Battery system

Abstract

To equalize the charge levels of the individual cells in a battery. [Solution] The battery system is a system capable of charging and discharging power and comprises a battery, which is a battery pack in which multiple battery cells are connected in series; an equalization unit, which is an equalization circuit including a discharge resistor for each cell and a switch that discharges from each cell to the corresponding discharge resistor; a voltage detection unit that functions as a voltage sensor to detect the voltage of each cell; and an ECU as a processor. The ECU identifies the voltage of each cell from the signal from the voltage detection unit corresponding to each cell (step S122), and when charging the battery, controls the switch to discharge the cells to the discharge resistors sequentially as the cells reach a predetermined voltage, thereby equalizing the voltage of the cells (steps S123, step S125). This makes it possible to adjust the timing of equalizing the voltage of all cells by discharging the cells that reach the predetermined voltage first.

Description

Technical Field

[0001] This disclosure relates to a battery system, particularly to a battery system capable of charging and discharging electric power.

Background Art

[0002] Conventionally, equalization of a battery pack in which battery cells are connected in series has been performed by cutting off the input of electric power from the outside to the battery pack and consuming electric power with a discharge resistor from a battery cell with a high voltage (see, for example, paragraph 0054 of Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] A lithium iron phosphate ion battery (hereinafter referred to as "LFP battery"), which is a type of secondary battery, has a region in its SOC (State Of Charge) - OCV (Open Circuit Voltage) characteristics where the change in OCV accompanying the increase and decrease of SOC is flat. According to the technology of Patent Document 1, in a battery pack of LFP batteries, equalization can only be performed near a power shortage or near full charge, resulting in fewer opportunities for equalization. Also, the battery cells are equalized at a voltage lower than the full charge voltage.

[0005] This disclosure has been made to solve the above - described problems, and its object is to provide a battery system capable of equalizing battery cells of a battery pack at full charge.

Means for Solving the Problems

[0006] The battery system described in this disclosure is a system capable of charging and discharging power. The battery system comprises a battery pack in which multiple battery cells are connected in series, a discharge resistor for each battery cell, an equalization circuit including a switch that discharges from each battery cell to its corresponding discharge resistor, a voltage sensor that detects the voltage of each battery cell, and a processor. The processor identifies the voltage of each battery cell from the signals from the voltage sensors corresponding to each battery cell, and when charging the battery pack, controls the switches to discharge the battery cells sequentially to the discharge resistors as they reach a predetermined voltage, thereby equalizing the voltage of the battery cells.

[0007] With this configuration, the timing of voltage equalization across all battery cells can be adjusted by discharging the battery cells that reach a predetermined voltage first. As a result, a battery system can be provided that enables the battery cells of a battery pack to be fully charged and equalized.

[0008] The processor may terminate charging when the voltage of the final battery cell, which is the last battery cell to reach a predetermined voltage, reaches that predetermined voltage, and after equalization of the final battery cell has begun, the voltage of the battery cells being equalized meets the full charge condition.

[0009] This configuration allows for the timing of the voltage equalization of all battery cells to be synchronized. As a result, the battery cells in the battery pack can be equalized at full charge.

[0010] The processor may also be configured to control the voltage of the power supplied to the battery pack so as not to exceed the sum of the allowable voltages of the battery cells being charged.

[0011] With this configuration, even if there are battery cells that are discharged during the equalization process, the sum of the allowable voltages of the battery cells that are being charged, excluding the battery cells that are being discharged, can be kept from exceeding the allowable voltage. [Effects of the Invention]

[0012] According to this disclosure, it is possible to provide a battery system that can equalize the battery cells of a battery pack at full charge. [Brief explanation of the drawing]

[0013] [Figure 1] This figure shows an example of a battery system according to an embodiment of this disclosure. [Figure 2] This flowchart shows the flow of the equalization control process in this embodiment. [Figure 3] This figure shows the SOC-OCV curve of a single cell in this embodiment. [Figure 4] This figure shows an example of the voltage change during charging of each individual cell in this embodiment. [Modes for carrying out the invention]

[0014] The embodiments of this disclosure will be described in detail below with reference to the drawings. In the drawings, identical or corresponding parts are denoted by the same reference numerals, and their descriptions will not be repeated.

