JP7669196B2 - State determination device, secondary battery system, and state determination method - Google Patents

Description

This disclosure relates to a state determination device, a secondary battery system, and a state determination method.

In recent years, secondary batteries such as lithium-ion secondary batteries have been used as on-board power sources for aircraft, ships, and vehicles, as power sources for storing electricity in smart houses, and in mobile communication terminals, and efforts are being made to use energy efficiently. The characteristics of secondary batteries deteriorate due to charging and discharging and storage. In particular, in the above applications, it is expected that they will be used as a power source for a long period of time. For this reason, in order to continue to use products equipped with secondary batteries stably, it is preferable to appropriately determine the deterioration state of the secondary battery.

Mobile communication terminals such as smartphones employ a rapid charging method in which a secondary battery is charged with a large current in order to shorten charging times. Such rapid charging rapidly deteriorates the characteristics of the secondary battery. Therefore, in order to determine the deterioration state of the secondary battery, it is preferable to appropriately evaluate the deterioration of the characteristics of the secondary battery caused by rapid charging.

The deterioration of secondary battery characteristics is due to the deterioration of the active material contained in the electrodes of the secondary battery. The active material is one of the materials that make up the electrodes of secondary batteries, and plays a role in promoting the charge and discharge reactions of secondary batteries by storing and releasing ions. For example, in the case of a lithium-ion secondary battery, lithium ions are released from the positive electrode active material during charging, and the released lithium ions are stored in the negative electrode active material, thereby promoting the charging reaction. The discharge reaction follows this reverse process.

Claim 1 of Patent Document 1 describes a "state estimation method for a rechargeable battery, comprising: determining, from the battery current, that the battery has stopped charging and discharging from a charging and discharging state; measuring the change over time in the voltage drop at time t from the stop of charging and discharging; expressing the measured voltage relaxation behavior as an approximation curve using a relaxation function Fv(t); estimating the state of charge of the battery from the relaxation function Fv(t); and characterizing the relaxation function Fv(t) as a sum of exponential functions having multiple different time constants, each of which is related to a different event occurring in the battery."

JP 2014-196943 A

An electrode is usually constructed by carrying an active material on the surface of a current collector. The active material deteriorates with charging and discharging, but the degree of deterioration varies depending on the frequency of absorption and release of lithium ions. For example, deterioration is more likely to progress on the electrode surface side (the side farther from the current collector) where absorption and release occur frequently, but deterioration is less likely to progress on the inner side of the electrode (the side closer to the current collector) where absorption and release occur less frequently. This causes uneven deterioration within the electrode.

The technology described in Patent Document 1 does not take uneven deterioration of the electrodes into consideration, which poses a problem in terms of accuracy in determining the state of the secondary battery.
The problem to be solved by the present disclosure is to provide a state determination device, a secondary battery system, and a state determination method that are capable of determining a state with high accuracy.

The state determination device of the present disclosure includes a parameter determination unit that determines a distribution state of a time constant in a relaxation process of a response function as a parameter of a first function that indicates the state of the secondary battery based on at least one of first time-varying data of voltage, which is a measurable response function of the secondary battery , from the start to the end of charging/discharging with a constant current to the secondary battery, and second time-varying data of voltage, which is the response function from the end of charging/discharging with a constant current until the voltage reaches a steady state; and a parameter determination unit that determines a distribution state of a time constant in a relaxation process of a response function as a parameter of a first function that indicates the state of the secondary battery based on the parameter determined by the parameter determination unit. and a charge/discharge condition determination unit that determines a charge/discharge condition, which is at least one of a charge/discharge current and a charge/discharge time of the secondary battery, so that the charge/discharge condition is included in a range, and the first function is expressed as V(t)=V0+∫f(τ)exp(-t/τ)dτ, where V0 is an open circuit voltage of the secondary battery, τ is the time constant, and the first function is expressed as V(t)=V0+∫f(τ)exp(-t/τ)dτ, where V0 is an open circuit voltage of the secondary battery, τ is the time constant, and the function f(τ) is a second function that represents the shape of a histogram created for the time constant, which changes into a wide shape according to an increase in unevenness in deterioration of an active material in the secondary battery. Other solutions will be described later in the description of the embodiment of the invention.

The present disclosure provides a state determination device, a secondary battery system, and a state determination method that can determine the state with high accuracy.

1 is a block diagram of a secondary battery system including a state determination device of the present disclosure. FIG. 2 is a schematic diagram of an electrode of an undegraded secondary battery. FIG. 2 is a schematic diagram of an electrode of a deteriorated secondary battery. 1 is a histogram showing the distribution state of the relaxation time constant in a deteriorated secondary battery. 1 is a histogram showing the distribution state of the relaxation time constants in an undegraded secondary battery. 11 is a graph showing the relationship between the shape and parameters of a Gaussian function as an example of a second function. 13 is a graph showing the relationship between the shape of a function indicating Levy's stable distribution as an example of a second function and parameters. 4 is a flowchart illustrating a state determination method according to the present disclosure. 1 is a graph showing the results of Example 1. 1 is a graph showing the results of Example 2.

Below, a form for implementing the present disclosure (referred to as an embodiment) will be described with reference to the drawings. In the following description of one embodiment, other embodiments that can be applied to the one embodiment will also be described as appropriate. The present disclosure is not limited to the one embodiment below, and different embodiments can be combined with each other, or modified as desired without significantly impairing the effects of the present disclosure. In addition, the same symbols will be used for the same components, and duplicate descriptions will be omitted. Furthermore, components having the same functions will be given the same names. The contents shown are merely schematic, and for the sake of illustration, changes may be made from the actual configuration without significantly impairing the effects of the present disclosure, and some components may be omitted or modified between drawings.

