CN116203428A - Self-discharge detection method based on constant voltage charging to calculate equivalent model parameters of lithium battery - Google Patents

Abstract

The invention discloses a self-discharge detection method based on constant-voltage charging to calculate the equivalent model parameters of lithium batteries, comprising the following steps: S1, constructing a lithium-ion battery self-discharge equivalent circuit model and a self-discharge current mathematical model; S2, using The multimeter measures the open circuit voltage of the battery; S3, measures the relationship curve between the state of charge of the battery and the open circuit voltage, and obtains the equivalent capacitance corresponding to the current state of charge; S4, measures the self-discharge resistance: apply a programmable constant voltage source to both sides of the battery terminal, the two are connected through a current-sensing resistor; monitor the real-time line current, and obtain the self-discharge resistance according to the mathematical relationship between the self-discharge resistance and the line current; S5, substitute the parameters measured by S2 to S4 into the self-discharge current mathematical model get the self-discharge current. When the invention measures the self-discharge equivalent model parameters, it does not need to wait for the line current to be stable, and uses the process quantity to solve the self-discharge resistance, which can effectively shorten the measurement time.

Description

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

technical field

The 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 technique

As the international community pays more and more attention to energy crisis and environmental pollution, lithium-ion batteries have been widely used as a kind of pollution-free, high energy density, long life and other characteristics; at present, lithium-ion batteries have been widely used in mobile phones, watches, Electronic products such as video cameras and children's toys.

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; and the power battery system requires each single battery in terms of capacity, internal resistance, Voltage, self-discharge and other aspects have a high degree of consistency.

Battery self-discharge will not only affect the capacity of a single battery, but also affect 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 quality assurance of power battery systems.

Battery self-discharge refers to the capacity loss caused by the reflexive reaction in the battery when the battery is not connected to an external circuit; self-discharge can be divided into reversible self-discharge and irreversible self-discharge. Reversible self-discharge is due to impurities in the battery piercing the separator, and electrons pass through the separator to form an internal short circuit. Reversible self-discharge is also a physical self-discharge. The self-discharge caused by the micro-short circuit has a great impact on the long-term performance of the battery; the irreversible self-discharge is caused by the chemical side reaction inside the battery. The loss of lithium ions directly leads to capacity loss. Irreversible self-discharge is chemical self-discharge. High-temperature storage is often used to accelerate chemical side reactions to screen for irreversible self-discharge of batteries. Compared with irreversible self-discharge, reversible self-discharge has a greater impact on the long-term performance of the battery.

At present, common self-discharge detection methods include direct measurement method, open circuit voltage method, and capacity retention method.

The specific steps of the direct measurement method are as follows: first charge the battery, use the charging and discharging equipment to obtain the current capacity Q ₀ of the battery, and then let it stand for a period of time at room temperature. /T 31486) stipulates that the standing time is 28 days, and then charge the battery at room temperature to obtain the capacity Q ₁ after standing, and the self-discharge rate of the lithium-ion battery can be obtained (Q ₀ -Q ₁ ) /Q ₀ ; charge the battery again, and then discharge all the power to obtain the battery capacity Q ₂ ; the reversible self-discharge of the battery can be obtained as Q _rev =Q ₂ -Q ₁ ; the irreversible self-discharge is Q _irr =Q ₀ -Q ₂ .

The open circuit voltage method is based on the unique relationship between the open circuit voltage OCV of the battery and the state of charge of the battery, and the power loss of the battery is represented by the open circuit voltage loss. Use a multimeter to measure the open circuit voltage OCV ₁ of the battery before standing still, and let it stand at room temperature for a period of time t. Generally, the standing time stipulated by the enterprise is 2 to 10 days, and then measure the open circuit voltage OCV ₂ after the battery stands still; open circuit voltage method The evaluation index is the K value, which refers to the reduction of the open circuit voltage per unit time, then

K = (OCV ₁ -OCV ₂ )/t.

The capacity retention method is to equate the battery to an equivalent circuit model composed of capacitance, internal resistance and self-discharge resistance. According to the circuit model, the mathematical model of the self-discharge current of the battery can be inferred. Apply a programmable constant voltage source to the battery, and set the voltage of the programmable constant voltage source to be equal to or close to the open circuit voltage of the battery as much as possible. Use the constant voltage power supply to charge the battery with a small current until the charging current and the battery terminal voltage are stable and stable. The charging current mathematical model is consistent with the self-discharging current mathematical model, so the charging current can just supplement the internal self-discharging current loss of the battery, that is, the current is equal to the self-discharging current; use a multimeter and other equipment to monitor and read the charging current, Take the stable measurement result as the final value of the self-discharge current.

