Battery system

Abstract

It suppresses the degradation of lithium iron phosphate batteries. [Solution] The battery system 100 includes an auxiliary battery 5 which is a lithium-ion secondary battery having a positive electrode containing lithium iron phosphate and a negative electrode containing graphite, a voltage sensor 61 which detects the voltage of the auxiliary battery 5, a DC / DC converter 3 configured to discharge the auxiliary battery 5, and an ECU 7 which controls the DC / DC converter 3 so that the discharge of the auxiliary battery 5 ends when the State of Charge (SOC) of the auxiliary battery 5 reaches the discharge termination SOC. When the voltage detected by the voltage sensor 61 falls below a threshold voltage, the ECU 7 acquires the SOC of the auxiliary battery 5 and sets the discharge termination SOC according to the acquired SOC.

Description

Technical Field

[0001] The present disclosure relates to a battery system, and more particularly to a battery system including a lithium iron phosphate ion battery.

Background Art

[0002] The control circuit of the battery module disclosed in International Publication No. 2015 / 080285 (Patent Document 1) is configured to control the charge and discharge of a lithium ion secondary battery based on the discharge cut-off voltage of the lithium ion secondary battery.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As an auxiliary battery mounted on a vehicle, a lithium iron phosphate ion battery may be adopted. In the case of such a lithium iron phosphate ion battery, from the viewpoint of stably supplying and maintaining the voltage required by the auxiliary load, it is preferable to adopt a combination of a positive electrode containing lithium iron phosphate and a negative electrode containing graphite.

[0005] Like other secondary batteries, a lithium iron phosphate ion battery deteriorates due to repeated charge and discharge. In particular, when a lithium iron phosphate ion battery is used as an auxiliary battery, the scene where the lithium iron phosphate ion battery discharges (that is, supplies power to the auxiliary load) may frequently occur, so the number of charge and discharge cycles required for the lithium iron phosphate ion battery may increase. It is desirable to suppress the deterioration of the lithium iron phosphate ion battery.

[0006] This disclosure was made to solve the above-mentioned problems, and one of the purposes of this disclosure is to suppress the degradation of lithium iron phosphate batteries. [Means for solving the problem]

[0007] (1) A battery system relating to a certain aspect of the present disclosure comprises a battery which is a lithium-ion secondary battery having a positive electrode containing lithium iron phosphate and a negative electrode containing graphite; a voltage sensor for detecting the voltage of the battery; a power converter configured to discharge the battery; and a control device that controls the power converter so that the discharge of the battery ends when the State of Charge (SOC) of the battery reaches the discharge termination SOC. The control device acquires the State of Charge of the battery when the voltage detected by the voltage sensor falls below a threshold voltage and sets the discharge termination SOC according to the acquired SOC.

[0008] (2) The threshold voltage is higher than the voltage when the negative electrode stage structure is stage 3L.

[0009] (3) The threshold voltage is 3.20V or higher and 3.28V or lower.

[0010] In the configurations described in (1) to (3) above, the control device sets the discharge termination SOC instead of the battery discharge termination voltage. The threshold voltage is higher than the voltage when the negative electrode stage structure is at stage 3L (3.20V or higher and 3.28V or lower). Therefore, the discharge termination SOC is set to the SOC corresponding to the voltage at which the negative electrode stage structure does not transition to stage 3L. As will be explained in more detail later, this suppresses large volume changes in the negative electrode. Thus, according to the configurations described in (1) to (3) above, battery degradation caused by volume changes in the negative electrode can be suppressed.

[0011] (4) The control device sets the discharge termination SOC to be higher than the acquired SOC by the amount of error in the acquired SOC.

[0012] In the configuration described in (4) above, the control device takes into account the error in the State of Charge (SOC) and sets the discharge termination SOC higher by the amount of the SOC error. In other words, the battery terminates discharge earlier by the amount of the SOC error. As a result, even if an SOC error occurs, the stage structure of the graphite negative electrode will not reach stage 3L. Therefore, the configuration described in (4) above can more reliably suppress battery degradation.