Figure 1 is a block diagram of a secondary battery system 400 including a state determination device 100 of the present disclosure. The secondary battery system 400 includes the state determination device 100, a secondary battery 200, and a charge/discharge control device 300. Of these, the secondary battery 200 is composed of a single or multiple secondary battery cells, such as, but not limited to, a lithium ion secondary battery. In addition, the secondary battery 200 may be either an undegraded one (Figure 2) or a degraded one (Figure 3).

The state determination device 100 is a device that determines the state of the secondary battery 200. The state here includes, for example, the degree of deterioration of the secondary battery 200, taking into account the uneven deterioration of the active material supported on the electrode 201 of the secondary battery 200. The uneven deterioration will be described with reference to Figures 2 and 3.

Figure 2 is a schematic diagram of an electrode 201 (positive or negative electrode) of an undegraded secondary battery 200. The electrode 201 of the secondary battery 200 is, for example, a current collector foil 11 carrying active material 12 (positive or negative active material), conductive additive 13, etc. Therefore, the inside of the electrode 201 is filled with active material 12.

Figure 3 is a schematic diagram of the electrode of a deteriorated secondary battery 200. When a large current is charged and discharged to the secondary battery 200, such as in rapid charging, the active material 12 in the electrode 201 deteriorates unevenly due to the expansion and contraction of the active material 12, Joule heat, and the like. For example, in addition to the deteriorated active material 121, active material 122 in which deterioration progresses more slowly is formed, and other active materials 12 in different states of deterioration are formed. In other words, the electrode 201 of the secondary battery 200 (Figure 1) deteriorates unevenly depending on the method of use, such as rapid charging.

Deterioration of the active material 12 causes deterioration of characteristics, such as an increase in the resistance of the secondary battery 200 and a decrease in the battery capacity (amount of electricity that can be stored and discharged) of the secondary battery 200. Therefore, in order to accurately grasp the deterioration of the characteristics of the secondary battery 200, it is preferable to evaluate the non-uniformity of the deterioration state formed in the electrode 201, i.e., the uneven deterioration, with high accuracy.

Returning to FIG. 1, the state determination device 100 includes a measurement unit 110, a determination unit 120, a memory unit 130, a parameter determination unit 140, an output unit 150, a charge/discharge condition determination unit 160, and a remaining life prediction unit 170.

The measurement unit 110 measures the changes over time (corresponding to the first time-varying data described below) of the voltage, current, and temperature of the secondary battery 200 from time t1 when charging/discharging of the secondary battery 200 starts to time t2 when charging/discharging ends, and stores the data in the memory unit 130. In addition, the measurement unit 110 measures the changes over time (corresponding to the second time-varying data described below) of the voltage, current, and temperature of the secondary battery 200 from time t2 to time t3 when charging/discharging resumes, which is the relaxation time, and stores the data in the memory unit 130. The measurement times from t1 to t2 and from t2 to t3 are not particularly limited, but are preferably, for example, independently, 1 minute or more, and more preferably 10 minutes or more.

The determination unit 120 determines whether or not the measurement results of the voltage and temperature during charging and discharging (from time t1 to time t2, which is equivalent to when electricity is flowing) acquired by the measurement unit 110 are to be used in the processing by the parameter determination unit 140. The determination unit 120 first calculates the average value μI and the variance value σI of the current value during charging and discharging using the time-dependent change data of the current value measured during charging and discharging. Next, the determination unit 120 approximates the change in the charging rate of the secondary battery 200 before and after charging and discharging as ΔSOC = μI × (t2 - t1). Then, the determination unit 120 compares each of σI and ΔSOC with a predetermined value set in advance. If it is greater than the predetermined value, the determination unit 120 determines that only the measurement results during non-charging and discharging (times t2 to t3) are to be used in the processing by the parameter determination unit 140. Conversely, if it is smaller than the predetermined value, the determination unit 120 determines that at least one of the measurement results during charging/discharging or non-charging/discharging should be used for processing by the parameter determination unit 140.

The memory unit 130 stores measurement data such as time-varying data of voltage, current, and temperature acquired by the measurement unit 110. The memory unit 130 also stores at least one type of a first function described below and a second function for acquiring information on uneven deterioration of the electrode 201 (FIG. 1) from the measurement data. The second function will be described later.

The parameter determination unit 140 determines the distribution state of the time constant (relaxation time constant τ) in the voltage relaxation process as a parameter of a first function indicating the state of the secondary battery 200 based on the time change data stored in the memory unit 130. The time change data is at least one of first time change data of the voltage from the start to the end of charging and discharging with a constant current to the secondary battery 200, or second time change data of the voltage from the end of charging and discharging with a constant current until the voltage reaches a steady state. The voltage is an example of a response function that can be measured for the secondary battery. The relaxation process is a state in which the voltage behavior does not follow Ohm's law after the start or end of charging and discharging. Details will be described later, but based on the first function using the determined parameters, for example, the charging and discharging status, remaining life, etc. can be determined.

The first function is a predetermined formula including f(τ), which is a second function that indicates the distribution state of the relaxation time constant τ (time constant in the relaxation process). In this way, the distribution state of the time constant in the voltage relaxation process can be determined using the second function stored in the memory unit 130. The specific determination method by the parameter determination unit 140 will be explained below, along with the differences from the technology described in Patent Document 1.

