Discharge voltage curve prediction method and battery system using the method

Abstract

A method for predicting a constant-current discharge curve of a battery cell according to a feature of the present invention includes: measuring a first time required for the battery cell voltage to drop to a first discharge limit voltage through discharge with a first constant current; measuring a second time required for the battery cell voltage to drop to a second discharge limit voltage through discharge with a second constant current; and calculating a proportionality constant and an exponential parameter in the relationship between the constant current and the discharge time during the discharge period based on the first constant current and the first time, and the second constant current and the second time. The first discharge limit voltage is obtained by subtracting a first voltage drop caused by the first constant current and the internal resistance of the battery cell from the discharge reference voltage when the discharge current is 0, and the second discharge limit voltage is obtained by subtracting a second voltage drop caused by the second constant current and the internal resistance of the battery cell from the discharge reference voltage.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2021-0002661, filed on January 8, 2021, with the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference.

[0003] This disclosure relates to a method for predicting discharge voltage curves and a battery system using the method. Background Technology

[0004] In the existing technology, in the absence of a discharge voltage curve prediction technology for lithium-ion secondary batteries, a discharge voltage curve is obtained by directly experimenting with each discharge current, thus obtaining a discharge voltage curve for the lithium ions that can reach the battery for the corresponding discharge current. The discharge voltage curve is a graph showing the change in individual cell voltage over time when discharged at a predetermined constant current, requiring measurement of the discharge limit current, discharge resistance, or discharge power over a predetermined time. Summary of the Invention

[0005] [Technical Issues]

[0006] This disclosure provides a method for predicting the discharge voltage curve under arbitrary constant current discharge conditions without experimentally obtained information on the discharge voltage curve, and a battery system using this method.

[0007] [Technical Solution]

[0008] A method for predicting a constant-current discharge curve of a battery cell according to a feature of the present invention includes: measuring a first time required for the battery cell voltage to drop to a first discharge limit voltage through discharge with a first constant current; measuring a second time required for the battery cell voltage to drop to a second discharge limit voltage through discharge with a second constant current; and calculating a proportionality constant and an exponential parameter in the relationship between the constant current and the discharge time during discharge, based on the first constant current and the first time, and the second constant current and the second time. The first discharge limit voltage is obtained by subtracting a first voltage drop caused by the first constant current and the internal resistance of the battery cell from a discharge reference voltage when the discharge current is 0, and the second discharge limit voltage is obtained by subtracting a second voltage drop caused by the second constant current and the internal resistance of the battery cell from the discharge reference voltage.

[0009] The method for predicting the constant current discharge curve of a battery cell may further include: predicting the time required for the voltage of the battery cell to reach a third discharge limit voltage by using a proportionality constant and an exponential parameter when the battery cell is discharged with a third constant current, and the third discharge limit voltage may be obtained by subtracting a third voltage drop caused by the third constant current and the internal resistance of the battery cell from the discharge reference voltage.

[0010] The method for predicting the constant current discharge curve of a battery cell may further include: changing the discharge reference voltage; measuring the third time required for the battery cell voltage to drop to the fourth discharge limit voltage through the fourth constant current discharge; measuring the fourth time required for the battery cell voltage to drop to the fifth discharge limit voltage through the fifth constant current discharge; and calculating a proportionality constant and an exponential parameter in the relationship between discharge current and time based on the fourth constant current and the third time, and the fifth constant current and the fourth time, wherein the fourth discharge limit voltage may be obtained by subtracting a fourth voltage drop caused by the third constant current and the internal resistance of the battery cell from the changed discharge reference voltage, and the fifth discharge limit voltage may be obtained by subtracting a fifth voltage drop caused by the fourth constant current and the internal resistance of the battery cell from the changed discharge reference voltage.

[0011] The method for predicting the constant current discharge curve of a battery cell may further include: predicting the time required for the battery cell voltage to reach the sixth discharge limit voltage by using a proportionality constant and an exponential parameter when discharging the battery cell with the sixth constant current, and the sixth discharge limit voltage being obtained by subtracting the sixth voltage drop caused by the sixth constant current and the internal resistance of the battery cell from the changed discharge reference voltage.

