JP7141012B2 - Secondary battery system - Google Patents

Description

The present invention relates to a secondary battery system, and more particularly to a secondary battery system having a function of estimating a deterioration index indicating the degree of deterioration of a secondary battery.

As a deterioration index representing the degree of deterioration of a secondary battery, SOH (State of Health), which is the ratio of the current full charge capacity to the new full charge capacity, is widely known. This deterioration index SOH can be obtained, for example, by charging a secondary battery from a state of zero remaining battery capacity to full charge and measuring the actual full charge capacity. However, in order to estimate the deterioration index, it is necessary to charge the battery from a state where the remaining battery capacity is 0 to a fully charged state, which poses a problem that the time required for estimation is significantly lengthened.

Therefore, using the fact that a differential curve having differential characteristics with parameters of battery capacity and battery voltage during charging and discharging of the secondary battery changes in shape according to the deterioration state of the secondary battery, the characteristics appearing on the differential curve Various methods have been proposed for estimating the deterioration index of a secondary battery based on the point transition, the amount of change, and the like.
For example, in the secondary battery system described in Patent Document 1, the battery capacity Q is calculated when the secondary battery is charged, and the differential value dQ, which is the ratio of the change amount dQ of the battery capacity Q to the change amount dV of the battery voltage V, is calculated. /dV, and from the relationship between the differential value dQ/dV and the battery voltage V, a differential curve V-dQ/dV is calculated.

Then, the voltage value of the characteristic point appearing on the differential curve V-dQ/dV within the range of the predetermined battery voltage V, more specifically, the voltage value of the maximum point, which is the apex portion of the peak shape, is used as a deterioration index of the secondary battery. We are looking for the rate of capacity decrease.
Further, in the secondary battery system described in Patent Document 2, from the characteristic points appearing on the differential curve V-dQ/dV, more specifically, the differential value dQ/dV (maximum value) of the maximum point of the differential curve V-dQ/dV The deterioration index of the secondary battery is obtained from

JP 2013-19709 A JP 2014-139897 A

However, in the secondary battery system in Patent Document 1 or Patent Document 2, the deterioration index of the secondary battery is estimated from the differential value dQ/dV at the maximum point of the differential curve V-dQ/dV and the battery voltage V at the maximum point. Therefore, when the local maximum point fluctuates due to noise or the like, there is a risk of an estimation error of the deterioration index.
As described above, the secondary battery systems described in Patent Literatures 1 and 2 are not sufficiently satisfactory in terms of estimation accuracy of the deterioration index.

In particular, in the case of secondary batteries installed in electric vehicles, the state of deterioration of the secondary batteries affects the cruising range of the vehicle, so it is required to be able to estimate the deterioration index with as high accuracy as possible. ing.
SUMMARY OF THE INVENTION The present invention has been made to solve such problems, and an object of the present invention is to provide a secondary battery system capable of estimating a deterioration index of a secondary battery with high accuracy. .

In order to achieve the above object, the secondary battery system of the present invention includes a charge/discharge control unit for controlling charge/discharge of a secondary battery, a battery voltage V of the secondary battery, and a change amount dV of the battery voltage V. a differential curve calculator for calculating a differential curve V-dQ/dV indicating a relationship with dQ/dV, which is a ratio of a change amount dQ of the battery capacity Q of the secondary battery; A feature point that specifies an intersection point between a peak portion of the differential curve V-dQ/dV and a predetermined straight line that slopes so that the differential value dQ/dV decreases as the battery voltage V increases. and a deterioration index estimator for estimating a deterioration index of the secondary battery based on the battery voltage at the characteristic point.

According to the secondary battery system configured in this way, when the secondary battery is charged or discharged, the differential curve V-dQ/dV is calculated by the differential curve calculation unit, and the battery voltage V is calculated by the feature point identification unit. An intersection point between a peak appearing in the differential curve V-dQ/dV in the region and a predetermined straight line that slopes so that the differential value dQ/dV decreases as the battery voltage V increases is specified as a characteristic point. be. Then, the deterioration index estimating unit estimates the deterioration index of the secondary battery based on the battery voltage at this feature point.

