Battery management system, battery pack including the same, and method for diagnosing whether or not secondary batteries have deteriorated

Abstract

The present invention provides a battery management system for diagnosing whether a secondary battery having a positive electrode containing lithium iron phosphate as a positive electrode active material has degraded, the battery management system including: a sensing unit configured to measure a voltage and an open circuit voltage of the secondary battery; a memory unit configured to store the open circuit voltage measured by the sensing unit; and a control unit configured to determine that the secondary battery has degraded when an amount of change in the open circuit voltage according to the charge / discharge cycles of the secondary battery satisfies the following condition 1 for five or more charge / discharge cycles. [Condition 1] V n-1 -V n <0.002V (In the above condition 1, V n-1 is the open circuit voltage of the battery measured in the n-1th charge / discharge cycle, and V n is the open circuit voltage of the battery measured at the nth charge / discharge cycle, where n is any integer.

Description

[Technical Field]

[0001] This application claims the benefit of priority from Korean Patent Application No. 10-2022-0150244, filed on November 11, 2022.

[0002] The present invention relates to a battery management system for diagnosing whether a secondary battery having a positive electrode containing lithium iron phosphate has deteriorated, a battery pack including the same, and a method for diagnosing whether a secondary battery has deteriorated. [Background technology]

[0003] In recent years, the demand for portable electronic products such as laptops, video cameras, and mobile phones has increased dramatically, and as the development of electric vehicles, energy storage batteries, robots, satellites, and other products has progressed in earnest, research into high-performance batteries that can be repeatedly charged and discharged is actively underway.

[0004] Currently commercially available batteries include nickel-cadmium batteries, nickel-metal hydride batteries, nickel-zinc batteries, and lithium secondary batteries. Among these, lithium secondary batteries are attracting attention due to their advantages of being free to charge and discharge, having almost no memory effect compared to nickel-based batteries, a very low self-discharge rate, and a high energy density.

[0005] Such lithium secondary batteries degrade, such as by reducing capacity and energy, through repeated charge and discharge. When a battery degrades, its cycle performance, such as its capacity retention rate, decreases, shortening its lifespan and reducing its usability. Therefore, it is necessary to diagnose whether a secondary battery in use is degraded and to restore batteries that are diagnosed as degraded.

[0006] There are various techniques for diagnosing the degradation state of a battery. For example, the following patent document discloses the use of differential voltage analysis (DVA) to obtain information about the degradation state of a battery from the Q-dV / dQ curve of the battery. The Q-dV / dQ curve can be represented as a graph having a Q axis and a dV / dQ axis, where Q is the amount of charge, V is the voltage, dV is the amount of change in V over a predetermined time, dQ is the amount of change in Q over the predetermined time, and dV / dQ is the ratio of dV to dQ.

[0007] However, this technique is not suitable for determining the degradation of secondary batteries having a positive electrode using lithium iron phosphate as the positive electrode active material. Figure 1 is a graph showing the voltage as a function of capacity for a secondary battery having a positive electrode using lithium iron phosphate as the positive electrode active material. Referring to Figure 1, it can be seen that secondary batteries containing lithium iron phosphate as the positive electrode active material exhibit a flat voltage profile even when the charge / discharge capacity changes due to the structural characteristics of lithium iron phosphate. This phenomenon occurs due to the stability of the olivine-structured lithium iron phosphate structure itself and the similarity in the crystalline structure between LiFePO4 in the discharged state and FePO4 in the charged state. Therefore, since the differential voltage analysis method cannot be used to determine the degradation and degree of degradation of secondary batteries containing lithium iron phosphate as the positive electrode active material, technological development to address this issue is necessary. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention aims to provide a battery management system and method for diagnosing whether a secondary battery containing lithium iron phosphate as a positive electrode active material is degraded due to repeated charging and discharging, and a battery pack including the same. [Means for solving the problem]

[0009] The present invention provides a battery management system for diagnosing whether a secondary battery having a positive electrode containing lithium iron phosphate as a positive electrode active material has degraded, the battery management system including: a sensing unit configured to measure the voltage and open circuit voltage of the secondary battery; a memory unit configured to store the open circuit voltage measured by the sensing unit; and a control unit configured to determine that the secondary battery has degraded if the amount of change in the open circuit voltage according to the charge / discharge cycles of the secondary battery satisfies the following condition 1 for five or more charge / discharge cycles:

[0010] [Condition 1] V n-1 -V n <0.002V

[0011] (In the above condition 1, V n-1 is the open circuit voltage of the battery measured at the n-1th charge / discharge cycle, and V n is the open circuit voltage of the battery measured at the nth charge / discharge cycle, where n is any integer.

