Method for estimating full charge capacity, and control device.

Abstract

To efficiently determine the full charge capacity of a unit cell in an LFP battery of a bipolar type.SOLUTION: A ΔQ calculation unit 81 calculates a capacity increase amount ΔQ when the voltage of a unit cell 10N (N is an integer of 1 to M) increases from a first voltage Vb1 to a second voltage Vb2 while the unit cell 10N is being charged in an LFP battery of a bipolar type in which unit cells 101 to 10M using iron lithium phosphate are laminated as a positive electrode active material layer 1. A full charge capacity estimation unit 83 determines a full charge capacity Cf of the unit cell 10N as a parameter of the capacity increase amount ΔQ by using a full charge capacity Cf map.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a fully charged capacity estimation method and a control device.

Background Art

[0002] Japanese Unexamined Patent Application Publication No. 2021-58071 (Patent Document 1) discloses determining that constant current constant voltage (CCCV) charging has been performed using the battery voltage after charging of a battery (secondary battery) is completed, and setting the state of charge (SOC) of the battery to 100%.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Bipolar batteries have been proposed for reducing internal resistance and improving output density. A bipolar battery is formed, for example, by laminating bipolar electrodes each composed of a current collector, a positive electrode formed on one surface of the current collector, and a negative electrode formed on the other surface of the current collector, with separators interposed therebetween. In a unit cell (single battery) composed of a positive electrode, a separator, and a negative electrode, the basis weight of the unit cell (for example, the density of the positive electrode and the negative electrode) may differ due to variations during manufacturing or the like, and the capacity of the unit cell may vary.

[0005] In bipolar batteries, unit cells are stacked and electrically connected in series. Therefore, to prevent overcharging of unit cells, if charging of a bipolar battery is stopped when a unit cell with a smaller capacity (full charge capacity) is fully charged, the unit cell with a larger capacity will not be fully charged. In lithium iron phosphate batteries (LFP batteries), which use lithium iron phosphate as the positive electrode active material, the OCV (Open Circuit Voltage)-SOC characteristics are known to exhibit a wide range of flat regions (voltage flat regions). Figure 6 illustrates the characteristics of an LFP battery. Figure 6(A) shows the OCV-SOC characteristics of an LFP battery, where a wide region exists in which the change in OCV accompanying the change in SOC is small (voltage flat region).

[0006] Figure 6(B) shows the characteristics of LFP batteries (unit cells) with different capacities when charged to full capacity. In Figure 6(B), the horizontal axis represents capacity [Ah], and the vertical axis represents the voltage of the LFP battery (unit cell). The charging method is either CC (Constant Current) charging or CCCV charging. In Figure 6(B), the dashed line shows the characteristics of a unit cell with a full charge capacity of C1, and the solid line shows the characteristics of a unit cell with a full charge capacity of C2 (>C1). In bipolar batteries, charging is stopped when the unit cell with a capacity (full charge capacity) of C1 is fully charged to prevent overcharging of the unit cells. As a result, charging of the unit cell with a capacity of C2 is also stopped, and in the unit cell with a capacity of C2, the area enclosed by the dashed line becomes the unused region, and the voltage of the unit cell with a capacity of C2 does not rise to the foreign matter dissolution potential shown by the double dashed line. In particular, with LFP batteries, the lower battery voltage compared to ternary lithium-ion batteries means that the positive electrode potential does not rise to the foreign matter dissolution potential in a unit cell with a capacity of C2. Therefore, there is a concern that foreign matter will not dissolve in a unit cell with a capacity of C2, potentially leading to short-circuit failures. To eliminate this concern about potential short-circuit failures, one could consider processing such as fully charging all unit cells (raising the voltage to the foreign matter dissolution potential), but for this to work, it is desirable to efficiently determine the capacity (full charge capacity) of each unit cell.

[0007] The purpose of this disclosure is to efficiently determine the full charge capacity of a unit cell in a bipolar LFP battery. [Means for solving the problem]

[0008] (1) The full charge capacity estimation method of this disclosure is a method for estimating the full charge capacity of a bipolar battery in which a plurality of unit cells using lithium iron phosphate as the positive electrode active material are stacked in the stacking direction. The full charge capacity estimation method determines the full charge capacity of a unit cell by using the amount of capacity increase when the voltage of a unit cell rises from a first voltage to a second voltage in a predetermined region during charging in which the voltage of a unit cell rises due to an increase in the positive electrode potential.