In the present invention, the intersection of the peak of the differential curve V-dQ/dV and a predetermined straight line that slopes so that the differential value dQ/dV decreases as the battery voltage V increases is specified as a characteristic point. Even if a detection error occurs in the differential value dQ/dV at the peak of the differential curve V-dQ/dV or in the peak battery voltage due to noise, etc., the feature point can be specified with high accuracy, and the deterioration index of the secondary battery can improve the estimation accuracy of

Preferably, the feature point specifying unit specifies, as the feature point, a point of contact of a circle with a predetermined radius inscribed in the peak portion of the differential curve V-dQ/dV.
As a result, it is possible to easily specify the feature point, which is the intersection point between the peak portion of the differential curve V-dQ/dV and the predetermined straight line.
Preferably, the feature point specifying unit specifies, as the feature point, a point of contact on the low voltage side of points of contact between the peak portion and the circle of the predetermined radius.

As a result, it is possible to accurately estimate the deterioration index by using the low-voltage side contacts, which are relatively clear and can be easily identified, as feature points.
Also, the differential curve calculation unit may calculate the differential curve V-dQ/dV by charging the secondary battery from a battery voltage lower than the predetermined range.
As a result, of the two contact points of the peak portion and the circle of a predetermined radius, the point of contact on the low voltage side is specified as a feature point, and the feature point is relatively early in the predetermined region of the battery voltage during charging. can be specified to estimate the deterioration index of the secondary battery at an early stage.

Further, a temperature detection unit that detects the temperature of the secondary battery is provided, and the deterioration index estimation means detects the deterioration of the secondary battery based on the battery voltage V and the temperature of the secondary battery at the feature point. Estimate a metric.
This makes it possible to improve the estimation accuracy of the deterioration index of the secondary battery over a wide temperature range.

According to the secondary battery system of the present invention, the deterioration index of the secondary battery can be estimated with high accuracy.

1 is a schematic configuration diagram showing a secondary battery system according to an embodiment of the invention; FIG. FIG. 4 is a characteristic diagram showing an example of a differential curve V-dQ/dV at 100% SOH; FIG. 4 is a characteristic diagram showing an example of a differential curve V-dQ/dV at SOH 86%; FIG. 4 is a characteristic diagram showing an example of a differential curve V-dQ/dV at SOH 73%; FIG. 4 is a characteristic diagram showing an example of a differential curve V-dQ/dV at SOH 62%; FIG. 6 is a characteristic diagram summarizing the differential curves V−dQ/dV of FIGS. 2 to 5; FIG. 4 is a characteristic diagram showing an example of a relationship between a differential value dQ/dV and a voltage value V at a characteristic point of a differential curve V-dQ/dV; FIG.

An embodiment of a secondary battery system embodying the present invention will be described below.
FIG. 1 is a schematic configuration diagram showing the secondary battery system of this embodiment.
The secondary battery system 1 of this embodiment is mounted on an electric vehicle, and supplies electric power to a running motor, which is a power source for running the vehicle. As a whole, the secondary battery system 1 is composed of a main controller 2 for integrated control of the whole, and a plurality of secondary battery modules 3 connected in parallel to the main controller 2 .

The secondary battery module 3 includes an assembled battery 4 (secondary battery), a sub-controller 5 and a charge/discharge control section 6 .
The assembled battery 4 is configured by combining a plurality of cells in order to achieve desired battery capacity and output voltage. The assembled battery 4 of the present embodiment contains LiMn ₂ O ₄ and LiMO ₂ (M is a transition metal element containing at least one of Co, Ni, Al, Mn, and Fe) in its positive electrode plate. there is

A voltage sensor 7 , a current sensor 8 and a temperature sensor 9 (temperature detection section) are connected to the assembled battery 4 . The voltage sensor 7 detects the battery voltage V of the assembled battery 4, the current sensor 8 detects the input/output current I of the assembled battery 4, the temperature sensor 9 detects the battery temperature T of the assembled battery 4, and the detection information thereof. is input to the sub-controller 5.
The sub-controller 5 includes an input/output device (not shown), a storage device (ROM, RAM, etc.) for storing control programs, control maps, etc., a central processing unit (CPU), a timer counter, and the like. The sub-controller 5 has a function of controlling charging and discharging of the assembled battery 4 by driving the charging/discharging control unit 6. During charging/discharging control, the maximum allowable current and maximum allowable voltage are controlled according to the deterioration index of the assembled battery 4. to adjust. In this embodiment, SOH (State of Health), which is the ratio of the current full charge capacity to the new full charge capacity, is used as the deterioration index of the assembled battery 4 .