Of the above three measurement methods, the direct measurement method has high measurement accuracy, but the measurement time is the longest, requiring a one-month standing time; the open circuit voltage method takes advantage of the high resolution of the multimeter to shorten the test time to a few days, but the measurement time remains the same. Very long; the capacity retention method directly measures the capacity loss per unit time, that is, the self-discharge current; the test time of this method is determined by the time constant of the test circuit. Generally, after three times the time constant, the change of the self-discharge current is less than the final value. 5%, that is, the self-discharge current can be considered to be stable. However, since the final value of the self-discharge current is an unknown parameter, it usually takes a test time greater than three times the time constant to determine that the self-discharge current has reached stability, and the waiting time is uncertain and long.

Contents of the invention

The purpose of the present invention is to overcome the low efficiency of the self-discharge test method in the prior art and the problem of long test time, and provide a method that utilizes the open circuit voltage curve of the multimeter, the battery, and the constant voltage charging line current in the established measurement loop to measure Open-circuit voltage, equivalent capacitance and self-discharge resistance of the battery, and then substitute these parameters into the mathematical model to obtain the self-discharge current self-discharge detection method based on constant voltage charging to solve the equivalent model parameters of lithium batteries. This method does not need to wait for constant The current of the voltage charging line is stable, and the self-discharge resistance is calculated by using its process value, so the measurement time can be shortened.

The object of the present invention is achieved through the following technical solutions: the self-discharge detection method based on constant voltage charging to solve the equivalent model parameters of lithium batteries comprises the following steps:

S1. Construct the lithium-ion battery equivalent circuit model and self-discharge current mathematical model: the battery equivalent circuit model includes equivalent capacitance C _eff , self-discharge resistance R _sd and internal resistance R _S , equivalent capacitance C _eff and self-discharge resistance R After _sd is connected in parallel, it is connected in series with the internal resistance R _S. In the open circuit state, the voltage at both ends of the series circuit is the open circuit voltage OCV, then the mathematical model of the battery self-discharge current I _sd is:

S2. Use a multimeter to measure the open circuit voltage OCV of the battery;

S3. Measure the relationship curve between the state of charge of the battery and the open circuit voltage to obtain the equivalent capacitance C _eff corresponding to the current state of charge;

S4. Measure the self-discharge resistance R _sd : Apply a programmable constant voltage source to both ends of the battery, and connect the two through the current-sensing resistor R _line ; monitor the real-time line current I _m through the voltmeter and the current-sensing resistor R _line , and According to the mathematical relationship between the self-discharge resistance R _sd and the line current I _m , the self-discharge resistance R _sd is obtained;

S5. Substitute the open-circuit voltage OCV measured by S2 to S4, the equivalent capacitance C _eff corresponding to the current state of charge, and the self-discharge resistance R _sd into the formula (1), and obtain the mathematics of the self-discharge current I _sd of the battery changing with time Model.

The specific implementation method of the step S3 is:

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

S32. Discharge at a constant current of 1C to the lower cut-off voltage, and measure the battery capacity Q;

S33. Charge the battery with a constant current of 1C to the upper cut-off voltage, and then switch to constant voltage charging until the current is less than 0.05C to stop charging;

S34. The measurement area of the state of charge is 5%-95%, the interval of the state of charge of the 10%-90% section is set to 5%, and the interval of the rest of the state of charge is set to 1%, a total of 27 points; discharge at 1C, according to The discharge time is calculated by the state of charge, and each time the discharge reaches a state of charge measurement point, stand still for 2 hours, and measure the open circuit voltage of the battery after standing; repeat the discharge process until all points are measured;

S35. After the measurement is completed, three Hermit interpolation is 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), obtain the equivalent capacitance under the current state of charge;

Among them, ΔQ is the capacity corresponding to ΔSOC of the total discharge capacity of the battery in 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 battery at the current state of charge. After ΔSOC of the total discharge capacity in the state of charge, the difference between the initial open circuit voltage and the open circuit voltage after discharge, that is, ΔOCV, can be obtained from the relationship between the state of charge and the open circuit voltage.