[0009] This method determines the full charge capacity of a unit cell in a bipolar battery (bipolar LFP battery) where multiple unit cells using lithium iron phosphate as the positive electrode active material are stacked in a stacking direction. The full charge capacity of a unit cell is determined by using the increase in capacity when the voltage of the unit cell rises from a first voltage to a second voltage during charging. The increase in capacity from the first voltage to the second voltage is within a predetermined region where the voltage of the unit cell rises due to an increase in the positive electrode potential during charging. In a unit cell using lithium iron phosphate as the positive electrode active material, there is a positive linear correlation between the increase in capacity in the predetermined region and the full charge capacity. Therefore, by setting the second voltage to a value less than the voltage at full charge, the full charge capacity can be estimated in a relatively short time by determining the increase in capacity in the predetermined region without fully charging the unit cell, thus efficiently determining the full charge capacity of the unit cell.

[0010] (2) In the full charge capacity estimation method described in (1) above, when the ratio of the increase in capacity to the increase in voltage of a unit cell is dQ / dV, the predetermined region may be the region from 0% to 100% of SOC where the voltage is above the second minimum value on the dQ / dV-voltage curve.

[0011] The second minimum on the dQ / dV-voltage curve is the second minimum value on the dQ / dV-voltage curve when the SOC increases from 0% to 100%. During charging, the voltage value at which the voltage of a unit cell rises due to the rise in the positive electrode potential coincides with the voltage value of the second minimum on the dQ / dV-voltage curve. Therefore, this method allows a predetermined range to be set without measuring the positive electrode potential of the unit cell.

[0012] Charging is preferably performed at a rate of 0.1C or less, and is preferably CC charging or CCCV charging.

[0013] When charging at a rate greater than 0.1C, the increase in capacity in a predetermined region may not correlate with the full charge capacity. For this reason, it is preferable to charge at a rate of 0.1C or less. Furthermore, by performing CCCV charging, it becomes possible to reliably perform charging in the predetermined region (the region where the voltage is higher than the second minimum value in the dQ / dV-voltage curve).

[0014] The control device of the present disclosure is a control device for a bipolar battery in which a plurality of unit cells using lithium iron phosphate as the positive electrode active material are stacked in the stacking direction. The control device comprises a capacity estimation unit that determines the full charge capacity of each unit cell by the full charge capacity estimation method of (1) or (2) above, and an equalization control unit that equalizes the unit cells using the full charge capacity of each unit cell determined by the capacity estimation unit.

[0015] With this configuration, the capacity estimation unit of the control device can determine the capacity increase in a predetermined range, thereby estimating the full charge capacity in a relatively short time without fully charging each unit cell. Using this full charge capacity, the equalization control unit can then equalize the amount of stored energy or SOC of each unit cell. Therefore, in a bipolar LFP battery, it becomes possible to efficiently determine the full charge capacity of each unit cell and equalize the amount of stored energy or SOC of each unit cell. [Effects of the Invention]

[0016] According to the present disclosure, in a bipolar LFP battery, it becomes possible to efficiently obtain the full charge capacity of a unit cell.

Brief Description of the Drawings

[0017] [Figure 1] It is a diagram showing a schematic configuration of a bipolar LFP battery and a capacity inspection system according to the present embodiment. [Figure 2] (A), (B), and (C) show the characteristics during charging of a unit cell. [Figure 3] (A), (B), and (C) are diagrams showing the relationship between the full charge capacity and the capacity increase amount of a unit cell. [Figure 4] It is a flowchart showing the processing steps of a full charge capacity estimation method performed by a capacity inspection system. [Figure 5] It is a diagram showing a schematic configuration of a control device of a bipolar LFP battery provided with an equalization circuit. [Figure 6] It is a diagram for explaining the characteristics of an LFP battery.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and their description will not be repeated.