[0015] Figure 1 shows an example of a battery system S according to an embodiment of this disclosure. Referring to Figure 1, the battery system S comprises a battery 100, an equalization unit (equalization circuit) 250, a monitoring unit 200, and an electronic control unit (hereinafter referred to as "ECU (Electronic Control Unit)") 300. The ECU 300 includes a CPU (Central Processing Unit) 301 and a memory (including, for example, ROM (Read Only Memory) and RAM (Random Access Memory)) 302. The ECU 300 controls the battery system S based on signals received from the monitoring unit 200, signals from various sensors, and information such as maps and programs stored in the memory 302.

[0016] Battery 100 is a rechargeable DC power source (secondary battery), and is a battery pack composed of multiple (n) single cells 101A to 101N stacked and electrically connected in series, for example. The single cells 101A to 101N may be composed of lithium-ion batteries, for example. In this embodiment, LFP batteries using lithium iron phosphate as the positive electrode active material are used as the single cells 101A to 101N.

[0017] The monitoring unit 200 includes a voltage detection unit 210, a current sensor, and a temperature sensor. The voltage detection unit 210 detects the voltage VB of the individual cells 101A to 101N (the voltage VB between each terminal of the individual cells 101A to 101N). The current sensor detects the current IB that is input to and output from the battery 100 (individual cells 101A to 101N). The temperature sensor detects the temperature TB of each individual cell 101A to 101N. Each detection unit outputs its detection result to the ECU 300.

[0018] The equalization unit 250 is incorporated as an equalization circuit into the voltage detection unit 210 of the monitoring unit 200. In the battery 100, multiple (n) single cells 101A to 101N (typically also called "single cell 101") are connected in series. The voltage detection unit 210 detects the voltages of the single cells 101A to 101N via multiple voltage detection lines L1, branch line L11, and branch line L12. The first voltage detection line L1 is connected to the positive terminal of single cell 101A. The second to (n+1)th voltage detection lines L1 are connected between adjacent single cells 101A to 101N, between the negative terminal of one single cell and the positive terminal of the other single cell.

[0019] The voltage detection line L1 is equipped with a fuse F and a chip bead Cb. The fuse F blows when an overcurrent occurs, protecting the circuit. The chip bead Cb reduces applied stress when a surge voltage is applied instantaneously.

[0020] Each of the single cells 101A to 101N has a Zener diode D connected in parallel via an adjacent voltage detection line L1. The cathode of the Zener diode D is connected to the positive terminal side of the corresponding single cell, and the anode is connected to the negative terminal side of the corresponding single cell. When an overvoltage is applied from the battery 100 (single cell 101) to the voltage detection unit 210, a current flows through the Zener diode D to protect the voltage detection unit 210 from the overvoltage.

[0021] The voltage detection line L1 branches into a branch line L11 and a branch line L12 on the side of the monitoring unit 200 from the Zener diode D. The branch line L11 is connected to the comparator 211 via the switch So, and the branch line L12 is connected to the comparator 211 via the switch Sh. The switches So and Sh can use, for example, a photo MOS (Metal Oxide Semiconductor) relay. Note that the branch line L11 branched from the voltage detection line L1 connected to the positive terminal of the single cell 101A arranged on the positive output terminal side of the battery 100 is not connected to the comparator 211. Also, the voltage detection line L1 connected to the negative terminal of the single cell 101N arranged on the negative output terminal side of the battery 100 does not include the branch line L12.

[0022] A resistor R1 is provided in the branch line L12. A capacitor (flying capacitor) C is provided between the branch line L12 connected to the positive terminal of each single cell and the branch line L11 connected to the negative terminal. In the branch line L12, the capacitor C is connected between the resistor R1 and the switch Sh, and the resistor R1 and the capacitor C form an RC low-pass filter. Each of the capacitors C is connected in parallel with the corresponding single cells 101A to 101N, and the charge of the corresponding single cells 101A to 101N is charged to the capacitor C, and the voltage value of the capacitor C becomes equal to the voltage value of the corresponding single cells 101A to 101N.

[0023] By turning ON (closing) switches Sh and So, which correspond to specific single cells 101A to 101N, the comparator 211 outputs the voltage (cell voltage) VB of the specific single cell 101A to 101N. As a result, the monitoring unit 200 can detect the voltage VB of each single cell 101A to 101N using the voltage detection unit 210 by sequentially turning ON switches Sh and So, which correspond to each single cell 101A to 101N. In addition, by turning ON (closing) switch Sh for single cell 101A and switch So, which is connected to the negative terminal of single cell 101N, the voltage Vb of the battery 100 can be detected.