The sub-controller 5 also includes a differential curve calculator 10 that calculates a differential curve necessary for estimating the SOH of the assembled battery 4 . In this embodiment, V-dQ/dV is used as the differential curve. The differential curve V-dQ/dV is the relationship between the battery voltage V of the assembled battery 4 and the differential value dQ/dV, which is the ratio of the variation dQ of the battery capacity Q to the variation dV of the battery voltage V of the assembled battery 4. is shown.

The differential curve calculation unit 10 sequentially calculates the battery capacity Q of the assembled battery 4 at predetermined time intervals during charging or discharging of the assembled battery 4, acquires the battery voltage V in synchronization with this calculation, and calculates the battery voltage of the assembled battery 4. A differential value dQ/dV, which is the ratio of the variation dQ of the battery capacity Q to the variation dV of the voltage V, is calculated. Then, a differential curve V-dQ/dV is calculated as a curve showing the relationship between the obtained differential value dQ/dV and the battery voltage V. FIG.

2 to 5 are characteristic diagrams showing examples of differential curves V-dQ/dV. 2 to 5, the differential curve V-dQ/dV is shown with the differential value dQ/dV on the vertical axis and the battery voltage V on the horizontal axis. As the assembled battery 4 is charged or discharged, the battery voltage V increases or decreases along with the state of charge (SOC) of the assembled battery 4, and the differential value dQ/dV changes accordingly. 5, a peak shape appears on the differential curve V-dQ/dV in a predetermined range of voltage value V (for example, around 3.85 V to 4.05 V). An entire peak shape appearing in this predetermined range will be referred to as a peak portion P hereinafter. 2, P2 in FIG. 3, P3 in FIG. 4, and P4 in FIG.

The differential curve calculator 10 outputs the calculated differential curve V−dQ/dV and the battery temperature T detected by the temperature sensor 9 (hereinafter referred to as measured data) to the main controller 2 . Since it is a characteristic part of the present invention to specify which feature point on the differential curve V-dQ/dV for estimating SOH and how to estimate it, the method will be described in detail later.
The sub-controller 5 calculates the SOC of the assembled battery 4 by integrating the input/output current I associated with charging and discharging of the assembled battery 4 every predetermined time, and outputs the information to the main controller 2 as well.

On the other hand, like the sub-controller 5, the main controller 2 includes an input/output device (not shown), a storage device (ROM, RAM, etc.) for storing control programs, control maps, etc., a central processing unit (CPU), a timer counter, etc. consists of
The main controller 2 includes an input/output unit 12 , a data storage unit 13 , a feature point identification unit 15 , an SOH estimation unit 16 (deterioration index estimation unit), and a charge/discharge command unit 17 .

The data storage unit 13 stores actual measurement data input from the sub-controller 5 of each secondary battery module 3 via the input/output unit 12 . The data storage unit 13 also stores data (hereinafter referred to as reference data) indicating the correlation between the voltage value Vt of a specific feature point Pt on the differential curve V-dQ/dV and the SOH of the assembled battery 4 in advance. stored for each region.
The processing for creating the reference data is as follows.

First, the assembled battery 4 of the same standard as that of the assembled battery 4 of the present embodiment is subjected to a deterioration test, and the unused assembled battery 4 is repeatedly charged and discharged to gradually deteriorate to the end of its life. At each SOH in the deterioration process, the assembled battery 4 is charged and discharged under a plurality of different temperature ranges.
Then, similar to the processing of the differential curve calculation unit 10 described above, the differential value dQ/dV is calculated based on the battery voltage V and the battery capacity Q obtained by charging and discharging, and the relationship between the battery voltage V and the differential value dQ/dV After calculating the differential curve V-dQ/dV indicating , the position (V, dQ/dV) of the specific feature point Pt appearing on the differential curve V-dQ/dV is obtained. As a result, the correlation between the voltage value Vt of the specific feature point Pt and the SOH of the assembled battery 4 is determined for each temperature range, and is stored in advance in the data storage unit 13 as common reference data for each secondary battery module 3. .

The SOH estimating unit 16 reads the reference data of each secondary battery module 3 stored in the data storage unit 13 and sequentially compares it with the actual measurement data to obtain the current SOH of the assembled battery 4 for each secondary battery module 3. presume. Specifically, a temperature range is specified based on the battery temperature T of the actual measurement data, and from among the reference data of each SOH corresponding to the temperature range, the feature point that matches or is closest to the position of the specific feature point of the actual measurement data is selected. and take the SOH of that reference data as an estimate. The feature point Pt in this embodiment is the intersection of the straight line L having a predetermined slope and the measured differential curve V-dQ/dV on the graph showing the relationship between the battery voltage V and the differential value dQ/dV. .