[0019] [Embodiment 1] FIG. 1 is a diagram showing a schematic configuration of a bipolar LFP battery 10 and a capacity inspection system 500 according to the present embodiment. The bipolar LFP battery 10 is formed by laminating unit cells 101, 102 to 10M (M is the number of unit cells) together with a frame member (sealing member) 5. The unit cells 101 to 10M are composed of a positive electrode active material layer (positive electrode) 1, a separator 2, and a negative electrode active material layer (negative electrode) 3. For example, a bipolar electrode 6 composed of a current collector 4, a positive electrode active material layer 1 formed on one surface (the lower surface in FIG. 1) of the current collector 4, and a negative electrode active material layer 3 formed on the other surface (the upper surface in FIG. 1) of the current collector 4 is laminated through the separator 2 to form the unit cells. The number of the unit cells 101 to 10M to be laminated may be arbitrary, for example, 20. Note that the current collector 4 disposed at one end (the upper end in FIG. 1) in the lamination direction also serves as a positive electrode terminal plate 41, and the current collector 4 disposed at the other end (the lower end in FIG. 1) also serves as a negative electrode terminal plate 42.

[0020] In the present embodiment, the unit cells 101 to 10M are lithium iron phosphate ion batteries (LFP batteries), the positive electrode active material layer 1 contains lithium iron phosphate (LiFePO4) as a positive electrode active material, and the negative electrode active material layer 3 contains graphite particles as a negative electrode active material. Then, the non-aqueous electrolyte is impregnated into the separator 2 or the like, the non-aqueous electrolyte is sealed, and the bipolar LFP battery 10 is formed.

[0021] The capacity inspection system 500 includes a charger 30, a current sensor 40, a voltage sensor 50, and a capacity measurement device 80. The charger 30 charges the bipolar LFP battery 10 by CC charging or CCCV charging. The current sensor 40 detects the charging current [A]. The voltage sensor 50 detects the voltage Vb [V] of the unit cell 10N (N is an integer from 1 to M).

[0022] The capacity measuring device 80 consists of a computer comprising a CPU (Central Processing Unit), memory, bus, etc., and a display. The capacity measuring device 80 includes, as functional blocks, a capacity increase calculation unit 81, a storage unit 82, a full charge capacity estimation unit 83, and a display unit 84.

[0023] Figure 2 shows the characteristics of a unit cell during charging. The charging method is CCCV charging, and the characteristics are shown when charging is performed with a termination voltage (switching voltage from CC charging to CV charging) set to 3.75V and a charging termination condition (current at the end of charging) set to 0.01C. Figure 2(A) shows the changes in the voltage and positive electrode potential of the unit cell. As shown in Figure 2(A), when the voltage of the unit cell exceeds 3.38[V], the voltage of the unit cell increases along with the increase in the positive electrode potential. Figure 2(B) shows the relationship between the voltage of the unit cell and dQ / dV when charging is performed at a C rate of 0.05C. dQ / dV is the ratio of the increase in the capacity [Ah] of the unit cell to the increase in the voltage of the unit cell. As shown by the dashed line in Figure 2(B), the dQ / dV-voltage curve shows a first and second minimum as the voltage of the unit cell 10N increases (the state of charge of the unit cell increases from 0% to 100%). The second minimum of the dQ / dV-voltage curve is the second minimum that appears when the state of charge of the unit cell increases from 0% to 100%. As shown in Figure 2(B), the voltage at the second minimum is 3.38[V], which is the same value as the voltage at which the voltage of the unit cell 10N begins to increase as the positive electrode potential increases in Figure 2(A). Figure 2(C) shows the relationship between the voltage of the unit cell and dQ / dV when charging is performed at a C rate of 0.1C. As shown by the dashed line in Figure 2(C), the voltage at the second minimum of the dQ / dV-voltage curve is 3.38[V], which is the same value as the voltage at which the voltage of the unit cell begins to rise as the positive electrode potential increases in Figure 2(A).

[0024] Figure 3 shows the relationship between the full charge capacity of a unit cell and the capacity increase. The capacity increase ΔQ is the increase in the capacity of a unit cell when the voltage of the unit cell rises from the first voltage Vb1 to the second voltage Vb2 due to charging. For example, if the capacity of the unit cell at the first voltage Vb1 is Q1 [Ah] and the capacity at the second voltage Vb2 is Q2 [Ah], then the capacity increase ΔQ is expressed as "ΔQ = Q2 - Q1". Note that the relationship shown in Figure 3 is for the case where the termination voltage is set to 3.75V, the charging termination condition is set to 0.01C, and CCCV charging is performed at a rate of 0.05C.