In the step S4, the equipment required to measure the self-discharge resistance R _sd includes a programmable constant voltage source, two seven and a half digital multimeters, a current detection resistor, a constant temperature device and a host computer; the constant temperature device is used to eliminate the influence of the temperature measurement test , the host computer is used for control, acquisition and calculation; multimeter 1 is used to measure the voltage across the programmable constant voltage source, multimeter 2 is used to measure the voltage V _cell across the battery under test, and the current detection resistor is located between the programmable constant voltage source and the battery Both ends, the measurement model of the line current I _m is shown in formula (3).

Before measuring the self-discharge resistance R _sd , 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 a programmable constant voltage source, and connect multimeter 2 to Battery, multimeter 2 measures the current open circuit voltage OCV of the battery, and sets 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 use the output voltage of the programmable constant voltage source fed back by the multimeter 1 The difference between V _s o _urce and the open-circuit voltage OCV of the battery fed back by the multimeter 2 is adjusted to the input of the programmable constant voltage source, and repeated several times so that the difference between the output voltage V _s o _urce of the programmable constant voltage source and the open-circuit voltage OCV of the battery under test is less than 5 microvolts to make the two match; connect the measurement circuit, and get the real-time line current I _m through the multimeter and formula (3); since the order of magnitude of the internal resistance R _S of the battery is much smaller than that of the current-sensing resistance R _line , the internal resistance R of the battery can be ignored The influence of _S , the mathematical relationship between the self-discharge resistance R _sd and the real-time line current _Im can be obtained from the measurement circuit, as shown in formula (4), and the host computer obtains the self-discharge resistance R _sd according to formula (4);

If the obtained self-discharge resistance R _sd changes less than 5% in the double time constant R _line C _eff of the measuring circuit, it is considered that the self-discharge resistance R _sd has reached stability, and its average value within this period of time is taken as the self-discharge resistance Final measurement of R _sd .

The beneficial effect of the present invention is: the present invention is by constructing lithium-ion battery self-discharging equivalent circuit model and self-discharging current mathematical model, respectively utilizes the multimeter, the open-circuit voltage curve of the battery, the process current in the established measurement circuit, obtains the battery The open-circuit voltage, equivalent capacitance and self-discharge resistance are substituted into the self-discharge current mathematical model to obtain the self-discharge current. This method does not need to wait for the constant voltage charging line current to stabilize, and uses its process value to solve the equivalent model parameter self-discharge resistance. , so the measurement time can be shortened. Compared with the traditional direct measurement method, open circuit voltage method and capacity retention method, this method greatly shortens the self-discharge measurement time and improves the measurement efficiency.

Description of drawings

Fig. 1 is the measurement flowchart of self-discharge detection method of the present invention;

Fig. 2 is the measurement schematic diagram of the self-discharge detection method of the present invention;

Fig. 3 is a schematic diagram of the relationship curve between the state of charge of the battery and the open circuit voltage;

Fig. 4 is a schematic diagram of the measurement principle of the model parameter self-discharge resistance of the battery.

Detailed ways

The technical solution of the present invention will be further described below in conjunction with the accompanying drawings.

As shown in Figure 1, the self-discharge detection method based on constant voltage charging of the present invention to solve the equivalent model parameters of lithium battery comprises the following steps:

S1. Construct 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 shown in the dotted line box in Figure 2. The model model includes the equivalent capacitance C _eff , self-discharge resistance R _sd and internal resistance R _S , the equivalent capacitance C _eff is connected in parallel with self-discharge resistance R _sd and then connected in series with internal resistance R _S . The mathematical model of the discharge current I _sd is:

S2. Use a multimeter to measure the open circuit voltage OCV of the battery;

S3. Measure the relationship curve between the state of charge of the battery and the open circuit voltage to obtain the equivalent capacitance C _eff corresponding to the current state of charge; the specific implementation method is:

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

S32. Discharge at a constant current of 1C to the lower cut-off voltage, and measure the battery capacity Q;

S33. Charge the battery with a constant current of 1C to the upper cut-off voltage, and then switch to constant voltage charging until the current is less than 0.05C to stop charging;

S34. The measurement area of the state of charge is 5%-95%, the interval of the state of charge of the 10%-90% section is set to 5%, and the interval of the rest of the state of charge is set to 1%, a total of 27 points; discharge at 1C, according to The discharge time is calculated by the state of charge, and each time the discharge reaches a state of charge measurement point, stand still for 2 hours, and measure the open circuit voltage of the battery after standing; repeat the discharge process until all points are measured;

S35. After the measurement is completed, perform Hermit interpolation three times 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 Shown in Fig. 3; Then according to the open-circuit voltage curve and formula (2) of battery, obtain the equivalent capacitance under the current state of charge;

Among them, ΔQ is the capacity corresponding to ΔSOC of the total discharge capacity of the battery in 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 battery at the current state of charge. After ΔSOC of the total discharge capacity in the state of charge, the difference between the initial open circuit voltage and the open circuit voltage after discharge, that is, ΔOCV, can be obtained from the relationship between the state of charge and the open circuit voltage.

