Battery Deterioration Diagnosis Method

Abstract

An object of the present invention is to provide a method for diagnosing battery deterioration that can easily estimate battery capacity in a short time. [Solution] 6) estimating that the internal temperature of the first battery is equal to the internal temperature of the second battery; 7) measuring the internal resistance of the second battery; and 8) estimating the SOH of the second battery from the internal temperature estimated in 6) above and the internal resistance measured in 7) above, using the temperature correction equation created in 2).

Description

[Technical Field]

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

[0002] The market for lithium-ion batteries is expanding rapidly for both industrial and household use, and as a result, the disposal of lithium-ion batteries is expected to increase. In recent years, there has been an increasing trend toward recycling and reusing lithium-ion batteries from the perspectives of 1) reducing environmental impact by realizing a circular economy, 2) resource risks for lithium-ion battery materials such as Co, Ni, and Li, and 3) reducing the life cycle costs of electric vehicles.

[0003] Used automotive lithium-ion batteries are discarded after about 5 to 10 years of use due to a decrease in battery capacity, but they still retain performance that makes them usable for other uses (such as stationary storage batteries). Therefore, the reuse of used automotive lithium-ion batteries for other purposes is being considered. However, this is not limited to automotive lithium-ion batteries, but also residential and industrial storage batteries are being considered for reuse for other purposes as long as they retain usable performance.

[0004] Repeated charging and discharging of a lithium-ion battery reduces the usable battery capacity. SOH (State of Health) is used as an indicator of usable battery capacity. SOH is calculated using the following formula: (Formula) SOH (%) = (battery capacity after deterioration) / (initial battery capacity) x 100 From this formula, if the SOH is 50%, it means that even if the battery is fully charged, it will only have half of its initial capacity.

[0005] Regarding the battery capacity of in-vehicle lithium-ion batteries, Patent Documents 1 and 2 disclose techniques for improving the accuracy of estimating the degree of battery deterioration. Patent Documents 2 and 3 disclose techniques related to a battery state estimation device. Furthermore, Patent Documents 4 and 5 disclose techniques for accurately measuring the internal temperature of a power storage device, since the state of health (SOH) of the power storage device affects the temperature of the power storage device. Patent Document 6 also discloses a technique for accurately estimating the battery temperature used to estimate the battery state. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-148720 [Patent Document 2] International Publication No. 2021 / 045172 [Patent Document 3] International Publication No. 2022 / 255480 [Patent Document 4] International Publication No. 2013 / 018641 [Patent Document 5] International Publication No. 2014 / 073208 [Patent Document 6] Japanese Patent Application Publication No. 2018-170144 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present disclosure is to provide a method for diagnosing battery deterioration that can accurately estimate battery capacity in a short amount of time and easily. [Means for solving the problem]

[0008] The gist of the present disclosure is as follows. [1] a first battery for constructing a calibration curve; a second battery to be diagnosed, 1) constructing a calibration curve of the SOH and internal resistance of the first battery; 2) creating a temperature correction formula by correcting the calibration curve by temperature; 3) placing the first battery and the second battery in the same environment; 4) measuring the internal resistance of the first battery; 5) calculating the internal temperature of the first battery from the SOH of the first battery and the measured internal resistance using the temperature correction formula of 2); 6) estimating an internal temperature of the first battery to be substantially the same as an internal temperature of the second battery; 7) measuring the internal resistance of the second battery; 8) estimating the SOH of the second battery from the internal temperature estimated in 6) above and the internal resistance measured in 7) above using the temperature correction formula created in 2) above; A method for diagnosing battery deterioration, comprising: [2] a first battery for constructing a calibration curve; a second battery to be diagnosed for deterioration; A battery deterioration diagnosis method using a third battery whose SOH is known, the method comprising: 1) constructing a calibration curve of the SOH and internal resistance of the first battery; 2) creating a temperature correction formula by correcting the calibration curve by temperature; 3) placing the third battery in the same environment as the second battery; 4) measuring the internal resistance of the third battery; 5) calculating the internal temperature of the third battery from the SOH and internal resistance of the third battery using the temperature correction formula of 2); 6) estimating an internal temperature of the third battery to be substantially the same as an internal temperature of the second battery; 7) measuring the internal resistance of the second battery; 8) estimating the SOH of the second battery from the internal temperature estimated in 6) above and the internal resistance measured in 7) above using the temperature correction formula created in 2) above; A method for diagnosing battery deterioration, comprising: [3] In the above 2), the first battery is placed in a thermostatic chamber to make the external temperature and the internal temperature the same. [4] The battery deterioration diagnosis method according to any one of the above [1] to [3], wherein the SOH of the first battery and / or the third battery is measured using a charge / discharge method. [5] The battery deterioration diagnosis method according to any one of [1] to [4] above, wherein the battery is in the form of a cell, a module, or a pack. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide a method for diagnosing battery deterioration that can easily estimate battery capacity in a short time. [Brief explanation of the drawings]