[0025] Figure 3(A) plots the capacity increase ΔQ0 (ΔQ) when the voltage of a unit cell rises from a state of charge (SOC) of 0% to 3.50[V] against the full charge capacity of the unit cell. Due to variations in the basis weight of the unit cells caused by manufacturing variations, etc., variations in the full charge capacity occur as shown in Figure 3(A). Furthermore, no correlation is observed between the full charge capacity and the capacity increase ΔQ0.

[0026] Figure 3(B) plots the capacity increase ΔQ1, which occurs when the voltage of a unit cell rises from 3.38[V] to 3.50[V], against the full charge capacity of the unit cell. Due to variations in the basis weight of the unit cells caused by manufacturing variations, etc., there is variation in the full charge capacity as shown in Figure 3(B). However, there is a positive linear correlation between the full charge capacity and the capacity increase ΔQ1.

[0027] Figure 3(C) plots the capacity increase ΔQ2 (ΔQ) when the voltage of a unit cell rises from 3.38[V] to 3.60[V] against the full charge capacity of the unit cell. Due to variations in the basis weight of the unit cells caused by manufacturing variations, etc., there is variation in the full charge capacity as shown in Figure 3(C). However, there is a positive linear correlation between the full charge capacity and the capacity increase ΔQ2.

[0028] From Figures 2 and 3, in a unit cell 10N of a bipolar LFP battery 10, the capacity increase ΔQ in a predetermined region (a region of 3.38[V] or higher) where the voltage of the unit cell 10N increases due to an increase in the positive electrode potential during charging correlates with the full charge capacity of the unit cell 10N. Furthermore, the predetermined region is the region where the voltage is higher than the second minimum of the dQ / dV-voltage curve. In this embodiment, the full charge capacity of the unit cell 10N is estimated by utilizing the correlation between the capacity increase ΔQ in the predetermined region and the full charge capacity.

[0029] Referring to Figure 1, the capacity increase calculation unit (ΔQ calculation unit) 81 of the capacity measuring device 80 calculates the capacity increase ΔQ of a unit cell 10N in a predetermined region when CC charging or CCCV charging is started by the charger 30. The storage unit 82 stores a full charge capacity Cf calculation map. This full charge capacity Cf calculation map is created in advance through experiments or other means based on the correlation between the capacity increase ΔQ and the full charge capacity Cf, and is stored in the storage unit 82. The full charge capacity estimation unit 83 calculates the full charge capacity Cf from the full charge capacity Cf calculation map, using the capacity increase ΔQ calculated by the ΔQ calculation unit 81 as a parameter. The calculated full charge capacity Cf is displayed in the display unit 84.

[0030] Figure 4 is a flowchart showing the processing steps of the full charge capacity estimation method performed by the capacity inspection system 500. First, in step 10 (hereinafter, steps are abbreviated as "S"), the charger 30 starts CC charging or CCCV charging of the unit cell 10N (bipolar LFP battery 10). In the following step S11, the ΔQ calculation unit 81 determines whether the voltage Vb of the unit cell 10N detected by the voltage sensor 50 is greater than or equal to a predetermined value α. The predetermined value α is the voltage corresponding to the second minimum value in the dQ / dV-voltage curve, which is 3.38[V] for the unit cell exemplified in Figures 2 and 3. As charging progresses and the voltage Vb becomes greater than or equal to the predetermined value α, a positive determination is made and the process proceeds to S12.

[0031] In S12, the ΔQ calculation unit 81 calculates the capacity increase ΔQ when the voltage Vb of a unit cell 10N rises from a first voltage Vb1 to a second voltage Vb2, based on the detected values ​​of the current sensor 40 and the voltage sensor 50. For example, Vb1 may be 3.38[V] and Vb2 may be 3.50[V]. In the subsequent S13, the full charge capacity estimation unit 83 calculates the full charge capacity Cf of the unit cell 10N from the capacity increase ΔQ calculated in S12, using the full charge capacity Cf calculation map stored in the memory unit 82. By performing the processes in S11 to S13 for all unit cells 10N included in the bipolar LFP battery 10, the full charge capacity Cf of all unit cells 10N is calculated.

