Battery control device and control method

Abstract

This makes it easy to select the voltage of the most degraded cell, reducing the computational load in battery state estimation. [Solution] A battery control device 1 is configured by connecting a plurality of rechargeable battery cells 10a in series, comprising: a voltage detection unit 21 that detects the cell voltage of each of the plurality of battery cells; a battery management unit 30 that balances the voltages between the plurality of battery cells so that the open-circuit voltage detected by the voltage detection unit becomes uniform when the plurality of battery cells are at a predetermined SOC or higher, close to a fully charged state; and a calculation device 5 that performs a battery state estimation calculation using the monitoring result of the cell voltage with the lowest open-circuit voltage.

Description

Technical Field

[0001] The present invention relates to a battery control device and a control method.

Background Art

[0002] A battery (secondary battery) is discharged or charged according to its usage situation. However, over time and with repeated charge and discharge, deterioration progresses, and the charge and discharge-capable capacity gradually decreases. Methods for estimating the state of such a secondary battery have been conventionally known. For example, Patent Document 1 discloses a technique for estimating the SOH of a plurality of battery cells and estimating the deterioration state of the entire battery system.

[0003] By the way, in a battery pack formed by connecting a plurality of cells in series, the deterioration state varies individually in each cell, resulting in variations between cells. Due to this variation, if a voltage variation occurs between cells, a specific cell may be intensively overcharged, etc., promoting a decrease in battery life. To prevent this, for example, as in Patent Document 2, when a difference greater than or equal to a predetermined value occurs with respect to the average cell voltage, a process (balancing process) for equalizing the voltage or SOC (State Of Charge) of each cell is performed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a battery consisting of multiple battery cells connected in series, the battery life depends on the most degraded cell with the lowest charge capacity. Therefore, the state estimation result of the most degraded cell can be considered as the state estimation result of the entire battery. If the lowest cell voltage among the detected cell voltages could be selected and monitored as the voltage of the most degraded cell, the load on the state estimation calculation could be reduced. However, as mentioned above, when balancing is performed to eliminate variations between cells, the lowest voltage is not necessarily the voltage of the most degraded cell, making it difficult to select the voltage to monitor.

[0006] The purpose of this disclosure is to reduce the computational load in battery state estimation by making it easy to monitor the voltage of the most degraded cell by performing a balancing process so that the lowest OCV voltage becomes the voltage of the most degraded cell. [Means for solving the problem]

[0007] The first embodiment is a battery control device configured by connecting a plurality of rechargeable battery cells in series, A voltage detection unit for detecting the cell voltage of each of the plurality of battery cells, When the plurality of battery cells are at a predetermined SOC or higher, close to a fully charged state, the battery management unit balances the cell voltages among the plurality of battery cells so that the open-circuit voltage detected by the voltage detection unit becomes uniform. The system is characterized by comprising a computing device that performs battery state estimation calculations using the monitoring results of the cell voltage with the lowest open-circuit voltage.

[0008] When discharging from the same open-circuit voltage (OCV), the OCV of the most degraded cell will be the lowest, so it was thought that this could be used to identify the voltage of the most degraded cell. However, the most degraded cell, which has a small charge capacity, will have its cell voltage drop faster than other battery cells during discharge, but will also have its cell voltage rise faster than other battery cells during charging. For example, if voltage balancing is performed between multiple cells when the State of Charge (SOC) is incomplete, the lowest voltage will not necessarily be the voltage of the most degraded cell. In that case, estimating the battery state from the lowest voltage may lead to errors in the estimation results. According to the first embodiment, by performing voltage balancing between multiple battery cells at an SOC close to full charge, it can always be determined that the lowest open-circuit voltage is the cell voltage of the most degraded cell (most degraded cell), and the highest open-circuit voltage is the cell voltage of the least degraded battery cell.

