Vehicle and method for estimating degradation of battery

Abstract

This vehicle is provided with a motor generator for travel, a PCU that drives the motor generator, a main battery that is charged and discharged by the PCU during use of the vehicle, and a processor. The processor is configured to execute an energization degradation process using an energization amount of the main battery when the vehicle is used to calculate an energization degradation amount of the main battery, and to execute a placement degradation process using a placement time of the main battery when the vehicle is placed to calculate a placement degradation amount of the main battery. And estimates the degree of deterioration of the main battery on the basis of the sum of the power-on deterioration amount and the placement deterioration amount. Even when the vehicle is in use, the processor executes a placement degradation process when the energization amount is lower than a reference amount.

Description

Technical Field

[0001] This disclosure relates to a method for estimating the degradation of vehicles and batteries. Background Technology

[0002] Japanese Patent Application Publication No. 2019-53074 discloses a degradation estimation device capable of accurately estimating the degradation of an energy storage element. The degradation estimation device includes an acquisition unit and an estimation unit. The acquisition unit acquires time-series data of the State of Charge (SOC) of the energy storage element. The estimation unit uses a coefficient based on the magnitude of the variation in SOC in the time-series data acquired by the acquisition unit to calculate a current-charge degradation value representing degradation caused by energizing the energy storage element. The degradation of the energy storage element is estimated based on the sum of the calculated current-charge degradation value and a non-current-charge degradation value representing degradation not caused by energizing the energy storage element.

[0003] There has always been a need for highly accurate estimation of the degree of degradation of batteries installed in vehicles. The inventors have addressed the issue that, under certain conditions, the accuracy of estimating the degree of degradation can easily become low when estimating the degree of degradation by dividing it into degradation caused by electrical current and degradation not caused by electrical current. Summary of the Invention

[0004] This disclosure was made to solve the above-mentioned problems, and one of the purposes of this disclosure is to improve the accuracy of estimating the degree of degradation of batteries installed in vehicles.

[0005] The vehicle disclosed in the first aspect comprises an electric generator for driving, a drive unit for driving the electric generator, a battery that is charged and discharged by the drive unit when the vehicle is in use, and a processor. The processor is configured to: calculate the amount of battery degradation by performing a degradation process based on the amount of battery charge used when the vehicle is in use; and calculate the amount of battery degradation by performing a degradation process based on the storage time used when the vehicle is parked, and estimate the degree of battery degradation based on the sum of the degradation amount and the storage degradation amount. Even when the vehicle is in use, the processor performs a storage degradation process when the charge is below a reference amount.

[0006] The second aspect of this disclosure relates to a battery degradation estimation method for a vehicle-mounted battery, comprising a step of estimating the degree of battery degradation by a processor. The estimation steps include: when using the vehicle, performing a power-on degradation process based on the amount of power supplied to the battery to calculate the amount of power-on degradation; when the vehicle is parked, performing a parking degradation process based on the parking time supplied to calculate the amount of parking degradation; even when using the vehicle, performing a parking degradation process when the power supplied is below a reference amount; and estimating the degree of degradation based on the sum of the power-on degradation and the parking degradation.

[0007] According to this disclosure, the accuracy of estimating the degree of degradation of batteries installed in vehicles can be improved. Attached Figure Description

[0008] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which the same reference numerals denote the same elements, wherein,

[0009] Figure 1 This is a block diagram illustrating an example of the overall configuration of the vehicle according to this embodiment;

[0010] Figure 2 This is a diagram used to illustrate the degradation coefficients due to electrical contact and placement.

[0011] Figure 3 This is a diagram used to explain the reasons for the error that causes the amount of electrical degradation;

[0012] Figure 4 This is a flowchart illustrating an example of the deterioration estimation process in this embodiment.

[0013] Figure 5 This is a diagram showing the driving simulation modes used to determine the electrical degradation coefficient; and

[0014] Figure 6 This is a graph illustrating the degradation coefficients due to power-on and placement. Detailed Implementation

[0015] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Furthermore, identical or equivalent reference numerals will be used to label the same or equivalent parts in the drawings, and their descriptions will not be repeated.

