Battery system

Abstract

This invention provides a battery system that can suppress the deposition of lithium metal during charging of a lithium-ion secondary battery, even if the lithium-ion secondary battery has degraded. [Solution] The battery system comprises a battery 80 made of a lithium-ion secondary battery, a group of sensors 90 that detect the voltage, current, and temperature of the battery 80, and a control device 100 that controls the charging current of the battery 80 based on the values ​​detected by the group of sensors 90. The control device 100 acquires the degradation state of the battery 80 from the values ​​detected by the group of sensors 90 and sets a threshold voltage for which lithium metal is deposited on the electrodes of the lithium-ion secondary battery with respect to the voltage of the battery 80, according to the acquired degradation state of the battery 80. If the voltage of the battery 80 exceeds the threshold voltage during charging of the battery 80, the control device 100 reduces the charging current to a limiting current.

Description

Technical Field

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

Background Art

[0002] Japanese Patent Application Laid-Open No. 2014-163861 (Patent Document 1) discloses a technique for estimating the state of charge (SOC) of a lithium-ion secondary battery based on the SOC-CCV characteristics representing the correlation between the remaining capacity (SOC) and the closed-circuit voltage (CCV) of the lithium-ion secondary battery.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a lithium-ion secondary battery, the internal resistance increases as the degree of deterioration increases. As the internal resistance increases, the potential difference between the open-circuit voltage (OCV) and the CCV of the lithium-ion secondary battery expands. Therefore, depending on the deterioration state of the lithium-ion secondary battery, lithium metal may precipitate on the surface of the negative electrode of the lithium-ion secondary battery before the SOC reaches the SOC value indicating the full charge state during charging of the lithium-ion secondary battery.

[0005] The present disclosure has been made to solve the above problems, and an object thereof is to provide a battery system capable of suppressing the precipitation of lithium metal during charging of a lithium-ion secondary battery even when the lithium-ion secondary battery deteriorates.

Means for Solving the Problems

[0006] A battery system according to one embodiment of the present disclosure comprises a lithium-ion secondary battery, a group of sensors for detecting the voltage, current, and temperature of the lithium-ion secondary battery, and a control device for controlling the charging current of the lithium-ion secondary battery based on the values ​​detected by the sensor group. The control device obtains the degradation state of the lithium-ion secondary battery from the values ​​detected by the sensor group. The control device sets a threshold voltage for which lithium metal is deposited on the electrodes of the lithium-ion secondary battery, with respect to the voltage of the lithium-ion secondary battery, according to the acquired degradation state of the lithium-ion secondary battery. If the voltage of the lithium-ion secondary battery exceeds the threshold voltage during charging, the control device reduces the charging current of the lithium-ion secondary battery to a limiting current. [Effects of the Invention]

[0007] According to this disclosure, it is possible to provide a battery system that can suppress the deposition of lithium metal during charging of a lithium-ion secondary battery even when the lithium-ion secondary battery has deteriorated. [Brief explanation of the drawing]

[0008] [Figure 1] This figure shows an example of the overall configuration of an electric vehicle to which the battery system according to this embodiment is applied. [Figure 2] This diagram shows the battery and monitoring unit configuration in more detail. [Figure 3] This figure shows examples of SOC-OCP curves and SOC-CCP curves for LFP batteries. [Figure 4] This figure shows an example of the time-dependent change in the resistance of the positive and negative electrodes of an LFP battery. [Figure 5] This flowchart shows the processing procedure for battery charging control in this embodiment. [Figure 6] This figure illustrates an example of battery charging control according to this embodiment. [Modes for carrying out the invention]

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

[0010] <Overall configuration of electric vehicles> Figure 1 shows an example of the overall configuration of an electric vehicle to which the battery system according to this embodiment is applied. The electric vehicle 1 according to this embodiment is, for example, an electric vehicle. The electric vehicle 1 may also be a hybrid vehicle, a plug-in hybrid vehicle, or the like.

[0011] The electric vehicle 1 comprises a motor generator (MG) 10, a power transmission gear 20, drive wheels 30, a power control unit (PCU) 40, a system main relay (SMR) 50, a battery 80, a monitoring unit 90, and an electronic control unit (ECU) 100.

