CN110031772A - A kind of real-time estimating method of lithium ion battery equivalent internal resistance - Google Patents

Abstract

The invention discloses a real-time estimation method of the equivalent internal resistance of a lithium ion battery, which calculates the state of charge of the battery by monitoring the ambient temperature, discharge current and discharge duration of the battery on-line, and then selects a corresponding correction coefficient according to the monitoring value, The leading internal resistance of the battery is determined according to the state of charge, and finally the real-time equivalent internal resistance of the battery is estimated.

Description

A Real-time Estimation Method for Equivalent Internal Resistance of Li-ion Batteries

technical field

The present application relates to the field of electric vehicle battery thermal management, in particular to a real-time estimation method for the equivalent internal resistance of a lithium-ion battery.

Background technique

The power battery pack is the main energy storage component of electric vehicles. Lithium-ion batteries are widely used in power battery packs for many electric passenger vehicles due to their high energy density, no memory effect, and long cycle life.

The Equivalent Internal Resistance (EIR) of a lithium-ion battery is an important indicator for calculating the heat generation rate of a battery.

At present, the equivalent internal resistance of lithium-ion batteries is mainly measured by the constant current intermittent discharge method of the battery in an offline state. At the same time, a large number of experimental data prove that in the process of lithium-ion battery discharge, its equivalent internal resistance is affected by ambient temperature, discharge rate, discharge time, and the state of charge (SoC, State of Charge) of the battery and changes constantly. Moreover, the working state of electric vehicles is complex and changeable. During this process, the output power of the on-board lithium-ion battery pack changes all the time, which causes the battery heat generation rate to change in real time. In order to calculate the real-time heat generation rate of the lithium-ion battery so that the battery management system can make timely decisions, the method of offline testing the equivalent internal resistance of the battery is no longer applicable.

SUMMARY OF THE INVENTION

Aiming at the inconvenience in calculating the real-time heat generation rate of the battery in the offline test of the battery equivalent internal resistance, the present invention provides a real-time estimation method for the equivalent internal resistance of the lithium ion battery. By monitoring the ambient temperature, discharge current and discharge duration of the battery online, the state of charge of the battery is calculated, and then the corresponding correction coefficient is selected according to the monitoring value, the leading internal resistance of the battery is determined according to the state of charge, and finally the real-time equivalent internal resistance of the battery is determined. resistance is estimated.

The present invention achieves the above object in this way:

A real-time estimation method for the equivalent internal resistance of a lithium-ion battery, comprising the following steps:

a. Take a certain number of lithium-ion batteries as samples, test the available capacity C of the sample batteries by the constant current discharge method, use the test data as the data basis, and use the formula (1) to fit the available capacity of the battery and the battery temperature T:

(1)

where y ₀ , A and k are the three fitting parameters of the first-order decay equation.

Wherein, the specific steps of the constant current discharge test experiment performed on the sample lithium ion battery in the step a are as follows:

a1. Preprocess the sample lithium-ion battery, set the temperature of the incubator to 40°C, and keep the temperature of the battery consistent with the ambient temperature. The sample battery was fully charged and discharged for a total of 6 times (one full battery charge and one full battery discharge are regarded as one cycle), in order to obtain a stable battery capacity.

a2. Fully charge the battery, set the discharge cut-off voltage, make the battery discharge at a constant current rate at a certain rate, stop discharging when the voltage is lower than the discharge cut-off voltage, and the total discharge capacity is regarded as the available capacity C of the battery.

a3. Before the test, in order to keep the battery temperature consistent with the ambient temperature, let the battery stand in the incubator for 10 minutes.

a4. From -20°C to 50°C, set a temperature variable every 10°C for testing.

a5. Each test is repeated three times, and the average of the available battery capacity obtained from the three tests is taken as the final result of the test.

b. Read the temperature value T (t) and current value I (t) of the battery during the discharge process, calculate the available capacity C (t) at each moment of the discharge process according to formula (1), and read the state of charge SoC before discharge (t ₀ ), use the charge accumulation method to calculate the current state of charge SoC (t) of the battery:

(2)

c. Establish a parameter database for lithium-ion batteries, including pilot internal resistance and correction factor.

