Battery device for electric vehicle
Patent Information
- Application Number
- PCT/JP2025/012293
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2025-03-27
- Publication Date
- 2026-10-01
Smart Images
Figure JP2025012293_01102026_PF_FP_ABST
Abstract
Description
Battery device for electric vehicle
[0001] The present invention relates to a battery device for an electric vehicle.
[0002] Patent Document 1 describes, in a battery in which a plurality of reused storage batteries are connected in parallel, a switch capable of disconnecting a degraded reused storage battery from the parallel circuit of reused storage batteries.
[0003] Japanese Unexamined Patent Application Publication No. 2017-79131
[0004] In an electric vehicle, large electric power is consumed during acceleration, and large regenerative electric power is generated during deceleration. Accordingly, in a battery that stores electric power for traveling, a large discharge current and a large charge current may frequently flow repeatedly. In recent years, a battery that stores electric power for traveling may include a parallel circuit of a plurality of battery cells.
[0005] The inventors of the present invention found that in a parallel circuit of a plurality of battery cells, variation in the degree of deterioration may occur among the plurality of battery cells, and further, when discharge current and charge current repeatedly flow, the variation in the degree of deterioration may further expand.
[0006] An object of the present invention is to provide a battery device for an electric vehicle that can suppress an increase in variation in the degree of deterioration among a plurality of battery cells in a battery including a parallel circuit of the plurality of battery cells.
[0007] One aspect of the present invention is a battery device for an electric vehicle that is mounted on an electric vehicle and stores electric power for traveling, the battery device comprising: a battery including a plurality of battery cells connected in parallel; a plurality of switches respectively connected in series with the plurality of battery cells; and a controller that controls the plurality of switches, wherein the controller determines a target switch whose state is to be switched among the plurality of switches based on a comparison of the degree of deterioration of each of the plurality of battery cells, and performs switching of the target switch to an open state and switching to a closed state based on a state of the battery.
[0008] According to the present invention, in a battery including a parallel configuration of battery cells, it is possible to suppress large variations in the degree of degradation of multiple battery cells.
[0009] This block shows an electric vehicle equipped with a battery device according to an embodiment of the present invention. This is a circuit diagram showing the details of the battery in Figure 1. This is a flowchart showing the processing procedure for the capacity measurement mode. This is a flowchart showing the processing procedure for the main control mode. This is a diagram showing the three states of the first to third battery cells connected in parallel. This is a diagram showing an example of the state change during discharge of a group of battery cells with varying degrees of degradation. This is a diagram showing an example of the state change during charging of a group of battery cells with varying degrees of degradation.
[0010] Embodiments of the present invention will be described in detail below with reference to the drawings.
[0011] Figure 1 is a block showing an electric vehicle equipped with a battery device according to an embodiment of the present invention.
[0012] An embodiment of the present invention includes a battery device 10 that stores power for driving and a controller 12 that manages the inside of the battery 11. The electric vehicle 1 includes drive wheels 2, a drive motor 3 that drives the drive wheels 2, the battery device 10, an inverter 4 that converts power between the battery 11 and the drive motor 3, a driving operation unit 5 that receives driving operations from the driver, a vehicle controller 6 that receives operation signals from the driving operation unit 5 and controls the inverter 4, a battery management unit 7 that controls the charging and discharging of the battery 11, a navigation system 8 that registers the planned driving route and charging schedule of the electric vehicle 1, and a charger 9 that charges the battery 11 using an external power source taken in via a charging plug 9a.
[0013] The driving control unit 5 includes an acceleration control unit (accelerator pedal, etc.) 5a for accelerating the electric vehicle 1, a braking control unit (brake pedal, etc.) 5b for braking the electric vehicle 1, and a steering control unit (steering wheel, etc.) 5c for steering. The driving control unit 5 may be operated by an automated driving system rather than a driver.
[0014] The battery management unit 7 is an ECU (Electronic Control Unit) that manages the state of the battery 11, such as its SOC (State of Charge) and temperature, and determines the dischargeable power and rechargeable power of the battery 11. The battery management unit 7 and the controller 12 of the battery device 10 may be integrated.
[0015] The vehicle controller 6 is an ECU that receives driving operation signals from the driving operation unit 5 and communicates with the battery management unit 7 to control the movement of the electric vehicle 1. Specifically, the vehicle controller 6 controls the inverter 4 so that acceleration or deceleration of the electric vehicle 1 occurs in accordance with the driving operation, and so that the charging and discharging power of the battery 11 stays within the range of the dischargeable power and the rechargeable power. Furthermore, if the rechargeable power of the battery 11 is low and the vehicle controller 6 cannot meet the request for deceleration of the electric vehicle 1 through regenerative operation of the driving motor 3, the vehicle controller 6 activates a braking device (not shown) to decelerate the electric vehicle 1.
[0016] <Circuit Configuration of Battery 11> Figure 2 is a circuit diagram showing the details of the battery. The battery 11 includes a configuration in which multiple battery cell groups 11G are connected in series. Each battery cell group 11G includes a configuration in which multiple battery cells 111 are connected in parallel. This parallel-connected configuration is called a "parallel circuit of multiple battery cells 111". In the figure, an example is shown in which one battery cell group 11G includes three battery cells 111 connected in parallel, but there may be four or more battery cells 111 connected in parallel. Also, in the figure, an example is shown in which each battery cell group 11G has the same configuration, but multiple battery cell groups 11G may include multiple types of battery cell groups 11G with different battery cell connection configurations.
