Self-discharge detection method based on constant voltage charging to solve equivalent model parameters of lithium battery

Abstract

The present invention discloses a self-discharge detection method for calculating lithium battery equivalent model parameters based on constant voltage charging, comprising the following steps: S1, constructing a lithium-ion battery self-discharge equivalent circuit model and a self-discharge current mathematical model; S2, using a multimeter to measure the battery's open-circuit voltage; S3, measuring the relationship curve between the battery's state of charge and the open-circuit voltage to obtain the equivalent capacitance corresponding to the current state of charge; S4, measuring the self-discharge resistance: applying a programmable constant voltage source to both ends of the battery, connecting the two via a current-sense resistor; monitoring the real-time line current, and obtaining the self-discharge resistance based on the mathematical relationship between the self-discharge resistance and the line current; S5, substituting the parameters measured in S2 to S4 into the self-discharge current mathematical model to obtain the self-discharge current. When measuring the self-discharge equivalent model parameters, the present invention eliminates the need to wait for the line current to stabilize, and instead uses its process quantities to calculate the self-discharge resistance, effectively shortening the measurement time.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a self-discharge detection method for calculating equivalent model parameters of lithium batteries based on constant voltage charging. Background Art

[0002] As the international community pays more and more attention to energy crisis and environmental pollution, lithium-ion batteries, which are pollution-free, have high energy density and long life, have been widely used. Currently, lithium-ion batteries have been widely used in electronic products such as mobile phones, watches, cameras, and children's toys.

[0003] In the new energy vehicle industry, the power battery system composed of lithium-ion batteries provides driving power for new energy vehicles and is one of the most critical components of new energy vehicles. The power battery system requires that each single cell has a high degree of consistency in capacity, internal resistance, voltage, self-discharge, etc.

[0004] Battery self-discharge not only affects the capacity of single cells, but also affects the consistency of the lithium-ion battery power system, thereby affecting the performance and life of new energy vehicles. Therefore, self-discharge measurement is an important detection technology for power battery system quality assurance.

[0005] Battery self-discharge refers to the loss of capacity due to internal self-reactions when the battery is not connected to an external circuit. Self-discharge can be categorized as reversible self-discharge and irreversible self-discharge. Reversible self-discharge occurs when impurities within the battery pierce the separator, allowing electrons to pass through the separator and form an internal short circuit. Reversible self-discharge is also called physical self-discharge. Self-discharge caused by micro-short circuits has a significant impact on the long-term performance of the battery. Irreversible self-discharge is caused by chemical side reactions within the battery. The positive and negative electrode materials and some impurities react with the battery electrolyte, resulting in lithium ion loss and a direct loss of capacity. Irreversible self-discharge is a chemical self-discharge, and high-temperature storage is often used to accelerate chemical side reactions to screen for irreversible self-discharge. Compared to irreversible self-discharge, reversible self-discharge has a greater impact on the long-term performance of the battery.

[0006] Currently, common self-discharge detection methods include direct measurement method, open circuit voltage method, and capacity retention method.

[0007] The specific steps of the direct measurement method are: first charge the battery and use the charging and discharging equipment to obtain the current capacity of the battery Q0, then let it stand at room temperature for a period of time. The Chinese national standard "Performance Requirements and Test Methods for Power Batteries for Electric Vehicles" (GB / T 31486) stipulates that the standing time is 28 days. Then charge the battery at room temperature again and obtain the capacity Q1 after standing. The self-discharge rate of the lithium-ion battery can be obtained as (Q0-Q1) / Q0; then charge the battery and discharge all the electricity to obtain the battery capacity Q2; the reversible self-discharge of the battery can be obtained as Q rev =Q2-Q1; irreversible self-discharge is Q irr =Q0-Q2.

[0008] The open circuit voltage method is based on the unique correspondence between the open circuit voltage (OCV) and the state of charge of the battery. The open circuit voltage loss is used to characterize the battery's power loss. Use a multimeter to measure the open circuit voltage (OCV1) of the battery before it is left at rest. Let it rest at room temperature for a period of time (t). Generally, companies stipulate that the resting time is 2 to 10 days. Then measure the open circuit voltage (OCV2) of the battery after it is left at rest. The evaluation index of the open circuit voltage method is the K value, which refers to the decrease in open circuit voltage per unit time.