c1. Use the constant current intermittent discharge method to test the lithium-ion battery sample, set the temperature of the thermostat to 40°C, discharge the battery at a rate of 0.3C for 10 minutes (about 5% of the power is discharged), and then let the battery stand for 2 hours , until the battery voltage change rate is lower than 0.1mV/min, which is regarded as a “discharge-resting” cycle. Starting from the state of charge SoC = 100%, the above cycle is repeated until the battery voltage is lower than the cut-off voltage, and the equivalent internal resistance of the battery under different states of charge SoC is recorded to obtain an "EIR-SoC" spectrum. According to the state of charge SoC(t) obtained in b, the corresponding equivalent internal resistance of the battery is found from the EIR-SoC spectrum, which is used as the reference for determining the equivalent internal resistance R (t) of the battery at the current moment. Rp ( _SoC ), .

c2. Use the constant current intermittent discharge method to test the lithium-ion battery samples, set different temperature of the incubator, discharge the battery at a rate of 0.3C for 10 minutes (about 5% of the power is discharged), and then let the battery stand for 2 hours , until the battery voltage change rate is lower than 0.1mV/min, which is regarded as a “discharge-resting” cycle. Starting from SoC=100%, the above cycle is repeated until the battery voltage is lower than the cut-off voltage, and the battery EIR of the SoC at different states of charge at the temperature of each group of incubators is recorded to obtain an "EIR-SoC-ambient temperature" spectrum. Taking the corresponding pilot internal resistance R _p (SoC) under different states of charge SoC as the benchmark, then, with the fixed state of charge SoC unchanged, take the equivalent internal resistance EIR _T of the battery corresponding to different battery temperatures T, and define the temperature correction coefficient :

(3)

c3. Use the constant current intermittent discharge method to test the lithium-ion battery samples, set the temperature of the incubator to 40°C, align the battery state of charge SoC with each state of charge SoC in step c, and charge the battery at different rates for 10 minutes, and then the battery was allowed to stand for 2 hours until the rate of change of the battery voltage was lower than 0.1 mV/min, followed by 10 minutes of post-discharge, and then allowed to stand according to the aforementioned method. The EIR of the battery under different SoCs was recorded, and an "EIR-SoC-discharge rate" spectrum was obtained. Taking the corresponding pilot internal resistance R _p (SoC) under different states of charge SoC as the benchmark, then, with the fixed state of charge SoC unchanged, take the equivalent internal resistance EIR _r of the battery corresponding to different discharge rates r, and define the correction factor for the discharge rate :

(4)

c4. Use the constant current intermittent discharge method to test the lithium-ion battery sample, set the temperature of the incubator to 40°C, align the battery state of charge SoC with each state of charge SoC in step c, and charge the battery at a rate of 0.3C For different time lengths t, the battery was then allowed to stand for 2 hours until the battery voltage change rate was lower than 0.1 mV/min, and then discharged for t seconds, and then allowed to stand according to the aforementioned method. The equivalent internal resistance EIR of the battery under different states of charge SoC is recorded, and an "EIR-SoC-discharge duration" spectrum is obtained. Taking the corresponding pilot internal resistance R _p (SoC) under different states of charge SoC as the benchmark, then, with the fixed state of charge SoC unchanged, take the equivalent internal resistance EIR _t of the battery corresponding to different discharge durations t, and define the discharge duration correction factor :

(5)

c5 . So far, the parameter database of the selected type of lithium-ion battery including the " _EIR - _SoC " spectrum, kT , _Kr , and Kt has been obtained, and the relevant data can be searched according to the battery discharge conditions to calculate the real-time performance of the lithium-ion battery. Equivalent internal resistance EIR(t).

d. The method for testing the EIR of the battery by the constant current voltage discharge method described in steps c1 to c4 is to record the voltage E(t ₀ ) when the discharge starts and the voltage E( t) and discharge current I(t), then the equivalent internal resistance EIR(t) of the battery during t time can be calculated by the following formula (6):

(6)

e. The calculation method of real-time equivalent internal resistance EIR(t) of lithium-ion battery is proposed:

(7)

f. For the reference values that are not entered in the database (for example, the temperature correction coefficient corresponding to the ambient temperature of 25°C), the adjacent reference values (temperature correction coefficients corresponding to 20°C and 30°C) can be interpolated to obtain the required Reference.