[0012] In one embodiment of the present invention, the sensing unit may be configured to measure the voltage of the secondary battery during discharge, and measure the open circuit voltage of the secondary battery each time the measured voltage reaches a reference discharge voltage.

[0013] In one embodiment of the present invention, the sensing unit may be configured to measure the open circuit voltage of the secondary battery during rest after the secondary battery has reached a reference discharge voltage.

[0014] In one embodiment of the present invention, the sensing unit may be configured to measure the open circuit voltage of the secondary battery every time one charge / discharge cycle of the secondary battery is completed.

[0015] A battery management system according to another embodiment of the present invention may further include a switching unit that turns on and off an electrical connection between the secondary battery and the charger.

[0016] The present invention provides a battery pack including the battery management system and a plurality of secondary batteries.

[0017] In one embodiment of the present invention, the secondary battery may contain, as the negative electrode active material, 50 to 100% of a graphite-based negative electrode active material based on the total weight of the negative electrode active material.

[0018] The method for diagnosing secondary battery degradation according to the present invention is a method for diagnosing whether a secondary battery having a positive electrode containing lithium iron phosphate as a positive electrode active material is degraded, and includes the steps of: (A) measuring the voltage of the secondary battery during discharge while repeatedly charging and discharging the secondary battery, and measuring the open circuit voltage of the secondary battery each time the measured voltage reaches a reference discharge voltage; and (B) determining that the secondary battery has degraded if the amount of change in the open circuit voltage satisfies the following condition 1 for five or more charge and discharge cycles.

[0019] [Condition 1] V n-1 -V n <0.002V

[0020] (In the above condition 1, V n-1 is the open circuit voltage of the battery measured at the n-1th charge / discharge cycle, and V n is the open circuit voltage of the battery measured at the nth charge / discharge cycle, where n is any integer.

[0021] In one embodiment of the present invention, the step (A) may include a step of measuring the open circuit voltage of the secondary battery during rest after the secondary battery has reached a reference discharge voltage.

[0022] In one embodiment of the present invention, the step (A) may include measuring the open circuit voltage of the secondary battery every time one charge / discharge cycle of the secondary battery is completed. [Effects of the Invention]

[0023] In a secondary battery having a positive electrode containing lithium iron phosphate as the positive electrode active material, the potential difference of the negative electrode appears as the open circuit voltage of the secondary battery, which has the effect of making it possible to diagnose the degradation of the secondary battery from the amount of change in the open circuit voltage as the charge / discharge cycle progresses. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a graph showing voltage depending on capacity of a secondary battery including a positive electrode in which lithium iron phosphate is used as a positive electrode active material. [Figure 2] 1 is a diagram illustrating an example configuration of a battery pack including a battery management system according to an embodiment of the present invention. [Figure 3] 1 is a block diagram illustrating a battery management system according to an embodiment of the present invention; [Figure 4] 1 is a graph showing exemplary measurements of open circuit voltage as a function of charge / discharge cycles calculated by a battery management system 100 according to an embodiment of the present invention. [Figure 5] FIG. 2 is a schematic diagram of a battery pack including a battery management system according to another embodiment of the present invention. [Figure 6] 10 is a flowchart illustrating an exemplary method for diagnosing degradation of a secondary battery according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Before that, the terms and words used in the specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that are consistent with the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concepts of the terms to best describe his own invention.

[0026] Therefore, the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention and do not represent the entire technical idea of ​​the present invention, and there may be various equivalents and modifications that can replace them at the time of this application.

[0027] Throughout the specification, when a part "comprises" a certain element, this does not exclude other elements and means that other elements may be further included unless otherwise specified. Furthermore, terms such as "control unit" used in the specification refer to a unit that processes at least one function or operation, and may be embodied in hardware, software, or a combination of hardware and software.

[0028] Furthermore, throughout this specification, when a part is said to be "connected" to another part, this includes not only "directly connected" but also "indirectly connected" with another element in between.

[0029] FIG. 2 is a diagram illustrating an example of a configuration of a battery pack including a battery management system according to an embodiment of the present invention.