[0024] The equalization unit 250 consists of a discharge resistor Rd provided 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 ECU 300. In Figure 1, the dashed-dotted arrows indicate the current flow when equalization control is performed to eliminate the unevenness of the SOC of single cell 101. This shows the case where the SOC of single cell 101B is large, and discharge is performed from single cell 101B to perform equalization control. When the SOC of single cell 101B is large, the switch S1 corresponding to single cell 101B is turned ON (closed). When the switch S1 corresponding to single cell 101B is turned ON (closed), as shown by the dashed-dotted arrows, the current discharged from single cell 101B is consumed by the two discharge resistors Rd, the SOC of single cell 101B decreases, and SOC equalization is performed. In this way, the individual cells 101 of the battery 100 (battery pack) are equalized.

[0025] Conventionally, equalization of a battery pack 100, in which individual cells 101 are connected in series, was performed by cutting off the power input to the battery 100 from an external source and consuming power from the high-voltage individual cells 101 through a discharge resistor Rd. LFP batteries are secondary batteries in which a region of flat voltage exists in the SOC-OCV characteristic. According to conventional technology, in an LFP battery pack, equalization can only be performed near depletion or near full charge, resulting in fewer opportunities for equalization. Furthermore, equalization occurs when the individual cells 101 are at a voltage lower than their full charge voltage.

[0026] Therefore, the CPU 301 of the ECU 300 identifies the voltage of each cell 101 from the signal from the voltage sensor (voltage detection unit 210) corresponding to each cell 101. When charging the battery 100, the CPU 301 controls switch S1 to discharge the cell 101 that has reached a predetermined voltage into the corresponding discharge resistor Rd in order, thereby equalizing the voltage of the cell 101.

[0027] This allows the timing of voltage equalization for all individual cells 101 to be adjusted by discharging the cell 101 that has reached a predetermined voltage first. As a result, the individual cells 101 of battery 100 can be equalized at full charge.

[0028] Figure 2 is a flowchart showing the flow of the equalization control process in this embodiment. Referring to Figure 2, this equalization control process is called and executed by the CPU 301 of the ECU 300 at predetermined intervals from higher-level processing.

[0029] The CPU 301 of the ECU 300 determines whether or not it is time for a charging start command to be received from outside the battery system S (step S111). If it determines that it is time for a charging start command to be received (YES in step S111), the CPU 301 turns off the switch S1 of the equalization unit 250 corresponding to all individual cells 101 and starts charging all individual cells 101 (step S112).

[0030] After step S112, or if it is determined that it is not the time when a charging start instruction has been given (NO in step S111), the CPU 301 determines whether the adjustment flag is in the ON state (step S121). The adjustment flag is a flag that indicates whether the timing of the equalization of the full charge of the single cell 101 by the equalization unit 250 is being adjusted, and if it is in the ON state, it indicates that adjustment is in progress.

[0031] If the adjustment flag is not in the ON state (NO in step S121), the CPU 301 determines whether or not there is a single cell 101 that has reached a predetermined voltage (step S122).

[0032] Figure 3 shows the SOC-OCV curve of a single cell 101 in this embodiment. Referring to Figure 3, the vertical axis of this graph represents OCV (unit: V), and the horizontal axis represents SOC (unit: %). The solid line graph shows the SOC-OCV curve of a single cell 101 of the LFP battery used in this embodiment. The dashed line graph shows the SOC-OCV curve of a single cell of a conventionally used ternary battery. The SOC-OCV curve of the LFP battery includes first to third parts. The first part is the portion where the OCV rises sharply as the SOC increases from 0% to about 10%. The second part is the portion where the increase in OCV is gradual as the SOC increases from about 10% to about 98% (referred to as the "plateau region"). The third part is the portion where the OCV rises sharply as the SOC increases from about 98% to 100%. The first and third parts are easy to use as opportunities to equalize the SOC of multiple single cells 101 contained in the battery 100 because the increase or decrease in OCV with respect to the change in SOC is large. In this embodiment, the predetermined voltage in step S122 is a voltage within the range of the equalization opportunity of the third part, for example, the OCV of a fully charged battery corresponding to an SOC of 100%, but it may be any other OCV within this range.