The actual measurement data at this time does not need to be calculated for the entire area of the differential curve V-dQ/dV. can be estimated. The deterioration index estimator of the present invention shall also include a case of calculating such a partial differential curve V-dQ/dV.
Since the reference data cannot be specified between each SOH and between each temperature range, the reference data may be calculated by interpolation processing.

The charge/discharge command unit 17 outputs a charge/discharge control command to the sub-controller 5 of each secondary battery module 3 via the input/output unit 12 based on the SOH and the like estimated by the SOH estimation unit 16 . For example, for the secondary battery module 3 whose SOH is estimated to be less than a predetermined value, a command is output to limit the maximum allowable current and maximum allowable voltage during charging and discharging. The charge/discharge control by the sub-controller 5 based on this command protects the assembled battery 4 whose deterioration has progressed.

In addition, the charge/discharge command unit 17 displays a message prompting vehicle inspection on the display unit 18 provided in the driver's seat when the SOH is estimated to fall below the life limit of any of the secondary battery modules 3 . As a result, the vehicle is inspected at the sales company or the like, and the assembled battery 4 is replaced if necessary.
In the above description, the battery voltage V, the input/output current I, and the temperature T are detected, the differential curve V-dQ/dV is calculated, and the SOH is calculated for the entire assembled battery 4 of each secondary battery module 3. Although the estimation process was performed, it is not limited to this. For example, each process may be performed for each unit cell that constitutes the assembled battery 4, or each process may be performed for each unit cell group composed of a plurality of cells. Further, the differential curve calculation unit 10, the data storage unit 13, the feature point identification unit 15, and the SOH estimation unit 16 do not necessarily exist in the main controller 2 or the sub-controller 5, and these processes are performed by software of an external PC. may

The data storage unit 13 further stores, for each battery temperature T, a straight line L connecting the feature points Pt for each SOH. A method for setting the straight line L will be described below.
For the assembled battery 4, for example, when the SOH is 100% and the battery temperature T is 25 degrees Celsius, the differential curve V-dQ/dV shown in FIG. 2 is obtained. Then, in the secondary battery system of the present embodiment, among a plurality of feature points appearing on the differential curve V-dQ/dV, as feature points suitable for estimating SOH, in a predetermined range of the battery voltage V, A point of contact with a circle A inscribed in the appearing peak portion P was defined as a feature point Pt.

2 to 5 are examples of differential curves V-dQ/dV based on actual measurements when the battery temperature T is 25° C. and each SOH is charged from a fully discharged state. 2 is SOH 100%, FIG. 3 is SOH 86%, FIG. 4 is SOH 73%, and FIG. 5 is SOH 62%. FIG. 6 is a characteristic diagram summarizing the differential curves V-dQ/dV of FIGS. 2 to 5. In FIG. Furthermore, FIGS. 2 to 6 show a circle A with a predetermined radius R inscribed in the peak portions P (P1 to P4). The predetermined radius R is a constant value, and may be appropriately set according to the specifications of the assembled battery 4 so as to inscribe the differential curve V-dQ/dV at two points as shown in FIGS. 2, Pt2 in FIG. 3, Pt3 in FIG. 4, and Pt4 in FIG. 2, Vt2 in FIG. 3, Vt3 in FIG. 4, and Vt4 in FIG. 5, respectively.

The inventor measured the differential curve V-dQ/dV for the assembled battery 4 in each SOH state. As a result, as shown in FIGS. 2 to 5, it was confirmed that the peak portion P (P1 to P4) in the differential curve V-dQ/dV shifts to the high voltage side as the deterioration progresses. This is considered to be caused by an increase in the internal resistance of the assembled battery 4 and deterioration of the positive electrode and the negative electrode as the assembled battery 4 deteriorates.
Therefore, in the present embodiment, charging is started from a completely discharged state, and a circle A with a predetermined radius R inscribed in an upward peak portion P that appears in a predetermined range of the battery voltage V in the differential curve V-dQ/dV and the differential curve Of the two points of contact with V-dQ/dV, the point of contact with the lower battery voltage V was defined as the feature point Pt.