[0013] (5) The battery system further includes a temperature sensor for detecting the battery temperature. The control unit sets the discharge termination state of charge (SOC) when the temperature detected by the temperature sensor exceeds the threshold temperature and the voltage detected by the voltage sensor falls below the threshold voltage.

[0014] In the configuration described in (5) above, the control device determines whether the battery temperature detected by the temperature sensor exceeds the threshold temperature. In the low-temperature range, the battery resistance increases, and the voltage drop associated with discharge becomes larger, which can reduce the accuracy of voltage detection. According to the configuration described in (5) above, voltage detection is performed when the battery temperature is above the threshold temperature. Therefore, it is possible to determine with high accuracy whether the battery voltage detected by the voltage sensor has fallen below the threshold voltage. [Effects of the Invention]

[0015] According to this disclosure, the degradation of lithium iron phosphate batteries can be suppressed. [Brief explanation of the drawing]

[0016] [Figure 1] This is a block diagram showing the configuration of the battery system according to this embodiment. [Figure 2] This figure shows examples of SOC-OCV and SOC-OCP curves for lithium iron phosphate batteries. [Figure 3] This figure shows an example of the relationship between the lithium insertion rate in a graphite anode and the anode potential or anode volume change rate. [Figure 4] This flowchart shows an example of the initial processing steps. [Figure 5] It is a diagram showing an example of the SOC-OCV curve and the SOC-OCP curve of a lithium iron phosphate ion battery after years. [Figure 6] It is a flowchart showing an example of the processing procedure for after-treatment over time.

Mode for Carrying Out the Invention

[0017] 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 descriptions will not be repeated.

[0018] [Embodiment] <System Configuration> FIG. 1 is a block diagram showing the configuration of a battery system according to the present embodiment. The battery system 100 is mounted on an electric vehicle such as an electric vehicle, a hybrid vehicle, or a plug-in hybrid vehicle. The battery system 100 includes a main battery 1, a first monitoring unit 2, a DC / DC converter 3, an auxiliary load 4, an auxiliary battery 5, a second monitoring unit 6, and an ECU (Electric Control Unit) 7.

[0019] The main battery 1 is a battery pack for vehicle running. Each cell of the battery pack may be a lithium ion secondary battery, a nickel metal hydride battery, or a all-solid-state battery. The voltage of the main battery 1 is several hundred V, which is higher than the voltage of the auxiliary battery 5 (typically 12V).

[0020] The first monitoring unit 2 includes a voltage sensor 21, a current sensor 22, and a temperature sensor 23. The voltage sensor 21 detects the voltage of the main battery 1. The current sensor 22 detects the current flowing through the main battery 1. The temperature sensor 23 detects the temperature of the main battery 1. Each sensor outputs a signal indicating the detection result to the ECU 7.

[0021] The DC / DC converter 3 supplies power from the main battery 1 to the auxiliary load 4 or charges the auxiliary battery 5 according to control commands from the ECU 7. The DC / DC converter 3 can also discharge the auxiliary load 4 and charge the main battery 1 with the discharged power, according to control commands from the ECU 7. The DC / DC converter 3 corresponds to the "power converter" in this disclosure.

[0022] Auxiliary load 4 may include various loads (not shown) such as audio equipment, lights, heaters, air conditioning, electronic brakes, and electric steering.

[0023] The auxiliary battery 5 is a lithium iron phosphate battery (LFP battery) having a positive electrode containing lithium iron phosphate (LiFePO4) and a negative electrode containing graphite. The auxiliary battery 5 corresponds to the "battery" in this disclosure.