[0012] The MG10 is an AC rotating electric machine, for example, a three-phase AC synchronous motor with permanent magnets embedded in the rotor. The MG10 is powered by a battery 80 via a PCU 40. The driving force of the MG10 is transmitted to the drive wheels 30 via a power transmission gear 20, which includes a reduction gear and a differential gear.

[0013] When the electric vehicle 1 is braking or when acceleration is reduced on a downhill slope, the MG10 is driven by the drive wheels 30, and the MG10 acts as a generator to perform regenerative power generation. The electricity generated by the MG10 is stored in the battery 80 via the PCU 40.

[0014] The PCU40 is a power converter that converts power bidirectionally between the MG10 and the battery 80. The PCU40 includes, for example, an inverter and a converter that operate based on control signals from the ECU100.

[0015] The converter boosts the DC voltage supplied from the battery 80 and supplies it to the inverter when the battery 80 discharges. The inverter converts the DC power supplied from the converter into AC power and drives the MG10.

[0016] When the battery 80 is being charged, the inverter converts the AC power generated by the MG10 into DC power and supplies it to the converter. The converter steps down the DC voltage supplied from the inverter to a voltage suitable for charging the battery 80 and supplies it to the battery 80.

[0017] The SMR50 is electrically connected to the power line 45 connecting the battery 80 and the PCU40. When the SMR50 is closed (ON) in response to a control signal from the ECU100, power can be exchanged between the battery 80 and the PCU40. On the other hand, when the SMR50 is opened (OFF) in response to a control signal from the ECU100, the electrical connection between the battery 80 and the PCU40 is cut off.

[0018] The battery 80 stores power for driving the MG10. The battery 80 is a rechargeable DC power source (secondary battery), and is a battery pack in which a plurality of single cells are electrically connected in series. Each cell is a lithium-ion secondary battery. In the present embodiment, as the cell, a lithium iron phosphate ion battery (LFP battery) using lithium iron phosphate as the positive electrode active material is adopted.

[0019] The monitoring unit 90 is a device for monitoring the state of the battery 80, and includes a voltage sensor group 92, a current sensor 94, and a temperature sensor group 96. The voltage sensor group 92 includes a plurality of voltage sensors. Each of the plurality of voltage sensors detects the voltage V of the corresponding cell among the cells included in the battery 80. The current sensor 94 detects the current I input to and output from the battery 80. The temperature sensor group 96 includes at least one temperature sensor. Each of the at least one temperature sensors detects the temperature T of the corresponding cell among the cells included in the battery 80. The monitoring unit 90 outputs the detection values by each sensor to the ECU100.

[0020] The electric vehicle 1 includes an inlet 60 and is configured to be externally charged with the battery 80 using a charging facility (EVSE: Electric Vehicle Supply Equipment) 200. The inlet 60 is configured to be connectable to a connector 220 provided at the tip of the charging cable 210 of the EVSE 200. The inlet 60 is electrically connected to the power line 45 via a charging circuit 70. In the present embodiment, when the SMR 50 is closed, the inlet 60 and the battery 80 are electrically connected and external charging becomes possible.

[0021] The ECU 100 includes a CPU (Central Processing Unit) 102, a memory 104 such as a ROM (Read Only Memory) and a RAM (Random Access Memory), and an input / output port (not shown) for inputting and outputting various signals. The ECU 100 executes control related to vehicle travel and charging / discharging of the battery 80 based on signals received from each sensor of the monitoring unit 90 and programs and maps stored in the memory 104.

[0022] Further, when the battery 80 is being charged, the ECU 100 reduces the charging current of the battery 80 as a measure to protect the battery 80 from lithium metal being deposited on the negative electrode surface of each cell (hereinafter also referred to as "Li deposition").

[0023] The ECU 100 corresponds to an embodiment of the "control device". The ECU 100 may be divided into a plurality of ECUs for each function. Note that at least a part of the ECU 100 can also be constructed and processed with dedicated hardware (electronic circuit).

[0024] <Configuration of Battery and Monitoring Unit> FIG. 2 is a diagram showing the configuration of the battery 80 and the monitoring unit 90 in more detail. As shown in FIG. 2, the battery 80 includes M cells 801 to 80M connected in series. M may be an integer of 2 or more, and typically is a dozen to several dozen.