[0009] Performing state estimation for all cells is computationally very demanding. However, according to the first embodiment, the lowest voltage is considered to be the voltage of the most degraded cell, and state estimation can be performed using the monitoring results of the most degraded cell, thus reducing the computational load. Furthermore, while conventional calculations for all cells had to be performed in the calculation unit within the battery, according to the first embodiment, it is possible to output only the cell voltage information for one most degraded cell and perform state estimation calculations in a calculation device outside the battery. Because this calculation has a low computational load, it is also possible to use a more sophisticated state estimation logic than before.

[0010] In a second embodiment, in the first embodiment, the battery management unit balances the cell voltages such that the difference between the highest open-circuit voltage and the lowest open-circuit voltage among the cell voltages detected by the voltage detection unit is smaller when the difference between the highest open-circuit voltage and the lowest open-circuit voltage is greater than or equal to a predetermined threshold.

[0011] In a third embodiment, in the first embodiment, the battery management unit monitors the cell voltage with the lowest open-circuit voltage when, as a result of balancing the cell voltages, the difference between the highest open-circuit voltage and the lowest open-circuit voltage becomes smaller than a predetermined threshold, and transmits the monitoring result to the computing device.

[0012] Furthermore, the fourth aspect is, A method for controlling a battery configured by connecting multiple rechargeable battery cells in series, The steps include detecting the cell voltage of each of the plurality of battery cells, When the plurality of battery cells are at a predetermined SOC or higher, close to a fully charged state, the step of performing control to balance the voltages between the plurality of battery cells so that the open-circuit voltages become uniform, The method is characterized by comprising the step of performing a battery state estimation calculation using the monitoring result of the cell voltage with the lowest open-circuit voltage.

[0013] According to this embodiment, the voltage balancing process between multiple battery cells is performed when the State of Charge (SOC) is close to full charge, so that the lowest cell voltage can always be determined to be the voltage of the most degraded cell. Since the state estimation calculation can be performed using only the cell voltage information of the most degraded cell, it is possible to estimate the state of the entire battery while reducing the computational load. Furthermore, it is possible to output only the cell voltage information of the most degraded cell and perform the state estimation calculation on a computing device outside the battery, and because the computational load is low, it is possible to make the state estimation logic used in the calculation more sophisticated than in conventional methods. [Effects of the Invention]

[0014] As explained above, according to the technology of this disclosure, by performing a balance process in a state of charge (SOC) that is close to full charge, it becomes easy to determine that the lowest OCV voltage is always the voltage of the most degraded cell, and by using the voltage of the most degraded cell to estimate the state of the battery, the computational load can be reduced. [Brief explanation of the drawing]

[0015] [Figure 1] It is a block diagram showing an example of a drive system to which a control device for a secondary battery is applied. [Figure 2] It is a circuit diagram showing an example of a balance circuit. [Figure 3] It is a graph showing the relationship between the remaining capacity of a cell and the balance point. [Figure 4] It shows a comparative example and is a graph corresponding to FIG. 3. [Figure 5] It shows a comparative example and is a graph corresponding to FIG. 3. [Figure 6] It is a flowchart showing an example of a procedure for estimating the state of a battery.

Mode for Carrying Out the Invention

[0016] Hereinafter, embodiments for carrying out the present invention will be described based on the drawings. The following description of the preferred embodiments is merely illustrative in nature and is not intended to limit the present invention, its applications, or its uses.

[0017] <Configuration of Drive System> The control device 1 for the secondary battery of the present embodiment is mounted on an automobile such as a hybrid vehicle (HEV) or an electric vehicle (EV), and is incorporated as part of the drive system of the automobile, for example, as shown in FIG. 1. Specifically, for example, in the motor 2 of the automobile, the DC power supplied from the battery pack 3 is converted into three-phase AC power by the inverter 4 and supplied. The power supplied to the motor 2 is controlled based on the required torque output from the arithmetic unit 5.

[0018] <Configuration of Control Device for Secondary Battery> The control device 1 for the secondary battery is a device that can control the voltage balance between the battery cells constituting the battery 10 and can estimate the state of the battery 10. The control device 1 for the secondary battery includes a battery 10 including a plurality of battery cells 10a, a voltage detection unit 21 that detects each cell voltage of the battery cell 10a, a battery management unit 30 that balances the cell voltages, and an arithmetic unit 5 that performs state estimation calculation of the battery 10 outside the battery pack 3.