[0024] The auxiliary battery 5 is discharged when the DC / DC converter 3 is not operating or when there is insufficient power supplied from the main battery 1, and supplies power to the auxiliary load 4. When the State of Charge (SOC) of the auxiliary battery 5 decreases due to discharge and reaches the "discharge completion SOC", the DC / DC converter 3 is driven and fully charged. The auxiliary battery 5 is also charged by the surplus power remaining after supplying power from the main battery 1 to the auxiliary load 4.

[0025] The second monitoring unit 6 includes a voltage sensor 61, a current sensor 62, and a temperature sensor 63. The voltage sensor 61 detects the voltage V of the auxiliary battery 5. The current sensor 62 detects the current I flowing through the auxiliary battery 5. The temperature sensor 63 detects the temperature T of the auxiliary battery 5. Each sensor outputs a signal indicating its detection result to the ECU 7.

[0026] The ECU7 includes a processor 71 such as a CPU (Central Processing Unit) and memory 72 such as ROM (Read Only Memory) and RAM (Random Access Memory). The ECU7 controls the DC / DC converter 3 based on the input signals from the sensors of the first monitoring unit 2 and the second monitoring unit 6, as well as the maps and programs stored in the memory 72. In this embodiment, the main processing performed by the ECU7 is the discharge control of the auxiliary battery 5 using the DC / DC converter 3. This discharge control will be described in detail later. The ECU7 corresponds to the "control device" in this disclosure.

[0027] <Lithium iron phosphate battery> First, the characteristics of the lithium iron phosphate battery used as the auxiliary battery 5 in this embodiment will be described.

[0028] Figure 2 shows an example of the SOC-OCV curve and SOC-OCP curve for a lithium iron phosphate battery. The horizontal axis represents the capacity (SOC) of the positive electrode, negative electrode, or lithium iron phosphate battery (cell). The vertical axis represents the positive electrode potential, negative electrode potential, and the voltage of the lithium iron phosphate battery (cell voltage). The same applies to Figure 5, which will be described later.

[0029] Figure 2 shows the SOC-OCV curve of a lithium iron phosphate battery cell as a solid line. The SOC-OCV curve of the lithium iron phosphate battery corresponds to the difference between the SOC-OCP curve of the positive electrode (LFP), shown as a dashed line, and the SOC-OCP curve of the negative electrode (graphite), shown as a dashed line.

[0030] As can be seen from Figure 2, the SOC-OCP curve of the positive electrode containing LFP exhibits very high flatness, except in the high SOC region near full charge. Therefore, it can be said that the shape of the cell's SOC-OCV curve is determined by the shape of the SOC-OCP curve of the negative electrode containing graphite. The flatness of the SOC-OCP curve of the negative electrode containing graphite is also relatively high, except in the low SOC region. Consequently, the cell's SOC-OCV curve is almost flat, except in the low SOC region.

[0031] One approach is to set a discharge termination voltage for the auxiliary battery 5, and terminate the discharge of the auxiliary battery 5 when its voltage V reaches the discharge termination voltage. However, because the auxiliary battery 5 has a flat SOC-OCV curve as shown in Figure 2, the voltage change relative to the change in SOC is extremely small. Therefore, if, for example, a detection error occurs in the voltage V of the auxiliary battery 5, the SOC may drop significantly before the discharge of the auxiliary battery 5 is completed. In such a case, as explained below, a large volume change in the negative electrode occurs due to the stage structure of the graphite, and as a result, the deterioration of the auxiliary battery 5 may progress.

[0032] Figure 3 shows an example of the relationship between the lithium insertion rate in the graphite anode and the anode potential or anode volume change rate. The horizontal axis represents the lithium insertion rate in the anode (Li x This represents x) in terms of chemical composition ratio with C6. The left vertical axis represents the negative electrode potential relative to metallic lithium. When there are n (n=1~4) layers of graphite without inserted lithium ions between two graphite layers, each with inserted lithium ions, the structure of the graphite negative electrode is called stage n. The stage structure of the graphite negative electrode includes stage 1, stage 2, stage 2L, stage 3L, and stage 4L. The right vertical axis represents the rate of change of the negative electrode volume relative to the volume of stage 4L, which does not have inserted lithium ions. Figure 3 also shows the changes in the stage structure.