As a result, it was found that the center C of the circle A inscribed in the differential curve V-dQ/dV is aligned on the straight line L, as shown in FIG. That is, characteristic points Pt (Pt1 to Pt4), which are intersections of the differential curve V-dQ/dV and the circle A on the low voltage side, are aligned on the straight line L. FIG. Furthermore, it was found that as the SOH decreases, the circle A and the feature point Pt move to the lower right in FIG. 6, that is, the differential value dQ/dV decreases and the voltage value V (Vt1 to Vt4) increases .

In the differential curve V-dQ/dV of SOH 62% shown in FIG. Do not touch both voltage sides. It is presumed that this peak portion P4 is a combination of two peak portions Pd1 and Pd2 generated by two kinds of materials as indicated by broken lines. Therefore, it is conceivable that the circle A inscribed at two points in the peak portion Pd1 on the low voltage side lies on the straight line L connecting the center C of the circle A defined by the differential curve V-dQ/dV of the other SOH. .

FIG. 7 is a characteristic diagram showing the results of multiple measurements of the relationship between the voltage value Vt at the feature point Pt and the differential value dQ/dV in this embodiment. In FIG. 7, □ points indicate that the battery temperature T is 0 degrees Celsius, and ◇ points indicate that the battery temperature T is 25 degrees Celsius.
As shown in FIG. 7, the position of the characteristic point Pt (voltage value Vt and When the differential value dQ/dV) was actually measured, the characteristic points Pt were arranged substantially on a straight line for each battery temperature T.

As shown in FIGS. 6 and 7, since the feature points Pt are aligned on a straight line according to the SOH for each battery temperature T, it is possible to obtain the SOH from the voltage value Vt of the feature point Pt.
In this embodiment, for example, in a factory or the like, the differential curves V-dQ/dV are measured at a plurality of known SOHs for the assembled battery 4, and a certain inscribed circle A is set. A straight line L connecting C is calculated for each battery temperature T set appropriately. Then, this straight line L is stored in the data storage unit 13 as data indicating the relationship between the voltage value Vt of the characteristic point Pt and the differential value dQ/dV. It should be noted that this straight line L is the same for assembled batteries 4 having the same specifications and can be used in common.

When the assembled battery 4 is used, for example, when the assembled battery 4 is charged, the feature point identification unit 15 determines the intersection point between the straight line L stored in advance in the data storage unit 13 and the measured differential curve V-dQ/dV. A feature point Pt is obtained. Then, the SOH estimator 16 estimates the SOH from the voltage value Vt of the feature point Pt using the reference data.
Specifically, the charge/discharge command unit 17 charges the assembled battery 4 via the charge/discharge control unit 6 from the state where the assembled battery 4 is fully discharged or charged to a predetermined amount or less close to being completely discharged. Then, the differential curve V-dQ/dV is calculated in the differential curve calculator 10 . A filter may be used when calculating the differential curve V-dQ/dV.

The feature point specifying unit 15 reads out the straight line L close to the current battery temperature T from the data storage unit 13, and the intersection of this straight line L and the differential curve V-dQ/dV, more specifically, the predetermined battery temperature as described above. A low-voltage crossing point within the voltage range is specified as a feature point Pt.
The SOH estimation unit 16 reads the reference data based on the battery temperature T input from the temperature sensor 9 from the reference data stored in advance in the data storage unit 13, and uses the reference data to calculate the voltage value of the characteristic point Pt. Estimate SOH based on Vt. Since this battery temperature T is expected to change during charging, the battery temperature T is detected and stored together with the detection of the battery voltage V during charging. T can be used.

As described above, in this embodiment, the SOH is estimated from the voltage value Vt at the characteristic point Pt of the differential curve V-dQ/dV. can be estimated with high accuracy.
Further, as shown in FIG. 7, when the battery temperature T is 0° C. In the temperature range between 25° C., the reference data can be collected, the memory capacity of the data storage unit 13 can be suppressed, and the calculation load when estimating the SOH in the SOH estimating unit 16 can be suppressed. .

In a battery with a large difference in the correlation between the variation maximum voltage value Vs and the SOH depending on the battery temperature T, by using different reference data for each battery temperature T, the SOH can be estimated with high accuracy.
By the way, there is a possibility that the peak value of the differential value dQ/dV fluctuates due to noise superimposed on the peak portion of the differential value dQ/dV in the differential curve V−dQ/dV. Therefore, as in Patent Document 1, the SOH of the battery is estimated based on the voltage value at the peak of the differential value dQ/dV, or as in Patent Document 2, the SOH of the battery is estimated based on the peak value of the differential value dQ/dV. If it is estimated, there is a possibility that the estimation accuracy may be lowered.