[0032] According to this embodiment, after starting CC charging or CCCV charging, the capacity increase ΔQ is calculated when the voltage Vb of the unit cell 10N rises from the first voltage Vb1 to the second voltage Vb2 in a region where the voltage Vb of the unit cell 10N is above a predetermined value α (voltage corresponding to the second minimum value in the dQ / dV-voltage curve). Then, using the capacity increase ΔQ as a parameter, the full charge capacity Cf of the unit cell 10N is determined from the full charge capacity Cf calculation map. Therefore, since the second voltage Vb2 is less than the voltage when the unit cell 10N is fully charged, the full charge capacity Cf can be estimated in a relatively short time by calculating the capacity increase ΔQ in a predetermined region (a region where the voltage is above the second minimum value in the dQ / dV-voltage curve) without charging the unit cell 10N to full charge, thus efficiently determining the full charge capacity Cf of the unit cell 10N.

[0033] [Embodiment 2] Figure 5 shows a schematic configuration of a control device for a bipolar LFP battery 10 equipped with an equalization circuit. In Figure 5, the voltage detection circuit 20 detects the voltage of unit cells 101 to 10M via multiple voltage detection lines L1, branch line L11, and branch line L12. A fuse F and a chip bead Cb are provided on the voltage detection line L1 for circuit protection, and the voltage detection circuit 20 is protected from overvoltage by a Zener diode D provided in parallel with the unit cells 101 to 10M.

[0034] The voltage detection line L1 branches into branch lines L11 and L12 on the voltage detection unit VBc side of the Zener diode D. Branch line L11 is connected to the voltage detection unit VBc via switch So, and branch line L12 is connected to the voltage detection unit VBc via switch Sh. Switches So and Sh can be, for example, photoMOS (Metal Oxide Semiconductor) relays.

[0035] A resistor R1 is provided in branch line L12. A capacitor (flying capacitor) C is provided between branch line L12, which is connected to the positive terminal of each unit cell, and branch line L11, which is connected to the negative terminal. As a result, the voltage detection unit VBc can detect the voltage Vb of each unit cell 101 to 10M using the voltage detection circuit 20 by sequentially turning on switches Sh and So, which correspond to unit cells 101 to 10M, using the flying capacitor method.

[0036] The equalization circuit EQ consists of a discharge resistor Rd installed on the branch line L11 and a switch S1 that conducts (closes) / interrupts (opens) the connection between adjacent branch lines L11. Switch S1 switches between ON (closed) and OFF (open) in response to a control signal from the control device 200. For example, to reduce the amount of charge [Ah] stored in a unit cell 102, the switch S1 corresponding to the unit cell 102 is turned ON (closed). As shown by the dashed arrow, the current discharged from the unit cell 102 is consumed by the discharge resistor Rd, and the amount of charge stored in the unit cell 102 decreases.

[0037] The control device 200 is a computer consisting of a CPU, memory, bus, etc. The capacity estimation unit 201 of the control device 200 has the same function as the capacity measuring device 80. When CC charging or CCCV charging is performed by the charger 30a, the capacity estimation unit 201 calculates the full charge capacity Cf of unit cells 101 to 10M using the same full charge capacity estimation method as in Embodiment 1 above.

[0038] The equalization control unit 202 sets the smallest full charge capacity Cf among the full charge capacities Cf of unit cells 101 to 10M determined by the capacity estimation unit 201 as the reference capacity Cfm. Then, when charging of the bipolar LFP battery 10 is complete, the switches S1 corresponding to the unit cells having a full charge capacity Cf greater than the reference capacity Cfm are sequentially turned ON to discharge. The discharge amount from the unit cells may be set in advance through experiments or other means as a value proportional to the deviation between the reference capacity Cfm and the full charge capacity Cf. This equalizes the stored energy [Ah] of the unit cells 101 to 10M.

