and the resistance change rate based on the calculated value.

[0047] In step S206, the ECU refers to a map showing the relationship between the capacity maintenance rate and the resistance change rate of a secondary battery that has deteriorated over time, which is stored in the ROM, to determine the deterioration state of the secondary battery. Specifically, as shown in Fig. 5, when the resistance change rate at low capacity in the map showing the relationship acquired in step S205 is larger than the resistance change rate at low capacity in the map related to the secondary battery that has deteriorated over time, the ECU determines that the positive electrode has abnormally deteriorated (S207). In contrast, when the ECU compares the maps and there is no difference in the resistance change rate at low capacity, the ECU determines that the positive electrode has not abnormally deteriorated, and ends the process shown in Fig. 4.

[0048] The secondary battery may be, for example, a nickel-metal hydride battery or a lithium-ion battery. The secondary battery may be mounted in, for example, a vehicle, and the vehicle may be driven using the output of the secondary battery.

Claims

1. A method for determining deterioration of a secondary battery, including: (a) obtaining a first internal resistance value of the battery following continuous discharge for a predetermined period of time at a current value equal to or greater than a first current value; (b) evaluating the relationship between the first internal resistance value obtained in the step (a) and a second internal resistance value or a capacity retention rate of the battery when the current value is equal to or less than a second current value.

2. 2. The method according to claim 1, wherein in the step (b), a relationship between the first internal resistance value obtained in the step (a) and a second internal resistance value when the current value is equal to or less than a second current value is evaluated.

3. The method according to claim 1 , wherein in the step (b), a relationship between the first internal resistance value obtained in the step (a) and a capacity retention rate of the battery is evaluated.

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