[0009] K = (OCV1 - OCV2) / t.

[0010] The capacity retention method treats the battery as an equivalent circuit model consisting of capacitance, internal resistance, and self-discharge resistance. Based on this circuit model, the mathematical model of the battery's self-discharge current can be inferred. A programmable constant voltage source is applied to the battery, with the voltage set as close as possible to the battery's open-circuit voltage. This constant voltage source is used to charge the battery with a small current until both the charging current and the battery terminal voltage reach stability. The mathematical model of the charging current at this point is consistent with the mathematical model of the self-discharge current, so this charging current just compensates for the battery's internal self-discharge current loss, meaning it is equal to the self-discharge current. This charging current is then monitored and read using a device such as a multimeter, with the stable measurement result used as the final value of the self-discharge current.

[0011] Of the three measurement methods mentioned above, the direct measurement method offers high accuracy but takes the longest, requiring a month of static testing. The open-circuit voltage method leverages the high resolution of a multimeter to shorten the test time to a few days, but the measurement time is still very long. The capacity retention method directly measures the capacity loss per unit time, or self-discharge current. The test time for this method is determined by the time constant of the test circuit. Generally, after three times the time constant, the change in the self-discharge current is less than 5% of the final value, indicating that the self-discharge current has stabilized. However, since the final value of the self-discharge current is an unknown parameter, a test time of more than three times the time constant is usually required to confirm that the self-discharge current has stabilized, resulting in an uncertain and lengthy waiting time. Summary of the Invention

[0012] The purpose of the present invention is to overcome the problems of low efficiency and long test time in the existing self-discharge test methods. A self-discharge detection method based on constant voltage charging and calculating equivalent model parameters of lithium batteries is provided, which uses a multimeter, an open circuit voltage curve of a battery, and a constant voltage charging circuit current in an established measurement loop to measure parameters such as the open circuit voltage, equivalent capacitance, and self-discharge resistance of the battery, and then substitutes these parameters into a mathematical model to obtain the self-discharge current. The method does not need to wait for the constant voltage charging circuit current to reach stability, and uses its process quantity to calculate the self-discharge resistance, thereby shortening the measurement time.

[0013] The object of the present invention is achieved through the following technical solution: a self-discharge detection method for calculating equivalent model parameters of a lithium battery based on constant voltage charging, comprising the following steps:

[0014] S1. Constructing the lithium-ion battery equivalent circuit model and self-discharge current mathematical model: The battery equivalent circuit model includes the equivalent capacitance C eff , self-discharge resistor R sd and internal resistance R S , equivalent capacitance C eff With the self-discharge resistor R sd After parallel connection with internal resistance R S In series, in the open circuit state, the voltage across the series circuit is the open circuit voltage OCV, then the battery self-discharge current I sd The mathematical model is:

[0015]

[0016] S2. Use a multimeter to measure the open circuit voltage (OCV) of the battery.

[0017] S3. Measure the relationship curve between the battery's state of charge and open circuit voltage to obtain the equivalent capacitance C corresponding to the current state of charge. eff ;

[0018] S4. Measure the self-discharge resistance R sd :A programmable constant voltage source is applied to both ends of the battery, with a current sensing resistor R between them. line Connection; through the voltmeter and current detection resistor R line Monitor real-time line current I m , and according to the self-discharge resistance R sd With the line current I m The mathematical relationship between the self-discharge resistance R sd ;

[0019] S5, the open circuit voltage OCV measured by S2~S4 and the equivalent capacitance C corresponding to the current state of charge eff , self-discharge resistor R sdSubstituting into formula (1), we can get the battery self-discharge current I sd Mathematical models of time-varying phenomena.