[0017] The battery cell 111 is, for example, a lithium-ion secondary battery. However, the battery cell 111 may also be other types of secondary batteries, such as nickel-metal hydride secondary batteries.
[0018] The battery cell group 11G includes multiple switches SW connected in series with each of the multiple battery cells 111. The switches SW are semiconductor switching elements such as FETs (field-effect transistors) or bipolar transistors, but other components can be used as long as they can open and close the electrical circuit. When one switch SW is opened, the corresponding battery cell 111 can be disconnected from the parallel circuit of the multiple battery cells 111. Even when charge and discharge current flows through the battery cell group 11G, no current flows through the disconnected battery cell 111.
[0019] As shown in the battery cell group 11G in the upper left of Figure 2, the multiple battery cells 111 included in one battery cell group 11G will also be referred to as the first battery cell 111a to the third battery cell 111c below. In addition, the multiple switches SW corresponding to the first battery cell 111a to the third battery cell 111c will also be referred to as the first switch SW1 to the third switch SW3.
[0020] The battery 11 circuit is further provided with a discharge circuit 113 that can adjust the voltage of each battery cell group 11G, and a voltmeter 115 that measures the voltage across the terminals of each battery cell group 11G. The discharge circuit 113 is a circuit in which a discharge switch 113a and a discharge resistor 113b are connected in series, and is connected across the terminals of each battery cell group 11G. The voltmeter 115 is connected across the terminals of one battery cell group 11G. When adjusting the voltage of the battery cell group 11G, the discharge switch 113a is switched to the closed state, causing discharge from multiple battery cells 111 of the battery cell group 11G, thereby lowering the voltage of the battery cell group 11G.
[0021] The switch SW and the discharge switch 113a are controlled to open and close based on control signals sent from the controller 12. The sensor output of the voltmeter 115 is sent to the controller 12.
[0022] The controller 12 of the battery device 10 mainly performs cell balancing and variation reduction processing. Cell balancing is a process that equalizes the voltage among multiple battery cell groups 11G. Variation reduction processing is a process that suppresses large variations in the degree of degradation among multiple battery cells 111 included in one battery cell group 11G. The controller 12 may be a single ECU (Electronic Control Unit), or it may have multiple ECUs and multiple sensors that work in cooperation with each other. The controller 12 includes a storage unit 12a that stores a control program, and performs control operations by executing the control program.
[0023] <Battery Cell Capacity and Degradation> As the battery cell 111 degrades, its capacity (i.e., the remaining charge when fully charged) decreases. The degradation of the battery cell 111 is, for example, the State of Health (SOH). SOH represents the ratio of the current capacity to the initial capacity, which is set at 100%. In this embodiment, it is explained that the initial capacities of the first battery cell 111a to the third battery cell 111c are equal. In this case, the size of each capacity of the first battery cell 111a to the third battery cell 111c represents the respective degradation levels of the first battery cell 111a to the third battery cell 111c. Therefore, below, the size of the capacity may be used to indicate the degradation level. Note that the initial capacities of multiple battery cells 111 may be different.
[0024] <Controller Control Operation> The controller 12 has two operating modes: a main control mode that performs cell balancing and variation reduction processing, and a capacity measurement mode that measures the degree of degradation of each of the multiple battery cells 111 connected in parallel within a single battery cell group 11G. The main control mode corresponds to an example of the first control mode according to the present invention. The capacity measurement mode corresponds to an example of the second control mode according to the present invention.
[0025] <Processing in Capacity Measurement Mode> Figure 3 is a flowchart showing the procedure for processing in capacity measurement mode. The processing in capacity measurement mode is a mode in which the current capacity of each of the first battery cells 111a to the third battery cells 111c is measured. The following shows the processing for one battery cell group 11G, but the controller 12 performs the same processing for multiple (for example, all) battery cell groups 11G.
[0026] The capacity measurement mode is repeatedly executed by the controller 12 at relatively long intervals. When the process starts, the controller 12 first determines whether the electric vehicle 1 is in a idle state (not running and the battery 11 is not being charged or discharged) (step S1). If the answer is YES, the controller 12 executes the process from the following steps. If the answer is NO, the controller 12 terminates the capacity measurement mode.
[0027] If the determination result in step S1 is YES and the process moves to the next step, first the controller 12 performs a process to measure the capacity of the third battery cell 111c (step S2). In the process of step S2, first the controller 12 opens the first switch SW1 and the second switch SW2, and closes the third switch SW3 (step S3). Next, the controller 12 controls the discharge switch 113a to discharge current a1 from the third battery cell 111c for a time s1 (step S4). Subsequently, the controller 12 waits for a predetermined time for the polarization due to discharge to decrease (step S5), and estimates the OCV (Open Circuit Voltage) of the third battery cell 111c from the output of the voltmeter 115 (step S6).