[0025] The voltage sensor group 92 is realized by, for example, a voltage monitoring IC (Integrated Circuit) in which processing circuits of a number of voltage sensors are integrated and is integrally configured. The voltage sensor group 92 includes M voltage sensors 921 to 92M. The voltage sensor 921 detects the voltage V1 of the cell 801. The voltage sensor 922 detects the voltage V2 of the cell 802. The same applies to the other voltage sensors 923 to 92M.

[0026] The current sensor 94 detects the current I flowing through the battery 80 (that is, the current I flowing through the cells 801 to 80M in common).

[0027] The temperature sensor group 96 includes N temperature sensors 961 to 96N. N is an integer greater than or equal to 1 and less than M. That is, temperature sensors are not provided for all cells. In the example of FIG. 2, the temperature sensor 961 detects the temperature T1 of the cell 801. The temperature sensor 96N detects the temperature TN of the cell 80N. However, the cells to which the temperature sensors are installed are not limited to the cells at both ends as in this example and can be set as appropriate.

[0028] In addition, in FIG. 2, a configuration in which all the cells included in the battery 80 are connected in series is illustrated, but the configuration of the battery 80 is not limited to this. The battery 80 may include two or more groups of cells connected in series.

[0029] <Protective measures against Li precipitation> A lithium ion secondary battery discharges and charges through a chemical reaction (battery reaction) at the interface between each of the negative electrode active material and the positive electrode active material and the electrolyte. During charging, a battery reaction that releases lithium ions and electrons occurs on the interface of the positive electrode active material, and a battery reaction that absorbs lithium ions and electrons occurs on the interface of the negative electrode active material. During discharging, a battery reaction in which the release / absorption is reversed occurs. During charging of a lithium ion secondary battery, the absorption diffusion at the negative electrode may not catch up with the supply of lithium ions, and lithium metal may precipitate on the surface of the negative electrode.

[0030] The following describes the change in the negative electrode potential when Li deposition occurs in an LFP battery. Figure 3 shows an example of the SOC-OCP curve and SOC-CCP curve of an LFP battery. Figure 3(A) shows the SOC-OCP curve and SOC-CCP curve in the initial state of the LFP battery (e.g., when new). Figure 3(B) shows the SOC-OCP curve and SOC-CCP curve in a degraded state of the LFP battery.

[0031] In Figures 3(A) and 3(B), the horizontal axis represents the State of Charge (SOC) of the LFP battery, and the vertical axis represents the potentials of the positive and negative electrodes of the LFP battery. The dashed lines represent the relationship between the open-circuit potential (OCP) of the positive electrode of the LFP battery (OCPP) and the SOC (hereinafter referred to as the "SOC-OCCP curve"), and the relationship between the OCP of the negative electrode (OCPN) and the SOC (hereinafter also referred to as the "SOC-OCPN curve"). The solid lines represent the relationship between the closed-circuit potential (CCP) of the positive electrode of the LFP battery (CCCP) and the SOC (hereinafter also referred to as the "SOC-CCPP curve"), and the relationship between the CCP of the negative electrode (CCPN) and the SOC (hereinafter also referred to as the "SOC-CCPN curve").

[0032] Furthermore, in each figure, the open-circuit voltage (OCV) of the LFP battery corresponds to the difference between the open-circuit potential of the positive electrode (OCPP) and the open-circuit potential of the negative electrode (OCPN). The closed-circuit voltage (CCV) of the LFP battery corresponds to the difference between the closed-circuit potential of the positive electrode (CCPP) and the closed-circuit potential of the negative electrode (CCPN).

[0033] As shown in Figure 3(A), the SOC-OCPP curve exhibits extremely high flatness, except in the low SOC region and the high SOC state near full charge. Therefore, it can be said that the SOC-OCV curve of an LFP battery is determined by the shape of the SOC-OCPN curve.

[0034] The SOC-OCPN curve, like the SOC-OCCP curve, exhibits flatness except in the low SOC region. However, in the SOC-OCPN curve, a "step" is observed in the region where SOC is moderate, where the open-circuit potential OCPN of the negative electrode drops sharply as SOC increases. Consequently, OCV also rises sharply in this region as SOC increases.

[0035] During charging of an LFP battery, the closed-circuit potential CCPP of the positive electrode is higher than the open-circuit potential OCPP of the positive electrode by the voltage across the resistance of the positive electrode. If the charging current of the LFP battery is I and the resistance of the positive electrode is Rp, then the closed-circuit potential CCPP of the positive electrode is expressed as CCPP = OCPP + I × Rp. The SOC-CCPP curve, reflecting the SOC-OCPP curve, has very high flatness except for the high SOC state.