[0020] The specific implementation method of step S3 is:

[0021] S31, charge the battery at a constant current of 1C to the upper cut-off voltage, then switch to constant voltage charging until the current is less than 0.05C and stop charging;

[0022] S32, discharge at a constant current of 1C to the lower cut-off voltage, and measure the battery capacity Q;

[0023] S33, charging the battery at a constant current of 1C to the upper cut-off voltage, then switching to constant voltage charging until the current is less than 0.05C and stopping charging;

[0024] S34. The state of charge measurement range is 5%-95%, the state of charge interval in the 10%-90% range is set to 5%, and the remaining state of charge intervals are set to 1%, for a total of 27 points; discharge at 1C, calculate the discharge time based on the state of charge, and after each discharge to a state of charge measurement point, let it rest for 2 hours and measure the open circuit voltage of the battery after rest; repeat this discharge process until all points are measured;

[0025] S35. After the measurement is completed, three Hermite interpolations are performed on the points in the 10%-90% state of charge interval to improve the resolution, so that the state of charge interval is 1%, and the relationship curve between the state of charge and the open circuit voltage of the battery is obtained; then, according to the open circuit voltage curve of the battery and formula (2), the equivalent capacitance at the current state of charge is calculated;

[0026]

[0027] Among them, ΔQ is the capacity discharged corresponding to ΔSOC of the total capacity discharged by the battery at the current state of charge, which can be obtained from the state of charge interval ΔSOC and the total capacity Q on the relationship curve between the state of charge and the open circuit voltage; ΔV is the difference between the initial open circuit voltage and the open circuit voltage after the total capacity ΔSOC of the battery is discharged at the current state of charge, that is, ΔOCV, which can be obtained from the relationship curve between the state of charge and the open circuit voltage.

[0028] In step S4, the self-discharge resistance R is measured. sd The required equipment includes a programmable constant voltage source, two 7.5-digit digital multimeters, a current-sense resistor, a constant temperature device, and a host computer. The constant temperature device is used to eliminate the influence of temperature measurement, and the host computer is used for control, acquisition, and calculation. Multimeter 1 is used to measure the voltage across the programmable constant voltage source, and multimeter 2 is used to measure the voltage V across the battery under test. cell , the current-sense resistor is located between the programmable constant voltage source and the battery, then the line current Im The measurement model is shown in formula (3).

[0029]

[0030] When measuring the self-discharge resistance R sd Before testing, the battery under test needs to be placed in a constant temperature device for one day to remove the polarization of the battery and make the battery voltage fully stable; connect multimeter 1 to the programmable constant voltage source, connect multimeter 2 to the battery, and measure the current open circuit voltage OCV of the battery with multimeter 2, and set it as the input voltage of the programmable constant voltage source. Since the programmable constant voltage source has an output error, it is necessary to set the output voltage V of the programmable constant voltage source based on the feedback of multimeter 1. source The difference between the open circuit voltage OCV of the battery fed back by multimeter 2 is used to adjust the input of the programmable constant voltage source. Repeat this process many times to make the output voltage V source The difference between the open circuit voltage (OCV) of the battery under test is less than 5 microvolts, so that the two are matched; connect the measurement circuit and use the multimeter and formula (3) to obtain the real-time line current I m ; Due to the battery internal resistance R S The magnitude of the current sense resistor R line , so the battery internal resistance R can be ignored S The self-discharge resistance R can be obtained from the measurement circuit. sd and real-time line current I m The mathematical relationship between them is shown in formula (4). The host computer obtains the self-discharge resistance R according to formula (4): sd ;

[0031]

[0032] If the self-discharge resistance R sd The measurement circuit has a time constant R line C eff If the change is less than 5%, it is considered that the self-discharge resistance R sd It has reached stability, and the average value during this period is taken as the self-discharge resistance R sd The final measurement result.