The beneficial effects of the invention are: by collecting the ambient temperature, discharge current and discharge duration of the lithium ion battery online, the real-time equivalent internal resistance of the lithium ion battery is estimated online, and the purpose of calculating the lithium ion real-time heat generation rate is achieved. The invention is based on the fact that a large number of test experiments are carried out on the selected lithium ion battery, the results are reliable, the experimental method is mature, and the operability is strong.

Description of drawings

Below in conjunction with accompanying drawing and embodiment, the present invention is further described:

Fig. 1 is the flow chart of establishing lithium-ion battery parameter database;

Figure 2 is a database diagram of a lithium-ion battery sample;

Fig. 3 is a flow chart of a specific embodiment of the method of the present invention.

Detailed ways

The real-time estimation method of the equivalent internal resistance EIR of a lithium-ion battery provided by the present invention includes establishing a parameter database of the lithium-ion battery, and the process is shown in Figure 1; The process is shown in Figure 3.

First, a certain number of lithium-ion batteries are taken as samples, and the available capacity C of the sample batteries is tested by the constant current discharge method. Using the test data as the data basis, the available capacity of the battery and the battery temperature T are fitted by formula (1):

(1)

where y ₀ , A and k are the three fitting parameters of the first-order decay equation.

Among them, the specific steps of the constant current discharge test experiment on the sample lithium-ion battery are as follows:

a. Preprocess the sample lithium-ion battery, set the temperature of the incubator to 40°C, and keep the battery temperature consistent with the ambient temperature. The sample battery was fully charged and discharged for a total of 6 times (one full battery charge and one full battery discharge are regarded as one cycle), in order to obtain a stable battery capacity.

b. Fully charge the battery to the state of charge SoC=100%, set the discharge cut-off voltage to 2.5V, discharge at a rate of 0.5C, and then discharge at a rate of 0.1C, stop discharging when the voltage is lower than the discharge cut-off voltage, and the total The discharge capacity is regarded as the available capacity C of the battery.

c. Before testing, in order to keep the battery temperature consistent with the ambient temperature, let the battery stand in the incubator for 10 minutes.

d. From -20°C to 50°C, set 1 temperature variable for testing every 10°C.

e. Each test is repeated three times, and the average of the available battery capacity obtained from the three tests is taken as the final result of the test.

At this time, read the temperature value T (t) and current value I (t) of the battery during the discharge process, calculate the available capacity C (t) at each moment of the discharge process according to formula (1), and read the state of charge before discharge. SoC (t ₀ ), use the charge accumulation method to calculate the current state of charge SoC (t) of the battery:

(2)

Secondly, the equivalent internal resistance EIR of the lithium-ion battery under various discharge conditions is tested by the constant current intermittent discharge method, and the parameter database of the lithium-ion battery is established, including the pilot internal resistance and the correction coefficient. The specific steps are as follows:

a. Use the constant current intermittent discharge method to test the lithium-ion battery sample, set the temperature of the thermostat to 40°C, discharge the battery at a rate of 0.3C for 10 minutes (about 5% of the power is discharged), and then let the battery stand for 2 hours , until the battery voltage change rate is lower than 0.1mV/min, which is regarded as a “discharge-resting” cycle. Starting from the state of charge SoC=100%, repeat the above cycle until the battery voltage is lower than the cut-off voltage, record the battery EIR under different state of charge SoC, and obtain an "EIR-SoC" spectrum, as shown in the upper left corner of Figure 2. Show. According to the state of charge SoC(t) obtained in b, the corresponding battery equivalent internal resistance EIR is found from the EIR-SoC spectrum, which is used as the reference for determining the current equivalent internal resistance R (t) of the battery. Resistor R _p (SoC).