[0019] <Configuration of a secondary battery> The battery pack 3 contains a battery 10, a voltage detection unit 21, a temperature detection unit 22, a current detection unit 23, a battery management unit 30, and a balance circuit 40. The battery 10 is a rechargeable secondary battery, and is a battery pack composed of multiple battery cells 10a (multiple unit batteries that are secondary batteries) connected in series, for example, lithium-ion batteries or nickel-metal hydride batteries. Each battery cell 10a may be composed of multiple smaller battery cells connected in series and / or in parallel.

[0020] Each battery cell 10a of the battery 10 is connected to a series-connected discharge switch 40a and discharge resistor 40b, as shown in Figure 2, for example, forming a balance circuit 40 to eliminate variations in cell voltage between each battery cell 10a. The discharge switch 40a is controlled ON / OFF by the battery management unit 30, enabling the discharge of each battery cell 10a individually. The balancing process, which balances the cell voltages among multiple battery cells 10a, is performed using a passive method. In the passive balancing process, the battery cell with the highest voltage among the multiple battery cells is discharged, and the voltage is adjusted to match the battery cell with the lowest voltage.

[0021] The charging current and charging voltage values ​​of the battery 10 are detected by the voltage detection unit 21 and the current detection unit 23, and the detection signals are transmitted to the battery management unit 30. In addition, the cell voltage and temperature of the battery cells 10a are detected by the voltage detection unit 21 and the temperature detection unit 22 installed in each battery cell, and the detection signals are transmitted to the battery management unit 30.

[0022] The battery management unit 30 performs temperature sensitivity correction of battery performance, monitoring of abnormal heat generation, calculation of SOC and state estimation for each battery cell 10a, control of the balance circuit 40, and monitoring of cell voltage, based on signals detected by the voltage detection unit 21, temperature detection unit 22, and current detection unit 23. The battery management unit 30 transmits the calculation results and monitoring results to the calculation device 5 outside the battery pack 3.

[0023] Specifically, regarding the control of the balance circuit 40, the battery management unit 30 controls the balance circuit 40 to make the open-circuit voltages uniform when the ignition switch is OFF, and when multiple battery cells 10a are close to a fully charged state and at a predetermined SOC of 90% or higher (for example, SOC of 90% or higher), and when the difference between the highest open-circuit voltage (Vmax) and the lowest open-circuit voltage (Vmin) among the cell voltages detected by the voltage detection unit 21 is above a predetermined threshold (for example, 1% or higher), thereby balancing the cell voltages among the multiple battery cells 10a.

[0024] After the cell voltage balancing is complete and the ignition switch is turned ON, the battery management unit 30 monitors the cell voltage with the lowest open-circuit voltage. The battery management unit 30 transmits the monitoring results to the arithmetic unit 5. If the cell voltage balancing process is not completed, the battery management unit 30 uses the detection signals obtained from each battery cell 10a to perform a state estimation calculation for all battery cells 10a, and selects the most degraded cell and estimates the state of the battery 10 from the calculation results. The state estimation calculation is performed using calculations such as resistance increase rate (SOH_R), capacity retention rate (SOH_C), and input / output calculation (SOF), and known state estimation logic can be used.

[0025] The arithmetic unit 5 performs state estimation calculations for the battery 10 based only on the monitoring results of the cell voltage with the lowest open-circuit voltage. The arithmetic unit 5 can output the calculation results as the state estimation result for the entire battery 10. The state estimation calculation is performed using calculations such as resistance increase rate (SOH_R), capacity retention rate (SOH_C), and input / output calculation (SOF), and known state estimation logic can be used. Different state estimation logic can be used for the state estimation calculation performed by the arithmetic unit 5 and the state estimation calculation performed by the battery management unit 30.