[0033] The beneficial effects of the present invention are as follows: by constructing a lithium-ion battery self-discharge equivalent circuit model and a self-discharge current mathematical model, the present invention uses a multimeter, the battery's open-circuit voltage curve, and the process current in the established measurement loop to obtain the battery's open-circuit voltage, equivalent capacitance, and self-discharge resistance, respectively. These are then substituted into the self-discharge current mathematical model to obtain the self-discharge current. This method does not require waiting for the constant-voltage charging circuit current to stabilize, but uses its process value to solve the equivalent model parameter, the self-discharge resistance, thereby shortening the measurement time. Compared with traditional direct measurement methods, open-circuit voltage methods, and capacity retention methods, this method significantly shortens the self-discharge measurement time and improves measurement efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a measurement flow chart of the self-discharge detection method of the present invention;

[0035] Figure 2 Schematic diagram of the measurement principle of the self-discharge detection method of the present invention;

[0036] Figure 3 Schematic diagram of the relationship between the state of charge and the open circuit voltage of the battery;

[0037] Figure 4 Schematic diagram of the measurement principle of the battery model parameter self-discharge resistance. DETAILED DESCRIPTION

[0038] The technical solution of the present invention is further described below with reference to the accompanying drawings.

[0039] like Figure 1 As shown, the self-discharge detection method of the present invention based on constant voltage charging and calculating the equivalent model parameters of a lithium battery includes the following steps:

[0040] S1. Constructing the equivalent circuit model of lithium-ion battery and the mathematical model of self-discharge current: According to the self-discharge characteristics of lithium-ion battery, construct the equivalent model of lithium-ion battery as follows: Figure 2 As shown in the dotted box, the model includes the equivalent capacitor C eff , self-discharge resistor R sd and internal resistance R S , equivalent capacitance C eff With the self-discharge resistor R sd After parallel connection with internal resistance R S In series, in the open circuit state, the voltage across the series circuit is the open circuit voltage OCV, then the battery self-discharge current I sd The mathematical model is:

[0041]

[0042] S2. Use a multimeter to measure the open circuit voltage (OCV) of the battery.

[0043] S3. Measure the relationship curve between the battery's state of charge and open circuit voltage to obtain the equivalent capacitance C corresponding to the current state of charge. eff ; The specific implementation method is:

[0044] S31. Charge the battery at a constant current of 1C (1 hour rate discharge current) to the upper cut-off voltage, then switch to constant voltage charging until the current is less than 0.05C and stop charging;

[0045] S32, discharge at a constant current of 1C to the lower cut-off voltage, and measure the battery capacity Q;

[0046] S33, charging the battery at a constant current of 1C to the upper cut-off voltage, then switching to constant voltage charging until the current is less than 0.05C and stopping charging;

[0047] S34. The state of charge measurement range is 5%-95%, the state of charge interval in the 10%-90% range is set to 5%, and the remaining state of charge intervals are set to 1%, for a total of 27 points; discharge at 1C, calculate the discharge time based on the state of charge, and after each discharge to a state of charge measurement point, let it rest for 2 hours and measure the open circuit voltage of the battery after rest; repeat this discharge process until all points are measured;

[0048] S35. After the measurement is completed, three Hermite interpolations are performed on the points in the 10%-90% state of charge interval to improve the resolution, so that the state of charge interval is 1%, and the relationship curve between the state of charge and the open circuit voltage of the battery is obtained, such as Figure 3 As shown; then, according to the open circuit voltage curve of the battery and formula (2), the equivalent capacitance at the current state of charge is calculated;

[0049]

[0050] Among them, ΔQ is the capacity discharged corresponding to ΔSOC of the total capacity discharged by the battery at the current state of charge, which can be obtained from the state of charge interval ΔSOC and the total capacity Q on the relationship curve between the state of charge and the open circuit voltage; ΔV is the difference between the initial open circuit voltage and the open circuit voltage after the total capacity ΔSOC of the battery is discharged at the current state of charge, that is, ΔOCV, which can be obtained from the relationship curve between the state of charge and the open circuit voltage.

[0051] S4. Measure the self-discharge resistance R sd :A programmable constant voltage source is applied to both ends of the battery, with a current sensing resistor R between them. line Connection; through the voltmeter and current detection resistor R line Monitor real-time line current I m , and according to the self-discharge resistance R sd With the line current I mThe mathematical relationship between the self-discharge resistance R sd ;

[0052] Measuring devices such as Figure 4 As shown, measure the self-discharge resistance R sd The required equipment includes a programmable constant voltage source, two 7.5-digit digital multimeters, a current-sense resistor, a constant temperature device, and a host computer. The constant temperature device is used to eliminate the influence of temperature measurement, and the host computer is used for control, acquisition, and calculation. Multimeter 1 is used to measure the voltage across the programmable constant voltage source, and multimeter 2 is used to measure the voltage V across the battery under test. cell , the current-sense resistor is located between the programmable constant voltage source and the battery, then the line current I m The measurement model is shown in formula (3).