[0036] The closed-circuit potential CCPN of the negative electrode is lower than the open-circuit potential OCPN of the negative electrode by the voltage across the resistance of the negative electrode. If the charging current of the LFP battery is I and the resistance of the negative electrode of the LFP battery is Rn, then the closed-circuit potential CCPN of the negative electrode is expressed as CCPN = OCPN - I × Rn. As a result, in the SOC-CCPN curve, a "step" appears in the region where the SOC is moderate, where the closed-circuit potential CCPN of the negative electrode drops sharply as the SOC increases.

[0037] Here, if the closed-circuit potential CCPN of the negative electrode falls below the lithium metal deposition potential (hereinafter also referred to as "Li deposition potential") of 0V during charging of the LFP battery, lithium metal will be deposited on the surface of the negative electrode of the LFP battery. If an excessive amount of lithium metal is deposited, the deposited lithium metal may short-circuit the positive and negative electrodes of the LFP battery. Therefore, in the charging control of the LFP battery, it is necessary to suppress the deposition of lithium metal on the negative electrode surface.

[0038] As a protective measure against Li deposition, one approach is to set the critical voltage (CCV) at which Li deposition begins to a protective voltage to protect the lithium-ion secondary battery from Li deposition, and then reduce the charging current (charging power) of the lithium-ion secondary battery when the CCV of the lithium-ion secondary battery reaches the protective voltage. In this approach, the protective voltage corresponds to the "threshold voltage" at which lithium metal is deposited on the electrodes of the lithium-ion secondary battery, relative to the voltage of the lithium-ion secondary battery.

[0039] For example, by determining the protection voltage from the SOC-CCP curve shown in Figure 3(A) and controlling the charging current based on the protection voltage, it is possible to suppress the closed-circuit potential CCPN of the negative electrode from falling below the Li deposition potential of 0V, thereby suppressing Li deposition in LFP batteries.

[0040] However, as the LFP battery deteriorates with repeated charging and discharging cycles, the SOC-CCP curve of the LFP battery changes from the initial SOC-CCP curve, as shown in Figure 3(B). Note that the charging current of the LFP battery is the same in Figure 3(A) and Figure 3(B).

[0041] Comparing Figure 3(B) and Figure 3(A), no significant difference is observed in the SOC-OCCP curve and the SOC-OCPN curve. On the other hand, the potential difference between the SOC-CCPP curve and the SOC-OCPP curve is smaller than in the initial state due to the degradation of the LFP battery. Since this potential difference corresponds to the product I × Rp of the charging current I and the positive electrode resistance Rp, it can be seen that the positive electrode resistance Rp has decreased due to the degradation of the LFP battery.

[0042] In contrast, the potential difference between the SOC-CCPN curve and the SOC-OCPN curve is larger than in the initial state due to the degradation of the LFP battery. Since this potential difference corresponds to the product I × Rn of the charging current I and the negative electrode resistance Rn, it can be seen that the negative electrode resistance Rn has increased due to the degradation of the LFP battery.

[0043] Figure 4 shows an example of the temporal change in the positive electrode resistance Rp and negative electrode resistance Rn of an LFP battery. Figure 4 shows how the positive electrode resistance Rp and negative electrode resistance Rn change according to the usage time (number of charge / discharge cycles, etc.) of the LFP battery from its initial state. The vertical axis of Figure 4 represents the positive electrode resistance Rp, the negative electrode resistance Rn, and the overall resistance of the LFP battery. The overall resistance of the LFP battery corresponds to Rp + Rn.

[0044] As shown in Figure 4, initially, the resistance Rp of the positive electrode is greater than the resistance Rn of the negative electrode. As the usage time of the LFP battery increases, the overall resistance of the LFP battery increases. However, the resistance Rp of the positive electrode decreases as the usage time of the LFP battery increases, while the resistance Rn of the negative electrode increases as the usage time of the LFP battery increases. Therefore, in the example in Figure 4, a reversal of the relative magnitudes of the positive electrode resistance Rp and the negative electrode resistance Rn occurs. Thus, as the usage time of the LFP battery increases, the ratio of the positive electrode resistance Rp to the negative electrode resistance Rn of the LPF battery changes.