[0033] Returning to Figure 2, if it is determined that there is a single cell 101 that has reached a predetermined voltage (YES in step S122), the CPU 301 starts control to intermittently switch the switch S1 corresponding to that single cell 101 so that the voltage of the single cell 101 that has reached the predetermined voltage decreases (step S123).

[0034] Figure 4 shows an example of the voltage change during charging of each cell 101 in this embodiment. Referring to Figure 4, the vertical axis of this graph represents the voltage of the cell, and the horizontal axis represents time. At time T1, the first cell 101 reaches a predetermined voltage. Then, at time T2, the second cell 101 reaches a predetermined voltage. At time T3, the third cell 101 reaches a predetermined voltage. At time Tn, the nth cell 101 reaches a predetermined voltage.

[0035] Returning to Figure 2, after step S123, the CPU 301 determines whether the last (nth) cell 101 has reached the predetermined voltage (step S124). If it determines that it is the last cell 101 (YES in step S124), the CPU 301 starts the regulated intermittent control (step S125). The regulated intermittent control is a control that intermittently switches the switch S1 corresponding to each cell 101 while adjusting the SOC so that all cell 101 reach the predetermined voltage simultaneously.

[0036] Referring again to Figure 4, each graph from time Tn onward shows the voltage of each cell 101 as it changes by controlling the corresponding switch S1 while adjusting the SOC of each cell 101. By controlling the SOC while switching the switch S1 on and off, at time Te, the voltage of all cell 101 reaches a predetermined voltage simultaneously or almost simultaneously.

[0037] Referring again to Figure 2, after step S125, the CPU 301 turns on the adjustment flag (step S126). If it is determined that there are no single cells 101 that have reached the predetermined voltage (NO in step S122), or if it is determined that it is not the last single cell 101 (NO in step S124), or after step S126, the CPU 301 returns the processing to be executed to the higher-level processing that called this equalization control process.

[0038] If the adjustment flag is turned on (YES in step S121), the CPU 301 determines whether all single cells 101 have reached a predetermined voltage (step S131). If the CPU 301 determines that all single cells 101 have not reached a predetermined voltage (NO in step S131), the CPU 301 continues the intermittent adjustment control started in step S125 (step S132).

[0039] After step S132, the CPU 301 determines whether the applied voltage to each cell 101 exceeds the allowable voltage of the cell 101 (step S133). During the execution of the regulated intermittent control, some cells 101 are discharged, so if the voltage applied to the battery 100 is divided and applied to the cells 101 that are not being discharged, the applied voltage to each cell 101 may exceed the allowable voltage of the cell 101.

[0040] If the CPU 301 determines that the applied voltage to each individual cell 101 exceeds the allowable voltage of the individual cell 101 (YES in step S133), the CPU 301 requests the control unit that controls the external applied voltage of the battery system S to ensure that the total applied voltage applied to the battery 100 does not exceed the total allowable voltage of the individual cells 101 that are not being discharged (step S134). After step S134, the CPU 301 returns the processing to be executed to the higher-level processing that called this equalization control process.

[0041] If the CPU 301 determines that all individual cells 101 have reached a predetermined voltage (YES in step S131), it terminates the intermittent adjustment control started in step S125 (step S135) and turns off the adjustment flag (step S136). This turns off the switch S1 of the equalization unit 250 corresponding to all individual cells 101. After step S136, the CPU 301 returns the processing to be executed to the higher-level processing that called this equalization control process.

[0042] [Differentiation] (1) In the embodiment described above, all single cells 101 included in the battery 100 were assumed to be LFP batteries. However, the invention is not limited to this, and all or some of the single cells 101 included in the battery 100 may be ternary batteries. Referring again to Figure 3, the SOC-OCV curve of a ternary battery does not have the aforementioned plateau region, unlike that of an LFP battery. For this reason, for a single cell 101 that is a ternary battery, the predetermined voltage shown in step S122 of Figure 2 is not limited to a voltage within the range of the equalization opportunity of the third part, but may be any OCV corresponding to any SOC.

[0043] (2) In the embodiment described above, as shown in Figure 4, the regulating intermittent control is started in step S125 after the last cell 101 reaches a predetermined voltage. However, the control is not limited to this, and the following type of regulating intermittent control may be performed. This regulating intermittent control calculates the time required for the last cell 101 to reach a predetermined voltage before the last cell 101 reaches a predetermined voltage (for example, when the first cell 101 reaches a predetermined voltage, or at a predetermined timing earlier), and controls the system so that all cell 101 reach a predetermined voltage when that time has elapsed. This makes the time Tn when the last cell 101 reaches a predetermined voltage and the time Te when the regulating intermittent control is completed the same, as shown in Figure 4, that is, the regulating intermittent control can be completed when the last cell 101 reaches a predetermined voltage.