[0053]

[0054] When measuring the self-discharge resistance R sd Before testing, the battery under test needs to be placed in a constant temperature device for one day to remove the polarization of the battery and make the battery voltage fully stable; connect multimeter 1 to the programmable constant voltage source, connect multimeter 2 to the battery, and measure the current open circuit voltage OCV of the battery with multimeter 2, and set it as the input voltage of the programmable constant voltage source. Since the programmable constant voltage source has an output error, it is necessary to set the output voltage V of the programmable constant voltage source based on the feedback of multimeter 1. source The difference between the open circuit voltage OCV of the battery fed back by multimeter 2 is used to adjust the input of the programmable constant voltage source. Repeat this process many times to make the output voltage V source The difference between the open circuit voltage (OCV) of the battery under test is less than 5 microvolts, so that the two are matched; connect the measurement circuit and use the multimeter and formula (3) to obtain the real-time line current I m , the current data is updated every 10 seconds; due to the battery internal resistance R S The magnitude of the current sense resistor R line , so the battery internal resistance R can be ignored S The self-discharge resistance R can be obtained from the measurement circuit. sd and real-time line current I m The mathematical relationship between them is shown in formula (4). The host computer obtains the self-discharge resistance R according to formula (4): sd ;

[0055]

[0056] The host computer can measure a self-discharge resistance R according to each set of battery voltage and constant voltage source voltage data. sd , that is, a self-discharge resistance R can be obtained every 10 seconds sd , but due to the line current I mThere is a measurement error. In the initial stage, the line current I m Small, its relative error is large, the self-discharge resistance R sd The measured value fluctuates greatly. As time goes by, the line current I m increases, its relative error decreases, and the self-discharge resistance R sd The measured value is more accurate and stable if the self-discharge resistance R sd The measurement circuit has a time constant R line C eff (about 3 to 4 hours) If the change value is less than 5%, it is considered that the self-discharge resistance R sd It has reached stability, and the average value during this period is taken as the self-discharge resistance R sd The final measurement result.

[0057] S5, the open circuit voltage OCV measured by S2~S4 and the equivalent capacitance C corresponding to the current state of charge eff , self-discharge resistor R sd Substituting into formula (1), we can get the battery self-discharge current I sd Mathematical model that changes with time. Due to the time constant R sd C eff For tens of days, within a few hours of measurement, the change in self-discharge current caused by time can be ignored, and it can be considered that OCV / R sd It is the self-discharge current of the battery at its current state of charge.

[0058] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and it should be understood that the scope of protection of the present invention is not limited to such specific descriptions and embodiments. Those skilled in the art can make various other specific variations and combinations based on the technical teachings disclosed in the present invention without departing from the essence of the present invention, and such variations and combinations are still within the scope of protection of the present invention.