[0033] If the volume of a lithium iron phosphate battery changes repeatedly due to repeated charging and discharging, the lithium iron phosphate battery may degrade. To suppress such degradation due to volume changes, it is ideal to limit the charging and discharging of the lithium iron phosphate battery to between stage 2 and stage 2L. When a lithium iron phosphate battery is used as an auxiliary battery 5, it is particularly desirable to limit the volume change of the negative electrode on the discharge side in order to stabilize the supply voltage to the auxiliary load 4.

[0034] Therefore, in this embodiment, the discharge termination condition is set considering the discharge depth of the graphite, and the discharge termination condition is set based on the State of Charge (SOC) instead of the cell voltage. Specifically, the negative electrode potential of the graphite changes significantly around stage 2L (when the stage structure transitions from stage 2L to stage 3L), and as a result, the cell voltage also changes. Therefore, the ECU7 sets the SOC corresponding to this cell voltage as the discharge termination SOC.

[0035] The following describes, in order, the "initial treatment" performed in the early stages when the auxiliary battery 5 has not yet deteriorated, and the "post-treatment" performed periodically, for example, after several years.

[0036] <Initial processing> Figure 4 is a flowchart showing an example of the initial processing procedure. The processes shown in this flowchart are executed when predetermined conditions are met (for example, at predetermined intervals). Each step is implemented by software processing by the ECU7, but may also be implemented by hardware (electrical circuits) located within the ECU7. Hereinafter, each step will be abbreviated as S.

[0037] In S11, the ECU 7 determines whether the auxiliary battery 5 has been left unattended for a predetermined period of time. For example, before the completed vehicle is handed over to the user (during transport, long-term storage, etc.), the ECU 7 determines that the auxiliary battery 5 has been left unattended. This determination can be achieved, for example, by measuring the length of the shutdown period of the DC / DC converter 3 using a timer (not shown).

[0038] If the auxiliary battery 5 is not left unattended (NO in S11), the ECU 7 terminates processing without executing any further steps. If the auxiliary battery 5 is left unattended (YES in S11), the ECU 7 proceeds to S12 and obtains the voltage V of the auxiliary battery 5 from the voltage sensor 61.

[0039] In step S13, the ECU7 determines whether the voltage V of the auxiliary battery 5 is less than or equal to the threshold voltage Vth. The threshold voltage Vth is preferably set such that the graphite negative electrode remains in stage 2L and does not transition to stage 3L. In other words, the threshold voltage Vth is higher than the voltage when the graphite negative electrode's stage structure is in stage 3L. Specifically, the threshold voltage Vth is preferably between 3.20V and 3.28V.

[0040] If the voltage V is higher than the threshold voltage Vth (NO in S13), ECU7 returns to processing S12. If the voltage V is less than or equal to the threshold voltage Vth (YES in S13), ECU7 proceeds to processing S14.

[0041] In S14, the ECU7 obtains the State of Charge (SOC) of the auxiliary battery 5 when the voltage V reaches the threshold voltage Vth. The ECU7 can estimate the SOC of the auxiliary battery 5 using known methods such as current integration.

[0042] In S15, the ECU 7 sets the discharge termination SOC to (SOC + α), which is the SOC obtained in S14 with a certain margin α. The margin α is preferably set considering the estimation error of the SOC, more specifically the error in current integration, and the error caused by differences in ambient temperature. The margin α may be set to a specified percentage (for example, 1-10%) of the SOC obtained in S14. By setting the discharge termination SOC higher by margin α, the auxiliary battery 5 will terminate discharge earlier by margin α. This prevents the graphite negative electrode stage structure from reaching stage 3L, even if an SOC error occurs. Therefore, the deterioration of the auxiliary battery 5 can be suppressed more reliably. The ECU 7 nonvolatilously stores the set discharge termination SOC in memory 72.