[0012] A battery system according to another feature of the present invention includes: a plurality of battery cells; and a battery management system for predicting the discharge time required for each of the plurality of battery cell voltages to reach a corresponding discharge limit voltage during constant current discharge. The battery management system can store information about a proportionality constant and an exponential parameter defining the relationship between the constant current and the discharge time. After measuring a first time required for the battery cell voltage to drop to a first discharge limit voltage via a first constant current discharge and a second time required for the battery cell voltage to drop to a second discharge limit voltage via a second constant current discharge, the proportionality constant and the exponential parameter for one of the plurality of battery cells can be calculated based on the first constant current and the first time, and the second constant current and the second time. The first discharge limit voltage can be obtained by subtracting a first voltage drop caused by the first constant current and the internal resistance of the battery cell from a discharge reference voltage when the discharge current is 0. The second discharge limit voltage can be obtained by subtracting a second voltage drop caused by the second constant current and the internal resistance of the battery cell from a discharge reference voltage when the discharge current is 0.

[0013] When discharging a battery cell with a third constant current, the battery management system can predict the time required for the battery cell voltage to reach the third discharge limit voltage by using a proportional constant and an exponential parameter. The third discharge limit voltage can be obtained by subtracting the third voltage drop caused by the third constant current and the internal resistance of the battery cell from the discharge reference voltage.

[0014] The state of charge (SOC) and temperature of the battery cells can be the same at the start of discharge using the first constant current, the second constant current, and the third constant current.

[0015] The relationship between discharge current and time can be expressed as I = a * t b Where I can be the discharge current, t can be time, a can be a proportionality constant, and b can be an exponential parameter.

[0016] [Beneficial Effects]

[0017] If discharge occurs at an untested constant current, it is difficult to predict whether a single battery cell will have any type of discharge voltage profile. Exemplary embodiments of the present invention can predict the discharge voltage profile when discharging at any constant current. Attached Figure Description

[0018] Figure 1 This is a graph illustrating a method for predicting a discharge voltage curve according to an exemplary embodiment.

[0019] Figure 2 This is a flowchart illustrating a method for determining the proportionality constant and exponential parameter between a constant current and a discharge time according to an exemplary embodiment.

[0020] Figure 3 It is a discharge voltage curve predicted when discharging with a predetermined current according to an exemplary embodiment.

[0021] Figure 4 It is a graph comparing the test results and predicted results of the discharge voltage for each discharge current.

[0022] Figure 5 It is a graph comparing the test results and predicted results of the discharge voltage for each discharge current.

[0023] Figure 6 This is a schematic diagram illustrating a battery system to which a method for predicting discharge voltage curves according to an exemplary embodiment is applied. Detailed Implementation

[0024] The embodiments disclosed in this specification are described in detail below with reference to the accompanying drawings. In this specification, identical or similar components are indicated by identical or similar reference numerals, and repeated descriptions thereof are omitted. The terms "module" and "unit" used for components in the following description are merely for ease of writing. Therefore, these terms do not inherently distinguish one another from the other in meaning or function. Furthermore, in describing embodiments of this specification, detailed descriptions of well-known techniques associated with the invention will be omitted if it is determined that such descriptions may obscure the essential points of the invention. Moreover, the accompanying drawings are provided merely to facilitate understanding of the embodiments disclosed in this specification and are not intended to limit the spirit of the disclosure herein. It should be understood that the invention includes all modifications, equivalents, and substitutions without departing from the scope and spirit of the invention.

[0025] Terms including ordinal numbers such as first, second, etc., will only be used to describe the various components and will not be interpreted as limiting these components. Terms are only used to distinguish one component from other components.

[0026] It should be understood that when a component is referred to as "connected" or "coupled" to another component, it can be directly connected or coupled to the other component, or connected or coupled to the other component with other components in between. On the other hand, it should be understood that when a component is referred to as "directly connected or coupled" to another component, it can be connected or coupled to the other component without any other components in between.

