Battery system

Abstract

This suppresses short circuits caused by metallic foreign objects mixed into secondary batteries. [Solution] The battery system comprises a secondary battery containing an electrolyte and a control unit that controls the discharge of the secondary battery. The control unit calculates an index value indicating the amount of dissolved metallic foreign matter in the electrolyte based on the history of the secondary battery's temperature and state of charge (SOC) during the secondary battery's usage period. The control unit discharges the secondary battery when the index value exceeds a predetermined threshold.

Description

Technical Field

[0001] The present disclosure relates to a battery system.

Background Art

[0002] JP 2006-269245 A discloses that by performing at least one charge after the secondary battery is configured and then performing discharge, metallic foreign matter mixed into the secondary battery during assembly of the secondary battery is dissolved and diffused.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problem to be Solved by the Invention

[0004] If the metallic foreign matter exists in a state where it is difficult to dissolve during the aging treatment, sufficient conduction cannot be obtained for the metallic foreign matter, and the metallic foreign matter may remain when the secondary battery is shipped. As the secondary battery is used in the market, the metallic foreign matter gradually dissolves and precipitates on the negative electrode, and the precipitated metallic foreign matter reaches the positive electrode, which may cause a short circuit.

[0005] The present disclosure has been made to solve the above problem, and one of the objects of the present disclosure is to suppress a short circuit caused by metallic foreign matter mixed into a secondary battery.

Means for Solving the Problem

[0006] A battery system relating to a certain aspect of this disclosure comprises a secondary battery containing an electrolyte and a control unit that controls the discharge of the secondary battery. The control unit calculates an index value indicating the amount of metal foreign matter dissolved in the electrolyte based on the history of the secondary battery's temperature and State of Charge (SOC) during the secondary battery's service life. The control unit discharges the secondary battery when the index value exceeds a predetermined threshold. [Effects of the Invention]

[0007] According to this disclosure, it is possible to suppress short circuits caused by metallic foreign matter mixed into the secondary battery. [Brief explanation of the drawing]

[0008] [Figure 1] This is an overall configuration diagram of a vehicle equipped with the battery system according to this embodiment. [Figure 2] This is a flowchart showing the short-circuit suppression process according to this embodiment. [Figure 3] This figure shows the conditions and results of the evaluation tests that were conducted. [Modes for carrying out the invention]

[0009] Embodiments of this disclosure will be described in detail below with reference to the drawings. In the drawings, identical or corresponding parts are denoted by the same reference numerals, and their descriptions will not be repeated.

[0010] Figure 1 is an overall configuration diagram of a vehicle equipped with the battery system according to this embodiment. In this embodiment, vehicle 1 is, for example, an electric vehicle (BEV: Battery Electric Vehicle). Vehicle 1 may also be a hybrid vehicle equipped with an internal combustion engine and a battery. Vehicle 1 comprises a motor generator (MG: Motor Generator) 10, which is a rotating electric machine, a power transmission gear 20, drive wheels 30, a power control unit (PCU: Power Control Unit) 40, a system main relay (SMR: System Main Relay) 50, a battery 100, a monitoring unit 200, and an ECU (Electronic Control Unit) 300.

[0011] MG10 is, for example, an embedded permanent magnet synchronous motor (IPM motor) that has both the function of a motor and the function of a generator. The output torque of MG10 is transmitted to the drive wheels 30 via a power transmission gear 20 which includes a reduction gear and a differential gear.

[0012] When vehicle 1 is braked, the MG10 is driven by the drive wheels 30, and the MG10 operates as a generator. In this way, the MG10 also functions as a braking device that performs regenerative braking, converting the kinetic energy of vehicle 1 into electrical energy. The regenerative power generated by the regenerative braking force in the MG10 is stored in the battery 100.

[0013] The PCU40 is a power converter that converts power bidirectionally between the MG10 and the battery 100. The PCU40 includes, for example, an inverter and a converter that operate based on control signals from the ECU300.

[0014] The SMR50 is electrically connected to the power line connecting the battery 100 and the PCU 40. When the SMR50 is closed (ON) in response to a control signal from the ECU 300, power can be exchanged between the battery 100 and the PCU 40. On the other hand, when the SMR50 is opened (OFF) in response to a control signal from the ECU 300, the electrical connection between the battery 100 and the PCU 40 is interrupted.