As described above, when the secondary battery 200 is charged and discharged, an ion absorption/release reaction occurs in the active material 12 (FIG. 2) in the electrode 201 (FIG. 2). The absorption/release reaction does not occur instantaneously, and it takes a certain amount of time for the absorption/release reaction to proceed. This time is called the relaxation time or relaxation time constant, and when the relaxation time constant τ is used, the voltage V(t) of the secondary battery 200 when a time t has elapsed after the start or end of charging/discharging in the secondary battery 200 is expressed by the following formula (1).
V(t)=A+B×exp(-t/τ)...Formula (1)
Here, A and B are constants.

In the following, the secondary battery 200 with uneven deterioration is the main subject. When uneven deterioration occurs in the electrode 201 (FIG. 3) due to rapid charging or the like, the value of the relaxation time constant τ will take on different values depending on the deterioration state of the electrode 201. For example, the active material 121 (FIG. 3) present in the deteriorated part of the electrode 201 has a thick accumulation of decomposition products of the electrolyte (not shown) on its surface. Normally, the accumulated decomposition products hinder the movement of ions, so that the occlusion/release reaction takes a long time in the deteriorated active material 121. This means that the relaxation time constant τ increases in the deteriorated part of the electrode 201. Conversely, the value of the relaxation time constant τ is unlikely to change in the active material 122 in the electrode 201 where deterioration is not progressing.

In this way, when n (2≦n) parts with different degrees of deterioration are formed inside the electrode 201, the relaxation time constants at each part have different values such as τ1, ..., τn. Therefore, for example, if these relaxation time constants τ1, ..., τn are measured in some way and the distribution state of the relaxation time constants is obtained, the deterioration unevenness of the secondary battery 200 can be quantitatively evaluated.

Figure 4 shows a histogram of the distribution of the relaxation time constants τ in a deteriorated secondary battery 200. For comparison, Figure 5 shows a histogram of the distribution of the relaxation time constants τ in an undegraded secondary battery 200. The distribution of the relaxation time constants τ refers to a histogram in which the horizontal axis represents the values of the measured relaxation time constants τ1, ..., τn. A method for creating a histogram, for example, is to divide the relaxation time constants τ into predetermined intervals τa < τ < τb, τb < τ < τc, ..., and calculate the frequency of how many of the measured relaxation time constants τ1, ..., τn are included in each interval.

To associate the shape of the histogram with the unevenness of deterioration, for example, attention should be paid to the "histogram spread" indicated by the solid arrow in Figures 4 and 5. The reason for this is that as the unevenness of deterioration increases, multiple relaxation time constants τ are measured. This is because the distribution state of the relaxation time constants τ (or the shape of the corresponding histogram) changes accordingly to a wider distribution shape. Specifically, the shape of the histogram in Figure 4, which shows the secondary battery 200 in a deteriorated state, is wider than the shape of the histogram in Figure 5, which shows the secondary battery 200 in an undegraded state. Therefore, it can be said that the spread of the relaxation time constants τ corresponds to the degree of unevenness of deterioration.

Such differences occur for the following reason. For example, if the active material 12 (Figure 2) were to deteriorate uniformly, the resistance would increase overall, and the graph shape of Figure 5 would be maintained, with the entire graph moving to the right. However, the presence of deterioration unevenness creates a distribution in the resistance, and as a result, a distribution also occurs in the relaxation time constant τ. This causes the graph of the relaxation time constant τ to spread out, as shown in Figure 5.

The resistance and the relaxation time constant τ can be associated with each other using an equivalent circuit model of a lithium ion secondary battery (LIB). That is, by converting the resistance R using the relaxation time constant τ (relaxation time) = RC, the resistance R can be converted into the relaxation time constant τ. Then, by converting into the relaxation time constant τ, it is possible to derive equation (4), which expresses the relationship between the voltage V and the relaxation time constant τ as a function of time t, as will be described in detail later.

Next, a method for obtaining the relaxation time constants τ1, ..., τn from the characteristic quantities of the secondary battery 200 will be shown. To this end, we consider the change over time in the voltage V(t) when a time t has elapsed since the start of or the end of charging and discharging. The voltage V(t) of the secondary battery 200 in which uneven deterioration has occurred can be calculated by adding up the voltages V1(t), ..., Vn(t) of n parts with different degrees of deterioration, and specifically, the following formula (2) is established.

V(t)=V1(t)+V2(t)+...+Vn(t)...Formula (2)
= ΣmVm(t)
=ΣmAm+ΣmBm×exp(-t/τm)
Here, the mathematical symbol Σm means to sum up V1 to Vn, and to approximate V(t) with n polynomials. Note that in the transformation from the second line to the third line, based on the above formula (1), Vm(t) is given by the following formula (3) using constants Am and Bm.

Vm(t)=Am+Bm×exp(-t/τm)...Formula (3)
The above formulas (2) and (3) express the change over time in voltage in the secondary battery 200 that has experienced uneven deterioration. A similar formula relating to voltage is also presented in Japanese Patent Laid-Open No. 2003-233996.