[0016] Implementation

[0017] Vehicle composition

[0018] Figure 1 This is a block diagram illustrating an example of the overall configuration of the vehicle according to this embodiment. In this example, vehicle 1 is a battery electric vehicle. However, vehicle 1 is not limited to any type of vehicle equipped with a battery for driving. Vehicle 1 can be a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a fuel cell electric vehicle.

[0019] Vehicle 1 includes a socket 10, an AC / DC converter 20, a charging relay 30, a main battery 40, a monitoring unit 50, a DC / DC converter 60, an auxiliary battery 70, a PCU (Power Control Unit) 80, an electric generator 90, a battery ECU (Electronic Control Unit) 100, and an integrated ECU 110.

[0020] The socket 10 is configured to allow insertion of a charging connector located at the front end of the charging cable 901. The vehicle 1 is configured to perform "external charging" via the charging cable 901 using power supplied from an external power source (not shown) located outside the vehicle 1. Additionally, the vehicle 1 is also configured to perform "external power supply" by supplying power to an external load 902. The external load 902 is, for example, a house, but can also be various electrical devices. The socket 10 corresponds to the "power supply port" as disclosed herein.

[0021] The AC / DC converter 20 converts AC power supplied from an external power source via socket 10 into DC power, and uses this DC power to charge the main battery 40. Additionally, the AC / DC converter 20 converts the DC power supplied from the main battery 40 into AC power, and supplies this AC power to an external load 902 via socket 10. The AC / DC converter 20 is equivalent to the "power supply device" involved in this disclosure.

[0022] The charging relay 30 is electrically connected to the power line connecting the AC / DC converter 20 and the main battery 40. The charging relay 30 is opened and closed according to control commands from the integrated ECU 110.

[0023] The main battery 40 is a battery pack comprising multiple cells. Each cell is a rechargeable battery (secondary battery) such as a lithium-ion battery or a nickel-metal hydride battery. The main battery 40 stores electricity for driving the electric generator 90 and supplies electricity to the electric generator 90 through the PCU 80. The main battery 40 is equivalent to the "rechargeable battery" involved in this disclosure.

[0024] The monitoring unit 50 includes a voltage sensor 51, a current sensor 52, and a temperature sensor 53. The voltage sensor 51 detects the voltage V of the main battery 40. The current sensor 52 detects the current I flowing through the main battery 40. The temperature sensor 53 detects the temperature T of the main battery 40. Each sensor outputs a signal indicating its detection result to the battery ECU 100.

[0025] The DC / DC converter 60 charges the auxiliary battery 70 using power supplied from the main battery 40, according to control commands from the integrated ECU 110. The DC / DC converter 60 is equivalent to the "charging device" involved in this disclosure.

[0026] The auxiliary battery 70 is charged by the DC / DC converter 60 and supplies power appropriately to the auxiliary equipment (not shown).

[0027] PCU80 drives electric generator 90 according to control commands from integrated ECU110. PCU80 is equivalent to the "drive unit" involved in this disclosure.

[0028] The electric generator 90 is a driving electric generator that uses power supplied from the main battery 40 to rotate the drive shaft.

[0029] The battery ECU 100 includes a processor 101 such as a CPU (Central Processing Unit) and a memory 102 such as ROM (Read Only Memory) and RAM (Random Access Memory). The battery ECU 100 manages the main battery 40 based on input signals from various sensors of the monitoring unit 50, mappings stored in the memory 102, and programs. In this embodiment, a "degradation estimation process" that estimates the degree of degradation of the main battery 40 is a primary process performed by the battery ECU 100. The degree of degradation of the main battery 40 is the degree of reduction in its capacity (full charge capacity).

[0030] The integrated ECU 110, like the battery ECU 100, includes a processor and a memory (neither shown). The integrated ECU 110 controls the device classes (AC / DC converter 20, charging relay 30, DC / DC converter 60, and PCU 80) to bring the vehicle 1 into a desired state based on input signals from various sensors located in the vehicle 1, mappings stored in the memory, and programs.

[0031] Degradation Presumed Treatment

[0032] The battery ECU100 is configured to execute two logics in the degradation estimation process: power-on degradation logic and placement degradation logic. The power-on degradation logic and placement degradation logic are respectively equivalent to the "power-on degradation process" and "placement degradation process" involved in this disclosure.