[0043] Thus, the SOC-OCV curve of the auxiliary battery 5, which has a positive electrode containing lithium iron phosphate, is almost flat except in the low SOC region (see Figure 2). Therefore, if the discharge termination condition of the auxiliary battery 5 is set based on the voltage V of the auxiliary battery 5, the SOC of the auxiliary battery 5 may drop significantly due to sensor errors, etc., which may cause the auxiliary battery 5 to deteriorate. In this embodiment, the ECU 7 sets the discharge termination SOC instead of the discharge termination voltage as the discharge termination condition for the auxiliary battery 5.

[0044] Furthermore, in the auxiliary battery 5 having a negative electrode containing graphite, the voltage change of the auxiliary battery 5 (shape of the SOC-OCV curve) is substantially determined by the change in the negative electrode potential (shape of the negative electrode's SOC-OCP curve) (see Figure 2). When the stage structure of the negative electrode moves beyond stage 2L to stage 3L, a large volume change of the negative electrode occurs (see Figure 3), which can cause the auxiliary battery 5 to deteriorate. Therefore, in this embodiment, the ECU 7 sets the SOC corresponding to the voltage at which the stage structure of the negative electrode remains at stage 2L and does not reach stage 3L as the discharge termination SOC of the auxiliary battery 5. Thus, according to this embodiment, the deterioration of the auxiliary battery 5, which is a lithium iron phosphate battery, can be suppressed.

[0045] <Post-Year Processing> Figure 5 shows an example of the SOC-OCV curve and SOC-OCP curve of a lithium iron phosphate battery after several years. For comparison, Figure 5 also shows the SOC-OCV curve and SOC-OCP curve at the initial stage, as shown in Figure 2.

[0046] In lithium iron phosphate batteries that have been in use for a long time, the full charge capacity decreases compared to lithium iron phosphate batteries in their initial stages. Even if a lithium iron phosphate battery deteriorates, the capacity of the positive electrode itself does not decrease, nor does the capacity of the negative electrode itself decrease. The main cause of the decrease in full charge capacity is the capacity difference between the positive and negative electrodes due to the formation of an SEI (Solid Electrolyte Interphase) coating on the graphite negative electrode. However, as the full charge capacity of the auxiliary battery 5 decreases, the State of Charge (SOC) corresponding to that voltage changes, even if the voltage of the auxiliary battery 5 remains the same. Therefore, instead of continuing to use the discharge termination SOC set in the initial processing, it is desirable to update the discharge termination SOC after a certain period of time has elapsed.

[0047] Figure 6 is a flowchart showing an example of the processing procedure for post-period processing. The processes shown in this flowchart are executed when predetermined conditions are met (for example, at predetermined intervals since the execution of the initial processing or the previous execution of post-period processing).

[0048] In S21, the ECU 7 determines whether the auxiliary battery 5 has been left unused for a predetermined period of time. For example, when the vehicle is parked, the auxiliary load 4 is not used by the user. Therefore, the ECU 7 determines that the auxiliary battery 5 has been left unused when the vehicle has been parked for a predetermined period of time.

[0049] If the auxiliary battery 5 is not left unattended (NO in S21), the ECU 7 terminates processing without executing any further steps. If the auxiliary battery 5 is left unattended (YES in S21), the ECU 7 proceeds to S22, where it obtains the temperature T of the auxiliary battery 5 from the temperature sensor 63 and the voltage V of the auxiliary battery 5 from the voltage sensor 61.

[0050] In S23, the ECU7 determines whether the temperature T of the auxiliary battery 5 is above the threshold temperature Tth. In the low-temperature range, the resistance of the auxiliary battery 5 increases, and the voltage drop associated with discharge becomes larger, which can reduce the accuracy of voltage detection. Therefore, the threshold temperature Tth is set to a temperature higher than the low-temperature range (for example, 15°C). This makes it possible to perform the voltage determination in the subsequent S24 with high accuracy.