[0026] Figures 3 to 5 are graphs showing the relationship between the remaining capacity of a cell and the balance point. In the secondary battery control device of this disclosure, as shown in Figure 3, if the balance point (BP) at which the cell voltages (Vcell) of multiple battery cells are equal is set to a predetermined state of charge (SOC) or higher, close to full charge, it can be determined that at any SOC, the minimum cell voltage (Vmin) is the cell voltage of the battery cell with the smallest charge capacity (Small Cell), and the maximum cell voltage (Vmax) is the cell voltage of the battery cell with the largest charge capacity (Large Cell). As shown in Figures 4 and 5, if the balance point is set to an intermediate or low SOC, it cannot always be determined that the minimum cell voltage (Vmin) is the cell voltage of the battery cell with the smallest charge capacity (Small Cell).

[0027] <Control flow by the battery control device> Figure 6 is a flowchart showing an example of the procedure for estimating the state of the battery. In step S1 after the start, when normal charging of the battery 10 is completed, with the ignition switch still in the OFF state, in step S2, the voltage detection unit 21 detects the cell voltage (open circuit voltage) of each of the multiple battery cells, and the battery management unit 30 determines, based on the signal detected from the voltage detection unit 21, whether the SOC of each battery cell 10a is at or above a predetermined SOC(α) that is close to a fully charged state (SOC≧α). If the SOC of each battery cell 10a is at or above a predetermined SOC that is close to a fully charged state, the process proceeds to step S3.

[0028] In step S3, the battery management unit 30 determines whether the difference between the highest open-circuit voltage (Vmax) and the lowest open-circuit voltage (Vmin) among the cell voltages detected by the voltage detection unit 21 is greater than or equal to a predetermined threshold (β) (Vmax - Vmin ≥ β), and determines whether or not cell voltage balancing is necessary. If the difference between the highest open-circuit voltage and the lowest open-circuit voltage is greater than or equal to the predetermined threshold, it is determined that balancing is necessary and the process proceeds to step S4.

[0029] In step S4, the battery management unit 30 controls the balance circuit 40 to perform a balancing process. The battery management unit 30 discharges the battery cells 10a with high open-circuit voltages and adjusts them to match the battery cells 10a with low open-circuit voltages.

[0030] Once the balancing process is complete, in step S5, the battery management unit 30 determines whether the cell voltages have been equalized by determining whether the difference between the highest open-circuit voltage (Vmax) and the lowest open-circuit voltage (Vmin) among the cell voltages detected by the voltage detection unit 21 has become smaller than a predetermined threshold (β) (Vmax - Vmin < β). When the cell voltages among multiple battery cells are equalized, it becomes possible to consider the cell voltage with the lowest open-circuit voltage as the cell voltage of the most degraded cell, and the process proceeds to step S6. If the cell voltages among multiple battery cells are not equalized even after the balancing process is complete, the process proceeds to step S8.

[0031] In step S6, after the ignition switch is turned ON, the battery management unit 30 monitors the cell voltage with the lowest open-circuit voltage. The battery management unit 30 then transmits the monitoring result to the arithmetic unit 5, which performs a state estimation calculation of the battery 10 based only on the monitoring result of the cell voltage with the lowest open-circuit voltage. The state estimation calculation is performed using calculations such as resistance increase rate (SOH_R), capacity retention rate (SOH_C), and input / output calculation (SOF). Because the state estimation calculation in step S6, which is performed only from the information of the cell voltage with the lowest open-circuit voltage, has a low computational load, it is possible to perform calculations not only for short-term resistance increase rates but also for long-term resistance increase rates, which have a high computational load, and it is possible to use a more advanced state estimation logic than that described in step S8.

[0032] If the difference between the highest and lowest open-circuit voltages detected by the voltage detection unit does not fall below a predetermined threshold, and the cell voltage balancing process is not completed, the cell voltage with the lowest open-circuit voltage cannot be considered the cell voltage of the most degraded cell. Therefore, in step S8, after the ignition switch is turned ON, the battery management unit 30 performs a state estimation calculation for all battery cells 10a using the detection signals obtained from each battery cell 10a. The state estimation calculation is performed by calculations such as resistance increase rate (SOH_R), capacity retention rate (SOH_C), and input / output calculation (SOF).