[0033] The power-on degradation logic is essentially a process that calculates the degradation amount (hereinafter referred to as "power-on degradation amount") of the main battery 40 based on the amount of charge [unit: Ah] of the main battery 40 when using vehicle 1. The power-on degradation amount depends on the temperature of the main battery 40. The higher the temperature of the main battery 40, the greater the power-on degradation amount. Therefore, for each temperature of the main battery 40 during a specified period, the battery ECU 100 calculates a small degradation amount corresponding to the amount of charge of the main battery 40, and adds up the calculated small degradation amounts for all temperatures to calculate the power-on degradation amount d1.

[0034] More specifically, the battery ECU100 calculates the current degradation amount d1 according to the following equation (1) in the current degradation logic. In equation (1), the current degradation coefficient representing the current degradation rate (the amount of current degradation per unit time) is denoted by a. j The charge (current × time) is represented by Ah. j is a natural number used to distinguish the temperature of the main battery 40 (j = 1, 2, ..., J).

[0035]

[0036] The placement degradation logic refers to the process of calculating the degradation amount (hereinafter referred to as "placement degradation amount") of the main battery 40 based on the placement time of the main battery 40 when the vehicle 1 is placed (not powered on). The placement degradation amount depends on the temperature and SOC of the main battery 40. The higher the temperature of the main battery 40, the greater the placement degradation amount; the higher the SOC of the main battery 40, the greater the placement degradation amount. Therefore, for each combination of temperature and SOC of the main battery 40 (temperature, SOC), the battery ECU 100 calculates the minute degradation amount corresponding to the placement time of the main battery 40, and adds up the calculated minute degradation amounts for all combinations (temperature, SOC) to calculate the placement degradation amount d2.

[0037] More specifically, the battery ECU100 calculates the placement degradation amount d2 according to the following equation (2) in the placement degradation logic. In equation (2), the placement degradation coefficient, which represents the placement degradation rate (the amount of placement degradation per unit time), is represented by b. jk The storage time (e.g., number of days) is represented by t. j is a natural number used to distinguish the temperature of the main battery 40 (j = 1, 2, ..., J). k is a natural number used to distinguish the SOC of the main battery 40 (k = 1, 2, ..., K).

[0038]

[0039] Figure 2 It is used to measure the degradation coefficient a during electrical conduction. j and placement degradation coefficient b jkThe diagram is for illustration. In this example, within the temperature range of -45°C to 65°C for the main battery 40, the current-carrying degradation factor α is determined based on prior experimental results, at 1°C increments. j The summation symbol J = 111. Furthermore, similarly, within a temperature range from -45°C to 65°C, the placement degradation factor b is determined based on prior experimental results, with each 1°C increment representing a 10% to 20% SOC range within the 0% to 100% SOC range. jk The summation symbol J = 111, K = 7. Regarding the energization degradation coefficient a... j and placement degradation coefficient b jk The detailed decision-making process will be explained later. Figure 5 and Figure 6 Please provide an explanation.

[0040] Figure 2 The temperature and current degradation coefficient a of the main cell 40 as shown are described. j The relationships between these relationships are stored, for example, as a table (or a mapping, a relational expression) in the memory 102 of the battery ECU 100. Similarly, the temperature, SOC, and placement degradation factor b of the main battery 40 are also stored. jk The relationships between these relationships are stored, for example, as tables in the memory 102 of the battery ECU 100. By referring to these tables, the battery ECU 100 can calculate the current-carrying degradation coefficient α based on the temperature of the main battery 40. j Furthermore, it can calculate the placement degradation factor b based on the temperature and SOC of the main battery 40. jk .

[0041] Furthermore, the battery ECU100 uses an electrical degradation coefficient a. j To calculate the electrical degradation d1, and using the placement degradation factor b jk The placement degradation amount d2 is calculated. Then, the battery ECU100 calculates the overall degradation amount of the main battery 40, i.e. the total degradation amount D (refer to the following formula (3)) based on the sum of the cumulative value of the power-on degradation amount d1 over time from the past (starting point) to the present (cumulative power-on degradation amount D1) and the cumulative value of the placement degradation amount d2 over time from the past to the present (cumulative placement degradation amount D2).

[0042] D1+D2=D···(3)

[0043] Error in current degradation

[0044] The inventors have pointed out that, under certain conditions, when calculating the total degradation amount D as described above, errors in the electrical degradation amount d1 can easily occur, thereby reducing the accuracy of the calculation of the total degradation amount D.