[0015] Battery 100 stores power to drive the MG10. Battery 100 is a rechargeable DC power source (secondary battery) and is a battery pack in which multiple single cells 100a are electrically connected in series. Each single cell 100a is a secondary battery containing an electrolyte and may be composed of, for example, lithium-ion batteries. Vehicle 1 is equipped with an inlet (not shown), and external charging of battery 100 is possible by connecting the plug (charging cable) of the charging equipment to the inlet.

[0016] The monitoring unit 200 includes a voltage sensor 210, a current sensor 220, and a temperature sensor 230. The voltage sensor 210 detects the voltage VB of the battery 100. The current sensor 220 detects the current IB that is input to and output from the battery 100. The temperature sensor 230 detects the temperature TB of the battery 100. Each sensor outputs its detection result to the ECU 300.

[0017] The ECU 300 includes a CPU (Central Processing Unit) 301 and a memory 302. The ECU 300 corresponds to the "control unit" in this disclosure. Based on signals received from each sensor and maps and programs stored in the memory 302, the ECU 300 controls each device so that the vehicle 1 reaches a desired state. Major controls (processes) performed by the ECU 300 include charge / discharge control and short-circuit suppression processing of the battery 100. The battery system SY according to this embodiment includes a battery 100, a monitoring unit 200, and the ECU 300.

[0018] Figure 2 is a flowchart showing the short circuit suppression processing according to the present embodiment. This flowchart is called from a main routine and executed when a predetermined condition is satisfied. Each step (hereinafter abbreviated as "S") included in these flowcharts is basically implemented by software processing by the ECU 300, but may be implemented by dedicated hardware (electric circuit) fabricated in the ECU 300.

[0019] First, in S1, the ECU 300 acquires the temperature TB of the battery 100 and the SOC of the battery 100. The temperature TB can be acquired from a temperature sensor 230. The SOC can be calculated from the SOC-OCV (Open Circuit Voltage) characteristic of the battery 100 using the voltage VB detected by a voltage sensor 210.

[0020] Next, in S2, the ECU 300 updates the frequency data stored in a memory 302 based on the temperature TB and the SOC acquired in S1. The frequency data indicates the history of the temperature TB of the battery 100 and the SOC of the battery 100 during the usage period of the battery 100. The usage period of the battery 100 is the elapsed time from the start of usage (initial state) of the battery 100, and is, for example, the cumulative time from when the battery 100 is mounted on the vehicle 1. Specifically, the frequency data indicates the occurrence frequency for each combination of the temperature TB of the battery 100 and the SOC of the battery 100. More specifically, the frequency data indicates, for each combination of the temperature TB of the battery 100 and the SOC of the battery 100, the cumulative time during which the combination has occurred in the usage period of the battery 100.

[0021] Next, in S3, the ECU300 calculates an index value P indicating the amount of metal foreign matter dissolved in the electrolyte of battery 100 (single cell 100a) based on the history of the temperature TB and state of charge (SOC) of battery 100 during the usage period of battery 100. The index value P is, for example, the amount of metal foreign matter deposited on the negative electrode side of single cell 100a (amount of metal foreign matter deposited). There is a relationship between the amount of metal foreign matter deposited on the negative electrode side and the amount of metal foreign matter dissolved in the electrolyte. The ECU300 calculates the index value P using a deposition rate map that shows the correspondence between the temperature TB of battery 100, the SOC of battery 100, and the deposition rate of metal foreign matter. Specifically, for each combination of the temperature TB of battery 100 and the SOC of battery 100, the ECU300 calculates the amount of metal foreign matter deposited by multiplying the cumulative time shown by the frequency data by the deposition rate shown by the deposition rate map, and the sum of the deposition amounts for each combination is taken as the index value P.

[0022] The deposition rate map is stored in memory 302. The deposition rate map can be created by storage tests using a battery cell equivalent to that of the single cell 100a (hereinafter referred to as the "test cell"). Specifically, a metallic foreign object is placed on the positive electrode side of the test cell, and multiple storage tests with different storage periods are conducted for each combination of battery temperature and SOC. After the storage period of each storage test has elapsed, the test cell is disassembled, and the amount of metal present on the negative electrode side of the test cell is quantified by ICP (Inductively Coupled Plasma) analysis. The amount of metal obtained in this way corresponds to an index value indicating the amount of metallic foreign object dissolved in the electrolyte of the test cell. Based on the amount of metal obtained for each storage period for each combination of battery temperature and SOC, the deposition rate of the metallic foreign object corresponding to that combination is calculated, and a deposition rate map is created.