b. Use the constant current intermittent discharge method to test the lithium-ion battery samples. Discharge at a rate of 0.3C for 10 minutes (about 5% of the power is discharged), then let the battery stand for 2 hours until the battery voltage change rate is lower than 0.1mV/min, which is used as a "discharge-standby" cycle . Starting from the state of charge SoC = 100%, repeat the above cycle until the battery voltage is lower than the cut-off voltage, record the equivalent internal resistance EIR of the battery at different state of charge SoCs at the temperature of each group of incubators, and obtain an "EIR-SoC-environment" temperature” spectrum, as shown in the upper right line spectrum in Figure 2. Taking the corresponding pilot internal resistance R _p (SoC) under different states of charge SoC as the benchmark, then, with the fixed state of charge SoC unchanged, take the equivalent internal resistance EIR _T of the battery corresponding to different battery temperatures T, and define the temperature correction coefficient :

(3)

c. Use the constant current intermittent discharge method to test the lithium-ion battery samples, set the temperature of the incubator to 40 °C, align the battery state of charge SoC with each state of charge SoC in step c, and make the battery at different rates (0.1 C, 0.3C, 0.5C, 0.7C, 1.0C, 1.2C, 1.4C, 1.6C, 1.8C, 2.0C) charge for 10 minutes, then let the battery stand for 2 hours until the battery voltage change rate is less than 0.1mV /min, post-discharge for 10 minutes, and then stand still according to the above method. The EIR of the battery under different SoCs was recorded, and an "EIR-SoC-discharge rate" spectrum was obtained, as shown in the spectrum in the lower left corner of Figure 2. Taking the corresponding pilot internal resistance R _p (SoC) under different states of charge SoC as the benchmark, then, with the fixed state of charge SoC unchanged, take the equivalent internal resistance EIR _r of the battery corresponding to different discharge rates r, and define the correction factor for the discharge rate :

(4)

d. Use the constant current intermittent discharge method to test the lithium-ion battery sample, set the temperature of the incubator to 40°C, align the battery state of charge SoC with each state of charge SoC in step c, and charge the battery at a rate of 0.3C t seconds (t is 30, 60, 120, 240, 360, 480, 600), then let the battery stand for 2 hours until the battery voltage change rate is lower than 0.1mV/min, and then discharge for t seconds, and then follow the above method to static set. The EIR of the battery under different states of charge SoC is recorded, and an "EIR-SoC-discharge duration" spectrum is obtained, as shown in the spectrum in the lower right corner of Figure 2. Taking the corresponding pilot internal resistance R _p (SoC) under different states of charge SoC as the benchmark, then, with the fixed state of charge SoC unchanged, take the equivalent internal resistance EIR _t of the battery corresponding to different discharge durations t, and define the discharge duration correction factor :

(5)

The method for testing the equivalent internal resistance EIR of the battery by the constant current voltage discharge method described in steps a to d is to record the voltage E(t ₀ ) at the beginning of discharge and the voltage of the battery at the end of discharge within each discharge period t. E(t) and discharge current I(t), then the equivalent internal resistance EIR(t) of the battery within t time can be calculated by the following formula (6):

(6)

So far, the parameter database of the selected type of _lithium -ion battery including the " _EIR - SoC " spectrum, kT , _Kr , and Kt has been obtained, and the relevant data can be searched according to the battery discharge conditions to calculate the real-time equivalent of the lithium-ion battery. Internal resistance EIR(t).

Finally, the real-time equivalent internal resistance EIR(t) of the lithium-ion battery is calculated according to formula (7):

(7)

In this way, at any time t in the discharge process, the battery internal resistance temperature correction coefficient is determined according to the battery temperature T(t) ; Determine the discharge rate r(t) of the battery according to the battery discharge current I(t), and then determine the correction coefficient of the battery internal resistance discharge rate ;According to the battery's continuous discharge duration, determine the battery internal resistance discharge duration correction factor ; Determine the available capacity C(t) of the battery according to the battery ambient temperature T(t), and then determine the current state of charge SoC(t) of the battery, and then find the corresponding leading internal resistance R _p ( SoC).

For the reference values that are not entered in the database (for example, the temperature correction coefficient corresponding to the ambient temperature of 25°C), the adjacent reference values (temperature correction coefficients corresponding to 20°C and 30°C) can be interpolated to obtain the required reference value. .