[0045] Figure 3This diagram is used to explain the reason for the error in the electrical degradation amount d1. Here, we assume that on a certain day (24 hours), vehicle 1 is used for 8 hours and then left idle for 16 hours. In this case, as shown as a comparative example, we can consider calculating the electrical degradation amount d1 according to the electrical degradation logic for the 8 hours of vehicle 1 use, and calculating the placement degradation amount d2 according to the placement degradation logic for the 16 hours of vehicle 1 idle.

[0046] Here, the electrical degradation amount d1 (more specifically, the electrical degradation coefficient a used to calculate the electrical degradation amount d1) is calculated based on the premise that vehicle 1 is in motion (supplying the power required to realize the driving simulation mode described later from the main battery 40). j However, as in Figure 1 As explained, vehicle 1 has various functions that differ from those used when the main battery 40 is in motion. More specifically, the power supplied from the main battery 40 is sometimes used to charge the auxiliary battery 70 (hereinafter also referred to as "auxiliary draw"), and sometimes used for external power supply, referred to as V2H, V2L.

[0047] The charge (or charge per unit time, i.e., current) of the main battery 40 when the auxiliary machine draws power or when it is powered externally is significantly reduced compared to the charge during operation. For example, the current during operation is 0.2A compared to 50A, while the current during auxiliary machine draws power is 5A. That is, the current during operation is one order of magnitude larger than the current during external power supply, and two orders of magnitude larger than the current during auxiliary machine draws power. Therefore, when calculating the current degradation d1 over the entire 8-hour period based on operation, the current degradation d1 during auxiliary machine draws power or when it is powered externally may be overestimated. As a result, the total degradation D may be inaccurate.

[0048] Therefore, in this embodiment, when the charge level of the main battery 40 is lower than the reference level (hereinafter also referred to as "low-speed charging"), the battery ECU 100 calculates the placement degradation amount d2 according to the placement degradation logic (refer to...). Figure 3 (at the very bottom). That is, when the battery ECU100 is powered on at low speed, even during the use of the vehicle 1, it calculates the placement degradation amount d2 according to the placement degradation logic instead of the power-on degradation logic. This suppresses the generation of errors in the power-on degradation amount d1 when powered on at low speed. As a result, the calculation accuracy of the total degradation amount D can be improved.

[0049] Processing flow

[0050] Figure 4This is a flowchart illustrating an example of the processing steps in the degradation estimation process of this embodiment. The processing shown in this flowchart is executed when predetermined conditions are met (e.g., every predetermined control cycle). Each step is implemented by software processing of the battery ECU 100, but can also be implemented by hardware (electronic circuitry) configured within the battery ECU 100. Hereinafter, the steps will be abbreviated as S.

[0051] In S1, the battery ECU 100 determines whether the vehicle 1 is in use or in storage. If the main battery 40 is in a state where it can be energized (charged / discharged), the battery ECU 100 determines that the vehicle 1 is in use. Use of the vehicle 1 includes, but is not limited to, driving (which may include temporary stops), and can include auxiliary equipment drawing power, external charging, and external power supply. The battery ECU 100 can also determine that the vehicle 1 is in use if the vehicle 1 is in a ready-to-go state.

[0052] On the other hand, if the main battery 40 is not energized, the battery ECU 100 determines that the vehicle 1 is in a parked state. Parking of the vehicle 1 typically occurs when the main battery 40 is electrically disconnected from other devices, such as by disconnecting a system main relay (SMR) (not shown) located on the main battery 40. The battery ECU 100 can also determine that the vehicle 1 is in a parked state if the vehicle 1 is not in a ready-to-go state.

[0053] When vehicle 1 is in use ("in use" in S1), battery ECU 100 initiates processing in S2. On the other hand, when vehicle 1 is parked ("parked" in S1), battery ECU 100 initiates processing in S5.

[0054] In S2, the battery ECU 100 calculates the charge during a specified period based on the detection result of the current I obtained from the current sensor 52. In this example, the battery ECU 100 calculates the average current value [unit: A] during the specified period, but it can also calculate the cumulative value of the current I [unit: A·s, A·min, Ah, etc.] during the specified period as the charge.