[0028] Next, the controller 12 calculates the capacity of the third battery cell 111c based on the current a1, time s1, and OCV (step S7). OCV and SOC (state of charge) are correlated, and the controller 12 has a data table in advance for calculating SOC from OCV. The controller 12 also has information on the SOC of each battery cell 111 immediately before executing the capacity measurement mode processing. In step S6, the controller 12 first calculates the cumulative power during discharge based on the current a1 and time s1. Then, the controller 12 can calculate the amount of energy at 100% SOC, i.e., the capacity of the third battery cell 111c, based on the difference between the SOC immediately before discharge and the SOC obtained from the OCV after discharge, and the cumulative power.
[0029] Next, the controller 12 switches its target from the third battery cell 111c to the second battery cell 111b and performs the same processing as in steps S2 to S7 to determine the capacity of the second battery cell 111b (step S8). Then, the controller 12 switches its target from the second battery cell 111b to the first battery cell 111a and performs the same processing as in steps S2 to S7 to determine the capacity of the first battery cell 111a (step S9).
[0030] Next, the controller 12 compares the capacities of the first battery cell 111a to the third battery cell 111c. If the difference is greater than a threshold, it determines that there is variation in the degree of degradation of the target battery cell group 11G. If the difference is less than the threshold, it determines that there is no variation (step S10). The controller 12 then stores the result of the determination of whether or not there is variation (i.e., stores it in the storage unit 12a) (step S11). The controller 12 may also store the capacity values of the first battery cell 111a to the third battery cell 111c measured in steps S2, S8, and S9. The controller 12 then terminates the processing of the capacity measurement mode.
[0031] According to the processing in the capacity measurement mode described above, if there is a variation in the degree of degradation among the multiple battery cells 111 connected in parallel due to the operation of the electric vehicle 1, the controller 12 can detect the variation.
[0032] The capacity measurement mode processing may be executed only when there has been a change in the SOH of the battery 11 that exceeds a threshold, such as when the SOH of the battery 11 has decreased by a threshold (e.g., 5%) since the previous processing. The SOH of the battery 11 is estimated by the controller 12 or the battery management unit 7 in a separate process. In the capacity measurement mode processing, as described above, discharge is performed from multiple battery cells 111 for measurement. Therefore, it is undesirable for this processing to be executed frequently. On the other hand, the capacity of the multiple battery cells 111, as well as the presence or absence of variation in the degree of degradation, do not change frequently and often change in the same way as the change in the SOH of the battery 11. Therefore, by adopting the above execution conditions, the capacity measurement mode processing can be executed at a more favorable frequency.
[0033] <Processing in Main Control Mode> Figure 4 is a flowchart showing the procedure for processing in main control mode. Processing in main control mode is repeatedly executed in short cycles while the electric vehicle 1 is starting up. While the electric vehicle 1 is starting up, this includes when the electric vehicle 1 is running, when it is stopped in a state where it can run, and when the battery 11 is being charged using external power. The following mainly shows the processing for one battery cell group 11G, but the controller 12 performs similar processing for multiple (for example, all) battery cell groups 11G.
[0034] When processing in main control mode begins, the controller 12 first determines whether it is time to execute cell balancing (step S21). Various conditions may be applied to determine the execution timing, such as a predetermined cycle or when the usage time of the battery 11 exceeds a threshold time.
[0035] If the result of the determination in step S21 is NO, the controller 12 then determines whether the conditions for performing the process to reduce the variation in the degradation degree of the battery cells 111 are met (step S31). These conditions are when there is a group of battery cells 11G in which the degradation degrees of multiple battery cells 111 vary, based on the information stored in the memory unit 12a in step S11 of Figure 3.
[0036] If the result of step S31 is NO, the controller 12 terminates the main control mode processing and starts the main control mode processing again in the next cycle.
[0037] <<Cell balancing process including pre-processing>> If the result of the determination in step S21 is YES, the controller 12 starts the cell balancing process in steps S22 to S26. When the cell balancing process starts, the controller 12 first identifies a group of battery cells 11G that have been determined to have variations in the degree of degradation of multiple battery cells 111, based on the information in the storage unit 12a (step S22). If there is a group of battery cells 11G with the above variations, the controller 12 estimates the trend of increase or decrease in the SOC of the battery 11 thereafter (step S23). The estimation in step S23 can be performed by the controller 12 communicating with the navigation system 8 to read information on the planned driving route and charging schedule of the electric vehicle 1, and based on that information. If there is a planned charge, or if the planned driving route has a long downhill section, the controller 12 can estimate that the SOC of the battery 11 will increase thereafter. On the other hand, if it is a normal planned driving route, the controller 12 can estimate that the SOC of the battery 11 will decrease thereafter. Furthermore, if the planned route involves a long uphill climb, the controller 12 can estimate that the battery 11's SOC will decrease significantly thereafter. Based on this estimation of SOC changes and the current SOC value of the battery 11, the controller 12 can determine whether the battery 11's SOC will approach 0%, approach 100%, or fluctuate within a moderate range such as 20% to 80%.