To obtain the values of the relaxation time constants τ1...τn of the n deteriorated parts formed inside the electrode 201, the calculated voltage value V(t) is compared with the data on the change in voltage over time measured by the measurement unit 110 (Figure 1), and the values of the parameters A1,...,An, B1,...,Bn, τ1,...,τn included in the formulas (2) and (3) are optimized so that the two match. However, to execute this method, the number n of deteriorated parts must be measured in advance. To do this, the secondary battery 200 must be disassembled and the electrode 201 must be analyzed. However, since the disassembled secondary battery 200 cannot be reused, even if the number n of deteriorated parts is obtained, it is meaningless to determine the state of the secondary battery 200 based on the number.

In addition, the number n of degraded parts changes from moment to moment depending on the progress of degradation of the secondary battery 200, so it is difficult to determine the value of n in advance. In the above explanation, the voltage V(t) of the secondary battery 200 is used as an example, but the same difficulty applies to a method and device for expressing a measurable response function (e.g., voltage, current, etc.) of the secondary battery 200 as n polynomials.

Therefore, in this disclosure, a different method for evaluating uneven deterioration is proposed. To begin with, to evaluate uneven deterioration, the values of the relaxation time constants τ1...τn at each deteriorated portion are not necessary, but information on the distribution state of the relaxation time constants τ shown in Figures 4 and 5 (i.e., the shape of the histogram created for the relaxation time constants τ) can be obtained. Therefore, in this disclosure, instead of the above formula (2) as the formula for voltage V(t), the following formula (4) is used as the first function indicating the state of the secondary battery 200.

V(t)=V0+∫f(τ)exp(-t/τ)dτ...Formula (4)
Here, V0 is the open circuit voltage (OCV), and the mathematical symbol ∫dτ... indicates integration with respect to τ. The function f(τ) is the second function that indicates the distribution state of the relaxation time constant τ as described above, and may be referred to as the distribution function below. The second function represents the shape of the histogram created for the relaxation time constant τ.

In the present disclosure, the second function (distribution function) f(τ) has one or more parameters that correspond to the distribution state of the relaxation time constant τ. Therefore, there exists a relaxation time constant τ that matches the first function with the time-dependent change data (within a predetermined error range), and the parameter can be determined. It is preferable that such a second function is a differentiable continuous function. This makes it easier to calculate the area of the second function by integration, as described in detail below.

Below, we will provide examples of some functions that satisfy this condition, but the present disclosure is not limited to the functions exemplified below. In addition, the second function may be composed of a combination of multiple functions, in which case it is sufficient that at least one of the functions that constitute the second function includes a parameter that corresponds to the distribution state of the relaxation time constant τ.

Figure 6 is a graph showing the relationship between the shape of a Gaussian function as an example of the second function and the parameters. The second function is, for example, a Gaussian function. As described above, when the relaxation time constant τ of the electrode 201 (Figure 3) is shown as a histogram, the shape of the histogram changes to a wider structure as the unevenness of deterioration increases. One function that approximately represents such behavior is the Gaussian function defined by the following formula (5).

f(τ)=αexp{-(τ-γ) ² /β}...Formula (5)
α, β, and γ are constants. Among these, β represents the spread of the histogram, and corresponds to a parameter (a parameter of the first function indicating the state of the secondary battery) representing the distribution state of the time constant in the voltage relaxation process. As shown in Fig. 6, the larger β is, the wider the graph of the second function is, which indicates that the secondary battery 200 has deteriorated.

Another example of the second function is a function indicating a gamma distribution (gamma function) shown in the following formula (6).
f(τ)={α ^β /Γ(β)}τ ^α−1 exp(−β×τ) …Equation (6)
Here, Γ(β) is a gamma function. In formula (6), the spread of the histogram is expressed by α/ ^β2 . Therefore, α and β correspond to parameters representing the distribution state of the time constant in the voltage relaxation process (parameters of the first function indicating the state of the secondary battery).

7 is a graph showing the relationship between the shape of the function indicating the Levy stable distribution as the second function and the parameters. Another example of the second function is the function indicating the Levy stable distribution shown in the following formula (7).
f(1/τ)=(1/τ)Σn{(-1) ⁿ ×τ ^−βn }/{n! ×Γ(−βn)} …Formula (7)
In formula (7), the spread of the histogram is represented by β. Therefore, β corresponds to a parameter (a parameter of the first function indicating the state of the secondary battery) that represents the distribution state of the time constant in the voltage relaxation process. As shown in Fig. 7, the smaller β is, the wider the graph of the second function is, which indicates that the secondary battery 200 is degraded.

Returning to FIG. 1, we will continue to explain the parameter determination unit 140. Here, we will explain the operation of the parameter determination unit 140 with respect to the time change V(t) of the voltage of the secondary battery 200, but the following procedure also applies when the time change I(t) of the current is used instead of the voltage.

The parameter determination unit 140 reads at least one of the first time-course data or the second time-course data from the storage unit 130. However, it is preferable that the parameter determination unit 140 uses at least the second time-course data to determine the parameters. This can reduce external factors and improve the accuracy of the parameters. In the following, as an example, both the first time-course data and the second time-course data are used.

The parameter determination unit 140 reads the second function from the storage unit 130. The second function is stored in advance in the storage unit 130 as described above. If two or more types of second functions are stored, the subsequent calculations may be performed using each of the second functions and the result with the highest accuracy may be used, or any one of the second functions may be used to perform the subsequent calculations. Furthermore, if a second function using information such as voltage, current, and temperature stored in the storage unit 130 is stored, the second function corresponding to the information stored in the storage unit 130 may be used.

The parameter determination unit 140 determines the parameters based on the area of a graph with the horizontal axis representing time t and the vertical axis representing voltage V(t) (an example of the response function). This makes it possible to calculate information about the distribution state of the relaxation time constant τ.

