JP7488121B2 - Diagnostic Systems - Google Patents

Description

The present invention relates to a diagnostic system for a secondary battery.

In vehicles that use a secondary battery as a drive battery, control is performed based on the SOC (State Of Charge), which indicates the remaining capacity of the secondary battery. Conventionally, the SOC of a secondary battery has been estimated using various methods (see, for example, Patent Document 1).

The most common method for estimating SOC is to use current and voltage. For example, there is the current integration method, which estimates the capacity transition (i.e., SOC transition) relative to the full charge capacity using current integration, and the OCV method, which estimates the SOC from the open circuit voltage (OCV) of the battery, measured using the correlation between the OCV and the SOC.

JP 2012-88157 A

The SOC estimation is subject to various errors, such as current integration errors due to current sensing errors, individual differences in batteries, and changes in characteristics due to degradation (full charge capacity, changes in SOC-OCV correlation). Conventionally, such errors have made it impossible to estimate the SOC with high accuracy. For this reason, the vehicle is usually controlled taking into account a margin for the estimated SOC error.

The present invention was made in consideration of the above-mentioned circumstances, and aims to improve the accuracy of vehicle control (i.e., vehicle performance) and improve the marketability of the vehicle by improving the accuracy of SOC estimation.

The diagnostic system for a secondary battery according to one aspect of the present invention includes a first acquisition unit that acquires current and voltage values detected during charging or discharging, an SOC estimation unit that estimates an SOC value according to the current and voltage values acquired by the first acquisition unit and derives a differential curve of the charge/discharge curve, a storage unit that stores a differential curve of the charge/discharge curve of the secondary battery for each combination of temperature, deterioration state, and C rate, a second acquisition unit that acquires information on temperature, deterioration state, and C rate as a charge/discharge condition related to charge/discharge control of the secondary battery, an identification unit that identifies a condition-matching differential curve that is a differential curve that matches the charge/discharge condition acquired by the second acquisition unit from each differential curve stored in the storage unit, and a correction unit that derives an error between the peak of the condition-matching differential curve identified by the identification unit and the peak of the differential curve derived by the SOC estimation unit, corrects the SOC value estimated by the SOC estimation unit based on the error, and derives a corrected SOC value.

According to a secondary battery diagnostic system according to one aspect of the present invention, the SOC value is estimated according to the current and voltage values, a differential curve is derived, and the SOC value is corrected based on the error between the peak of the differential curve and the peak of a condition-matching differential curve (differential curve of bench data) specified according to the charge and discharge conditions. The SOC value estimated according to the current and voltage values may not be an accurate value due to, for example, current integration error due to current sensing error, individual battery differences, characteristic changes due to deterioration (full charge capacity, changes in SOC-OCV correlation), etc. In this regard, in the diagnostic system of the present invention, the estimated SOC value is corrected taking into account information on the differential curve of the bench data stored in advance according to the charge and discharge conditions, i.e., information that does not include information that affects the estimation of the SOC value as described above. Specifically, in the diagnostic system of the present invention, the error between the peak of the condition-matching differential curve (differential curve of the bench data according to the charge and discharge conditions) and the peak of the derived differential curve is derived, and the SOC value is corrected based on the error. This allows the estimated SOC value to be corrected to approach a value that eliminates various influences that reduce the estimation accuracy, improving the accuracy of the SOC estimation. As described above, according to the secondary battery diagnostic system of one aspect of the present invention, the accuracy of vehicle control (i.e., vehicle performance) can be improved by improving the accuracy of the SOC estimation, thereby improving the marketability of the vehicle.

According to the present invention, by improving the accuracy of SOC estimation, the accuracy of vehicle control (i.e., vehicle performance) can be improved, thereby improving the marketability of the vehicle.

FIG. 2 is a functional block diagram of a diagnostic system for a secondary battery. FIG. 2 is a diagram showing a schematic diagram of on-table data. FIG. 2 is a diagram showing an example of a charge/discharge curve of a lithium ion battery. FIG. 2 is a diagram showing an example of the differential characteristics of a charge/discharge curve of a lithium ion battery. FIG. 13 is a diagram illustrating an SOC estimation error. 4 is a flowchart showing a diagnostic process (diagnostic method) for a lithium ion battery.