[0038] As a result, if it is determined in step S22 that there are no battery cell groups 11G with variations, or if it is determined in step S23 that the SOC of the battery 11 is fluctuating within a moderate range, the controller 12 performs normal cell balancing (step S23). That is, the voltage of each of the multiple series-connected battery cell groups 11G is balanced using the discharge circuit 113.
[0039] On the other hand, if the controller 12 determines in step S23 that the SOC of the battery 11 is approaching 0%, it performs cell balancing processing so that the voltage of the battery cell group 11G with variations is higher than the voltage of the other battery cell groups 11G (step S25). This cell balancing processing is achieved by discharging more of the battery cell group 11G with variations using the discharge circuit 113.
[0040] Furthermore, if the controller 12 determines in step S23 that the State of Charge (SOC) of the battery 11 is approaching 100%, it performs cell balancing so that the voltage of the battery cell group 11G with the above-mentioned variations is lower than the voltage of the other battery cell groups 11G (step S26). This cell balancing is achieved by discharging more of the other battery cell groups 11G using the discharge circuit 113.
[0041] The processes in steps S25 and S26 are preferable to perform in advance for the subsequent degradation variation reduction process, and are referred to as "pre-processing for variation reduction." The pre-processing in step S25 or S26 reduces the likelihood that a group of battery cells 11G with varying degradation levels will reach the lower or upper voltage limit before other battery cell groups 11G connected in series during the variation reduction process described later.
[0042] <<Variation Reduction Processing>> If the result of the determination in step S31 is YES, the controller 12 starts the degradation degree variation reduction processing in steps S32 to S37 for the battery cell group 11G in which there is variation in the degree of degradation among the multiple battery cells 111.
[0043] When variation reduction processing is started, the controller 12 first determines whether the battery cells 111 are highly degraded based on the state of the battery 11 (step S32), and if NO, it determines whether they are moving away from the highly degraded state (step S33).
[0044] The degree of progress of deterioration of the battery cells 111 increases in a range where the voltage of the battery 11 is close to the upper limit voltage and a range where the voltage of the battery 11 is close to the lower limit voltage. In addition, the degree of progress of the aforementioned deterioration becomes higher when the discharge current or charging current of the battery 11 is large, when the temperature of the battery 11 is close to the upper limit temperature, and when the temperature of the battery 11 is close to the lower limit temperature. Therefore, the controller 12 performs the aforementioned determination based on one or more parameters obtained by further adding any one or all of current magnitude, current direction, and temperature to the SOC, voltage, or SOC and voltage of the battery 11. In the determination processing of step S32, the aforementioned determination may be performed based on a comparison between each of the aforementioned parameters and a threshold value corresponding to each parameter, or based on a comparison between an index value calculated based on the plurality of aforementioned parameters and a threshold value corresponding to the index value.
[0045] As one specific example, in step S32, the controller 12 determines "YES" when the voltage of the battery 11 becomes equal to or higher than a first threshold voltage set close to the upper limit voltage, and when the voltage of the battery 11 becomes lower than a second threshold voltage set close to the lower limit voltage. Furthermore, in step S33, the controller 12 determines that the battery cell 111 is in a state departing from a state of high deterioration progress when the voltage of the battery 11 deviates by a predetermined voltage width or more from a range equal to or higher than the first threshold voltage and a range lower than the second threshold voltage.
[0046] The aforementioned first threshold voltage is set to a voltage that is lower than the upper limit voltage and closer to the upper limit voltage than to the lower limit voltage. The aforementioned second threshold voltage is set to a voltage that is higher than the lower limit voltage and closer to the lower limit voltage than to the upper limit voltage. The aforementioned predetermined voltage width is set to a voltage width that avoids chattering of the determination result.
[0047] Note that there is a correlation between the SOC, voltage, current, and temperature of the battery 11 and the SOC, voltage, current, and temperature of the battery cell group 11G. Therefore, any one or all of the SOC, voltage, current, and temperature used as parameters in the determination processing of steps S32 and S33 may be replaced with the SOC, voltage, current, and temperature of the battery cell group 11G.
[0048] Furthermore, in steps S32 and S33, the controller 12 is not limited to making the above determination based on the state of the battery 11 at the time of the determination. For example, the controller 12 may predict the future state of the battery 11 based on the traveling and charging schedules of the electric vehicle 1, and make the above determination based on the predicted state of the battery 11. The prediction can be made by the controller 12 communicating with the navigation system 8 to read information on the planned traveling route and charging schedule of the electric vehicle 1, and based on the information and the state of the battery 11 at that time point.
[0049] If it is determined as a result of the determination in step S32 that the degree of progression of deterioration is high, the controller 12 sets the switch SW corresponding to the battery cell 111 having a small capacity (that is, a large degree of deterioration) among the target battery cell group 11G as the target switch SW. Then, the controller 12 determines whether the target switch SW is in the closed state (step S34). If the switch SW is in the closed state, the controller 12 switches the switch SW to the open state (step S35). Through this process, the low-capacity battery cell 111 is disconnected from the battery cell group 11G.
[0050] Note that, in order to avoid chattering, when the switch SW is closed in step S38 described later, the process of step S35 may be omitted for a predetermined period of time after the switch SW is closed.