[0030] 2, a battery pack 1000 may be provided so as to be installable in an electrical system (e.g., an electric vehicle). The battery management system 100 according to the present invention is electrically connected to a battery module 10 including a plurality of secondary batteries 11, and can diagnose the state of each of the plurality of secondary batteries.

[0031] Furthermore, the battery management system 100 according to the present invention may be included in a battery pack 1000 together with the battery module 10. Although Fig. 2 shows an example in which one battery module 10 and one battery management system 100 are included in the battery pack 1000, the number of battery modules 10 and battery management systems 100 included in the battery pack 1000 is not limited to the number shown in Fig. 2. Similarly, the number of secondary batteries 11 included in the battery module 10 is not limited to the number shown in Fig. 2.

[0032] A secondary battery included in the battery module and battery pack of the present invention will now be described. The secondary battery that is the subject of degradation diagnosis by the battery management system 100 of the present invention includes lithium iron phosphate as the positive electrode active material of the positive electrode. A lithium secondary battery using lithium iron phosphate as the positive electrode active material includes a plateau section in a specific capacity range in a capacity-voltage graph in which voltage corresponds to capacity, where the voltage change is zero or close to zero despite capacity changes associated with charging and discharging. Conventional technology has not provided a method for diagnosing the degradation of a secondary battery using lithium iron phosphate. The present invention provides a battery management system capable of diagnosing the degradation of such a secondary battery, and a battery pack including the same.

[0033] In an exemplary embodiment, the lithium iron phosphate may be a compound of Formula 1:

[0034] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0035] (In the above chemical formula 1, M includes any one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes any one or more elements selected from the group consisting of F, S, and N; and a, b, and x are −0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

[0036] For example, the lithium iron phosphate may be LiFePO4.

[0037] In an exemplary embodiment, the negative electrode constituting the secondary battery of the present invention may include a graphite-based negative electrode active material as the negative electrode active material. Specifically, the graphite-based negative electrode active material may be included in an amount of 50 wt % to 100 wt % based on the total weight of the negative electrode active materials. The graphite-based negative electrode active material also changes the negative electrode potential in response to changes in the depth of charge or depth of discharge. That is, in the case of a negative electrode including a graphite-based negative electrode active material, a correlation graph between the state of charge (SOC) or depth of discharge (DOD) and the negative electrode potential does not include a plateau section in which the negative electrode potential does not change in response to changes in the depth of charge or depth of discharge. Therefore, a secondary battery including a negative electrode including a graphite-based negative electrode active material is suitable for application of the degradation diagnosis method of the present invention.

[0038] Specific examples of such graphite-based negative electrode active materials include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, low-crystalline carbon, and high-crystalline carbon, as well as composites containing metallic compounds and carbonaceous materials. Low-crystalline carbon includes soft carbon and hard carbon, while high-crystalline carbon includes natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0039] A specific configuration of the battery management system 100 will be described with reference to Fig. 3. Fig. 3 is a block diagram schematically illustrating a battery management system according to an embodiment of the present invention.

[0040] Referring to FIG. 3, the battery management system 100 may include a sensing unit 110, a memory unit 120, and a control unit 130.

[0041] The sensing unit 110 may be configured to measure the voltage of the secondary batteries 11 included in the battery module 10. That is, the sensing unit 110 may be configured to measure the voltage of each of the secondary batteries 11 included in the battery module 10.

[0042] 2, the sensing unit 110 may measure the voltages of the first secondary battery C1, the second secondary battery C2, the third secondary battery C3, and the fourth secondary battery C4 during discharge included in the battery module 10. Specifically, the sensing unit 110 may measure the voltage of the first secondary battery C1 via the first sensing line SL1 and the second sensing line SL2, and the voltage of the second secondary battery C2 via the second sensing line SL2 and the third sensing line SL3. The sensing unit 110 may also measure the voltage of the third secondary battery C3 via the third sensing line SL3 and the fourth sensing line SL4, and the voltage of the fourth secondary battery C4 via the fourth sensing line SL4 and the fifth sensing line SL5.

[0043] The sensing unit 110 is also configured to measure the open circuit voltage (OCV) of the secondary battery 11. That is, the sensing unit 110 is configured to measure both the voltage and the open circuit voltage of the secondary battery 11. The sensing unit 110 is also configured to measure the voltage of the secondary battery when the secondary battery is charged and discharged, and is configured to measure the open circuit voltage of the secondary battery 11 to determine degradation.