[0039] Furthermore, the equalization control unit 202 controls the equalization circuit EQ to equalize the SOC of the unit cell 101. When the SOC of unit cells 101 to 10M is uneven at the end of charging of the bipolar LFP battery 10, the smaller the full charge capacity Cf, the larger the SOC value. The largest full charge capacity Cf among the full charge capacities Cf of unit cells 101 to 10M obtained by the capacity estimation unit 201 is set as the reference capacity Cfx. Then, when charging of the bipolar LFP battery 10 is complete, the switches S1 corresponding to the unit cells having a full charge capacity Cf smaller than the reference capacity Cfx are sequentially turned ON to discharge. The discharge amount from the unit cells may be set in advance through experiments or other means as a value proportional to the deviation between the reference capacity Cfx and the full charge capacity Cf. This equalizes the SOC of unit cells 101 to 10M.

[0040] According to this embodiment, the capacity estimation unit 201 of the control device 200 determines the capacity increase ΔQ in a predetermined region, thereby estimating the full charge capacity Cf in a relatively short time without fully charging the unit cell 10N. Using this full charge capacity Cf, the equalization control unit 0202 can equalize the amount of stored energy or SOC of the unit cell 102. Therefore, in a bipolar LFP battery 10, it becomes possible to efficiently determine the full charge capacity Cf of the unit cell 10N and equalize the amount of stored energy or SOC of the unit cell 10N.

[0041] Furthermore, as shown by the dashed lines in Figure 5, multiple switches Sc may be provided so that charging power (charging current) supplied from the charger 30a can be supplied to every unit cell 101 to 10M. With this configuration, by turning ON (closing) the switches Sc corresponding to unit cells 101 to 10M, charging (CC charging, CCCV charging) can be performed for every unit cell 101 to 10M. After the charging of the bipolar LFP battery 10 is completed, the charger 30a and switches Sc may be controlled to perform additional charging on unit cells with large full charge capacities Cf determined by the capacity measuring device 80 or the capacity estimation unit 201, thereby raising the voltage of the unit cells with large full charge capacities Cf above the foreign matter dissolution potential and dissolving the foreign matter.

[0042] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the description of the embodiments above, and all modifications within the meaning and scope equivalent to the claims are intended to be included. [Explanation of symbols]

[0043] 1 Positive electrode active material layer (positive electrode), 2 Separator, 3 Negative electrode active material layer (negative electrode), 4 Current collector, 5 Frame member (sealing member), 6 Bipolar electrode, 10 Bipolar LFP battery, 10¹~10M, 10N unit cell, 20 Voltage detection circuit, 30, 30A charger, 40 Current sensor, 50 Voltage sensor, 80 Capacity measuring device, 81 Capacity increase calculation unit (ΔQ calculation unit), 82 Memory unit, 83 Full charge capacity estimation unit, 200 Control device, 201 Capacity estimation unit, 202 Equalization control unit, 500 Capacity inspection system, EQ Equalization circuit.

Claims

1. A method for estimating the full charge capacity of a bipolar battery in which multiple unit cells using lithium iron phosphate as the positive electrode active material are stacked in the stacking direction, A method for estimating the full charge capacity of a bipolar battery, which involves determining the full charge capacity of a unit cell by using the increase in capacity when the voltage of the unit cell rises from a first voltage to a second voltage in a predetermined region during charging of the bipolar battery, due to an increase in the positive electrode potential.

2. When the ratio of the increase in capacitance to the increase in voltage of the unit cell is denoted as dQ / dV, The method for estimating full charge capacity according to claim 1, wherein the predetermined region is a region from 0% to 100% of the SOC where the voltage is greater than or equal to the second minimum value in the dQ / dV-voltage curve.

3. The method for estimating full charge capacity according to claim 2, wherein the charging is performed at a rate of 0.1C or less.

4. The method for estimating full charge capacity according to any one of claims 1 to 3, wherein the charging is CC charging or CCCV charging.

5. A control device for a bipolar battery in which multiple unit cells using lithium iron phosphate as the positive electrode active material are stacked in the stacking direction, The control device is A capacity estimation unit that determines the full charge capacity of each unit cell by the full charge capacity estimation method described in claim 1 or claim 2, A control device comprising: an equalization control unit that equalizes the unit cells using the full charge capacity of each unit cell obtained by the capacity estimation unit;

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