[0045] In Figure 3(B), the SOC-CCPN curve is shifted to a lower potential compared to the initial SOC-CCPN curve, due to the increased resistance Rn of the negative electrode of the LPF battery. Therefore, the SOC at which the closed-circuit potential CCPN of the negative electrode reaches the Li deposition potential is lower than the SOC at which CCPN reaches the Li deposition potential in the initial state. As shown in Figure 4, the resistance Rn of the negative electrode increases as the LFP battery deteriorates, so it is expected that the SOC at which the closed-circuit potential CCPN of the negative electrode reaches the Li deposition potential will become even lower as the LFP battery deteriorates further. Therefore, if the charging current of the LFP battery is controlled based on the same protection voltage as in the initial state, there is a concern that Li deposition may not be suppressed depending on the deterioration state of the LFP battery.

[0046] To address these concerns, this embodiment sets the protection voltage to be variable according to the degradation state of the lithium-ion secondary battery. For example, in the case of an LFP battery, the SOC-OCP curve and SOC-CCP curve shown in Figure 3 are determined experimentally for each degree of LFP battery degradation, and according to the degree of LFP battery degradation, the CCV at which the closed-circuit potential CCPN of the negative electrode reaches the Li deposition potential can be calculated as the protection voltage from the corresponding SOC-OCP curve and SOC-CCP curve. In a certain scenario, the protection voltage is set to be less than or equal to the CCV at which the closed-circuit potential CCPN of the negative electrode of the LFP battery reaches the Li deposition potential in the SOC-CCP curve.

[0047] Furthermore, as the temperature of the lithium-ion secondary battery decreases, the resistance Rn of the negative electrode increases, and therefore the potential difference I × Rn between the SOC-CCPN curve and the SOC-OCPN curve also increases. As a result, at low temperatures, the closed-circuit potential CCPN of the negative electrode tends to fall below the Li deposition potential. In addition, at low temperatures, the absorption and diffusion of lithium ions at the negative electrode slows down considerably, making Li deposition more likely.

[0048] Therefore, in this embodiment, the protection voltage is set to be variable according to the degradation state and temperature of the lithium-ion secondary battery. Specifically, for each temperature of the lithium-ion secondary battery, a map or formula that specifies the relationship between the degree of degradation of the lithium-ion secondary battery and the protection voltage is generated in advance through experiments or other means and stored in the memory 104 of the ECU 100. The ECU 100 refers to this map or formula to calculate the protection voltage from the temperature and degree of degradation of the lithium-ion secondary battery.

[0049] <Processing Flow> Figure 5 is a flowchart showing the charging control procedure for the lithium-ion secondary battery in this embodiment. This flowchart is executed when predetermined conditions are met by the ECU 100 during the charging of the battery 80 (for example, at each control cycle). Each step is implemented by software processing by the ECU 100, but may also be implemented by hardware (electrical circuits) located within the ECU 100. Hereinafter, each step will be abbreviated as S.

[0050] As shown in Figure 5, first, in S01, the ECU 100 determines whether the detection preconditions are met. The "detection preconditions" refer to the fact that all sensors for detecting the state of the battery 80 (voltage V, current I, and temperature T), the ECU 100 itself, and the power supply for the ECU 100 are all functioning normally.

[0051] If the detection preconditions are met (YES determination in S01), the ECU 100 executes a process to protect the battery 80 from lithium deposition. Specifically, in S02, the ECU 100 obtains voltages V1 to VM from the voltage sensor group 92 (voltage sensors 921 to 92M) and current I from the current sensor 94. The ECU 100 also obtains temperatures T1 to TN from the temperature sensor group 96 (temperature sensors 961 to 96N).

[0052] In S02, the ECU100 obtains a value indicating the degradation level of cells 801 to 80M using at least one of the following: voltage V1 to VM, current I, and temperature T1 to TN. The value indicating the degradation level of each cell is, for example, SOH (State of Health).

[0053] Specifically, the ECU100 calculates an estimated value of SOH (hereinafter referred to as "estimated SOH") [%] using, for example, the estimated full charge capacity Ces [Ah], the initial full charge capacity Co [Ah], and a predetermined estimation formula. The estimation formula is, for example, expressed as "estimated SOH = Ces / Co × 100".