[0027] Furthermore, it should be understood that the terms "comprising" or "having" as used in this specification specify the presence of the said features, numbers, steps, operations, components, parts or combinations thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, operations, components, parts or combinations thereof.

[0028] Figure 1This is a graph illustrating a method for predicting a discharge voltage curve according to an exemplary embodiment.

[0029] Figure 1 This shows how the cell voltage changes over time when discharged at different constant currents (CC) under predetermined starting SOC (state of charge) and predetermined starting temperature conditions.

[0030] first, Figure 1 Discharge voltage curve 1 is a graph showing the change of the cell voltage (VC) when discharged with a constant current I1, while discharge voltage curve 2 shows the change of the cell voltage (VC) when discharged with a constant current I2.

[0031] exist Figure 1 In this configuration, "VCO" can be arbitrarily chosen as the discharge reference voltage when the discharge current is 0. "VCO1" is the voltage obtained by subtracting the voltage drop (VIR1 = R * I1) from the discharge reference voltage (VCO) when a constant current I1 flows through the battery cell (VCO - VIR1). "VCO2" is the voltage obtained by subtracting the voltage drop (VIR2 = R * I2) from the discharge reference voltage (VCO) when a constant current I2 flows through the battery cell (VCO - VIR2). In other words, VCO1 is the discharge limit voltage when the discharge current is I1, and VCO2 is the discharge limit voltage when the discharge current is I2. Under the same initial SOC and initial temperature conditions, when performing CC discharge, VCO1, VCO2, and VCO have the relationship shown in Equation 1. The discharge limit voltage refers to the minimum voltage at which the battery cell voltage can be reduced during discharge. When a battery cell is discharged to a voltage below the discharge limit voltage, the battery cell may be damaged.

[0032] [Equation 1]

[0033] VCO1 + R*I1 = VCO2 + R*I2 = VCO

[0034] like Figure 1 As shown, when discharge begins, the individual cell voltage VC rapidly decreases from the open-circuit voltage (OCV) VOCV to the voltage drop caused by the cell's resistance and constant current, and then decreases over time. Due to the constant current I1 and the cell resistance R, the individual cell voltage drops by a voltage drop R*I1 at the beginning of discharge, and decreases over time, reaching the discharge limit voltage VCO1 when time t1 has elapsed. Due to the constant current I2 and the cell resistance R, the individual cell voltage drops by a voltage drop R*I2 at the beginning of discharge, and decreases over time, reaching the discharge limit voltage VCO2 when time t2 has elapsed.

[0035] When a battery cell discharges, the relationship between the constant current "I" and the discharge time "t" satisfies the following equation 2.

[0036] [Equation 2]

[0037] I = a * t b

[0038] In Equation 2, a and b are the proportionality constant and exponential parameter between the constant current and the discharge time during the discharge period.

[0039] If Equation 2 is described relative to time, then it is as shown in Equation 3.

[0040] [Equation 3]

[0041]

[0042] Figure 2 This is a flowchart illustrating a method for determining the proportionality constant and exponential parameter between a constant current and a discharge time according to an exemplary embodiment.

[0043] First, set two constant currents I1 and I2 (S0).

[0044] Next, select the discharge reference voltage VCO(S1).

[0045] When the battery is discharged with a constant current I1, the voltage VC of the individual cell drops, and then the time t1 (S2) required to reach the discharge limit voltage (VCO1=VCO-R*I1) is measured.

[0046] Then, when discharging with a constant current I2, the cell voltage VC drops, and the time t2 (S3) required to reach the discharge limit voltage (VCO2=VCO-R*I2) is measured.

[0047] By substituting I1 and t1, and I2 and t2 obtained through steps (S2) and (S3) into Equation 2, two simultaneous equations are obtained. By solving the two simultaneous equations, the proportionality constant a and the exponential parameter b are obtained (S4).