[0033] In step S9, the battery management unit 30 selects the state estimation result for the most degraded cell from the state estimation results of multiple battery cells based on the calculation results of step S8.

[0034] The results of the state estimation in steps S6 and S9 are output in step S7 as the estimated performance of the entire battery 10.

[0035] As described above, with the control device, the voltage balancing process between multiple battery cells 10a is performed when the State of Charge (SOC) is close to full charge, making it possible to always consider the lowest open-circuit voltage as the voltage of the most degraded cell. The battery management unit 30 monitors the cell voltage showing the lowest open-circuit voltage, and the arithmetic unit 5 can perform state estimation using only the monitoring results, resulting in a low computational load and the ability to use a more sophisticated state estimation logic than before. Furthermore, while calculations for all cells had to be performed in the battery management unit 30 inside the battery in the past, with the control device, it is possible to output only the cell voltage information for the most degraded cell and perform state estimation calculations in the arithmetic unit 5 outside the battery.

[0036] <Other Embodiments> In the above embodiment, the state estimation calculation, which is performed using the monitoring results of the cell voltage with the lowest open-circuit voltage, is performed by the calculation device 5 outside the battery pack, but it may also be performed by the battery management unit 30 inside the battery pack.

[0037] When the cell voltages are equalized in a state of charge (SOC) close to full charge, the cell voltage with the lowest open-circuit voltage can be considered the cell voltage of the most degraded cell, and the cell voltage with the highest open-circuit voltage can be considered the cell voltage of the least degraded battery cell. In the above embodiment, only the cell voltage with the lowest open-circuit voltage is monitored, and the state is estimated using the monitoring results for one battery cell. However, it is also possible to monitor both the cell voltage with the lowest open-circuit voltage and the cell voltage with the highest open-circuit voltage, and estimate the state using the monitoring results for two battery cells. [Explanation of symbols]

[0038] 1. Control device 2 motors 3 Battery pack 4 Inverters 5 Computing device 10 batteries 10A battery cell 21 Voltage detection unit 22 Temperature detection unit 23 Current detection unit 30 Battery Management Department 40 Balanced Circuit

Claims

1. A battery control device configured by connecting multiple rechargeable battery cells in series, A voltage detection unit for detecting the cell voltage of each of the plurality of battery cells, When the plurality of battery cells are at a predetermined SOC or higher, close to a fully charged state, the battery management unit balances the cell voltages among the plurality of battery cells so that the open-circuit voltage detected by the voltage detection unit becomes uniform. A battery control device comprising a computing device that performs battery state estimation calculations using the monitoring results of the cell voltage with the lowest open-circuit voltage.

2. In claim 1, The battery management unit is a battery control device that, when the difference between the highest open-circuit voltage and the lowest open-circuit voltage among the cell voltages detected by the voltage detection unit is greater than or equal to a predetermined threshold, balances the cell voltages so that the difference between the highest open-circuit voltage and the lowest open-circuit voltage becomes smaller.

3. In claim 1, The aforementioned battery management unit is a battery control device that, when the difference between the highest open-circuit voltage and the lowest open-circuit voltage becomes smaller than a predetermined threshold as a result of balancing the cell voltages, monitors the cell voltage with the lowest open-circuit voltage and transmits the monitoring result to the calculation device.

4. A method for controlling a battery configured by connecting multiple rechargeable battery cells in series, The steps include detecting the cell voltage of each of the plurality of battery cells, When the plurality of battery cells are at a predetermined SOC or higher, close to a fully charged state, the step of balancing the cell voltages among the plurality of battery cells so that the open-circuit voltage becomes uniform, A battery control method characterized by comprising the step of performing a battery state estimation calculation using the monitoring result of the cell voltage with the lowest open-circuit voltage.

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