[0044] Since secondary batteries deteriorate due to repeated charging and discharging, the sensing unit 110 may be configured to measure the voltage of the secondary battery 11 while the secondary battery 11 is being charged and discharged, and to measure the open circuit voltage of the secondary battery 11 after each charge and discharge cycle in order to collect data for deterioration diagnosis. Here, one charge and discharge cycle may be defined as a process of charging the discharged secondary battery 11 and discharging the charged secondary battery 11.

[0045] The sensing unit 110 may measure the open-circuit voltage of each secondary battery 11 each time the voltage of the secondary battery measured during N (N is an integer greater than or equal to 2) charge / discharge cycles of the secondary battery reaches a reference discharge voltage. Here, the reference discharge voltage may be a voltage preset and stored by a user or the like to enable the sensing unit 110 to measure the open-circuit voltage. That is, the reference discharge voltage is a reference value for measuring the open-circuit voltage of the secondary battery 11 by the sensing unit 110 and may provide a timing for the sensing unit 110 to measure the open-circuit voltage of the secondary battery 11. For example, the reference discharge voltage may be 2.5 V. However, the reference discharge voltage is not limited to this.

[0046] The sensing unit 110 may be configured to measure the voltages of a plurality of secondary batteries 11 during discharge and measure the open-circuit voltage of the secondary battery 11 each time the measured voltage of the secondary battery 11 reaches the reference discharge voltage. The sensing unit 110 may be configured to measure the open-circuit voltage of the secondary battery 11 during a rest period after the secondary battery 11 reaches the reference discharge voltage. Here, "during a rest period" refers to a period of time after the secondary battery has finished discharging and before it is charged for the next charge / discharge cycle. As a non-limiting example, the open-circuit voltage may be measured 1 second after the end of discharge, specifically within 1 minute after 3 seconds, and more specifically within 30 seconds after 5 seconds.

[0047] For example, in the embodiment shown in FIG. 2, assume that the reference discharge voltage for each of the secondary batteries 11 is set to Vs [V]. In this case, when the voltage of the first secondary battery C1 reaches Vs due to discharge, the sensing unit 110 may measure the open-circuit voltage of the first secondary battery C1. Similarly, when the voltages of the second secondary battery C2, the third secondary battery C3, and the fourth secondary battery C4 reach Vs, the sensing unit 110 may measure the open-circuit voltages of the secondary batteries that have reached Vs.

[0048] In one specific example, the sensing unit 110 may include a voltage sensor (not shown). The voltage sensor is electrically connected to the positive and negative terminals of the secondary battery 11. The voltage sensor may be installed in a charge / discharge path of the secondary battery. The sensing unit 110 may also include a current sensor (not shown). The current sensor may be configured to measure the current of the secondary battery at predetermined time intervals while the secondary battery is being charged or discharged.

[0049] The memory unit 120 may be operably coupled to the sensing unit 110. The memory unit 120 may be configured to store open circuit voltage measurements measured by the sensing unit 110 and various other sensing information. The memory unit 120 may also store data and programs required for each component of the battery management system 100 to operate and function, or data generated during the operation and function.

[0050] The memory unit 120 may be configured to store the open circuit voltage of the secondary battery 11 measured each time a charge / discharge cycle is completed while repeatedly charging and discharging. For example, the memory unit 120 may be configured to store the open circuit voltage value measured each time a charge / discharge cycle is performed, in such a manner that the memory unit 120 stores the open circuit voltage V1 measured when the secondary battery 11 reaches a reference discharge voltage during the discharge process of a first charge / discharge cycle, and stores the open circuit voltage V2 measured when the secondary battery 11 reaches a reference discharge voltage during the discharge process of a second charge / discharge cycle.

[0051] The memory unit 120 is not particularly limited in type as long as it is a known information storage means capable of recording, erasing, and updating data. For example, the memory unit 120 may include at least one type of storage medium selected from the group consisting of flash memory, hard disk, solid state disk (SSD), silicon disk drive (SDD), multimedia card micro, random access memory (RAM), static random access memory (SRAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), and programmable read-only memory (PROM).

[0052] The control unit 130 may receive the open circuit voltage measured by the sensing unit 110. The control unit 130 is configured to transmit and receive electrical signals to and from the memory unit 120 within the battery management system 100, and may receive open circuit voltage data corresponding to the charge / discharge cycle stored in the memory unit 120.