On the other hand, in the present embodiment, since the point of contact of the inscribed circle that inscribes the peak portion P is the feature point Pt, even if the peak value changes due to noise on the peak portion P, the influence of the noise can be avoided. , SOH can be accurately estimated. Thus, in this embodiment, by estimating the SOH based on the feature point Pt specified from the overall shape of the peak portion P, rather than the peak value or the voltage value at the peak, it is difficult to be affected by noise, and the accuracy is improved. SOH can be well estimated.

In the assembled battery 4 of the present embodiment, for example, LiMn ₂ O ₄ and LiMO ₂ (M is a transition metal element containing at least one of Co, Ni, Al, Mn, and Fe) are used as active materials in the positive electrode plate. Among them, the characteristic that is considered to appear due to LiMn ₂ O ₄ appears as a clear peak shape in a predetermined voltage range on the differential curve V-dQ/dV, and the peak part of this clear peak shape Using the points of contact of the inscribed circles as the feature points Pt, it is possible to perform highly accurate estimation from the voltage values of the feature points Pt.

Further, in the present embodiment, SOH is estimated based not only on the variation maximum voltage value Vs, but also on the battery temperature T, so the SOH estimation accuracy can be improved over a wide temperature range.
Although the description of the embodiment is finished above, the aspect of the present invention is not limited to this embodiment. For example, in the above embodiment, when calculating the differential curve V-dQ/dV, the assembled battery 4 is charged from a fully discharged state or a charged state close to a fully discharged state. should be charged with For example, in the assembled battery 4 of the present embodiment having a differential curve V-dQ/dV as shown in FIGS. appear. This eliminates the need for complete discharge when estimating the SOH, thereby shortening the SOH estimation time.

In the above embodiment, the straight line L used to specify the characteristic point Pt is a line connecting the center C of the circle A having a predetermined radius R inscribed in the peak portion P, but it is not limited to this. do not have. Regarding the straight line L, as shown in FIG. 6, as the SOH decreases, the peak portion P changes to the lower right, and as the battery voltage V increases, the differential value dQ/dV decreases. It may be appropriately set to a straight line that slopes downward to the right.

However, by making the straight line L a line connecting the center C of the circle A with a predetermined radius R inscribed in the peak portion P as in the above embodiment, the straight line L can be easily set and the feature point Pt can be easily found. can be specified.
In addition, in the present embodiment, the present invention is embodied as the secondary battery system 1 mounted on an electric vehicle, but the present invention is not limited to vehicles, and is used in factories, stores, etc., for example. It may be embodied in a stationary secondary battery system.

4 assembled battery (secondary battery)
6 charge/discharge control unit 9 temperature sensor (temperature detection unit)
10 differential curve calculator 15 feature point identifier 16 SOH estimator (degradation index estimator)

Claims (5)

a charge/discharge control unit that controls charge/discharge of the secondary battery;
A differential curve V-dQ/ showing the relationship between the battery voltage V of the secondary battery and the differential value dQ/dV, which is the ratio of the variation dQ of the battery capacity Q of the secondary battery to the variation dV of the battery voltage V. a differential curve calculator that calculates dV;
A peak portion of the differential curve V-dQ/dV that appears in a predetermined region of the battery voltage V, and a predetermined straight line that slopes so that the differential value dQ/dV decreases as the battery voltage V increases. a feature point identifying unit that identifies the intersection of as a feature point ;
a deterioration index estimation unit that estimates a deterioration index of the secondary battery based on the battery voltage at the feature point;
A secondary battery system comprising: 2. The secondary battery according to claim 1, wherein the feature point specifying unit specifies, as the feature point, a point of contact of a circle with a predetermined radius inscribed in the peak portion of the differential curve V-dQ/dV. system. 3. The secondary according to claim 2, wherein the characteristic point identifying unit identifies, as the characteristic point, the point of contact on the low voltage side of two points of contact between the peak portion and the circle of the predetermined radius. battery system. 4. The secondary battery system according to claim 3, wherein the differential curve calculation unit charges the secondary battery from a battery voltage lower than the predetermined region and calculates the differential curve V-dQ/dV . . A temperature detection unit that detects the temperature of the secondary battery,
5. The deterioration index estimator estimates the deterioration index of the secondary battery based on the battery voltage at the feature point and the temperature of the secondary battery. The secondary battery system described in .

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