[0051] In S24, the ECU7 determines whether the voltage V of the auxiliary battery 5 is less than or equal to the threshold voltage Vth. The threshold voltage Vth is preferably set so that the graphite negative electrode remains in stage 2L and does not move to stage 3L, similar to the initial processing. Therefore, the threshold voltage Vth is preferably higher than the voltage when the graphite negative electrode's stage structure is in stage 3L, and is between 3.20V and 3.28V.

[0052] If the voltage V is higher than the threshold voltage Vth (NO in S24), ECU7 returns to processing S22. If the voltage V is less than or equal to the threshold voltage Vth (YES in S24), ECU7 proceeds to processing S25.

[0053] In S25, the ECU7 obtains the State of Charge (SOC) of the auxiliary battery 5 when the voltage V reaches the threshold voltage Vth. The ECU7 can estimate the SOC of the auxiliary battery 5 using known methods such as current integration.

[0054] In S26, ECU7 sets the discharge end SOC to (SOC + α), which is the SOC obtained in S25 with a certain margin α. The margin α is set considering the estimation error of the SOC, similar to the initial processing. ECU7 nonvolatilously stores the set discharge end SOC in memory 72. As a result, the discharge end SOC is updated to a value that reflects the capacity shift due to aging.

[0055] As described above, in this embodiment, the ECU 7 sets the discharge termination SOC instead of the discharge termination voltage as the discharge termination condition for the auxiliary battery 5. The discharge termination SOC is set to the SOC corresponding to the voltage at which the negative electrode stage structure remains at stage 2L and does not transition to stage 3L. Therefore, according to this embodiment, the degradation of the auxiliary battery 5, which is a lithium iron phosphate battery, can be suppressed.

[0056] Furthermore, as explained in Figures 5 and 6, the ECU 7 takes into account the capacity shift between the positive and negative electrodes due to aging and resets the discharge end SOC at regular intervals. This updates the discharge end SOC to an appropriate value that reflects the capacity shift. Therefore, according to this embodiment, the deterioration of the auxiliary battery 5 can be continuously suppressed.

[0057] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure 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]

[0058] 100 Battery System, 1 Main Battery, 2 First Monitoring Unit, 21 Voltage Sensor, 22 Current Sensor, 23 Temperature Sensor, 3 Converter, 4 Auxiliary Load, 5 Auxiliary Battery, 6 Second Monitoring Unit, 61 Voltage Sensor, 62 Current Sensor, 63 Temperature Sensor, 7 ECU, 71 Processor, 72 Memory.

Claims

1. A lithium-ion secondary battery having a positive electrode containing lithium iron phosphate and a negative electrode containing graphite, A voltage sensor for detecting the voltage of the aforementioned battery, A power converter configured to discharge the aforementioned battery, The system includes a control device that controls the power converter so that when the State of Charge (SOC) of the battery reaches the discharge termination SOC, the discharge of the battery is terminated. The control device, when the voltage detected by the voltage sensor falls below a threshold voltage, acquires the State of Control (SOC) of the battery and sets the discharge termination SOC according to the acquired SOC, in a battery system.

2. The battery system according to claim 1, wherein the threshold voltage is higher than the voltage when the negative electrode stage structure is stage 3L.

3. The battery system according to claim 2, wherein the threshold voltage is 3.20V or more and 3.28V or less.

4. The battery system according to any one of claims 1 to 3, wherein the control device sets the discharge termination SOC to be higher than the acquired SOC by an amount equal to the error of the acquired SOC.

5. The system further includes a temperature sensor for detecting the temperature of the battery, The battery system according to any one of claims 1 to 3, wherein the control device sets the discharge termination SOC when the temperature detected by the temperature sensor exceeds a threshold temperature and the voltage detected by the voltage sensor falls below the threshold voltage.

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