[0051] On the other hand, if it is determined as a result of the determination in step S33 that the state has left the state where the degree of progression of deterioration is high, the controller 12 determines whether the target switch SW is in the open state (step S36). If the switch SW is in the open state, the controller 12 switches the switch SW to the closed state (step S37). Through this process, the low-capacity battery cell 111 that has been disconnected from the battery cell group 11G is reconnected.
[0052] On the other hand, if the determination in step S34 indicates that the switch SW is in the open state, the controller 12 determines whether the elapsed time since switching the switch SW to the open state has reached a threshold time (step S38). In step S38, the controller 12 may determine whether the elapsed time during charging or discharging has reached a threshold time, or whether the integrated value of the charging current or the integrated value of the discharging current has reached a threshold integrated value. If the determination result in step S38 is YES, the controller 12 switches the target switch SW to the closed state (step S37).
[0053] Then, in step S35 or step S37, once the state of the target switch SW has been switched, the controller 12 finishes processing in one main control mode cycle and repeats the processing from step S21 in the following cycle.
[0054] Furthermore, if the determination process in step S33 is NO (meaning the state is between a state of high deterioration and a state far from that state), the controller 12 terminates the main control mode process while maintaining the state of the target switch SW. Also, if the determination process in steps SS36 and S38 is NO, the controller 12 terminates the main control mode process while maintaining the state of the target switch SW. Then, in the next cycle, the controller 12 repeats the process from step S21 again.
[0055] The program for the capacity measurement mode and the program for the main control mode are stored in a non-transient computer-readable medium, such as the storage unit 12a of the controller 12. The controller 12 may be configured to read a program stored on a portable non-transient recording medium and execute the program. The portable non-transient storage medium may store the program for the capacity measurement mode and the program for the main control mode.
[0056] <Explanation of the effect of the variation reduction treatment> <<Problems caused by variations in the degree of degradation>> Figure 5 shows the three states J1 to J3 of the first battery cell 111a to the third battery cell 111c connected in parallel. In Figure 5, the capacity and SOC of the first battery cell 111a to the third battery cell 111c in each state are shown on the left, and the OCV at that time is shown on the right.
[0057] The State of Charge (SOC) of battery 11 refers to the current percentage of remaining charge when the battery 11 is fully charged, with the remaining charge at SOC being 100%. The SOC of each individual battery cell 111 (any of the first battery cell 111a to the third battery cell 111c) refers to the current percentage of remaining charge when the battery cell 111 is fully charged, with the remaining charge at SOC being 100%. The unit of remaining charge corresponds to the unit of energy. When battery 11 deteriorates, the remaining charge when SOC is 100% decreases compared to before deterioration. Similarly, when individual battery cells 111 deteriorate, the remaining charge when SOC is 100% decreases compared to before deterioration.
[0058] The OCV of each battery cell 111 (any of the first battery cell 111a to the third battery cell 111c) is the open-circuit voltage of that battery cell 111 after the voltage division is resolved. When a charging current or a discharging current is flowing, the closed-circuit voltages of the first battery cell 111a to the third battery cell 111c, which are connected in parallel, are equal. However, if their internal resistances are different, the OCVs of the first battery cell 111a to the third battery cell 111c will be different. The OCV of a battery cell 111 corresponds approximately one-to-one with the State of Control (SOC) of that battery cell 111.
[0059] State J1 in Figure 5 shows the state when there is no deterioration in the three battery cells 111 of the battery cell group 11G. In state J1, the capacity of each of the first battery cell 111a to the third battery cell 111c is the initial capacity, and the internal resistance of each is the initial internal resistance. That is, the capacity and internal resistance of each of the first battery cell 111a to the third battery cell 111c are equivalent. Therefore, when the battery 11 is discharged or charged, the same current flows through the first battery cell 111a to the third battery cell 111c in the parallel circuit of the battery cell group 11G, and the OCV and SOC of the first battery cell 111a to the third battery cell 111c are always equivalent. When battery 11 discharges a certain amount of energy from a full charge, the SOC of the first battery cell 111a becomes 50% and the OCV becomes 3.7V. At the same time, the SOC of the second battery cell 111b and the third battery cell 111c also become 50% and the OCV becomes 3.7V. Therefore, when the OCV of the battery cell group 11G is measured comprehensively by measuring the OCV of the first battery cell 111a to the third battery cell 111c which are connected in parallel, the OCV of the battery cell group 11G becomes 3.7V.
[0060] State J2 in Figure 5 shows the state when there is variation in the degree of degradation among the first battery cells 111a to the third battery cells 111c of the battery cell group 11G, and the capacity of the third battery cell 111c decreases. In state J2, the internal resistance of the third battery cell 111c is greater than that of the first battery cell 111a and the second battery cell 111b. Also, the capacity of the third battery cell 111c is smaller than that of the first battery cell 111a and the second battery cell 111b. Therefore, when the battery 11 discharges a certain amount of energy from a full charge, and the SOC of the first battery cell 111a becomes 50% and the OCV becomes 3.7V, if a similar discharge occurs in the second battery cell 111b, the SOC will also become 50% and the OCV will become 3.7V. However, in the third battery cell 111c, due to its smaller capacity, the same discharge occurs, resulting in a low SOC of 40% and a low OCV of 3.5V. At this time, when the OCV of the battery cell group 11G is measured comprehensively by combining the OCVs of the first battery cells 111a to the third battery cells 111c, which are connected in parallel, the OCV of the battery cell group 11G is approximately the average of the OCVs of the first battery cells 111a to the third battery cells 111c. This value is 3.63V.