[0048] When the proportional constant a and the exponential parameter b are applied to Equation 3, and discharge is performed with an arbitrary constant current Ix, the time tx (S5) for reaching the discharge limit voltage (VCOx) that is the result of subtracting the voltage drop (R*Ix) from the discharge reference voltage (VCO) is calculated.

[0049] Change the discharge reference voltage (VCO) (S6), and repeat steps (S2 to S5) again.

[0050] Figure 3 It is a discharge voltage curve predicted when a discharge occurs with a predetermined current according to an exemplary embodiment.

[0051] To compare the time to reach the discharge limit voltage for different constant currents I1 and I2, and for any current Ix. Figure 3 The discharge voltage curves for each of the constant currents I1 and I2 are also shown.

[0052] like Figure 3 As shown, the discharge voltage curve is based on an arbitrary constant current (Ix). Figure 3 If discharge begins, due to the corresponding constant current and the resistance of the battery cell, the battery cell voltage (VC) drops rapidly from the open circuit voltage (OCV) (VOCV) (VIRx = R*Ix), and the battery cell voltage decreases as time passes, reaching the discharge limit voltage (VCOx) when time tx has passed.

[0053] Figure 4 This is a schematic diagram comparing the test results and predicted results of the discharge voltage for each discharge current.

[0054] exist Figure 4 In the diagram, thin solid lines 41-46 show the discharge voltage curves based on the test results, and thick solid lines 47-50 show the predicted discharge voltage curves.

[0055] The initial SOC and initial temperature were the same: SOC 60% and 25°C.

[0056] exist Figure 4 In this context, "C" represents the "C rate," where the current corresponding to the reference capacity of a battery cell corresponds to the 1C rate. For example, in the case of a battery cell with a reference capacity of 100 ampere-hours (Ah), 1C represents 100A, and 2C represents 200A. Figure 4 The discharge voltage curves shown in Figures 42 and 45 are for discharge experiments with constant currents of 3C and 4.5C, respectively. The proportionality constant a and the exponential parameter b are calculated according to the method described above.

[0057] exist Figure 4 In the experiment, when discharged with constant currents of 2.5C, 3.5C, 4C, and 5C respectively, the predicted discharge voltage results exhibit an average error of 1mV-3mV and a maximum error range of 3mV-8mV, obtained through actual experiments on discharge voltage. In other words, as... Figure 4 As shown, the prediction error compared to the individual cell voltage is quite low.

[0058] Figure 5 This is a schematic diagram comparing the test results and predicted results of the discharge voltage for each discharge current.

[0059] exist Figure 5In the diagram, thin solid lines 51-56 show the discharge voltage curves based on the test results, and thick solid lines 57-60 show the predicted discharge voltage curves.

[0060] The initial SOC and initial temperature were the same: SOC 25% and 0℃.

[0061] based on Figure 5 The discharge voltage curves shown in Figures 53 and 55 are for discharge experiments with constant currents of 2.5C and 3.5C, respectively. The proportionality constant a and the exponential parameter b are calculated according to the method described above.

[0062] exist Figure 5 In the experiment, when discharged with constant currents of 2.5C, 3.5C, 4C, and 5C respectively, the predicted discharge voltage exhibits an average error of 1mV-3mV and a maximum error range of 7mV-10mV, obtained through actual experiments on discharge voltage. In other words, in... Figure 5 As shown in the graph, the prediction error compared to the individual cell voltage is quite low.

[0063] This method reduces the number of experiments required to obtain the discharge voltage curve and shortens the experimental duration. Furthermore, because a constant current discharge voltage curve can be predicted, the discharge limiting current, discharge resistance, or discharge power at any given time (x seconds elapsed since the start of discharge) can be predicted and measured. The discharge limiting current is the constant current in the discharge voltage curve when the cell voltage reaches the discharge limiting voltage after x seconds from its initial voltage. The discharge resistance is calculated by dividing the cell voltage obtained by subtracting the cell voltage at x seconds from the initial discharge voltage by the discharge current. The discharge power is calculated by dividing the area in the discharge voltage curve up to x seconds by x seconds.