[0010] [Figure 1] 10 is an example of a calibration curve of SOH and internal resistance. [Figure 2] 10 is an example of a temperature correction formula obtained from a calibration curve for each temperature. [Figure 3] FIG. 3 is a reference diagram showing a storage state of the battery in the first embodiment. [Figure 4] FIG. 10 is a reference diagram showing a storage state of the battery in the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] As lithium-ion batteries (sometimes simply referred to as batteries) continue to be used, their usable battery capacity decreases due to degradation. State of health (SOH) is an index used to determine the battery capacity of used lithium-ion batteries (including unused ones that have deteriorated) and determine whether they can be reused. The charge-discharge method is also known as one method for measuring battery capacity. The charge-discharge method is a method defined by standards such as JIS, and because it directly measures capacity, it allows for accurate measurement of SOH.

[0012] On the other hand, the charge-discharge method requires about a day of cycles: leave to stand, discharge to a certain extent, leave to stand, fully charge, leave to stand, discharge, and so on, making it unsuitable for measuring the battery capacity of a large number of batteries. Furthermore, battery capacity is affected by the internal temperature of the battery, so the internal temperature must be kept constant. Therefore, to determine the battery capacity of a large number of lithium-ion batteries, it is necessary to find a simple alternative method that can be performed quickly. Furthermore, it is preferable for the alternative method to be close to the SOH (true value) measured by the charge-discharge method, be independent of the measurement environment (temperature), or be able to be corrected.

[0013] Alternative methods for measuring the battery capacity of large numbers of batteries include charge curve analysis and AC impedance analysis (AC method). However, while AC curve analysis does not require a discharge operation compared to the charge / discharge method, it has drawbacks such as a long measurement time. The AC method calculates the internal resistance when an AC current is applied to the battery and estimates the SOH from a calibration curve created in advance. However, because the internal resistance changes with temperature, temperature correction is required to accurately estimate the SOH.

[0014] When estimating SOH from internal resistance using a calibration curve, the internal resistance changes with temperature, so it is necessary to accurately measure the battery temperature. However, the temperature of a battery left stationary indoors varies depending on the measurement location (surface temperature, internal temperature, etc.) and the time of measurement (morning, noon, night, etc.), making it difficult to measure a temperature appropriate for estimating SOH. While storing batteries in a thermostatic chamber could ensure that the surface and internal temperatures are the same, storing a large number of batteries requires a large thermostatic chamber, which raises concerns about increased costs for equipment and electricity.

[0015] In light of this situation, the inventors have conducted extensive research into methods for accurately estimating battery capacity, and have devised a new method for accurately estimating the temperature inside a battery (internal temperature), which is difficult to measure directly. They then discovered that this estimated internal temperature can be applied to other batteries placed in the same environment, and discovered that for other batteries, the SOH can be estimated with high accuracy by using a calibration curve from the internal resistance and further correcting for temperature, which led to the present invention.

[0016] (First embodiment) The battery deterioration diagnosis method according to the first embodiment will be described in detail below. The first embodiment is a battery degradation diagnosis method that uses a first battery for constructing a calibration curve and a second battery to be diagnosed. The first battery is, for example, a battery used to create a calibration curve with SOH on the horizontal axis and internal resistance on the vertical axis. The first battery is a battery whose SOH is known, as measured using a method such as a charge / discharge method, and multiple batteries with different SOHs are used to create the calibration curve.