Various embodiments will be described in detail below with reference to the drawings. Note that the same reference numerals will be used to refer to the same or corresponding parts in each drawing, and duplicate descriptions of the same or corresponding parts will be omitted.

Figure 1 is a functional block diagram of a diagnostic system 10 for a secondary battery. The secondary battery in this embodiment is, for example, an in-vehicle lithium ion battery, but may be other secondary batteries. The secondary battery in this embodiment functions, for example, as a driving battery for the vehicle. In this embodiment, the secondary battery is described as a lithium ion battery that functions as a driving battery for the vehicle. More specifically, the secondary battery is a lithium ion battery that uses a graphite-based negative electrode.

First, an overview of the lithium-ion battery diagnostic system 10 according to this embodiment will be described with reference to Figs. 2 to 5. The diagnostic system 10 is, for example, a function of a BMS (Battery Management System) that estimates and monitors the state of a lithium-ion battery functioning as a drive battery. The diagnostic system 10 is a system that non-destructively estimates the internal state of a lithium-ion battery, and more specifically, estimates the SOC (State Of Charge) value, which represents the remaining capacity of the lithium-ion battery. The diagnostic system 10 estimates the SOC value using well-known techniques based on the current, voltage, and surface temperature of the lithium-ion battery.

The diagnostic system 10 calculates the charge/discharge curve (see FIG. 3) of the lithium ion battery based on the current and voltage of the lithium ion battery measured during charging and discharging, and further calculates the differential characteristic of the charge/discharge curve (differential curve, see FIG. 4).

Figure 3 is a diagram showing an example of a charge/discharge curve of a lithium-ion battery. As shown in Figure 3, the charge/discharge curve has the horizontal axis representing capacity (mAh) and the vertical axis representing voltage (V), and is derived based on the current and voltage of the lithium-ion battery during charging and discharging. Figure 4 is a diagram showing an example of the differential characteristic (differential curve) of the charge/discharge curve of a lithium-ion battery. As shown in Figure 4, the differential curve has the horizontal axis representing capacity (SOC (%)) and the vertical axis representing dV/dQ (V is voltage, Q is capacity), and is derived based on the charge/discharge curve.

As shown in FIG. 4, peaks (locally convex portions of the curve) occur in the differential curve for both charging and discharging. That is, in the example of FIG. 4, two peaks P1 and P2 occur in the charging curve, and two peaks P3 and P4 occur in the discharging curve. Such peak information (peak shape and position) is used in the SOC value correction process described below.

Here, the diagnostic system 10 prestores bench data (see FIG. 2) that records the differential curve of the charge/discharge curve for each combination of the temperature (specifically, the internal temperature), the degradation state, and the C rate of the lithium ion battery. The C rate is the magnitude of the current when charging and discharging the battery, and is derived based on the current. The C rate indicates the speed of charging and discharging. The diagnostic system 10 then compares the differential curve derived based on the acquired current and voltage, etc., with a differential curve according to the charge/discharge conditions (a condition-matching differential curve identified from the bench data), and corrects the estimated SOC value based on the comparison result. Specifically, the diagnostic system 10 derives the error between the peak of the condition-matching differential curve and the peak of the derived differential curve, and corrects the estimated SOC value based on the error to derive the corrected SOC value.

Figure 5 is a diagram for explaining the SOC estimation error. Figure 5(a) shows a differential curve derived based on the current and voltage, etc., and Figure 5(b) shows a differential curve (condition-matching differential curve specified from bench data) according to the same charge/discharge conditions as those under which the differential curve of Figure 5(a) was derived. Now, as shown in Figure 5(a), for example, in the derived charge differential curve, the first peak appears at about SOC: 40%. And, as shown in Figure 5(b), in the condition-matching differential curve specified according to the charge/discharge conditions, the first peak appears at about SOC: 60% in the charge differential curve. The condition-matching differential curve can be said to be correct data that is acquired in advance according to the charge/discharge conditions. Therefore, based on the positions of the peaks, the difference (60%-40%=20%) is derived as the SOC estimation error, and the SOC estimation error is added to the estimated SOC value to derive the corrected SOC value, thereby improving the accuracy of the SOC estimation.