Claims

1. A self-discharge detection method based on constant voltage charging to calculate equivalent model parameters of lithium batteries, characterized in that: The following steps are involved: S1. Constructing the lithium-ion battery equivalent circuit model and self-discharge current mathematical model: The battery equivalent circuit model includes the equivalent capacitance C eff , self-discharge resistor R sd and internal resistance R S , equivalent capacitance C eff With the self-discharge resistor R sd After parallel connection with internal resistance R S In series, in the open circuit state, the voltage across the series circuit is the open circuit voltage OCV, then the battery self-discharge current I sd The mathematical model is: S2. Use a multimeter to measure the open circuit voltage (OCV) of the battery. S3. Measure the relationship curve between the battery's state of charge and open circuit voltage to obtain the equivalent capacitance C corresponding to the current state of charge. eff ; The specific implementation method is: S31, charge the battery at a constant current of 1C to the upper cut-off voltage, then switch to constant voltage charging until the current is less than 0.05C and stop charging; S32, discharge at a constant current of 1C to the lower cut-off voltage, and measure the battery capacity Q; S33, charging the battery at a constant current of 1C to the upper cut-off voltage, then switching to constant voltage charging until the current is less than 0.05C and stopping charging; S34. The state of charge measurement range is 5%-95%, the state of charge interval in the 10%-90% range is set to 5%, and the remaining state of charge intervals are set to 1%, for a total of 27 points; discharge at 1C, calculate the discharge time based on the state of charge, and after each discharge to a state of charge measurement point, let it rest for 2 hours and measure the open circuit voltage of the battery after rest; repeat this discharge process until all points are measured; S35. After the measurement is completed, three Hermite interpolations are performed on the points in the 10%-90% state of charge interval to improve the resolution, so that the state of charge interval is 1%, and a curve showing the relationship between the state of charge and the open circuit voltage of the battery is obtained; then, based on the open circuit voltage curve of the battery and formula (2), the equivalent capacitance at the current state of charge is calculated; Wherein, ΔQ is the capacity discharged corresponding to ΔSOC of the total capacity discharged at the current state of charge, which is obtained from the state of charge interval ΔSOC and the total capacity Q on the state of charge vs. open circuit voltage curve; ΔV is the difference between the initial open circuit voltage and the open circuit voltage after the battery discharges ΔSOC of the total capacity at the current state of charge, i.e., ΔOCV, which is obtained from the state of charge vs. open circuit voltage curve; S4. Measure the self-discharge resistance R sd :A programmable constant voltage source is applied to both ends of the battery, with a current sensing resistor R between them. line Connection; through the voltmeter and current detection resistor R line Monitor real-time line current I m , and according to the self-discharge resistance R sd With the line current I m The mathematical relationship between the self-discharge resistance R sd ; S5, the open circuit voltage OCV measured by S2~S4 and the equivalent capacitance C corresponding to the current state of charge eff , self-discharge resistor R sd Substitute into formula (1) to obtain the battery self-discharge current I sd Mathematical models of time-varying phenomena.

2. The self-discharge detection method for calculating lithium battery equivalent model parameters based on constant voltage charging according to claim 1, characterized in that: In step S4, the self-discharge resistance R is measured. sd The required equipment includes a programmable constant voltage source, two 7.5-digit digital multimeters, a current-sensing resistor, a constant temperature device, and a host computer. The constant temperature device is used to eliminate the influence of temperature test, and the host computer is used for control, collection, and calculation. Multimeter 1 is used to measure the voltage V across the programmable constant voltage source. source , Multimeter 2 is used to measure the voltage V across the battery under test cell , the current-sense resistor is located between the programmable constant voltage source and the battery, then the line current I m The measurement model is shown in formula (3): When measuring the self-discharge resistance R sd Before testing, the battery under test needs to be placed in a constant temperature device for one day to remove the polarization of the battery and make the battery voltage fully stable; connect multimeter 1 to the programmable constant voltage source, connect multimeter 2 to the battery, and measure the current open circuit voltage OCV of the battery with multimeter 2, and set it as the input voltage of the programmable constant voltage source. Since the programmable constant voltage source has an output error, it is necessary to set the output voltage V of the programmable constant voltage source based on the feedback of multimeter 1. source The difference between the open circuit voltage OCV of the battery fed back by multimeter 2 is used to adjust the input of the programmable constant voltage source. Repeat this process many times to make the output voltage V source The difference between the open circuit voltage (OCV) of the battery under test is less than 5 microvolts, so that the two are matched; connect the measurement circuit and use the multimeter and formula (3) to obtain the real-time line current I m ; Due to the battery internal resistance R S The magnitude of the current sense resistor R line , so the battery internal resistance R is ignored S The influence of the self-discharge resistance R is obtained from the measurement circuit sd and real-time line current I m The mathematical relationship between them is shown in formula (4). The host computer obtains the self-discharge resistance R according to formula (4): sd ; If the self-discharge resistance R sd The measurement circuit has a time constant R line C eff If the change is less than 5%, it is considered that the self-discharge resistance R sd It has reached stability, and the average value during this period is taken as the self-discharge resistance R sd The final measurement results.

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