[0054] Here, the initial full charge capacity Co is the full charge capacity when no battery degradation has occurred. The initial full charge capacity Co is, for example, a value predetermined by the type of battery 80, and may be stored in the memory 104 of the ECU 100, obtained from the storage device of the monitoring unit 90, or obtained from an external server.

[0055] The estimated full charge capacity Ces is, for example, an estimate of the current full charge capacity. The ECU100 estimates the full charge capacity Ces from, for example, the open-circuit voltage OCV at the start of charging, the open-circuit voltage OCV at the end of charging, and the integrated value of the charging current from the start to the end of charging. The ECU100 may also calculate the SOH1 to SOHM of cells 801 to 80M when charging the battery 80 using the EVSE200.

[0056] In S03, the ECU100 calculates the protection voltage based on the SOH1 to SOHM and temperatures T1 to TN of cells 801 to 80M. In S03, the ECU100 calculates a representative value (e.g., average, median, minimum, etc.) of the battery 80 temperature from the temperatures T1 to TN obtained in S02. In this embodiment, the ECU100 uses the lowest value among the temperatures T1 to TN as the representative value of the battery 80 temperature. This takes into account that Li deposition is more likely to occur at lower temperatures.

[0057] Furthermore, ECU100 uses the lowest value among SOH1 to SOHM as the representative value of the SOH of battery 80. This takes into account that as the SOH decreases, i.e., as the degree of degradation increases, the closed-circuit potential CCPN of the negative electrode decreases, making Li deposition more likely.

[0058] The memory 104 of the ECU 100 stores a map or formula that identifies the relationship between the state of heat (SOH) and the protection voltage of the lithium-ion secondary battery for each temperature of the lithium-ion secondary battery. The ECU 100 refers to this map or formula and calculates the protection voltage based on typical values ​​for the temperature and SOH of the battery 80. For example, the ECU 100 calculates the CCV as the protection voltage when the closed-circuit potential CCPN of the negative electrode reaches the Li deposition potential.

[0059] In S04, the ECU100 calculates the maximum cell voltage, which is the maximum value among the voltages V1 to VM obtained in S02, and compares the calculated maximum cell voltage with the protection voltage obtained in S03.

[0060] If the maximum cell voltage is above the protection voltage (YES determination in S04), the ECU 100 proceeds to S05 and calculates the limiting current, which is the upper limit of the charging current of the battery 80, based on the temperatures T1 to TN of cells 801 to 80M. The ECU 100 sets the calculated limiting current as the target current in the charging control of the battery 80. In S05, the ECU 100 calculates a representative value of the battery 80 temperature (e.g., average value, median value, minimum value, etc.) from the temperatures T1 to TN obtained in S02. In this embodiment, considering that Li deposition is more likely to occur at lower temperatures, the ECU 100 uses the lowest value among the temperatures T1 to TN as the representative value of the battery 80 temperature.

[0061] The memory 104 of the ECU 100 stores a map or formula that identifies the relationship between the temperature of the lithium-ion secondary battery and the limiting current. The ECU 100 refers to this map or formula to calculate the limiting current from a typical value of the battery 80 temperature.

[0062] In S06, the ECU 100 determines whether the "limiting flag" stored in memory 104 is set to ON. The limiting flag is set to either ON or OFF. When the limiting flag is ON, it means that the charging current of the battery 80 is being limited as a protective measure against Li deposition. When the limiting flag is OFF, it means that the charging current of the battery 80 is not being limited.

[0063] When the "Restricted" flag is OFF (when S06 is judged as YES), ECU100 calculates the command current I*(n-1), which is the control command value for the charging current in the previous control cycle, in S07. In S07, ECU100 sets the command current I*(n-1) for the previous control cycle to the larger of the target current calculated in S05 and the current I obtained in S02. Note that when the "Controlled" flag is ON (when S06 is judged as NO), ECU100 skips S07.

[0064] In S08, ECU100 sets the control in progress flag to ON. Then, in S09, ECU100 calculates the command current I*(n) for the current control cycle.

[0065] In S09, the ECU100 calculates the value obtained by reducing the command current I*(n-1) from the previous control cycle according to the maximum limit rate. The "limit rate" is the rate of change over time (i.e., the rate of decrease) when reducing the charging current, and is expressed as -dI / dt. The "maximum limit rate" means the maximum value of the limit rate. The maximum limit rate is preset, for example, taking into account the electrical characteristics of the EVSE200 and the charging circuit 70 (such as the tracking speed with respect to the command current).