S4. Measure the self-discharge resistance R _sd : Apply a programmable constant voltage source to both ends of the battery, and connect the two through the current-sensing resistor R _line ; monitor the real-time line current I _m through the voltmeter and the current-sensing resistor R _line , and According to the mathematical relationship between the self-discharge resistance R _sd and the line current I _m , the self-discharge resistance R _sd is obtained;

The measurement device is shown in Figure 4. The equipment required to measure the self-discharge resistance R _sd includes a programmable constant voltage source, two 7.5-digit multimeters, a current-sensing resistor, a constant temperature device, and a host computer; the constant temperature device is used to exclude temperature test The host computer is used for control, acquisition and calculation; multimeter 1 is used to measure the voltage across the programmable constant voltage source, multimeter 2 is used to measure the voltage V _cell across the battery under test, and the current-sensing resistor is located at the programmable constant voltage source and both ends of the battery, the measurement model of the line current I _m is shown in formula (3).

Before measuring the self-discharge resistance R _sd , 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 a programmable constant voltage source, and connect multimeter 2 to Battery, multimeter 2 measures the current open circuit voltage OCV of the battery, and sets 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 use the output voltage of the programmable constant voltage source fed back by the multimeter 1 The difference between V _s o _urce and the open-circuit voltage OCV of the battery fed back by the multimeter 2 is adjusted to the input of the programmable constant voltage source, and repeated several times so that the difference between the output voltage V _s o _urce of the programmable constant voltage source and the open-circuit voltage OCV of the battery under test is less than 5 microvolts to make the two match; connect the measurement circuit, get the real-time line current I _m through the multimeter and formula (3), and the current data is updated every 10 seconds; because the order of magnitude of the internal resistance R _S of the battery is much smaller than the current-sensing resistance R _line , so the influence of the battery internal resistance R _S can be ignored, and the mathematical relationship between the self-discharge resistance R _sd and the real-time line current I _m can be obtained from the measurement circuit, as shown in formula (4). Discharge resistance R _sd ;

The host computer can measure a self-discharge resistance R _sd according to each set of battery voltage and constant voltage source voltage data, that is, a self-discharge resistance R _sd can be obtained every 10 seconds, but due to the measurement error of the line current I _m , in In the initial stage, the line current I _m is small, its relative error is large, and the measured value of the self-discharge resistance R _sd fluctuates greatly. As time increases, the line current I _m increases, and its relative error decreases, and the self-discharge resistance R _sd The measured value is more accurate and stable. If the obtained self-discharge resistance R _sd changes less than 5% in the double time constant R _line C _eff (about 3 to 4 hours) of the measurement circuit, it is considered that the self-discharge resistance R _sd has reached stability. Take its average value during this period as the final measurement result of the self-discharge resistance R _sd .

S5. Substitute the open-circuit voltage OCV measured by S2 to S4, the equivalent capacitance C _eff corresponding to the current state of charge, and the self-discharge resistance R _sd into the formula (1), and obtain the mathematics of the self-discharge current I _sd of the battery changing with time Model. Since the time constant R _sd C _eff is tens of days, the time-induced change in self-discharge current is negligible within a few hours of measurement, and OCV/R _sd can be considered to be the self-discharge current at the current state of charge of the battery.

Those skilled in the art will appreciate that the embodiments described here are to help readers understand the principles of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical revelations disclosed in the present invention without departing from the essence of the present invention, and these modifications and combinations are still within the protection scope of the present invention.