[0064] Figure 6 This is a schematic diagram illustrating a battery system to which a method for predicting discharge voltage curves according to an exemplary embodiment is applied.

[0065] like Figure 6 As shown, the battery system 100 includes: a battery 110 comprising a plurality of battery cells 110_1 to 110_n connected in series, a battery management system (BMS) 111, a current sensor 112, a relay 113, and a temperature sensor 114.

[0066] The current sensor 112 can detect the current flowing through the battery 110 (hereinafter referred to as battery current) and transmit a current detection signal SC indicating the detected battery current to the BMS 111. Figure 6 In this configuration, the current sensor 112 is connected between the negative terminal of the battery 110 and the output terminal (P-) of the battery 110, but... Figure 6Unlike the diagram, it can be connected between the positive terminal of battery 110 and the output terminal (P+) of battery 110.

[0067] Temperature sensor 114 can be positioned inside battery 110 to measure or estimate the temperature of each of the multiple battery cells. Temperature sensor 114 can transmit a signal indicating the temperature of each of the multiple battery cells to BMS 111.

[0068] BMS 111 can measure the individual cell voltages of multiple battery cells 110_1 to 110_n, and measure the battery voltage (the voltage between the two terminals of battery 110), the temperature of each of the multiple battery cells 110_1 to 110_n, and can predict the state of charge (SOC) and internal resistance of each of the multiple battery cells 110_1 to 110_n based on the individual cell voltages, battery current, and battery cell temperatures. Methods for estimating SOC and internal resistance are known techniques, and various methods can be applied to this invention. BMS 111 can control charging and discharging based on the estimated SOC, control the equalization operation of the multiple battery cells based on the individual cell voltages and battery cell temperatures, and control protection operations in the event of overvoltage, overcurrent, or high temperature.

[0069] Relay 114 is connected between the output terminal (P+) of battery 110 and the positive terminal of battery 110, and opens or closes according to the relay control signal (RCS) of BMS 111. Relay 114 can close according to the relay control signal (RCS) at the on level and open according to the relay control signal (RCS) at the off level.

[0070] Based on the prediction method described above using the discharge voltage curve of constant current discharge, BMS 111 can predict the time required to reach the discharge limit voltage (VCO_i, where i is a natural number from 1 to n) corresponding to each of the plurality of battery cells 110_1 to 110_n. For this purpose, BMS 111 can store a lookup table 115, which stores information about the proportionality constants and exponential parameters for each SOC and battery temperature at the start of the discharge operation.

[0071] When discharging any one of the multiple battery cells 110_1 to 110_n with an arbitrary constant current (Ix), the BMS 111 can predict the time required to reach the discharge limit voltage (VCOx) for the corresponding battery cell voltage by using the stored proportional constant and exponential parameters and Equation 3. In this case, the BMS 111 can read the proportional constant and exponential parameters corresponding to the same SOC and temperature as the corresponding battery cell voltage from lookup table 115.

[0072] While the invention has been described in conjunction with what are now considered to be practical exemplary embodiments, it should be understood that the invention is not limited to the disclosed embodiments. Rather, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A method for predicting the constant current discharge curve of a single battery cell, comprising: The first time required for the voltage of a single battery cell to drop to the first discharge limit voltage under a first constant current discharge is measured. The second time required for the voltage of the individual battery cell to drop to the second discharge limit voltage under a second constant current discharge is measured. as well as Based on the first constant current and the first time, and the second constant current and the second time, calculate the proportionality constant and exponential parameter in the relationship between the constant current and the discharge time during the discharge period. The first discharge limit voltage is obtained by subtracting a first voltage drop caused by the first constant current and the internal resistance of the battery cell from the discharge reference voltage when the discharge current is 0, and the second discharge limit voltage is obtained by subtracting a second voltage drop caused by the second constant current and the internal resistance of the battery cell from the discharge reference voltage.