[0053] The control unit 130 may be configured to calculate the amount of change in each open circuit voltage measured according to the charge / discharge cycle from the memory unit 120, and may be configured to determine that the secondary battery has degraded if the amount of change in the open circuit voltage satisfies the following condition 1 and continues to do so for five or more charge / discharge cycles.

[0054] [Condition 1] V n-1 -V n <0.002V

[0055] (In the above condition 1, V n-1 is the open circuit voltage of the battery measured at the n-1th charge / discharge cycle, and V nis the open circuit voltage of the battery measured at the nth charge / discharge cycle, where n is any integer.

[0056] That is, the control unit 130 calculates the open circuit voltage (V n-1 ) and the open circuit voltage (V n ) is less than 0.002 V, and if such a change in open circuit voltage satisfies the above condition 1 for at least five charge / discharge cycles, it can be determined that the secondary battery has degraded.

[0057] 4 is a graph illustrating an example of measured values ​​of open circuit voltage according to charge / discharge cycles calculated by the battery management system 100 according to an embodiment of the present invention. The control unit 130 may generate the graph shown in FIG. 4 from open circuit voltage data stored in the memory unit 120. The graph of FIG. 4 shows the relationship between the number of charge / discharge cycles and the open circuit voltage. As the charge / discharge cycles progress, the open circuit voltage gradually decreases, and the amount of change in voltage gradually decreases, until the amount of change in open circuit voltage approaches zero after approximately 1,500 cycles.

[0058] Lithium iron phosphate with an olivine structure is more stable in terms of structural changes due to degradation or deterioration than positive electrode active materials with a spinel or layered structure. Therefore, lithium iron phosphate exhibits less voltage change during charging and discharging. Therefore, in secondary batteries with a positive electrode containing lithium iron phosphate, the voltage of the secondary battery is often determined by the potential difference at the negative electrode. In other words, it is easy to determine the potential difference at the negative electrode from the measured open-circuit voltage of lithium iron phosphate-based secondary batteries.

[0059] Therefore, in order to diagnose degradation of a secondary battery having a lithium iron phosphate positive electrode, the present invention determines whether the negative electrode has degraded if the change in open circuit voltage of the secondary battery measured during the charge-discharge cycle at a rest stage after discharge is less than 0.002 V during a certain number of cycles, thereby determining whether the secondary battery has degraded.

[0060] In a lithium secondary battery including a positive electrode using the lithium iron phosphate as the positive electrode active material, as charge / discharge cycles accumulate, lithium may be charged in the non-facing portion of the negative electrode that does not face the positive electrode due to the difference in charge / discharge rates between the positive and negative electrodes. The non-facing portion of the negative electrode that does not face the positive electrode exists because the negative electrode is cut larger than the positive electrode, and is typically the edge of the negative electrode. If this phenomenon of lithium charging in the non-facing portion occurs continuously, lithium required for charge / discharge accumulates in the non-facing portion, resulting in a decrease in battery capacity and potentially safety issues due to lithium precipitation.

[0061] In a secondary battery using lithium iron phosphate as the positive electrode active material, the potential difference of the negative electrode can be determined through the behavior of the open circuit voltage. When the non-facing portion of the negative electrode, which does not face the positive electrode, is degraded due to the decrease in soluble lithium caused by charging lithium, the potential of the negative electrode does not change beyond a certain number of charge / discharge cycles. Therefore, the present invention can diagnose whether or not the secondary battery is degraded due to charging lithium in the non-facing portion through the behavior of the open circuit voltage of the secondary battery.

[0062] The control unit 130 may optionally include a processor, an application-specific integrated circuit (ASIC), other chipsets, logic circuits, registers, a communication modem, a data processing device, or the like known in the art for executing various control logics performed in the battery management system 100 according to an embodiment of the present invention. When the control logic is implemented in software, the control unit 130 may be implemented as a collection of program modules. In this case, the program modules may be stored in a memory and executed by a processor. For example, the control unit 130 may be a processor included in the battery management system 100 according to an embodiment of the present invention, and may provide a user with the determined deterioration status of the secondary battery 11 via an output device such as a display device. The control unit 130 may also provide a user with a recommendation to replace the secondary battery 11 or a warning alarm via an external notification device based on the determination result regarding the deterioration status of the secondary battery 11.