[0066] If the magnitude of the maximum limiting rate is dIres / dt and the length of the control cycle is dt, the value obtained by decreasing the command current I*(n-1) in the previous control cycle according to the maximum limiting rate is expressed as I*(n-1)-dIres. The ECU100 uses the larger of this value and the target current (i.e., the limiting current) obtained in S05 as the command current I*(n) for the current control cycle.

[0067] In this manner, if the maximum cell voltage exceeds the protection voltage (when S04 is judged as YES), the ECU100 reduces the command current I*(n) in each control cycle at the maximum limiting rate, within a range that does not fall below the limiting current.

[0068] Returning to S04, if the maximum cell voltage is below the protection voltage (resulting in a NO judgment in S04), the ECU 100 proceeds to S10 and sets the target current for battery 80 charging control to the maximum current. This "maximum current" is calculated based on the input allowable power Win, which is the upper limit of the power (charging power) input to battery 80.

[0069] In S11, the ECU100 sets the limiting flag to OFF. Then, in S12, the ECU100 calculates the command current I*(n) for the current control cycle. In S12, the ECU100 finds the value obtained by increasing the command current I*(n-1) from the previous control cycle according to the maximum relaxation rate. The "relaxation rate" is the rate of change over time (i.e., the rate of increase) when increasing the charging current, and is expressed as +dI / dt. The "maximum relaxation rate" means the maximum value of the relaxation rate. The maximum relaxation rate is set in advance, for example, taking into account the electrical characteristics of the EVSE200 and the charging circuit 70.

[0070] If the magnitude of the maximum relaxation rate is dIrel / dt and the length of the control period is dt, the value obtained by increasing the command current I*(n-1) in the previous control period according to the maximum relaxation rate is expressed as I*(n-1)+dIrel. ECU100 sets the command current I*(n) for the current control period to be the smaller of this value and the target current (i.e., the maximum current) for the current control period.

[0071] In this manner, when the maximum cell voltage falls below the protection voltage (when NO is determined in S04), the ECU100 increases the command current I*(n) in each control cycle at the maximum relaxation rate, within a range that does not exceed the maximum current determined by the input allowable power Win.

[0072] Returning to S01, if the detection prerequisite is not met (resulting in a NO determination in S01), the ECU 100 proceeds to S13 and sets the target current for battery 80 charging control to the maximum current. As mentioned above, the "maximum current" is calculated based on the input allowable power Win.

[0073] In S15, the ECU100 sets the command current I*(n) for the current control cycle to the target current (i.e., the maximum current) determined in S13.

[0074] <An example of battery charging control> Figure 6 is a diagram illustrating an example of battery 80 charge control according to this embodiment. The horizontal axis of Figure 6 represents time, and the vertical axis represents the maximum cell voltage of the battery 80 and the current I flowing through the battery 80. Figure 6 shows an example of the time progression of the detected precondition fulfillment flag, the maximum cell voltage, and the current I.

[0075] The detection prerequisite fulfillment flag is set to either ON or OFF. When the detection prerequisite fulfillment flag is ON, it means that the above-mentioned detection prerequisites are met. When the detection prerequisite fulfillment flag is OFF, it means that the detection prerequisites are not met. In Figure 6, the detection prerequisite flag is set to ON at time t1.

[0076] In the time evolution of current I shown in Figure 6, the solid line represents the waveform of current I detected by the current sensor 94 (i.e., the actual current of the battery 80). The dotted line represents the waveform of the target current calculated based on the temperature of the battery 80. The dashed line represents the waveform of the command current I* calculated for each control cycle.

[0077] When the detection prerequisite flag is turned ON at time t1, the ECU 100 sets a protection voltage to suppress Li deposition according to the temperature of the battery 80 (the lowest value between temperatures T1 and TN) and the SOH (the lowest value between SOH1 and SOHM). The ECU 100 determines the maximum cell voltage from the voltages V1 to VM obtained from the voltage sensor group 92 and compares the maximum cell voltage with the protection voltage.

[0078] During the period from time t1 to t2, the maximum cell voltage is below the protection voltage, so the target current and command current are set to the maximum current determined by the allowable input voltage Win.