[0017] The second battery is a battery with unknown SOH, which is the target of the deterioration diagnosis of the present disclosure. The second battery may be one or more batteries, but the more batteries there are, the more efficient the deterioration diagnosis and the shorter the work time can be. In addition, the second battery may be a battery with unknown SOH, which is the target of the deterioration diagnosis of the present disclosure. The second battery is a battery to which the calibration curve created using the above formula can be applied, and is preferably the same type of battery as the first battery.

[0018] The first step is to construct a calibration curve between the SOH and internal resistance of the first battery. There are no particular restrictions on the method for measuring the SOH of the first battery, and any known method can be used. For example, the SOH can be measured using a charge / discharge method. There are no particular restrictions on the method for measuring the internal resistance of the first battery, and any known method can be used. For example, the internal resistance can be measured by applying an alternating current to a standard battery. A calibration curve can be constructed by measuring the SOH and internal resistance of each of multiple batteries with different SOHs. Figure 1 shows an example of a calibration curve (horizontal axis: SOH [%], vertical axis: internal resistance [mΩ]). For a battery with unknown SOH, measuring its internal resistance and substituting the internal resistance into the calibration curve in Figure 1 makes it possible to back-calculate (estimate) the SOH.

[0019] However, because the internal resistance of a battery varies with its temperature, the calibration curve must be corrected for temperature to estimate the SOH with high accuracy. Therefore, in the second step, a temperature correction formula is created in which the calibration curve created in the first step is corrected for the internal temperature of the battery. There are no particular restrictions on the method for creating the temperature correction formula, and any known method can be used. For example, the first battery is placed in a temperature-controllable thermostatic bath to create a calibration curve (horizontal axis: SOH, vertical axis: internal resistance). By placing the battery in the thermostatic bath, the internal and external temperatures of the battery can be made the same. The temperature of the thermostatic bath is then changed to create a calibration curve for each temperature. The slope and intercept of the calibration curve are highly correlated with temperature and can be approximated by a linear equation (temperature correction formula). Figure 2 shows calibration curves corresponding to temperatures of 10°C, 25°C, and 40°C. The calibration curve and temperature correction formula vary depending on the type of first battery.

[0020] In the third step, the first battery and the second battery used to create the calibration curve and the temperature correction formula are placed in the same environment. "In the same environment" refers to an environment in which the internal temperatures of multiple batteries stored in a warehouse or the like, such as that shown in Fig. 3, can be estimated to be substantially the same. Specifically, in order to place the batteries in the same environment, the following items (a) to (e) can be adjusted: (a) Surface temperature difference: The difference in surface temperature between the first battery and the second battery is within ±3°C. (b) Ambient temperature difference: The difference in air temperature between the first battery and the second battery must be within ±2°C. (c) Shading conditions: Avoid direct sunlight (d) Air conditioning conditions: - Keep at least 3m away from heat sources (heaters, air conditioners, windows) Air flow rate of 0.5 m / s or less (e) Humidity difference: The relative humidity difference between the storage locations of the first battery and the second battery is within 10% RH

[0021] It was confirmed that when at least the above conditions are met, the error in the internal temperature of the battery is within ±1°C. With an error of this magnitude, it can be assumed that the internal temperatures of the batteries are substantially the same. As mentioned above, multiple first batteries are used to construct the calibration curve, and one of these batteries can be selected as the first battery to be placed in the same environment as the second battery.

[0022] In the fourth step, the internal resistance of the first battery is measured. The internal resistance of the first battery has already been measured to construct a calibration curve. However, since the internal resistance is affected by the internal temperature of the battery, it is necessary to measure the internal resistance again when the environment is changed. The internal resistance of the first battery can be measured using, for example, an alternating current. Since the battery's SOH has already been measured using the charge / discharge method, there is no need to measure it again.