The following describes in detail the functions of the lithium-ion battery diagnostic system 10 with reference to FIG. 1. As shown in FIG. 1, the diagnostic system 10 includes a memory unit 11, an acquisition unit 12 (first acquisition unit, second acquisition unit), an SOC estimation unit 13, an identification unit 14, and a correction unit 15.

The memory unit 11 is a database that stores bench data (see FIG. 2) that records the differential curves of the charge/discharge curves of the lithium ion battery for each combination of the temperature, deterioration state, and C rate of the lithium ion battery. Such bench data is prepared in advance, for example, for each type of battery.

The acquisition unit 12 acquires the current and voltage values detected during charging or discharging. The acquisition unit 12 acquires the value of the current flowing through the lithium ion batteries 22 from a current sensor 21 of the battery module 2, which is composed of multiple lithium ion batteries 22. The acquisition unit 12 also acquires the value of the voltage applied to the lithium ion batteries 22 from a voltage sensor that measures the voltage between the terminals of the lithium ion batteries 22 that compose the battery module 2.

The acquisition unit 12 acquires information on temperature, deterioration state, and C rate as charge/discharge conditions related to charge/discharge control of the lithium ion battery. The temperature is the internal temperature of the lithium ion battery, and is derived, for example, according to the surface temperature of the lithium ion battery and the current detected by the current sensor 21 of the battery module 2. The deterioration state is the deterioration state of the lithium ion battery estimated, for example, from the driving history of the vehicle. The C rate may be a predetermined value, or may be derived according to the current detected by the current sensor 21.

The SOC estimation unit 13 estimates the value of the SOC according to the current and voltage values acquired by the acquisition unit 12. The SOC estimation unit 13 estimates the value of the SOC based on the current, voltage, and surface temperature of the lithium-ion battery, using conventionally known technology. The SOC estimation unit 13 also derives a differential curve of the charge/discharge curve according to the current and voltage values acquired by the acquisition unit 12. The SOC estimation unit 13 derives the differential curve until at least one peak in the charge/discharge curve can be identified.

The identification unit 14 identifies, from among the differential curves stored in the memory unit 11, a condition-matching differential curve that is a differential curve that matches the charge/discharge conditions acquired by the acquisition unit 12. That is, the identification unit 14 identifies, as a condition-matching differential curve, a differential curve of the on-table data in which all of the temperature, deterioration state, and C rate match (or are most similar to) the charge/discharge conditions acquired by the acquisition unit 12.

The correction unit 15 derives the error between the peak of the condition-matching differential curve identified by the identification unit 14 and the peak of the differential curve derived by the SOC estimation unit 13, and corrects the SOC value estimated by the SOC estimation unit 13 based on the error to derive the corrected SOC value. As shown in FIG. 5, the correction unit 15 compares the SOC values of the corresponding peaks of the condition-matching differential curve and the derived differential curve, and derives the difference as the error. The correction unit 15 then adds the error to the SOC value estimated by the SOC estimation unit 13 to derive the corrected SOC value. For example, in the example shown in FIG. 5, since the error in the charge differential curve is about 20%, the correction unit 15 sets the value of the SOC estimated by the SOC estimation unit 13 + 20% as the corrected SOC value.

Next, the diagnostic process (diagnosis method) for a lithium-ion battery according to this embodiment will be described with reference to FIG. 6. FIG. 6 is a flowchart showing the diagnostic process (diagnosis method) for a lithium-ion battery, and more specifically, a flowchart showing the SOC estimation process when charging a lithium-ion battery.

As shown in FIG. 6, the diagnostic system 10 first acquires the current and voltage values during charging of the lithium-ion battery (step S1). Next, the diagnostic system 10 estimates the SOC value based on the acquired current and voltage values, and derives a differential curve of the charge/discharge curve (step S2).