Specifically, the parameter determination unit 140 calculates the change in voltage over time V(t)calc as a first function indicating the state of the secondary battery 200 based on the read second function and the above formula (4). At this time, the values of the parameters included in the second function and formula (4) may be set arbitrarily. For example, in the first calculation, the parameter values of an undegraded secondary battery 200 may be used for convenience. In addition, in the second and subsequent calculations, for example, the parameter values obtained in the previous calculation may be used. Furthermore, the parameter values may be the same or different when the first time-varying data is used and when the second time-varying data is used.

The parameter determination unit 140 determines the parameters by comparing the read time-varying data with V(t)calc, which is the first function obtained by calculation. That is, the parameters can be determined by comparing the actual measured value with the calculated value. Specifically, the parameter determination unit 140 compares V(t)calc (first function) into which a predetermined value is substituted as the parameter with the time-varying data. Then, the parameter determination unit 140 determines the predetermined value as the parameter at which the difference between V(t)calc and the time-varying data is equal to or smaller than a predetermined value or is minimized.

That is, the parameter determination unit 140 optimizes the parameters so that, for example, the graph of Vcalc(t) approaches the graph of the time-dependent change data. Specifically, the parameter determination unit 140 continues calculations until Vcalc(t) and the time-dependent change data V(t) match within a predetermined error range (for example, within 1%). If they do not match, the parameter values are changed and V(t)calc is recalculated, and the parameter values are continued to be changed until V(t)calc and the time-dependent change data match within a predetermined error range or until a parameter set with the smallest error value is found.

The parameter determination unit 140 transmits the determined parameters to the output unit 150. The parameter determination unit 140 also transmits values such as the change in voltage over time V(t) obtained from the determined parameters to the output unit 150. The transmitted parameters need only include at least a parameter related to the distribution state of the relaxation time constant τ. If necessary, the parameter determination unit 140 may also calculate the response characteristics of the secondary battery 200 to a specified load using the determined parameter values.

The output unit 150 transmits the information received from the parameter determination unit 140 to an apparatus, device, etc. that uses the information. The information here includes, for example, the parameters determined by the parameter determination unit 140, the response characteristics of the secondary battery 200 calculated based on these, such as the change over time in the voltage of the secondary battery 200 when a certain arbitrary current value is passed through it, etc. The apparatus, device, etc. include, for example, an external memory and an external monitor (neither of which are shown).

The charge/discharge condition determination unit 160 determines the charge/discharge conditions, which are at least one of the charge/discharge current and the charge/discharge time of the secondary battery 200, based on the parameters determined by the parameter determination unit 140 so that the charge/discharge conditions are within a predetermined range indicating the appropriate operating voltage range of the secondary battery 200. This makes it possible to prevent the life of the secondary battery 200 from being excessively shortened. The charge/discharge condition determination unit 160 is provided when the response function is voltage. The charge/discharge condition determination unit 160 transmits the determined charge/discharge conditions to the charge/discharge control device 300 (described below).

The charge/discharge condition determination unit 160 creates a fourth function relating to the change over time of the voltage of the secondary battery 200 based on the third function relating to the change over time of the parameter. At the same time, the charge/discharge condition determination unit 160 determines the charge/discharge conditions so that the voltage calculated based on the fourth function falls within a predetermined range indicating the appropriate voltage range. In this way, it is possible to determine the charge/discharge conditions under which the secondary battery 200 can be used within the appropriate voltage range. The fourth function can be created, for example, by substituting the determined parameters into the above formula (4). In this embodiment, the charge/discharge condition determination unit 160 determines either the upper limit of the charge/discharge current or the upper limit of the charge/discharge time when the voltage of the secondary battery 200 reaches the upper or lower limit of the predetermined range.

The remaining life prediction unit 170 predicts the remaining life of the secondary battery 200 based on the parameters determined by the parameter determination unit 140. This makes it possible to predict the remaining life of the secondary battery 200 connected to the state determination device 100, and allows the secondary battery 200 to be used according to the predicted remaining life under specified usage conditions. The remaining life prediction unit 170 is provided when the response function is voltage.

The remaining life prediction unit 170 creates a fourth function relating to the change over time of the voltage of the secondary battery 200 based on the third function relating to the change over time of the parameter. The fourth function can be created, for example, by substituting the determined parameters into the above formula (4) as described above. In addition, the remaining life prediction unit 170 determines, as the remaining life, the time when the voltage calculated based on the fourth function falls outside a predetermined range representing the appropriate voltage range of the secondary battery 200 when the secondary battery 200 is used under predetermined usage conditions. Below, as an example, a method for predicting the remaining life of the secondary battery 200 used in a predetermined first period is shown.

As a premise of this method, the state quantities of the secondary battery 200 are measured at least twice during the first period. The state quantities refer to quantities that can be measured from the target secondary battery, such as the capacity and resistance of the secondary battery 200, and include at least parameters related to the distribution state of the relaxation time constant τ. The measured state quantities are stored in the memory unit 130. Consider a case in which these state quantities are measured at times t1, t2, ... tn (n ≥ 2) during the first period, and consider a case in which the usage status of the secondary battery at each time is measured in addition to the state quantities. Here, examples of the usage status of the secondary battery include the usage period, the number of charge/discharge cycles, the cumulative charge/discharge electricity amount, temperature, battery voltage, current, and electrode potential.