[0023] In the fifth step, the internal temperature of the first battery is calculated using the temperature correction formula in the second step from the (known or measured) SOH of the first battery and the internal resistance measured in the fourth step. Specifically, the internal temperature of the first battery can be calculated by substituting the SOH and internal resistance of the first battery into the temperature correction formula created in the second step. Here, while it is possible to estimate the battery temperature using, for example, the room temperature of the warehouse where the battery is stored or the surface temperature of the battery, it has been confirmed that using room temperature or surface temperature results in large errors. It is also possible to store the battery in a thermostatic bath to make the internal and surface temperatures of the battery the same, but as mentioned above, storing multiple batteries in a thermostatic bath is costly. It has been confirmed that the internal temperature obtained in the fifth step has an error level equivalent to that of measurements using a thermostatic bath.

[0024] Conventionally, it has been difficult to directly measure the internal temperature of a battery, and even if it were possible, it would be inefficient to measure the internal temperature of all batteries from an operational standpoint. In this embodiment, the internal resistance of a battery whose SOH is known (or has been measured) is measured, and the internal temperature is calculated using a temperature correction formula. Then, by placing a battery whose internal temperature has been calculated (corresponding to the first battery) and a battery whose SOH is unknown (or has not been measured) (corresponding to the second battery) in the same environment, it is assumed that the internal temperatures of all batteries in the same environment are substantially the same.

[0025] That is, in the sixth step, the internal temperature of the second battery placed in the same environment as the first battery is estimated to be substantially the same as the internal temperature of the first battery. While an identical environment would ideally be something like a thermostatic bath, it has been confirmed that there is no problem in estimating that the internal temperatures are substantially the same if the batteries are placed in the same indoor environment, such as a warehouse. There is no particular limit to the period of time the batteries are placed in the same environment, as long as the initial temperature of the batteries (the temperature before being placed) is not an unusual temperature.

[0026] In a seventh step, the internal resistance of the second battery is measured in the same environment as the first battery. The internal resistance of the second battery can be measured using, for example, an alternating current.

[0027] In the eighth step, the SOH of the second battery is estimated using the temperature correction formula created in the second step from the internal temperature estimated in the sixth step and the internal resistance measured in the seventh step (i.e., battery degradation is diagnosed). By going through the first to eighth steps described above, it is possible to estimate the battery capacity (SOH) of a battery (corresponding to the second battery) whose SOH is unknown (or has not been actually measured) without actually measuring the SOH using a charge / discharge method or the like, and without using a thermostatic bath or the like to estimate the internal temperature, making it possible to simply estimate the battery capacity (SOH) in a short time.

[0028] (Second embodiment) The battery deterioration diagnosis method according to the second embodiment will be described in detail below. The second embodiment is a method for diagnosing battery degradation using a first battery for constructing a calibration curve, a second battery for degradation diagnosis, and a third battery whose SOH is known (including cases where it is actually measured). The first battery and the second battery play the same roles as the first battery and the second battery according to the first embodiment, respectively. The third battery differs from the first battery in that it is not used to construct a calibration curve, and also differs from the second battery in that its SOH is known. In the first embodiment, the first battery and the second battery are placed in the same environment, and the internal temperature of the second battery is estimated using the first battery, but in the second embodiment, the third battery is used instead of the first battery. The third battery and the second battery are placed in the same environment, and the internal temperature of the second battery is estimated using the third battery.

[0029] In the second embodiment, the first step of constructing the calibration curve and the second step of creating the temperature correction equation can both be the same as in the first embodiment. However, there are no particular restrictions on the method of actually measuring the SOH or the method of measuring the internal resistance, and methods other than those used in the first embodiment can be used.

[0030] In the third step, as illustrated in FIG. 4, a third battery with a known SOH and the second battery are placed in the same environment. Alternatively, one battery may be selected from multiple batteries with unknown SOH in the same environment as the third battery, and the remaining batteries may be designated as the second batteries. However, if the third battery is selected from multiple batteries with unknown SOH, the SOH of the third battery must be measured using a charge / discharge method or the like. As in the first embodiment, the term "in the same environment" refers to an environment in which the internal temperatures of multiple batteries stored in a warehouse or the like can be estimated to be substantially the same.

[0031] In a fourth step, the internal resistance of the third battery is measured. The internal resistance of the third battery can be measured using, for example, an alternating current.