[0055] In S3, the battery ECU 100 determines whether the average current value (or cumulative current value) calculated in S2 is above a reference value. The reference value is a current value small enough to be considered low-speed energization (e.g., 5A), which is set sufficiently small compared to the average current value when the vehicle 1 is in motion (e.g., 50A). If the average current value is above the reference value (yes in S3), the battery ECU 100 moves the process to S4 and executes the energization degradation logic. Conversely, if the average current value is less than the reference value (no in S3), the battery ECU 100 moves the process to S5 and executes the placement degradation logic. Thus, the battery ECU 100 executes the energization degradation logic based on the case where the amount of electricity (average current value) flowing through the vehicle 1 while it is in motion (when the PCU 80 is operating) is above the reference value, and executes the placement degradation logic based on the case where the amount of electricity (average current value) flowing through the auxiliary machine or when the external power supply is less than the reference value.

[0056] In the power-on degradation logic of S4, the battery ECU 100 obtains the temperature of the main battery 40 (S41). The battery ECU 100 obtains the power-on degradation coefficient 'a' corresponding to the temperature of the main battery 40 by referring to the mapping stored in the memory 102. j (S42). The battery ECU100 calculates the current by multiplying the square root of the applied current Ah by the applied current degradation coefficient a according to the above formula (1). j The current-current degradation amount d1 is calculated (S43). Then, as shown in equation (4) below, the battery ECU100 calculates (updates) the current (nth) cumulative current-current degradation amount D1(n) by adding the new current-current degradation amount d1 to the cumulative current-current degradation amount D1(n-1) up to the last (n-1th) time. Then, the battery ECU100 proceeds to S6.

[0057] D1(n)=D1(n-1)+d1···(4)

[0058] In the placement degradation logic of S5, the battery ECU 100 obtains the temperature and SOC of the main battery 40 (S51). The battery ECU 100 obtains the placement degradation coefficient b corresponding to the temperature and SOC of the main battery 40 by referring to the mapping stored in the memory 102. jk (S52). The battery ECU100 calculates the time t by multiplying the square root of the time t by the placement degradation factor b according to the above formula (2). jk The placement degradation amount d2 is calculated (S53). Then, as shown in equation (5) below, the battery ECU100 calculates (updates) the current cumulative placement degradation amount D2(n) by adding the new placement degradation amount d2 to the cumulative placement degradation amount D2(n-1) up to the last time (S54). Then, the battery ECU100 proceeds to S6.

[0059] D2(n)=D2(n-1)+d2···(5)

[0060] In S6, the battery ECU100 calculates the sum of the cumulative power-on degradation D1 and the cumulative placement degradation D2 as the total degradation D (refer to the above formula (3)).

[0061] In S7, the battery ECU100 calculates the capacity retention rate Q [unit: %] based on the total degradation amount D. Specifically, the battery ECU100 calculates the capacity retention rate Q according to the following formula (6).

[0062] Q = 100 - √D···(6)

[0063] Deterioration coefficient

[0064] Figure 5 This is a diagram illustrating the driving simulation modes (i.e., the driving modes of batteries of the same type as the main battery 40) used to determine the power-on degradation coefficient. In this example, the upper part illustrates a driving simulation mode with one cycle of power-on quantity when vehicle 1 is a pure electric vehicle (BEV). The lower part illustrates a driving simulation mode with one cycle of power-on quantity when vehicle 1 is a hybrid electric vehicle (HEV). The horizontal axis represents time. The left vertical axis represents the current value flowing in the battery, and the right vertical axis represents the state of charge (SOC) of the battery. A power-on durability test is performed while maintaining a constant ambient temperature for the battery. That is, the battery is charged and discharged repeatedly in a driving simulation mode with varying current values ​​and SOC as shown in the diagram at a constant temperature. Then, the capacity retention rate of the battery after the test is measured.

[0065] Figure 6 This is a graph illustrating the degradation coefficient *a* and the degradation coefficient *b*. Generally, the degradation of a battery due to charging (the decrease in capacity retention caused by charging) is proportional to the square root of the amount of charge charged. Therefore, as shown in the graph above, if the horizontal axis represents the charge charged by the main battery 40 and the vertical axis represents the capacity retention of the main battery 40, then a straight line represents the relationship between capacity retention and the square root of the charge charged. The slope of this line corresponds to the degradation coefficient *a*. In this example, the degradation coefficient *a* (25°C) is 0.0005 at 25°C, 0.0007 at 40°C, and 0.001 at 60°C.