[0061] State J3 in Figure 5 indicates a state in which the first battery cells 111a to the third battery cells 111c of the battery cell group 11G have uniformly deteriorated, and their respective capacities and internal resistances are equivalent. In state J3, when the battery 11 discharges a certain amount of energy from a full charge, and the SOC of the first battery cell 111a becomes 42% and the OCV becomes 3.63V, the second battery cell 111b and the third battery cell 111c will also have equivalent SOC and OCV. Therefore, when the OCV of the first battery cells 111a to the third battery cells 111c, which are connected in parallel, is measured comprehensively as the OCV of the battery cell group 11G, the OCV of the battery cell group 11G is 3.63V.
[0062] Here, let's assume that the OCV of battery cell 111 is set to 3.6V as the first-stage limiting voltage, and that some kind of limiting control is performed. In this case, in the battery cell group 11G in state J3, each battery cell 111 is not below the limiting voltage, but in the battery cell group 11G in state J2, the third battery cell 111c is below the limiting voltage. On the other hand, since the controller 12 measures the OCV for each individual battery cell group 11G, it cannot distinguish between state J2 and state J3. Therefore, in state J3, where there is variation in the degree of degradation, a situation may occur where the third battery cell 111c is below the first-stage limiting voltage, but limiting control at that limiting voltage is not performed.
[0063] Furthermore, in state J2, where the capacity of the third battery cell 111c is reduced, the internal resistance of the third battery cell 111c increases. Therefore, when the discharge current or charging current increases, the voltage drop due to the internal resistance in the third battery cell 111c increases. When the battery 11 undergoes repeated discharge and small charges (e.g., regenerative charging), the voltage drop described above can cause the voltage of the third battery cell 111c to become significantly lower. Conversely, during periods when the battery 11 is fully charged, a similar effect can cause the voltage of the third battery cell 111c to become significantly higher.
[0064] <<When variation reduction processing is performed>> Figure 6 is a diagram showing an example of the state change during discharge of a battery cell group 11G having variations in the degree of degradation. In the battery cell group 11G in Figure 6, the capacity of the third battery cell 111c is smaller than that of the first battery cell 111a and the second battery cell 111b (i.e., the degree of degradation is higher). Figure 6 shows that the state of the battery cell group 11G transitions from state J11 to J13 in chronological order. State J11 indicates that the battery 11 is fully charged. State J12 indicates the state when the voltage of the battery 11 approaches the lower limit voltage due to discharge during driving and the degree of degradation exceeds the threshold. State J13 indicates the state when the third switch SW3 is closed after a predetermined time has elapsed since the third switch SW3 was opened.
[0065] When the battery 11 is fully charged (J11), and after driving for a relatively long period, it repeatedly undergoes discharge and regenerative charging, causing the State of Charge (SOC) of the battery 11 to become low, the voltage of the battery 11 decreases, and the battery cells 111 enter a state (J12) where the degree of degradation is high. At this time, the SOCs of the first battery cells 111a to the third battery cells 111c also decrease. Furthermore, the difference between the SOC of the third battery cell 111c, which has a smaller capacity (e.g., 16%), and the SOCs of the first battery cell 111a and the second battery cell 111b (e.g., 22%) becomes large. At this time, in the variation reduction processing steps S32, S34, and S35, the third switch SW3 is switched to the open state, and the discharge from the third battery cell 111c is stopped.
[0066] Subsequently, as the vehicle continues to run, the battery 11 continues to discharge. When the elapsed time from state J12 reaches the threshold time, in state J13, the third switch SW3 is switched to the closed state by steps S32, S34, S38, and S37 of the variation reduction process. During the period from state J12 to state J13, the discharge of the third battery cell 111c is stopped, while the discharge of the first battery cell 111a and the second battery cell 111b progresses, so the difference in SOC between the first battery cell 111a to the third battery cell 111c becomes smaller. Therefore, as long as discharge or short regenerative charging is performed thereafter, the difference in SOC between the first battery cell 111a to the third battery cell 111c does not become large. Thus, it is possible to avoid the third battery cell 111c alone dropping further down to near the lower limit voltage and degrading further.
[0067] Furthermore, in the battery cell group 11G to which the above variation reduction processing is performed, the third battery cell 111c is disconnected during the period from state J12 to state J13. Therefore, the capacity used by the battery cell group 11G during this period becomes smaller compared to the other battery cell groups 11G. Consequently, as discharge progresses during this period, the voltage of the target battery cell group 11G changes significantly compared to the voltage of the other battery cell groups 11G connected in series. However, the voltage of the target battery cell group 11G is adjusted to a higher level in advance by the pre-processing of the variation reduction processing described above (steps S25, S26). Therefore, even if battery cell 111 is disconnected during the variation reduction processing, the voltage difference between the multiple battery cell groups 11G connected in series can be reduced. Consequently, the power stored in the multiple battery cell groups 11G can be used efficiently, and the reduction in the driving range of the electric vehicle 1 can be suppressed.