[0032] In the fifth step, the internal temperature of the third battery is calculated using the temperature correction formula in the second step from the third battery's (known or measured) SOH and the internal resistance measured in the fourth step. Specifically, the internal temperature of the third battery can be calculated by substituting the third battery's SOH and internal resistance into the temperature correction formula created in the second step. Here, while it is possible to estimate the battery temperature using, for example, the room temperature of the warehouse where the batteries are stored or the surface temperature of the battery, it has been confirmed that using room temperature or surface temperature results in large errors. It is also possible to store the battery in a thermostatic bath to make the internal and surface temperatures of the battery the same, but as mentioned above, storing multiple batteries in a thermostatic bath is costly. It has been confirmed that the internal temperature obtained in the fifth step has an error level equivalent to that of measurements using a thermostatic bath.

[0033] In the past, it was difficult to directly measure the internal temperature of a battery, and even if it were possible, it would be inefficient to measure the internal temperature of all batteries. In this embodiment, the internal resistance of a battery whose SOH is known (already measured) is measured, and the internal temperature is calculated using a temperature correction formula. Then, by placing a battery whose internal temperature has been calculated (corresponding to the third battery in this embodiment) and a battery whose SOH is unknown (unmeasured) in the same environment, it is assumed that the internal temperatures of all batteries in the same environment are substantially the same.

[0034] That is, in the sixth step, the internal temperature of the second battery placed in the same environment as the third battery is estimated to be substantially the same as the internal temperature of the third battery. While an identical environment would ideally be something like a thermostatic bath, it has been confirmed that there is no problem in estimating that the internal temperatures are substantially the same if the batteries are placed in the same indoor environment, such as a warehouse. There is no particular limit to the time the batteries are placed in the same environment, as long as the initial temperature of the batteries (the temperature before being placed) is not an unusual temperature.

[0035] In a seventh step, the internal resistance of the second battery is measured in the same environment as the third battery. The internal resistance of the second battery can be measured using, for example, an alternating current.

[0036] In the eighth step, the internal temperature estimated in the sixth step and the internal resistance measured in the seventh step are used to calculate the internal temperature. The SOH of the second battery is estimated using the temperature correction formula created in the second step (i.e., a battery deterioration diagnosis is performed). By going through the above-mentioned steps 1 to 8, it is possible to estimate the battery capacity (SOH) of a battery (corresponding to the second battery) whose SOH is unknown (or has not been actually measured) without actually measuring the SOH using a charge / discharge method or the like, and without using a thermostatic bath or the like to estimate the internal temperature, making it possible to simply estimate the battery capacity (SOH) in a short amount of time.

[0037] In the first and second embodiments, the battery includes any of the forms of a cell, a module, and a pack. [Example]

[0038] Next, examples of the present invention and comparative examples will be described. Note that the following examples are representative examples, and the present invention is not necessarily limited by these examples, and should be interpreted within the scope of the technical ideas described in the specification.

[0039] Deterioration diagnosis was performed on the following lithium-ion batteries. Battery: Lithium-ion battery for electric vehicles Battery voltage: 29.6V Initial capacity: 40Ah (1,184Wh) Battery dimensions: W180 mm ×D305 mm ×H130 mm Battery weight: 12kg Charge rate: SOC (State of charge) 50% First, to construct a calibration curve, multiple batteries (corresponding to the first battery) were placed in a thermostatic chamber, and the temperature of the chamber was changed to measure the SOH (actual value) and internal resistance using a charge / discharge method. A calibration curve was then constructed based on the measured SOH and internal resistance. Figure 2 (left) shows the calibration curves when the temperature was changed to 10°C, 25°C, and 40°C. Figure 2 (right) also shows the correlation between the slope and intercept of each calibration curve and temperature. The following temperature correction formula was derived from the correlation in Figure 2 (right). (Equation 1) Internal resistance = a × (SOH) + b (a: slope, b: intercept) (Formula 2)a=0.0039T-0.29 (Formula 3)b=-0.34T+28

[0040] Next, one battery was selected from the batteries (corresponding to the first battery) for which the calibration curve had been constructed, and was moved to a warehouse where batteries (corresponding to the second battery) with unknown SOH were stored, as shown in Figure 3, and left to stand for approximately one day. After that, the internal resistance of the first battery was measured every hour (the actual measured SOH was 64.5%). The measured internal resistance and SOH were then substituted into the above (Equations 1) to (Equations 3) to calculate the internal temperature of the selected first battery.