Claims (3)

1. a real-time estimation method of lithium ion battery equivalent internal resistance, is characterized in that comprising the following steps: a. Take a certain number of lithium-ion batteries as samples, test the available capacity C of the sample batteries by the constant current discharge method, use the test data as the data basis, and use the formula (1) to fit the available capacity of the battery and the battery temperature T: (1) where y ₀ , A and k are three fitting parameters; b. Read the temperature value T (t) of the battery during the discharge process, and calculate the available capacity C (t) at each moment of the discharge process according to formula (1); at the same time, read the state of charge SoC (t ₀ ) and The current value I (t) of the discharge process is calculated using the charge accumulation method to calculate the state of charge SoC (t) of the battery at the current moment: (2) c. Use the constant current intermittent discharge method to establish the parameter database of the lithium-ion battery, including the pilot internal resistance R _p (SoC) and the correction coefficient , and ; d. The real-time equivalent internal resistance of lithium-ion battery can be obtained by formula (7): (7) e. For the reference values that are not entered in the database, interpolate the adjacent reference values to obtain the required reference values. 2. The real-time estimation method of the equivalent internal resistance of a lithium ion battery according to claim 1, characterized in that: in the step a, the concrete steps of carrying out a constant current discharge experiment to the lithium ion battery of the sample include: a1. Pre-process the lithium-ion battery, set a certain temperature of the incubator, keep the temperature of the battery consistent with the ambient temperature, and perform multiple “fill-empty” cycles for the sample battery; a2. Fully charge the battery to discharge the battery at a certain rate, stop discharging when the voltage is lower than the discharge cut-off voltage, and the total discharge capacity is regarded as the available capacity C of the battery; a3. Before the test, in order to keep the battery temperature consistent with the ambient temperature, let the battery stand for a period of time in the incubator; a4. Set different incubator temperatures, repeat a2, a; a5. Each test is repeated three times, and the average of the available battery capacity obtained from the three tests is taken as the final result of the test. 3. The real-time estimation method of the equivalent internal resistance of a lithium-ion battery according to claim 2, wherein the specific steps of performing a constant current intermittent discharge method on the lithium-ion battery of the sample in the step c include: c1. Set a series of incubator temperatures, discharge the battery at different rates for a certain period of time, and then let the battery stand for 2 hours until the battery voltage change rate is lower than 0.1mV/min, which is regarded as a "discharge-standstill" cycle; c2. Set different incubator temperatures, different discharge rates and different discharge durations, and repeat c1 with the experimental principle of the control variable method to obtain the determined pilot internal resistance R _p (SoC) and correction coefficient , , The required equivalent internal resistance data base of lithium-ion batteries; c3. The method for obtaining the equivalent internal resistance data of each lithium-ion battery in the step c2 is to record the voltage E(t ₀ ) at the beginning of discharge and the voltage E of the battery at the end of discharge in each section of discharge duration t (t) and discharge current I(t), then the equivalent internal resistance (t) of the battery within t time can be calculated by formula (6): (6) c4. The method for determining the pilot internal resistance R _p (SoC) in the step c2 is as follows: the temperature of the selected temperature chamber is 40°C, the discharge rate of the selected lithium-ion battery is 0.3C, the selected discharge time is 10 minutes, and the measured The corresponding EIR of the battery under different states of charge SoC is the leading internal resistance R _p (SoC); c5. Determine the temperature correction coefficient in the step c2 The method is to take the corresponding pilot internal resistance R _p (SoC) under different SoCs as the benchmark, the state of charge SoC of the fixed battery remains unchanged, and the equivalent internal resistance EIR _T of the battery corresponding to different battery temperatures T is taken to define the temperature correction coefficient. : (3) c6. Determine the discharge rate correction coefficient in the step c2 The method is to take the corresponding pilot internal resistance R _p (SoC) under different SoCs as the benchmark, the state of charge SoC of the fixed battery is unchanged, and the equivalent internal resistance EIR _r of the battery corresponding to different battery discharge rates r is taken to define the temperature correction. coefficient : (4) c7. Determine the correction factor for the discharge duration in the step c2 The method is to take the corresponding pilot internal resistance R _p (SoC) under different SoCs as the benchmark, fix the state of charge of the battery SoC unchanged, take the battery equivalent internal resistance EIR _{t corresponding to the different battery discharge time t} , and define the temperature correction coefficient : (5).