[0068] Figure 7 shows an example of the state changes during charging of a battery cell group 11G having variations in the degree of degradation. In the battery cell group 11G in Figure 7, the capacity of the third battery cell 111c is smaller than that of the first battery cell 111a and the second battery cell 111b (i.e., the degree of degradation is higher). Figure 7 shows that the state of the battery cell group 11G transitions from state J21 to J23 in chronological order. State J21 indicates that the battery 11 is in a low state of charge (SOC) state. State J22 indicates the state when the voltage of the battery 11 approaches the upper limit voltage due to charging and the degree of degradation exceeds the threshold. State J23 indicates the state when the third switch SW3 is closed after a predetermined time has elapsed since the third switch SW3 was opened.
[0069] When the battery 11 moves from a low SOC state J21 to a high SOC state due to increased charging by external power or regenerative charging, the voltage of the battery 11 rises, and the battery cells 111 enter a state J22 where the degree of degradation is high. At this time, the SOC of the first battery cells 111a to the third battery cells 111c of the battery cell group 11G also rises. Furthermore, the difference between the SOC of the third battery cell 111c, which has a smaller capacity (e.g., 84%), and the SOC of the first battery cell 111a and the second battery cell 111b (e.g., 78%) becomes large. At this time, in steps S32, S34, and S35 of the variation reduction process, the third switch SW3 is switched to the open state, and charging to the third battery cell 111c is stopped.
[0070] Next, as the battery 11 continues to charge, when the elapsed time from state J22 reaches the threshold time, in state J23, the third switch SW3 is switched to the closed state by steps S32, S34, S38, and S37 of the variation reduction process. During the period from state J22 to state J23, charging of the third battery cell 111c is stopped, while charging of the first battery cell 111a and the second battery cell 111b progresses, so the difference in SOC between the first battery cell 111a to the third battery cell 111c becomes smaller. Therefore, as long as charging to full charge or short regenerative charging is performed thereafter, the difference in SOC between the first battery cell 111a to the third battery cell 111c will not increase. Thus, it is possible to avoid the third battery cell 111c rising further to near the upper limit voltage and degrading.
[0071] Furthermore, in the battery cell group 11G to which the above variation reduction processing is performed, the third battery cell 111c is disconnected during the period from state J22 to state J23. Therefore, the capacity used by the battery cell group 11G during this period becomes smaller compared to the other battery cell groups 11G. Consequently, as charging progresses during this period, the voltage of the target battery cell group 11G changes significantly compared to the voltage of the other battery cell groups 11G connected in series. However, the voltage of the target battery cell group 11G is adjusted to a lower level in advance by the pre-processing of the variation reduction processing described above (steps S25, S26). Therefore, even if battery cell 111 is disconnected during the variation reduction processing, the voltage difference between multiple battery cell groups 11G connected in series can be reduced. Consequently, it becomes possible to charge multiple battery cell groups 11G to near full charge, and the reduction in the driving range of the electric vehicle 1 compared to when it is not possible to charge to near full charge can be suppressed.
[0072] As described above, according to the battery device 10 of this embodiment, the controller 12 compares the capacity (i.e., degree of degradation) of each of the first battery cells 111a to the third battery cells 111c. Based on this comparison, the controller 12 determines which of the first to third switches SW1 to SW3 is the target switch SW3 to switch its state. The controller 12 then switches the target switch SW3 to the open state or the closed state based on the state of the battery 11. For example, as a specific example, when the voltage of the battery 11 becomes equal to or above a first threshold voltage close to the upper limit voltage, the controller 12 switches the target switch SW3 to the open state, and switches it to the closed state when the voltage moves away from the first threshold voltage. Therefore, it is possible to use the degraded battery cell 111 for charging and discharging while suppressing further degradation of the degraded battery cell 111 compared to the other battery cells 111. Thus, the battery 11 can be used without waste, and even if the battery 11 degrades, the reduction in the driving range of the electric vehicle 1 can be kept to a small extent.
[0073] Furthermore, according to the battery device 10 of this embodiment, the battery state used to determine whether or not to switch the target switch SW includes the future state of the battery 11 predicted by the controller 12 based on the planned driving schedule of the electric vehicle 1. Therefore, the timing of disconnecting or reconnecting deteriorated battery cells 111 can be further optimized to match the future state of the battery 11.
[0074] Furthermore, according to the battery device 10 of this embodiment, the operating modes of the controller 12 include a main control mode that switches the open / closed state of the target switch SW, and a capacity measurement mode that measures the capacity (i.e., degree of degradation) of each of the multiple battery cells 111. Therefore, the capacity measurement mode, which functions as an operating mode separate from the main control mode, makes it possible to accurately determine whether or not there is variation in the degree of degradation among the multiple battery cells 111 connected in parallel.
[0075] Furthermore, according to the battery device 10 of this embodiment, the controller 12 selects the switch SW to be switched between open and closed states by connecting in series with the battery cell 111 that has the smallest capacity (i.e., the highest degree of degradation). Therefore, the battery cell 111 with the lowest degree of degradation is used normally, and the capacity of the battery 11 can be used without waste.