2. The method for predicting the constant current discharge curve of a single battery cell according to claim 1, further comprising: When discharging the battery cell with a third constant current, the time required for the battery cell's voltage to reach the third discharge limit voltage is predicted by using the proportional constant and the exponential parameter. The third discharge limit voltage is obtained by subtracting the third voltage drop caused by the third constant current and the internal resistance of the battery cell from the discharge reference voltage.

3. The method for predicting the constant current discharge curve of a single battery cell according to claim 2, wherein... At the start of discharge by the first constant current, the second constant current, and the third constant current, the state of charge (SOC) of the battery cell and the temperature of the battery cell are the same.

4. The method for predicting the constant current discharge curve of a single battery cell according to claim 2, further comprising: Change the discharge reference voltage; The third time required for the voltage of the battery cell to drop to the fourth discharge limit voltage under a fourth constant current discharge is measured. The fourth time required for the voltage of the battery cell to drop to the fifth discharge limit voltage under the fifth constant current discharge is measured. as well as Based on the fourth constant current and the third time, and the fifth constant current and the fourth time, calculate the proportionality constant and the exponential parameter in the relationship between the discharge current and time. The fourth discharge limit voltage is obtained by subtracting a fourth voltage drop caused by the third constant current and the internal resistance of the battery cell from the changed discharge reference voltage, and the fifth discharge limit voltage is obtained by subtracting a fifth voltage drop caused by the fourth constant current and the internal resistance of the battery cell from the changed discharge reference voltage.

5. The method for predicting the constant current discharge curve of a single battery cell according to claim 4, further comprising: When discharging the battery cell with the sixth constant current, the time required for the battery cell's voltage to reach the sixth discharge limit voltage is predicted by using the proportional constant and the exponential parameter. The sixth discharge limit voltage is obtained by subtracting the sixth voltage drop caused by the sixth constant current and the internal resistance of the battery cell from the changed discharge reference voltage.

6. The method for predicting the constant current discharge curve of a single battery cell according to claim 1, wherein... The relationship between the discharge current and time is the same as that shown in Equation 1. [Equation 1] I=a*t b in, In Equation 1, I is the discharge current, t is time, a is a proportionality constant, and b is an exponential parameter.

7. A battery system, comprising: Multiple battery cells; as well as A battery management system is used to predict the discharge time required for each of a plurality of individual cell voltages to reach its corresponding discharge limit voltage during constant current discharge. The battery management system stores information about the proportionality constant and exponential parameters relating to the relationship between a constant current and discharge time. After measuring the first time required for the voltage of the individual battery cell to drop to a first discharge limit voltage under a first constant current discharge and the second time required for the voltage of the individual battery cell to drop to a second discharge limit voltage under a second constant current discharge, the proportionality constant and the exponential parameter are calculated for one of the plurality of battery cells based on the first constant current and the first time, and the second constant current and the second time. The first discharge limit voltage is obtained by subtracting a first voltage drop caused by the first constant current and the internal resistance of the battery cell from the discharge reference voltage when the discharge current is 0, and the second discharge limit voltage is obtained by subtracting a second voltage drop caused by the second constant current and the internal resistance of the battery cell from the discharge reference voltage.

8. The battery system according to claim 7, wherein When discharging the battery cell with a third constant current, the battery management system predicts the time required for the battery cell voltage to reach the third discharge limit voltage by using the proportional constant and the exponential parameter. The third discharge limit voltage is obtained by subtracting the third voltage drop caused by the third constant current and the internal resistance of the battery cell from the discharge reference voltage.

9. The battery system according to claim 8, wherein At the start of discharge by the first constant current, the second constant current, and the third constant current, the state of charge (SOC) of the battery cell and the temperature of the battery cell are the same.

10. The battery system according to claim 7, wherein The relationship between the discharge current and time is the same as that shown in Equation 1. [Equation 1] I=a*t b in, In Equation 1, I is the discharge current, t is time, a is a proportionality constant, and b is an exponential parameter.

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