Next, the diagnostic system 10 acquires the charging conditions (step S3). The charging conditions are information on the temperature, deterioration state, and C rate of the lithium-ion battery. Next, the diagnostic system 10 identifies a condition-matching differential curve, which is a differential curve that matches the acquired charging conditions, from among the differential curves of the stored bench data (step S4).

Next, the diagnostic system 10 derives the error (difference) between the peak of the condition-matching differential curve and the peak of the derived differential curve (step S5). Then, the diagnostic system 10 corrects the estimated SOC value based on the derived error (difference) to derive the corrected SOC value (step S6).

Finally, we will explain the effects of the lithium-ion battery diagnostic system 10 according to this embodiment.

The diagnostic system 10 includes an acquisition unit 12 that acquires the current and voltage values detected during charging or discharging, an SOC estimation unit 13 that estimates the SOC value according to the current and voltage values acquired by the acquisition unit 12 and derives a differential curve of the charge/discharge curve, a memory unit 11 that stores the differential curve of the charge/discharge curve of the secondary battery for each combination of temperature, deterioration state, and C rate, the acquisition unit 12 that acquires information on the temperature, deterioration state, and C rate as a charge/discharge condition related to the charge/discharge control of the secondary battery, an identification unit 14 that identifies a condition-matching differential curve that is a differential curve that matches the charge/discharge condition acquired by the acquisition unit 12 from each differential curve stored in the memory unit 11, and a correction unit 15 that derives the error between the peak of the condition-matching differential curve identified by the identification unit 14 and the peak of the differential curve derived by the SOC estimation unit 13, corrects the SOC value estimated by the SOC estimation unit 13 based on the error, and derives a corrected SOC value.

According to such a diagnostic system 10, the value of SOC is estimated according to the values of current and voltage, and a differential curve is derived, and the value of SOC is corrected based on the error between the peak of the differential curve and the peak of a condition-matching differential curve (differential curve of bench data) specified according to the charging and discharging conditions. The value of SOC estimated according to the values of current and voltage may not be an accurate value due to, for example, current integration error due to current sensing error, individual battery differences, characteristic changes due to deterioration (full charge capacity, changes in SOC-OCV correlation), etc. In this regard, in the diagnostic system 10, the estimated value of SOC is corrected taking into account information on the differential curve of the bench data stored in advance according to the charging and discharging conditions, that is, information that does not include information that affects the estimation of the value of SOC as described above. Specifically, in the diagnostic system 10, the error between the peak of the condition-matching differential curve (differential curve of the bench data according to the charging and discharging conditions) and the peak of the derived differential curve is derived, and the value of SOC is corrected based on the error. This allows the estimated SOC value to be corrected to approach a value that eliminates various influences that reduce the estimation accuracy, improving the accuracy of the SOC estimation. As described above, according to the diagnostic system 10, the accuracy of vehicle control (i.e., vehicle performance) can be improved by improving the accuracy of the SOC estimation, thereby improving the marketability of the vehicle.

10...diagnosis system, 11...storage unit, 12...acquisition unit (first acquisition unit, second acquisition unit), 13...SOC estimation unit, 14...identification unit, 15...correction unit.

Claims (1)

A first acquisition unit that acquires current and voltage values detected during charging or discharging;
an SOC estimation unit that estimates an SOC value according to the current and voltage values acquired by the first acquisition unit and derives a differential curve of a charge/discharge curve;
A storage unit that stores a differential curve of a charge/discharge curve of a secondary battery for each combination of temperature, deterioration state, and C rate;
a second acquisition unit that acquires information on a temperature, a degradation state, and a C rate as charge/discharge conditions related to charge/discharge control of the secondary battery;
an identification unit that identifies a condition-matching differential curve, which is a differential curve that matches the charge/discharge condition acquired by the second acquisition unit, from among the differential curves stored in the storage unit;
a correction unit that derives an error between the peak of the condition-matching differential curve identified by the identification unit and the peak of the differential curve derived by the SOC estimation unit, and corrects the value of the SOC estimated by the SOC estimation unit based on the error, to derive a corrected SOC value.

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