As a procedure for predicting the remaining life of a secondary battery, first, the remaining life prediction unit 170 constructs a third function so as to reproduce the change over time of the parameter related to the distribution state of the relaxation time constant τ measured at each time t1, t2, ... tn. The third function is stored in the storage unit 130. It is preferable that the variables of the third function include the usage status of the secondary battery. The third function may be a function prepared in advance, such as a straight line, a quadratic function, an exponential function, or a logarithmic function, or may be a regression model constructed using the measured values at each time t1, t2, ... tn.

Next, the remaining life prediction unit 170 uses the constructed third function to calculate the value of the parameter at a time point in the future than the first period. Furthermore, the remaining life prediction unit 170 predicts a fourth function related to the change over time of the voltage of the secondary battery 200 during charging and discharging based on the calculated parameters. The remaining life prediction unit 170 then determines whether the predicted fourth function is larger or smaller than the upper and lower limits of a pre-set appropriate voltage range. If the voltage exceeds a predetermined upper limit value or falls below a predetermined lower limit value at a certain point in time, the remaining life prediction unit 170 determines that this point in time is the life of the secondary battery 200. This makes it possible to predict the remaining life of the secondary battery 200 based on the determined parameters. The predicted remaining life makes it possible to determine the appropriate time to replace the secondary battery 200, taking into account how the secondary battery 200 is used.

The remaining life prediction unit 170 outputs to the output unit 150 at least one of the time when the voltage exceeds a predetermined upper limit, the time when the voltage falls below a predetermined lower limit, or the time from the current time to these times. If the time from the current time to these times is shorter than a predetermined shortest time, the remaining life prediction unit 170 outputs a warning to the output unit 150.

The state determination device 100 is configured with, for example, a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), and the like, none of which are shown in the figure. The state determination device 100 is realized when a predetermined control program stored in the ROM is expanded into the RAM and executed by the CPU.

The charge/discharge control device 300 charges and discharges the secondary battery 200 under the charge/discharge conditions determined by the charge/discharge condition determination unit 160. This prevents the life of the secondary battery 200 from being excessively shortened, allowing the secondary battery 200 to be used appropriately.

As described above, the state determination device 100 determines parameters related to the distribution state of the relaxation time constant τ based on the voltage of the secondary battery 200, etc. As an example, the state determination device 100 reads the time change of the voltage measured using a predetermined constant current value (first time change data and second time change data). Then, the state determination device 100 provisionally sets parameters related to the distribution state of the relaxation time constant τ (which may be a parameter group including multiple parameters) and calculates a first function that is the time change of the voltage based on the provisional parameters. Next, the state determination device 100 determines the parameters based on a comparison between the first function calculated based on the provisional parameters and the time change of the measured voltage (first time change data and second time change data). This allows the parameters related to the distribution state of the relaxation time constant τ in the secondary battery 200 to be grasped non-destructively and with high accuracy.

Figure 8 is a flowchart showing the state determination method of the present disclosure. The state determination method of the present disclosure can be executed by the state determination device 100 (Figure 1). Therefore, the explanation of Figure 8 will be made with reference to Figure 1 as appropriate.

The state determination method disclosed herein includes a measurement step S1, a determination step S2, a parameter determination step S3, a charge/discharge condition determination step S4, and a remaining life prediction step S5.

The measurement step S1 is a step of measuring the changes over time (corresponding to the first change over time data) of the voltage, current, and temperature of the secondary battery 200 (Figure 1) from time t1 when charging/discharging of the secondary battery 200 (Figure 1) starts to time t2 when charging/discharging ends, and storing the data in the memory unit 130 (Figure 1). The measurement step S1 is executed by the measurement unit 110 (Figure 1), and the contents described above for the measurement unit 110 can also be applied to the measurement step S1.

The determination step S2 is a step for determining whether or not the measurement results of the voltage and temperature during charging and discharging (from time t1 to time t2) acquired by the measurement unit 110 are to be used for processing by the parameter determination unit 140 (Figure 1). The determination step S2 is executed by the determination unit 120 (Figure 1), and the contents described above for the determination unit 120 can also be applied to the determination step S2.

The parameter determination step S3 is a step of determining the distribution state of the time constant (relaxation time constant τ) in the voltage relaxation process as a parameter of a first function indicating the state of the secondary battery 200, based on the time change data stored in the memory unit 130 (Figure 1). The time change data is at least one of the above-mentioned first time change data or the second time change data. The parameter determination step S3 is executed by the parameter determination unit 140 (Figure 1), and the contents described above for the parameter determination unit 140 can also be applied to the parameter determination step S3.

The charge/discharge condition determination step S4 is a step of determining the charge/discharge conditions based on the parameters determined in the parameter determination step S3 so that the charge/discharge conditions are within a predetermined range indicating the appropriate operating voltage range of the secondary battery 200. The charge/discharge conditions are at least one of the charge/discharge current and the charge/discharge time of the secondary battery 200. The charge/discharge condition determination step S4 is executed by the charge/discharge condition determination unit 160 (Figure 1), and the contents described above for the charge/discharge condition determination unit 160 can also be applied to the charge/discharge condition determination step S4.

The remaining life prediction step S5 is a step of predicting the remaining life of the secondary battery 200 based on the parameters determined in the parameter determination step S3. The remaining life prediction step S5 is executed by the remaining life prediction unit 170 (FIG. 1), and the contents described above for the remaining life prediction unit 170 can also be applied to the remaining life prediction step S5.