[0041] Next, the internal temperature of the battery with unknown SOH (corresponding to the second battery) was estimated to be substantially the same as the internal temperature of the first battery calculated above. Then, as shown below, the internal resistance of each of the second batteries (No. 2 to No. 13) was measured, and the SOH of the second battery was estimated by substituting the internal temperature and internal resistance into the above (Equations 1) to (Equations 3). The internal resistance and estimated SOH of each battery are shown in Table 1 below. The difference between the SOH (measured value) measured using the charge / discharge method and the SOH (estimated value) estimated using the internal temperature was within ±5%, and the root mean square error (RMSE) was 2.2%, demonstrating good results. Although not shown in Table 1, the difference in estimated SOH (maximum value minus minimum value) depending on the measurement time for the same sample was up to 0.6%. For reference, the SOH was estimated using the surface temperature and compared with the SOH (actual measured value). The RMSE value was 2.8%, and the difference in estimated SOH (maximum value - minimum value) depending on the measurement time for the same sample was a maximum of 1.5%.

[0042] [Table 1] [Industrial Applicability]

[0043] As described above, this disclosure proposes a new method for determining internal temperature, and by applying that internal temperature to other batteries in the same environment, it is possible to determine the temperature-corrected SOH with high accuracy. This disclosure provides a degradation diagnosis method for quickly and easily estimating the battery capacity of a lithium-ion battery (battery) placed stationary indoors. The development of such a diagnostic method with low error makes it possible to accurately and quickly determine whether a lithium-ion battery can be reused. Furthermore, because temperature adjustment using a thermostatic bath is not required, there are cost benefits.

Claims

1. a first battery for constructing a calibration curve; a second battery to be diagnosed with deterioration, 1) constructing a calibration curve of the SOH and internal resistance of the first battery; 2) creating a temperature correction formula by correcting the calibration curve by temperature; 3) placing the first battery and the second battery in the same environment; 4) measuring the internal resistance of the first battery; 5) calculating an internal temperature of the first battery from the SOH of the first battery and the measured internal resistance using the temperature correction formula of 2); 6) estimating the internal temperature of the first battery to be substantially the same as the internal temperature of the second battery; 7) measuring the internal resistance of the second battery; 8) estimating the SOH of the second battery from the internal temperature estimated in 6) above and the internal resistance measured in 7) above using the temperature correction formula created in 2) above; A method for diagnosing battery deterioration, comprising:

2. a first battery for constructing a calibration curve; a second battery to be diagnosed for deterioration; a third battery having a known SOH; and a method for diagnosing deterioration of a battery, the method comprising: 1) constructing a calibration curve of the SOH and internal resistance of the first battery; 2) creating a temperature correction formula by correcting the calibration curve by temperature; 3) placing the third battery and the second battery in the same environment; 4) measuring the internal resistance of the third battery; 5) calculating the internal temperature of the third battery from the SOH and internal resistance of the third battery using the temperature correction formula of 2); 6) estimating the internal temperature of the third battery to be substantially the same as the internal temperature of the second battery; 7) measuring the internal resistance of the second battery; 8) estimating the SOH of the second battery from the internal temperature estimated in 6) above and the internal resistance measured in 7) above using the temperature correction formula created in 2) above; A method for diagnosing battery deterioration, comprising:

3. 3. The method for diagnosing deterioration of a battery according to claim 1, wherein in the above 2), the first battery is placed in a thermostatic chamber to make the external temperature and the internal temperature the same.

4. 3. The battery deterioration diagnosis method according to claim 1, wherein the SOH of the first battery and / or the third battery is measured using a charge / discharge method.

5. 3. The method for diagnosing deterioration of a battery according to claim 1, wherein the battery is in the form of a cell, a module, or a pack.

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