[0023] Subsequently, in S4, the ECU 300 determines whether or not the index value P is larger than a discharge start threshold. The discharge start threshold is predetermined through experiments. For example, the maximum value of the deposition amount of metallic foreign matter when no short circuit occurs can be obtained through experiments, and the maximum value can be set as the discharge start threshold. If the index value P is larger than the discharge start threshold (YES in S4), the ECU 300 advances the process to S5. If the index value P is not larger than the discharge start threshold (NO in S4), the ECU 300 returns the process to S1.

[0024] In S5, the ECU 300 starts a first discharge. The discharge current value in the first discharge is, for example, 50A. Note that the discharge current value in the first discharge only needs to be a value smaller than the discharge current value in a second discharge described later.

[0025] Subsequently, in S6, the ECU 300 determines whether or not the SOC of the battery 100 has become equal to or less than a predetermined SOC. The predetermined SOC may be, for example, the lower limit value of the SOC during vehicle travel, or may be lower than the lower limit value of the SOC during vehicle travel. If the SOC has become equal to or less than the predetermined SOC (YES in S6), the ECU 300 advances the process to S7. If the SOC has not become equal to or less than the predetermined SOC (NO in S6), the ECU 300 returns the process to S5 and continues the first discharge.

[0026] In S7, the ECU 300 starts a second discharge. The discharge current value in the second discharge is larger than the discharge current value in the first discharge, and is, for example, 100A. Note that the discharge current value in the second discharge may be larger than 100A. By S7, a short-time discharge with a large current is performed.

[0027] Next, in S8, the ECU 300 measures the OCV of the battery 100. Then, in S9, the ECU 300 determines whether the OCV of the battery 100 measured in S8 is below the degradation threshold. The degradation threshold is predetermined by experiment. If the OCV of the battery 100 is below the degradation threshold (YES in S9), the ECU 300 terminates the short-circuit suppression process. If the OCV of the battery 100 is not below the degradation threshold (NO in S9), the ECU 300 returns to S7 and restarts the second discharge. Note that if the OCV of the battery 100 is not below the degradation threshold (NO in S9), the ECU 300 may terminate the short-circuit suppression process without restarting the second discharge. In this embodiment, the first and second discharges are performed in a negative electrode potential range of 2.0V or higher.

[0028] Figure 3 shows the conditions and results of the evaluation tests conducted. The type of battery cell used in the evaluation tests (hereinafter referred to as "test battery") was a prismatic HEV (Hybrid Electric Vehicle) cell. The evaluation tests were conducted for Comparative Examples 1 to 5 and Example 1 under the following evaluation conditions.

[0029] <Evaluation Criteria> (1) Preservation test In the storage test, the test batteries were stored for 30 days at a temperature of 60°C with a State of Charge (SOC) of 80%.

[0030] (2) Over-discharge treatment In the over-discharge test, the test battery was discharged once every three days in a zero-degree Celsius environment until the State of Charge (SOC) reached 15%.

[0031] (3) Volume measurement In capacity measurement, the discharge capacity was measured by discharging the test battery at a current rate of 1 / 3C within a voltage range of 4.1V to 3.0V. "C" is a symbol representing the current rate. At a rate of "1C," the battery's rated capacity is supplied over one hour. If the capacity reduction compared to Comparative Example 1 was less than a predetermined amount, it was evaluated as being within the acceptable range ("A" in Figure 3). If the capacity reduction compared to Comparative Example 1 was greater than or equal to a predetermined amount, it was evaluated as being outside the acceptable range ("B" in Figure 3).

[0032] (4) Short circuit inspection After storing the test battery for 30 days, it was recharged until the State of Charge (SOC) reached 80%, and the voltage change was observed after 10 days. If the voltage drop was below a predetermined value compared to Comparative Example 1, it was evaluated as being within the acceptable range ("A" in Figure 3). If the voltage drop was above a predetermined value compared to Comparative Example 1, it was evaluated as being outside the acceptable range ("B" in Figure 3).

[0033] <Comparative Example 1> In Comparative Example 1, no stainless steel foreign material was placed inside the test battery, and capacity measurement and short-circuit testing were performed after the storage test.

[0034] <Comparative Example 2> In Comparative Example 2, a stainless steel foreign object made of SUS304 with a diameter of 100 μm and a thickness of 5 μm was placed inside the winding of the test battery, and after a storage test, capacity measurement and short-circuit testing were performed.