[0066] Generally, under the condition of equal SOC, the amount of storage degradation of a battery (the decrease in capacity retention caused by storage) is proportional to the square root of the elapsed time. Therefore, as shown in the figure below, if the horizontal axis represents the square root of the elapsed time (number of days) and the vertical axis represents the capacity retention of the main battery, then a straight line can be used to represent the relationship between capacity retention and the square root of the elapsed time. The slope of this line corresponds to the storage degradation coefficient b. In this example, at SOC = 90%, the storage degradation coefficient b (25°C) is 0.006 at 25°C, 0.009 at 40°C, and 0.01 at 60°C. Although not illustrated, the same straight line can be obtained even under conditions of equal temperature but different SOCs.

[0067] As described above, in this embodiment, even when using vehicle 1, if the charge level (average current value or cumulative current value over a specified period) of the main battery 40 is less than a reference value, the battery ECU 100 executes placement degradation logic instead of power-on degradation logic. The degradation mode of the main battery 40 during low-speed power-on when the charge level is less than the reference value is closer to placement degradation than the power-on degradation assumed during vehicle 1's operation. Therefore, by executing placement degradation logic, the generation of error in the power-on degradation amount d1 during low-speed power-on is suppressed. Therefore, according to this embodiment, the estimation accuracy of the capacity retention rate Q of the main battery 40 mounted on vehicle 1 can be improved.

[0068] All points in the embodiments disclosed herein should be considered illustrative and not intended to limit the invention. The scope of this disclosure is not limited by the description of the embodiments above, but is defined by the technical solutions and is intended to include equivalents and all modifications within that scope.

Claims

1. A vehicle, characterized in that, The vehicle has the following features: Electric generator for driving; The drive unit drives the electric generator; The battery is charged and discharged by the drive unit while the vehicle is in motion; as well as The processor performs a power-on degradation process (calculating the amount of power supplied to the battery) when the vehicle is in use to calculate the amount of power-on degradation, and performs a storage degradation process (calculating the amount of storage time supplied to the battery) when the vehicle is parked to calculate the amount of storage degradation, and estimates the degree of battery degradation based on the sum of the power-on degradation and the storage degradation. Even when the vehicle is in use, the processor performs the placement degradation process when the charge level is below a reference level.

2. The vehicle according to claim 1, characterized in that, The processor is configured as follows: In the aforementioned electrical degradation treatment, for each temperature of the battery, the square root of the electrical charge is multiplied by the electrical degradation rate of the battery. For all temperatures, the calculated amounts are summed to determine the amount of electrical degradation. In the aforementioned storage degradation treatment, for each temperature and SOC of the battery, the amount obtained by multiplying the square root of the storage time by the storage degradation rate of the battery is calculated. For all temperatures and all SOCs, the calculated amounts are added together to calculate the storage degradation amount.

3. The vehicle according to claim 2, characterized in that, The processor calculates the current capacity retention of the battery as the degree of degradation by subtracting the square root of the sum from the capacity retention before degradation.

4. The vehicle according to any one of claims 1 to 3, characterized in that, The vehicle also features: Auxiliary battery; A charging device that uses power supplied from the storage battery to charge the auxiliary battery; Power supply port; as well as The power supply device supplies power to the outside of the vehicle via the power supply port using the power supplied from the battery. The processor is configured to perform power-on degradation processing when the power supply is above the reference amount when the drive device is operating, and to perform placement degradation processing when the power supply is below the reference amount when the charging device or the power supply device is operating.

5. A method for estimating the degradation of a storage battery, wherein the storage battery is mounted in a vehicle, characterized in that, The battery degradation estimation method includes the step of estimating the degree of degradation of the battery through a processor. The steps for the estimation include: When using the vehicle, a step is performed to calculate the amount of battery degradation by performing a power degradation process on the amount of power supplied by the battery. When the vehicle is placed, a step is performed to calculate the amount of storage degradation of the battery by using the storage time of the battery. Even when the vehicle is in use, the placement degradation treatment step is performed when the charge level is lower than a reference level; and The step of estimating the degree of degradation based on the sum of the electrical degradation and the placement degradation.

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