[0079] When the maximum cell voltage reaches the protection voltage at time t2, the ECU 100 reduces the charging current of the battery 80. Specifically, the ECU 100 calculates the limiting current from the temperature of the battery 80 (for example, the lowest value of temperature T1 to TN) and sets the calculated limiting current as the target current. Furthermore, the ECU 100 sets the command current at time t2 to the larger of the target current and the actual current of the battery 80, and reduces the command current in each control cycle after time t2 toward the target current (limiting current) at the maximum limiting rate.

[0080] As the command current begins to decrease at time t2, the actual current of battery 80 also begins to decrease at a delayed time (time t3), following the command current. Along with this decrease in actual current, the maximum cell voltage also decreases.

[0081] At time t4, when the maximum cell voltage drops to the protection voltage, ECU100 changes the target current from the limiting current to the maximum current. Then, ECU100 increases the commanded current towards the new target current (maximum current) at the maximum relaxation rate. The actual current of battery 80 is fixed at the limiting current from time t5 onward, after time t4. However, it begins to increase at a delayed timing (time t6) following the increase in the commanded current.

[0082] From time t6 onward, the maximum cell voltage increases as the actual current increases. When the maximum cell voltage reaches the protection voltage again at time t7, the ECU 100 reduces the charging current of the battery 80 again. That is, the ECU 100 changes the target current from the maximum current to the limiting current. Furthermore, the ECU 100 sets the command current at time t7 to the larger of the target current (limiting current) and the actual current, and reduces the command current in each control cycle from time t7 onward towards the target current (limiting current) at the maximum limiting rate. As a result, the actual current of the battery 80 also starts to decrease at time t8, which is delayed from time t7, and is eventually fixed at the limiting current.

[0083] As described above, in this embodiment, the threshold voltage for protecting the lithium-ion secondary battery from Li deposition is set variably according to the degradation state of the lithium-ion secondary battery. This allows the protection voltage to be appropriately set, taking into account the degradation state of the lithium-ion secondary battery, even when the ratio of the resistance of the positive electrode to the resistance of the negative electrode of the lithium-ion secondary battery changes as the degradation progresses (see Figure 4). Then, by controlling the charging current of the lithium-ion secondary battery based on this set protection voltage, the deposition of lithium metal can be suppressed even if the lithium-ion secondary battery degrades.

[0084] 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 description of the embodiments described above, and is intended to include all modifications in the sense and scope equivalent to the claims. [Explanation of symbols]

[0085] 1 Electric vehicle, 10 MG, 20 Power transmission gears, 30 Drive wheels, 45 Power lines, 50 SMR, 60 Inlet, 70 Charging circuit, 80 Battery, 801-80M cells, 90 Monitoring unit, 92 Voltage sensor group, 94 Current sensor, 96 Temperature sensor group, 100 ECU, 102 CPU, 104 Memory, 200 EVSE, 210 Charging cable, 220 Connector.

Claims

1. Lithium-ion rechargeable batteries and A group of sensors for detecting the voltage, current, and temperature of the lithium-ion secondary battery, The system includes a control device that controls the charging current of the lithium-ion secondary battery based on the detected values ​​of the sensor group, The control device is The degradation state of the lithium-ion secondary battery is obtained from the detected values ​​of the aforementioned sensor group. Regarding the voltage of the lithium-ion secondary battery, the threshold voltage at which lithium metal is deposited on the electrodes of the lithium-ion secondary battery is set according to the acquired degradation state of the lithium-ion secondary battery. A battery system that, when the voltage of the lithium-ion secondary battery exceeds the threshold voltage during charging, reduces the charging current of the lithium-ion secondary battery to a limiting current.

2. The battery system according to claim 1, wherein the control device sets the threshold voltage according to the degradation state and temperature of the lithium-ion secondary battery.

3. The battery system according to claim 1, wherein the threshold voltage is set to be less than or equal to the closed-circuit voltage of the lithium-ion secondary battery at which the closed-circuit potential of the negative electrode of the lithium-ion secondary battery becomes the deposition potential of lithium metal.

4. The battery system according to claim 1, wherein the control device sets the limiting current based on the temperature of the lithium-ion secondary battery using a map or formula capable of identifying the limiting current from the temperature of the lithium-ion secondary battery.

5. The battery system according to any one of claims 1 to 4, wherein the lithium-ion secondary battery is a lithium iron phosphate battery.

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