The state determination method disclosed herein can accurately grasp parameters related to the distribution state of the relaxation time constant τ in the secondary battery 200, similar to the state determination device 100 (Figure 1).

In the above explanation, the degraded secondary battery 200 was the subject, but the state determination device 100 and the state determination method can also be applied to an undegraded secondary battery 200. The reason is as follows. As described in Patent Document 1, in addition to fast reactions such as ion absorption and release reactions in the active material, reactions with multiple relaxation time constants τ contribute to the reaction of the secondary battery 200, such as slow reactions such as ion diffusion inside the active material. Therefore, since there is a certain distribution state of the relaxation time constant τ even in an undegraded secondary battery 200, the state determination device 100 and the state determination method via the distribution state of the relaxation time constant τ as described above can be applied.

Furthermore, as described above, the secondary battery 200 can be composed of multiple battery cells. In this case, the present disclosure can also be used to detect differences in the deterioration state between the battery cells.

Furthermore, in the above example, the voltage (V(t)) of the secondary battery 200 is used as an example of a measurable response function for the secondary battery 200. However, the present disclosure is not only applicable to the time change in voltage V(t), but can also be applied to, for example, the current (I(t)) of the secondary battery 200. For example, by using Ohm's law V=IR, the voltage V(t) in the above formula (4) can be replaced with the current I(t), and the present disclosure can also be applied to the current value I(t).

Figure 9 is a graph showing the results of Example 1. Using a degraded lithium ion secondary battery, data on the change in voltage over time and a graph of the calculated first function were plotted on the same graph in accordance with the following method.

The negative electrode of the secondary battery 200 (FIG. 1) was graphite, the positive electrode was LiNiCoMnO ₂ (molar ratio Ni:Co:Mn=1:1:1), and the separator was polypropylene. First, the secondary battery 200 was repeatedly charged and discharged, and the capacity of the secondary battery 200 was deteriorated from an undegraded state to about 20%. Next, the deteriorated secondary battery 200 was placed in a thermostatic chamber at 10° C., and the charge rate was adjusted so that the voltage of the secondary battery 200 was 3.6 V. Next, the secondary battery 200 was discharged for 8 seconds at a current value of 9 CA, and then rested for 30 minutes (CA: unit of current value. 1 CA is the amount of current that can fully charge a battery in 1 hour), and the second time-dependent change data V(t)exp (actual value) regarding the voltage of the secondary battery 200 was measured.

Next, the graph of Example 1 was determined as follows. Based on the measured V(t)exp, the parameters included in the above formula (4) were determined by the above method. The function shown in the above formula (7) was used as the second function. Finally, the determined parameters were used to find the change in voltage over time from formula (4), and the graph of Example 1 was obtained. The error between the graph of Example 1 and the measured value V(t)exp was evaluated.

For Comparative Example 1, the parameters in Equation (2) (similar to the formula described in Patent Document 1) were determined from the second time-dependent change data V(t)exp using the above formula (2). The determined parameters were then used to determine the time-dependent change in voltage, which was then used to create the graph of Comparative Example 1. The error between the graph of Comparative Example 1 and the measured value V(t)exp was evaluated. Note that in the calculation of Equation (2), cases where n=2 and n=3 were considered, but the results were nearly identical, so only the results for n=2 are shown in Figure 9.

As shown in FIG. 9, Example 1 and the actual measured values show almost the same tendency, with almost no error. However, there is a large deviation between Comparative Example 1 and the actual measured values, with a large error. This indicates that the method of Comparative Example 1 is unable to properly parameterize and extract the internal state of the secondary battery 200. In other words, even if the parameters extracted by the method of Comparative Example 1 are used, the voltage, open circuit voltage (OCV), etc. cannot be predicted with high accuracy. On the other hand, the method of Example 1 takes into account uneven deterioration of the electrodes, and can predict the change in voltage over time with high accuracy.

Figure 10 is a graph showing the results of Example 2. Using an undegraded lithium ion secondary battery, data on the change in voltage over time was plotted on the same graph as the calculated first function V(t) in accordance with the following method.

Graphite electrodes and metallic lithium electrodes were used for the electrodes of the secondary battery 200, and polyethylene was used for the separator. This secondary battery 200 was placed in a thermostatic chamber at 25°C, and the charging state was adjusted so that the open circuit voltage of the secondary battery 200 was 0.095 V. This secondary battery 200 was charged at a current value of 1 CA for 72 seconds and then rested for 30 minutes, during which the change in voltage over time V(t) was measured. The measurement method was the same as in Example 1 above, and then the graph of Example 2 was created and the error was evaluated in the same manner as in Example 1.

In addition, a graph was created and the error was evaluated for Comparative Example 2 in the same manner as for Comparative Example 1 above. However, since the results were almost identical, only the results for n=2 are shown in Figure 10.

As shown in FIG. 10, Example 2 and the actual measured values show almost the same tendency, with almost no error. However, there is a large deviation between Comparative Example 2 and the actual measured values, with a large error. Therefore, as in the case described with reference to FIG. 9 above, it is possible to predict the change in voltage over time with high accuracy even for an undegraded secondary battery 200, taking into account uneven electrode deterioration. Therefore, the present disclosure can be widely applied to secondary batteries 200, including undegraded batteries, regardless of the deterioration state of the secondary battery 200.