[0063] The battery management system of the present invention has the effect of utilizing the characteristic that in a secondary battery having a positive electrode containing lithium iron phosphate as the positive electrode active material, the potential difference of the negative electrode appears as the open circuit voltage of the secondary battery, and determining that deterioration of the negative electrode is deterioration of the secondary battery from the amount of change in the open circuit voltage that accompanies the progress of the charge / discharge cycle, thereby making it possible to diagnose deterioration of the secondary battery.

[0064] 5 is a schematic diagram of a battery pack 2000 including a battery management system according to another embodiment of the present invention. Referring to FIG. 5, the battery management system 200 may further include a switching unit 240 and an interface unit 250 for turning on and off an electrical connection between the secondary battery 11 and the charger.

[0065] The switching unit 240 may include a switch 241 and a switch driver 242. The switch 241 is disposed in a current path for charging and discharging the secondary battery 11. Charging and discharging of the secondary battery 11 is possible while the switch 241 is turned on. The switch 241 may be a mechanical relay that is turned on and off by the magnetic force of a coil, or a semiconductor switch such as a metal oxide semiconductor field effect transistor (MOSFET). Charging and discharging of the secondary battery 11 is suspended while the switch 241 is turned off. In one embodiment, the switch 241 may be turned on in response to a first control signal and turned off in response to a second control signal.

[0066] The switch driver 242 may be electrically connected to the switch 241 and the control unit 230, and may be configured to selectively output a first control signal or a second control signal to the switch 241 in response to a command from the control unit 230. The control unit 230 may instruct the switch driver 242 to turn on or off the switch 241 when at least one of predetermined events occurs.

[0067] The interface unit 250 is configured to support wired or wireless communication between the control unit 230 and a host controller 2 (e.g., an Electronic Control Unit (ECU)) of the electrical system 1. The wired communication may be, for example, a Controller Area Network (CAN) communication, and the wireless communication may be, for example, ZigBee or Bluetooth (registered trademark) communication. Of course, the type of communication protocol is not particularly limited as long as it supports wired or wireless communication between the control unit 230 and the host controller 2.

[0068] The interface unit 250 may include an output device (not shown) such as a display or speaker that provides a user-recognizable result of the process related to the degradation state of the secondary battery 11 performed by the control unit 230. The interface unit 250 may include an input device (not shown) such as a mouse or keyboard that can receive and input data from the user.

[0069] 6 is a flowchart illustrating an exemplary method for diagnosing degradation of a secondary battery according to another embodiment of the present invention. Referring to FIG. 6, the method for diagnosing degradation of a secondary battery according to the present invention includes the steps of: (A) measuring the voltage of the secondary battery during discharge while repeatedly charging and discharging the secondary battery, and measuring the open circuit voltage of the secondary battery every time the measured voltage reaches a reference discharge voltage; and (B) determining that the secondary battery has degraded if the amount of change in the open circuit voltage satisfies the following condition 1 for five or more charge and discharge cycles:

[0070] [Condition 1] V n-1 -V n <0.002V

[0071] (In the above condition 1, V n-1 is the open circuit voltage of the battery measured at the n-1th charge / discharge cycle, and V n is the open circuit voltage of the battery measured at the nth charge / discharge cycle, where n is any integer.

[0072] The secondary battery to which the method for diagnosing secondary battery degradation according to the present invention is applied is as described above.

[0073] The process (A) is a process of collecting data for diagnosing whether a secondary battery has deteriorated. The deterioration diagnosis method according to the present invention is a process of measuring an open circuit voltage for each charge / discharge cycle while performing charge / discharge cycles of the secondary battery, and collecting open circuit voltages corresponding to the number of charge / discharge cycles.

[0074] Therefore, the above step (A) includes a step of measuring the open circuit voltage of the secondary battery every time one charge / discharge cycle is completed for the secondary battery, and the open circuit voltage may be measured during a pause before the next charge after the secondary battery reaches the reference discharge voltage.

[0075] The above-mentioned step (B) is a step of diagnosing whether the secondary battery has deteriorated based on the amount of change in the open circuit voltage according to the charge / discharge cycles collected through step (A). Specifically, it includes a step of determining that the secondary battery has deteriorated if the difference between the open circuit voltage of the battery measured in the (n-1)th charge / discharge cycle and the voltage of the battery measured in the nth charge / discharge cycle is less than 0.002 V for five charge / discharge cycles.

[0076] Referring to FIG. 6, the above-mentioned step (A) includes a step of measuring the voltage and open circuit voltage of the secondary battery while repeatedly charging and discharging the secondary battery.