[0035] <Comparative Example 3, Comparative Example 4, Example 1, Comparative Example 5> In Comparative Examples 3, 4, 1, and 5, stainless steel foreign material similar to that in Comparative Example 2 was placed inside the winding of the test battery. After a storage test, an over-discharge treatment was performed, followed by capacity measurement and short-circuit testing. In the over-discharge treatment of Comparative Example 3, the discharge current was set to 50A and the maximum potential of the negative electrode to 1.0V. In the over-discharge treatment of Comparative Example 4, the discharge current was set to 70A and the maximum potential of the negative electrode to 1.5V. In the over-discharge treatment of 1 Example, the discharge current was set to 100A and the maximum potential of the negative electrode to 2.1V. In the over-discharge treatment of Comparative Example 5, the discharge current was set to 150A and the maximum potential of the negative electrode to 3V. Furthermore, the OCV after the over-discharge treatment of Comparative Examples 3, 4, and 1 was above the degradation limit threshold, but the OCV after the over-discharge treatment of Comparative Example 5 was below the degradation limit threshold.

[0036] <Result> In Comparative Example 1, neither short circuit nor capacity reduction occurred. Furthermore, when the discharge current value during the over-discharge treatment was less than 100A and the maximum potential of the negative electrode was less than 2.0V (in Comparative Examples 3 and 4), the voltage reduction was greater than or equal to a predetermined value compared to Comparative Example 1. In contrast, when the discharge current value during the over-discharge treatment was 100A or more and the maximum potential of the negative electrode was 2.0V or more (in Example 1 and Comparative Example 5), the voltage reduction was less than a predetermined value compared to Comparative Example 1. Also, when the OCV after the over-discharge treatment was below the degradation limit threshold (in Comparative Example 5), the capacity reduction was greater than or equal to a predetermined amount compared to Comparative Example 1. In contrast, when the OCV after the over-discharge treatment was above the degradation limit threshold (in Comparative Examples 3, 4, and Example 1), the capacity reduction was less than a predetermined amount compared to Comparative Example 1. Therefore, evaluation tests revealed that by setting the discharge current to 100A or higher, the negative electrode potential to 2.0V or higher, and discharging within a range where the OCV does not fall below the degradation limit threshold, it is possible to suppress the reduction in secondary battery capacity while also suppressing the occurrence of short circuits caused by metallic foreign matter within the secondary battery.

[0037] As described above, in this embodiment, the ECU 300 calculates an index value P indicating the amount of metal foreign matter dissolved in the electrolyte of the battery 100 based on the history of the temperature TB and state of charge (SOC) of the battery 100 during the period of use of the battery 100. The ECU 300 discharges the battery 100 when the index value P exceeds a predetermined discharge start threshold (threshold). At least a portion of the metal foreign matter deposited on the negative electrode as the battery 100 is used is dissolved by discharge, thereby suppressing conductivity between the positive and negative electrodes. Therefore, according to this embodiment, it is possible to suppress the occurrence of short circuits caused by metal foreign matter mixed into the battery 100 (secondary battery).

[0038] Furthermore, in this embodiment, the discharge current value for the second discharge is 100A, and the discharge is performed with a high current. As a result, at least a portion of the metallic foreign matter deposited on the negative electrode is more easily dissolved. Therefore, the occurrence of short circuits caused by metallic foreign matter mixed into the battery 100 can be further suppressed. In addition, in this embodiment, when the OCV falls below the degradation limit threshold, the discharge is not restarted and the short-circuit suppression process is terminated. This suppresses the decrease in the capacity of the battery 100.

[0039] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended. [Explanation of Symbols]

[0040] 1 Vehicle, 100 Battery, 100A Single Cell, 200 Monitoring Unit, 210 Voltage Sensor, 220 Current Sensor, 230 Temperature Sensor, 300 ECU, 301 CPU, 302 Memory, P Index Value, SY Battery System, TB Temperature, VB Voltage.

Claims

[Claim 1] A secondary battery containing an electrolyte, The system includes a control unit that controls the discharge of the secondary battery, The control unit, Based on the history of the temperature of the secondary battery and the State of Charge (SOC) of the secondary battery during its usage period, an index value indicating the amount of dissolved metallic foreign matter in the electrolyte is calculated. A battery system that discharges the secondary battery when the aforementioned indicator value exceeds a predetermined threshold.

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