[0076] Furthermore, according to the battery device 10 of this embodiment, the controller 12 switches the target switch SW to the open state when the voltage of the battery 11 becomes equal to or greater than a first threshold voltage, and when it falls below a second threshold voltage. The first threshold voltage is set near the lower limit voltage, and the second threshold voltage is set near the upper limit voltage. Therefore, it is possible to suppress further deterioration of the battery cells 111 that have deteriorated during both discharge and charging.
[0077] Furthermore, according to the battery device 10 of this embodiment, the controller 12 switches the target switch SW to the closed state when the voltage of the battery 11 moves out of the range of a first threshold voltage or above, and the range of a second threshold voltage or below. Alternatively, the controller 12 switches the target switch SW to the closed state based on the elapsed time since the target switch SW was switched to the open state. By switching in this way, it is possible to reduce the situation in which the voltage difference between the degraded battery cell 111 and other battery cells 111 connected in parallel becomes large, making it difficult to release the disconnection without disconnecting the degraded battery cell 111. Therefore, the capacity of the battery 11, including the degraded battery cell 111, can be used without waste.
[0078] Here, we assume a case where, among multiple battery cell groups 11G connected in series, there is a battery cell group 11G (referred to as the first battery cell group 11G) where degradation variations exist and the target switch SW has been determined. In such a case, according to the battery device 10 of this embodiment, in the cell balancing process, the controller 12 adjusts the voltage of the first battery cell group 11G to a voltage that adds the difference to the voltages of the other battery cell groups 11G. Therefore, it is possible to reduce the widening of the voltage gap between multiple battery cell groups 11G during the period when the target switch SW is open and the degraded battery cell 111 is disconnected. Consequently, it is possible to reduce situations where the voltage of a small number of battery cell groups 11G approaches the upper or lower voltage limit, preventing charging or discharging from proceeding, including the many other battery cell groups 11G with sufficient capacity, and to use the battery capacity 11 without waste.
[0079] Embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. For example, in the above embodiments, the electric vehicle equipped with the battery device was shown to be an electric vehicle without an internal combustion engine, but the electric vehicle may be a HEV (Hybrid Electric Vehicle) or a PHEV (Plug-in Hybrid Electric Vehicle). In addition, details specifically shown in the embodiments, such as the conditions and timing for disconnecting deteriorated battery cells and the method for determining whether there is variation in the degree of deterioration of battery cells, can be appropriately modified without departing from the spirit of the invention.
[0080] This invention can be used in battery systems for electric vehicles.
[0081] 1 Electric vehicle 2 Drive wheels 3 Driving motor 4 Inverter 5 Driving control unit 6 Vehicle controller 7 Battery management unit 8 Navigation system 9 Charger 10 Battery device 11 Battery 11G Battery cell group 111 Battery cell 111a First battery cell 111b Second battery cell 111c Third battery cell SW Switch SW1 First switch SW2 Second switch SW3 Third switch 12 Controller 12a Memory unit 113 Discharge circuit 113a Discharge switch 113b Discharge resistor 115 Voltmeter
Claims
1. A battery device for an electric vehicle, which is mounted on an electric vehicle and stores power for driving, comprising: a battery including a plurality of battery cells connected in parallel; a plurality of switches connected in series with each of the plurality of battery cells; and a controller that controls the plurality of switches, wherein the controller determines a target switch from the plurality of switches to switch its state based on a comparison of the degree of degradation of each of the plurality of battery cells, and switches the target switch to an open state or a closed state based on the state of the battery.
2. The battery device for an electric vehicle according to claim 1, characterized in that the state of the battery includes the state of the battery predicted based on the planned driving schedule of the electric vehicle.
3. The battery device for an electric vehicle according to claim 1, characterized in that the operating modes of the controller include a first control mode for switching the open / closed state of the target switch and a second control mode for measuring the degree of degradation of each of the plurality of battery cells.
4. The battery device for an electric vehicle according to claim 1, characterized in that the target switch is a switch connected in series with the battery cell having the highest degree of degradation among the plurality of switches.
5. The battery device for an electric vehicle according to claim 1, characterized in that the controller switches the target switch to the open state when the voltage of the battery becomes equal to or greater than a first threshold voltage set closer to the upper limit voltage than the lower limit voltage, and when it falls below a second threshold voltage set closer to the lower limit voltage than the upper limit voltage.
6. The battery device for an electric vehicle according to claim 5, characterized in that the controller switches the target switch to the closed state when the voltage of the battery moves out of the range of the first threshold voltage or above, and the range of the second threshold voltage or below, or based on the elapsed time since the target switch was switched to the open state.
7. The battery includes a plurality of battery cell groups connected in series, each of the plurality of battery cell groups includes a plurality of battery cells connected in parallel, the controller performs a cell balancing process to equalize the voltages of the plurality of battery cell groups, and if the target switch is determined in a first battery cell group among the plurality of battery cell groups, in the cell balancing process, the controller adjusts the voltage of the first battery cell group to a voltage that adds a difference to the voltages of the other battery cell groups, as described in claim 1.