[0077] Repeated charging and discharging of a secondary battery means repeating such a charge-discharge cycle several times, with the process of charging a secondary battery in a discharged state and then discharging the secondary battery in a charged state being considered as one charge-discharge cycle.

[0078] In one charge / discharge cycle, the voltage of the secondary battery may be measured during the process of charging the secondary battery and then discharging the charged secondary battery, and the voltage measurement of the secondary battery may be performed at regular time intervals.

[0079] While monitoring whether the measured voltage of the secondary battery reaches the reference discharge voltage, the secondary battery continues to be discharged and the voltage measurement continues until the measured voltage of the secondary battery reaches the reference discharge voltage.

[0080] If the measured voltage reaches the reference discharge voltage, the discharge of the secondary battery is stopped and the secondary battery is put into a rest state. The rest state means that the supply of current for charging and discharging the secondary battery is stopped. Then, the open circuit voltage of the secondary battery is measured in this rest state after the discharge.

[0081] In an exemplary embodiment, the charge / discharge method in step (A) may be a charge / discharge method applicable to secondary batteries, including, but not limited to, a constant current (CC) method, a constant voltage (CV) method, a constant power (CP) method, and combinations thereof such as a CC-CV method and a CP-CV method. A suitable charge / discharge method may be selected depending on the type of secondary battery. For example, a CP charge / discharge method may be selected for a secondary battery for an ESS. The charge / discharge rate (C rate, CP rate) is not particularly limited, and a suitable range of charge / discharge rates may be selected taking into account the charge / discharge characteristics of the electrodes.

[0082] As shown in FIG. 4, as the secondary battery is cycled through charge and discharge, the amount of change in the open circuit voltage of the secondary battery gradually decreases, and as a result, the amount of change in the open circuit voltage becomes less than 0.002V.

[0083] The present invention determines whether a secondary battery has degraded based on the amount of change in open circuit voltage. Specifically, if the change in the open circuit voltage of the battery measured in the (n-1)th charge / discharge cycle and the open circuit voltage of the battery measured in the nth charge / discharge cycle is less than 0.002 V, the secondary battery is determined to have degraded if this condition is met for at least five charge / discharge cycles.

[0084] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0085] <Example>

[0086] (The process of collecting open circuit voltage for each charge / discharge cycle of a secondary battery) A battery cell was prepared having a negative electrode made of 100% artificial graphite as the negative electrode active material and a positive electrode made of 100% LiFePO4 as the positive electrode active material. The battery cell was charged to 3.65 V at a 1 CP rate and discharged to 2.5 V at a 1 CP rate. During discharge, the battery cell voltage was monitored and the discharge was terminated when the battery cell voltage reached 2.5 V. Five seconds after the end of discharge, the open circuit voltage of the battery cell was measured and the result was saved as V1.

[0087] After that, charge and discharge are repeated n times in the same manner as above, and the open circuit voltages measured at each charge and discharge cycle are V2, V3...V n Saved as.

[0088] (The process of determining whether a secondary battery has deteriorated or not) While repeatedly charging and discharging the battery cell, measure the open circuit voltage (V n-1 ) and the open circuit voltage (V n ) and calculated the difference between the open circuit voltage and the actual voltage. If the difference in open circuit voltage was less than 0.002V five times, charging and discharging of the battery cell was terminated and the battery cell was deemed to have deteriorated.

[0089] <Experimental Example 1: Confirmation of lithium in the negative electrode that does not face the positive electrode>

[0090] In the battery cell determined to be degraded in the above example, the battery case was opened, and the electrode assembly housed therein was disassembled to obtain the negative electrode. Observation of the negative electrode confirmed that lithium had been deposited in the non-facing portion. Based on these results, it is believed that the degradation diagnosis method according to the present invention can diagnose the accumulation of lithium in the portion of the negative electrode that does not face the positive electrode.

[0091] <Experimental Example 2: Confirmation of capacity retention rate>

[0092] The discharge capacity 1 of the battery cell in the above example was measured in the first charge / discharge cycle. The discharge capacity 2 of the battery cell in the charge / discharge cycle in which it was determined that the battery cell had degraded in the above example was measured. Then, the capacity retention rate was calculated according to the following Equation 1.

[0093] Formula 1: (discharge capacity 2×100) / discharge capacity 1

[0094] The calculated capacity retention rate was 83%. Generally, a secondary battery with a capacity retention rate of 80% is considered to be degraded, but the degradation diagnosis method according to the present invention is analyzed to be effective for diagnosing the degradation of secondary batteries that use lithium iron phosphate as the positive electrode active material.