Claims (3)

1. The self-discharge detection method based on constant voltage charging to solve the lithium battery equivalent model parameter is characterized in that, comprising the following steps: S1. Construct the lithium-ion battery equivalent circuit model and self-discharge current mathematical model: the battery equivalent circuit model includes equivalent capacitance C _eff , self-discharge resistance R _sd and internal resistance R _S , equivalent capacitance C _eff and self-discharge resistance R After _sd is connected in parallel, it is connected in series with the internal resistance R _S. In the open circuit state, the voltage at both ends of the series circuit is the open circuit voltage OCV, then the mathematical model of the battery self-discharge current I _sd is:

S2. Use a multimeter to measure the open circuit voltage OCV of the battery; S3. Measure the relationship curve between the state of charge of the battery and the open circuit voltage to obtain the equivalent capacitance C _e ff corresponding to the current state of charge; S4. Measure the self-discharge resistance R _sd : Apply a programmable constant voltage source to both ends of the battery, and connect the two through the current-sensing resistor R _line ; monitor the real-time line current I _m through the voltmeter and the current-sensing resistor R _line , and According to the mathematical relationship between the self-discharge resistance R _sd and the line current I _m , the self-discharge resistance R _sd is obtained; S5. Bring the open-circuit voltage OCV measured by S2 to S4, the equivalent capacitance C _eff corresponding to the current state of charge, and the self-discharge resistance R _sd into the formula (1) to obtain the self-discharge current I _sd of the battery that changes with time mathematical model. 2. The self-discharge detection method based on constant voltage charging to solve the lithium battery equivalent model parameters according to claim 1, wherein the specific implementation method of the step S3 is: S31. Charge the battery, charge with a constant current of 1C to the upper cut-off voltage, and then switch to constant voltage charging until the current is less than 0.05C, then stop charging; S32. Discharge at a constant current of 1C to the lower cut-off voltage, and measure the battery capacity Q; S33. Charge the battery with a constant current of 1C to the upper cut-off voltage, and then switch to constant voltage charging until the current is less than 0.05C to stop charging; S34. The measurement area of the state of charge is 5%-95%, the interval of the state of charge of the 10%-90% section is set to 5%, and the interval of the rest of the state of charge is set to 1%, a total of 27 points; discharge at 1C, according to The discharge time is calculated by the state of charge, and each time the discharge reaches a state of charge measurement point, stand still for 2 hours, and measure the open circuit voltage of the battery after standing; repeat the discharge process until all points are measured; S35. After the measurement is completed, perform Hermit interpolation three times 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 and formula (2) of the battery, the equivalent capacitance in the current state of charge is obtained;

Among them, ΔQ is the capacity corresponding to ΔSOC of the total discharge capacity of the battery in the current state of charge, which is 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 battery at the current state of charge. After ΔSOC of the total discharge capacity in the state of charge, the difference between the initial open circuit voltage and the open circuit voltage after discharge, that is, ΔOCV, is obtained from the relationship curve between the state of charge and the open circuit voltage. 3. The self-discharge detection method based on constant voltage charging to solve lithium battery equivalent model parameters according to claim 1, characterized in that, in the step S4, the required equipment for measuring the self-discharge resistance R _sd includes a programmable constant Pressure source, two seven and a half digital multimeters, current detection resistors, constant temperature equipment and host computer; the constant temperature equipment is used to eliminate the influence of temperature measurement and testing, and the host computer is used for control, acquisition and calculation; multimeter 1 is used to measure programmable constant temperature The voltage V _s o _urce at both ends of the voltage source, the multimeter 2 is used to measure the voltage V _cell at both ends of the battery under test, and the current detection resistor is located at both ends of the programmable constant voltage source and the battery, then the measurement model of the line current I _m is shown in the formula (3 ) as shown:

Before measuring the self-discharge resistance R _sd , 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 a programmable constant voltage source, and connect multimeter 2 to Battery, multimeter 2 measures the current open circuit voltage OCV of the battery, and sets 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 use the output voltage of the programmable constant voltage source fed back by the multimeter 1 The difference between V _s o _urce and the open-circuit voltage OCV of the battery fed back by the multimeter 2 is adjusted to the input of the programmable constant voltage source, and repeated several times so that the difference between the output voltage V _s o _urce of the programmable constant voltage source and the open-circuit voltage OCV of the battery under test is less than 5 microvolts to make the two match; connect the measurement circuit, and get the real-time line current I _m through the multimeter and formula (3); since the order of magnitude of the internal resistance R _S of the battery is much smaller than that of the current-sensing resistance R _line , the internal resistance R of the battery can be ignored The influence of _S , the mathematical relationship between the self-discharge resistance R _sd and the real-time line current _Im can be obtained from the measurement circuit, as shown in formula (4), and the host computer obtains the self-discharge resistance R _sd according to formula (4);

If the obtained self-discharge resistance R _sd changes less than 5% in the double time constant R _line C _eff of the measuring circuit, it is considered that the self-discharge resistance R _sd has reached stability, and its average value within this period of time is taken as the self-discharge resistance Final measurement of R _sd .

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