[0044] (3) The embodiments described above can be interpreted as disclosures of the battery system S shown in Figure 1 or a control device for the battery system S, such as the ECU 300. Alternatively, they can be interpreted as disclosures of a control method or control program shown in Figure 2 that is executed by the control device for the battery system S.

[0045] [summary] (1) As shown in Figure 1, the battery system S is a system capable of charging and discharging power and comprises a battery 100 which is a battery pack in which multiple battery cells, or single cells 101, are connected in series; an equalization unit 250 which is an equalization circuit including a discharge resistor Rd for each single cell 101 and a switch S1 that discharges from each single cell 101 to the corresponding discharge resistor Rd; a voltage detection unit 210 which functions as a voltage sensor that detects the voltage of each single cell 101; and an ECU 300 which is a processor. As shown in Figure 2, the ECU 300 identifies the voltage of each single cell 101 from the signal from the voltage detection unit 210 corresponding to each single cell 101 (for example, step S122), and when charging the battery 100, controls the switch S1 to discharge the single cells 101 to the discharge resistor Rd sequentially as they reach a predetermined voltage, thereby equalizing the voltage of the single cells 101 (for example, steps S123, step S125).

[0046] This allows the timing of voltage equalization for all individual cells 101 to be adjusted by discharging the cell 101 that has reached a predetermined voltage first. As a result, the individual cells 101 of battery 100 can be equalized at full charge.

[0047] (2) As shown in Figure 2, the ECU 300 may terminate charging (for example, steps S131, S135) when the voltage of the final battery cell, which is the last battery cell to reach the predetermined voltage, reaches the predetermined voltage and equalization of the final battery cell begins, and the voltage of the battery cells being equalized satisfies the full charge condition.

[0048] This allows the timing of the equalization of the voltage of all individual cells 101 to be synchronized. As a result, the individual cells 101 of battery 100 can be equalized at full charge.

[0049] (3) As shown in Figure 2, the ECU 300 may control the voltage of the power supplied to the battery 100 so as not to exceed the sum of the allowable voltages of the individual cells 101 being charged (for example, steps S133, S134).

[0050] This ensures that even if there is a single cell 101 that is being discharged during the equalization process, the sum of the allowable voltages of the single cells 101 that are being charged, excluding the single cell 101 that is being discharged, does not exceed the allowable voltage of the single cell 101 that is being charged.

[0051] 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]

[0052] 100 Battery, 101, 101A~101N Single Cell, 200 Monitoring Unit, 210 Voltage Detection Unit, 211 Comparator, 250 Equalization Unit, 300 ECU, 301 CPU, 302 Memory, C Capacitor, Cb Chip Bead, D Zener Diode, F Fuse, L1 Voltage Detection Line, L11, L12 Branch Lines, R1 Resistor, Rd Discharge Resistor, S Battery System, S1, Sh, So Switch.

Claims

1. A battery system capable of charging and discharging power, A battery pack in which multiple battery cells are connected in series, The discharge resistance of each of the aforementioned battery cells, An equalization circuit including a switch that discharges from each of the aforementioned battery cells to the corresponding respective discharge resistors, A voltage sensor for detecting the voltage of each of the aforementioned battery cells, Equipped with a processor, The aforementioned processor, The voltage of each battery cell is identified from the signal from the voltage sensor corresponding to each battery cell. A battery system that, when charging the battery pack, controls the switch to discharge the battery cells sequentially from the battery cells that have reached a predetermined voltage to the discharge resistor, thereby equalizing the voltage of the battery cells.

2. The aforementioned processor, The battery system according to claim 1, wherein charging is terminated when the voltage of the final battery cell, which is the last battery cell to reach the predetermined voltage, reaches the predetermined voltage, and after the equalization of the final battery cell has begun, the voltage of the battery cell performing the equalization satisfies the full charge condition.

3. The battery system according to claim 1 or 2, wherein the processor controls the voltage of the power supplied to the battery pack so as not to exceed the sum of the allowable voltages of the battery cells during charging.

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