[0095] The above-described embodiments of the present invention may be realized not only by the battery management system 100 and method, but also by a program that realizes functions corresponding to the configurations of the embodiments of the present invention or a recording medium on which the program is recorded. Such realization may be easily realized by a person skilled in the art based on the description of the above-described embodiments.

[0096] Although the present invention has been described above with reference to limited embodiments and drawings, the present invention is not limited thereto, and it is of course possible for a person having ordinary skill in the art to which the present invention pertains to make various modifications and variations within the technical spirit of the present invention and the equivalent scope of the claims set forth below.

[0097] Furthermore, the present invention described above is susceptible to various substitutions, modifications, and alterations by a person having ordinary knowledge in the technical field to which the present invention pertains, within the scope of not departing from the technical concept of the present invention, and therefore is not limited to the above-described embodiments and the accompanying drawings, and may be configured by selectively combining all or part of each embodiment so that various modifications can be made. [Explanation of symbols]

[0098] 1: Electrical system 2: Upper controller 10: Battery module 11: Secondary battery 1000, 2000: Battery pack 100, 200: Battery management system 110, 210: Sensing unit 120, 220: Memory section 130, 230: control unit 240: Switching section 250: Interface section

Claims

1. A battery management system for diagnosing whether a secondary battery having a positive electrode containing lithium iron phosphate as a positive electrode active material is degraded, a sensing unit configured to measure a voltage and an open circuit voltage of the secondary battery; a memory unit configured to store the open circuit voltage measured by the sensing unit; a control unit configured to determine that the secondary battery has degraded when an amount of change in open circuit voltage according to charge / discharge cycles of the secondary battery satisfies the following condition 1 for five or more charge / discharge cycles: [Condition 1] V ｎ－１ -V ｎ <0.002V (In the above condition 1, V ｎ－１ is the open circuit voltage of the battery measured in the n-1th charge / discharge cycle, and V ｎ is the open circuit voltage of the battery measured at the nth charge / discharge cycle, where n is any integer.

2. 2. The battery management system according to claim 1, wherein the sensing unit is configured to measure a voltage of the secondary battery during discharge and measure an open circuit voltage of the secondary battery each time the measured voltage reaches a reference discharge voltage.

3. The battery management system according to claim 2 , wherein the sensing unit is configured to measure the open circuit voltage of the secondary battery during rest after the secondary battery reaches a reference discharge voltage.

4. The battery management system according to claim 1 , wherein the sensing unit is configured to measure an open circuit voltage of the secondary battery every time one charge / discharge cycle of the secondary battery is completed.

5. The battery management system according to claim 1 , further comprising a switching unit that turns on and off an electrical connection between the secondary battery and the charger.

6. The battery management system of claim 1 ; A battery pack including a plurality of secondary batteries.

7. 7. The battery pack according to claim 6, wherein the secondary battery contains, as the negative electrode active material, 50 to 100% of a graphite-based negative electrode active material based on the total weight of the negative electrode active material contained in the negative electrode.

8. A method for diagnosing whether or not a secondary battery having a positive electrode containing lithium iron phosphate as a positive electrode active material is degraded, comprising: (A) a step of measuring the voltage of the secondary battery during discharge while repeatedly charging and discharging the secondary battery, and measuring the open circuit voltage of the secondary battery every time the measured voltage reaches a reference discharge voltage; (B) determining that the secondary battery has deteriorated if the amount of change in open circuit voltage satisfies the following condition 1 for five or more charge / discharge cycles: [Condition 1] V ｎ－１ -V ｎ <0.002V (In the above condition 1, V ｎ－１ is the open circuit voltage of the battery measured in the n-1th charge / discharge cycle, and V ｎ is the open circuit voltage of the battery measured at the nth charge / discharge cycle, where n is any integer.

9. 9. The method of claim 8, wherein step (A) comprises measuring an open circuit voltage of the secondary battery during rest after the secondary battery reaches a reference discharge voltage.

10. 9. The method of claim 8, wherein step (A) comprises measuring an open circuit voltage of the secondary battery every time one charge / discharge cycle of the secondary battery is completed.

11. 9. The method for diagnosing degradation of a secondary battery according to claim 8, wherein the secondary battery contains 50 to 100% of a graphite-based negative electrode active material as a negative electrode active material.

Images