REFERENCE SIGNS LIST 100 State determination device 11 Current collecting foil 110 Measurement unit 12 Active material 120 Determination unit 121 Active material 13 Conductive assistant agent 130 Memory unit 140 Parameter determination unit 150 Output unit 160 Charge/discharge condition determination unit 170 Remaining life prediction unit 200 Secondary battery 201 Electrode 300 Charge/discharge control device 400 Secondary battery system S1 Measurement step S2 Determination step S3 Parameter determination step S4 Charge/discharge condition determination step S5 Remaining life prediction step

Claims (13)

a parameter determination unit that determines a distribution state of a time constant in a relaxation process of a response function as a parameter of a first function indicating a state of the secondary battery, based on at least one of first time-varying data of a voltage that is a measurable response function of the secondary battery from the start to the end of charging/discharging with a constant current to the secondary battery, and second time-varying data of the voltage that is the response function from the end of charging/discharging with a constant current to the voltage reaching a steady state;
a charge/discharge condition determination unit that determines a charge/discharge condition, which is at least one of a charge/discharge current and a charge/discharge time of the secondary battery, based on the parameters determined by the parameter determination unit so that the charge/discharge condition is within a predetermined range indicating an appropriate operating voltage range of the secondary battery;
The first function is
V(t)=V0+∫f(τ)exp(-t/τ)dτ
where V0 is an open circuit voltage of the secondary battery, τ is the time constant, and function f(τ) is a second function that represents the shape of a histogram created with respect to the time constant, the shape of the histogram changing over a wide range in response to an increase in unevenness in deterioration of an active material in the secondary battery. The state determination device according to claim 1 , wherein the parameter determination unit determines the parameter based on an area of a graph in which the horizontal axis represents time and the vertical axis represents the response function. The state determination device according to claim 1 , wherein the second function is a differentiable continuous function. 3. The state determination device according to claim 1, wherein the second function is a Gaussian function, a function indicating a gamma distribution, or a function indicating a Levy stable distribution. The state determination device according to claim 1 or 2, wherein the parameter determination unit determines the parameter by comparing the time-varying data with the first function. 6. The state determination device according to claim 5, wherein the parameter determination unit compares the first function, into which a predetermined value is substituted as the parameter, with the time-varying data, and determines, as the parameter, the predetermined value at which a difference between the first function and the time-varying data is equal to or smaller than a predetermined value or is minimized. The state determination device according to claim 1 or 2, wherein the parameter determination unit determines the parameter by using at least the second time-varying data. The charge/discharge condition determination unit is
A fourth function relating to a change in voltage of the secondary battery over time is created based on a third function relating to the change in the parameter over time;
3 . The state determination device according to claim 1 , further comprising: determining the charge/discharge condition so that the voltage calculated based on the fourth function falls within the predetermined range. 4 . The state determination device according to claim 1 or 2, further comprising a remaining life prediction unit that predicts a remaining life of the secondary battery based on the parameters determined by the parameter determination unit. The remaining life prediction unit is
A fourth function relating to a change in voltage of the secondary battery over time is created based on a third function relating to the change in the parameter over time;
The state determination device according to claim 9, characterized in that the remaining life is determined as the time when, when the secondary battery is used under specified operating conditions, the voltage calculated based on the fourth function falls outside a specified range representing an appropriate voltage range for the secondary battery. A secondary battery;
a state determination device including a parameter determination unit that determines a distribution state of a time constant in a relaxation process of a response function as a parameter of a first function indicating a state of the secondary battery, based on at least one of first time-varying data of voltage, which is a measurable response function of the secondary battery, from the start to the end of charging/discharging with a constant current to the secondary battery, and second time-varying data of voltage, which is the response function, from the end of charging/discharging with a constant current to when the voltage reaches a steady state;
the state determination device further includes a charge/discharge condition determination unit that determines a charge/discharge condition, which is at least one of a charge/discharge current and a charge/discharge time of the secondary battery, based on the parameter determined by the parameter determination unit so as to be included in a predetermined range indicating an appropriate operating voltage range of the secondary battery;
The first function is
V(t)=V0+∫f(τ)exp(-t/τ)dτ
where V0 is an open circuit voltage of the secondary battery, τ is the time constant, and a function f(τ) is a second function that represents the shape of a histogram created with respect to the time constant, the shape of the histogram changing over a wide range in response to an increase in unevenness in deterioration of an active material in the secondary battery. The secondary battery system according to claim 11 , further comprising a charge/discharge control device that charges and discharges the secondary battery under the charge/discharge conditions determined by the charge/discharge condition determination unit. a parameter determination step of determining, based on at least one of first time-varying data of voltage, which is a measurable response function of the secondary battery, from the start to the end of charging/discharging with a constant current to the secondary battery, and second time-varying data of voltage, which is the response function, from the end of charging/discharging with a constant current to the secondary battery until the voltage reaches a steady state, a distribution state of a time constant in a relaxation process of the response function as a parameter of a first function indicating the state of the secondary battery;
a charge/discharge condition determination step of determining, based on the parameters determined in the parameter determination step, a charge/discharge condition, which is at least one of a charge/discharge current and a charge/discharge time of the secondary battery, so that the charge/discharge condition is included in a predetermined range indicating an appropriate operating voltage range of the secondary battery;
The first function is
V(t)=V0+∫f(τ)exp(-t/τ)dτ
where V0 is an open circuit voltage of the secondary battery, τ is the time constant, and function f(τ) is a second function that represents the shape of a histogram created with respect to the time constant, the shape of which changes over a wide range in response to an increase in unevenness in deterioration of an active material in the secondary battery.

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