Method for correcting SOC at moment of stopping charging and discharging of lithium ion electric vehicle

Abstract

The invention relates to the technical field of lithium ion electric vehicles, and discloses a method for realizing SOC correction at the moment of stopping charging and discharging of a lithium ion electric vehicle, and the method comprises the following steps: S1, equivalent circuit model establishment, S2, voltage data research, S3, numerical value determination, and S4, SOC correction. By further researching the standing process of the lithium ion battery (instantaneous ohmic internal resistance affects the voltage and continuous polarization effect enables the voltage to tend to a constant value for a long time) and combining OCV-SOC data on the basis of off-line and on-line parameter identification methods of a lithium ion battery equivalent circuit model, the lithium ion battery can be identified. The lithium ion battery polarization characteristics and equivalent circuit model parameter identification are utilized to realize the effect of correcting the SOC at the moment of stopping charging and discharging of the lithium ion electric vehicle, more original offline test data are not needed, the condition that sufficient standing is needed before the SOC is corrected by an open-circuit voltage method is eliminated, the correction efficiency is improved, and the correction cost is reduced. And the selective correction of any time point from the stop of driving or the end of charging to the BMS sleep period is also realized.

Description

Technical Field

[0001] The invention relates to the technical field of lithium-ion electric vehicles, and in particular to a method for realizing SOC correction at the moment when charging and discharging of a lithium-ion electric vehicle is stopped. Background Art

[0002] The state of charge (SOC) is a quantity that characterizes the percentage of remaining charge in a battery. It is the core state of a battery management system and is crucial for estimating the remaining mileage of electric vehicles, charging and discharging power limits, and overcharge and over-discharge protection. Currently, SOC estimation can be roughly divided into two categories based on engineering applications and laboratory research: one is engineering applications that use single or comprehensive estimation algorithms such as ampere-hour integration, open-circuit voltage correction, and Kalman filtering; the other is a laboratory research method based on electrochemical property modeling or driven by big data.

[0003] The invention patent with authorization announcement number CN108445422A proposes a battery state of charge estimation method based on polarization voltage recovery characteristics, including battery polarization voltage characteristic analysis and SOC estimation based on polarization voltage recovery characteristics; the first part: based on the battery static voltage curve under different battery SOCs, the battery residual polarization voltage under different static times is extracted, and the maximum deviation of the battery voltage allowed within the SOC error of 3% is obtained by derivation of the OCV-SOC curve. On the premise of ensuring that the SOC error is 3%, the minimum static time required to eliminate the polarization voltage under different SOCs is obtained; the second part: based on the minimum static time of the battery under different SOCs of batteries in the same batch and its aging law over time, the response surface model is used as an approximate model, and an accurate approximation function relationship is obtained in a local range through a small number of data points, so as to obtain the minimum static time to ensure that the SOC error is within 3% during the entire life cycle of the battery, and the corresponding battery SOC estimation is performed according to whether the static time reaches the minimum static time.

[0004] The above methods start from the electrochemical reaction characteristics of the battery, focus on studying the voltage change process after the battery stops charging and discharging, and propose improvements to the open circuit voltage correction method. However, the improved method does not change the essence of the strict condition that the open circuit voltage correction requires a static time. The actual situation of the vehicle is often that the key is turned off immediately after the car is parked, and the BMS goes into sleep mode in a few seconds. The sufficient static time during the sleep period cannot trigger the open circuit voltage correction, or the vehicle is parked for a few minutes and then drives again, or the vehicle drives within a few minutes after charging is completed. Under these working conditions, the short static time of the automotive lithium-ion battery cannot trigger the SOC correction. Therefore, a method for realizing the instantaneous correction of the SOC of lithium-ion electric vehicles when charging and discharging is stopped is proposed to solve the above problems. Summary of the invention

[0005] In order to solve the above-mentioned problems, the present invention provides a method for correcting the SOC of a lithium-ion electric vehicle when charging and discharging are stopped.

[0006] The present invention provides a method for realizing instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging stops, which adopts the following technical solution:

[0007] A method for realizing SOC correction at the moment when a lithium-ion electric vehicle stops charging and discharging, comprising the following steps:

[0008] S1. Establishment of equivalent circuit model

[0009] A first-order RC equivalent circuit model of lithium-ion batteries is established, and the zero-input response characteristics of polarization voltage and polarization resistance are used to characterize the slow voltage change process after the end of charging and discharging.

[0010] S2. Voltage data research

[0011] Study the voltage data of lithium-ion batteries at the end of charging and discharging, and further determine the exponential function expression of zero input response;

[0012] S3. Determination of numerical value

[0013] Based on the first-order RC equivalent circuit model established in step S1, the value of each data is determined by using an online identification method or an offline identification method;

[0014] S4. Correction of SOC

[0015] The solution is based on the general definition of OCV in the BMS industry to further correct the current SOC and ensure that its correction accuracy is equivalent to that of the traditional open circuit voltage method that requires a static state of more than 1 hour, but without the need for a static state waiting time.

[0016] Preferably, when the first-order RC equivalent circuit model of the lithium-ion battery in step S1 is constructed, Uoc represents the actual open circuit voltage of the battery, Ut represents the real-time voltage between the positive and negative electrodes of the battery, R0 represents the ohmic internal resistance that affects the instantaneous voltage rise and fall of the battery, and R1 and C1 represent the polarization resistance and polarization capacitance that affect the battery after charging and discharging stops.

[0017] Preferably, in step S2, the voltage data of the lithium-ion battery after charging and discharging are studied to determine the exponential function expression of the zero input response: U1 = U0*e -(t / (R1C1)) .

[0018] Preferably, U1 in the exponential function expression is the real-time voltage across the R1C1 ring, U0 is the voltage value across the R1C1 ring at the moment the battery stops charging and discharging, e is the natural logarithm, R1 is the polarization resistance, C1 is the polarization capacitance, and t is the time variable.

[0019] Preferably, in step S3, the values ​​of R1, C1 and U0 in the first-order RC equivalent circuit model are determined using an online identification method or an offline identification method.

[0020] Preferably, the offline identification method is relatively simple, firstly selects a limited number of SOC points for pulse charge and discharge tests, and uses the test data to determine the R1, C1 and U0 parameters corresponding to the selected SOC points. When applied in actual vehicles, based on the offline parameter values ​​of the limited SOC points, the approximate values ​​of R1, C1 and U0 in the current state are confirmed by interpolation and other methods.

[0021] Preferably, in step S4, according to the common definition of OCV in the BMS industry, U1 is obtained at t=1h. (t＝1h) It can be considered as OCV. When t = 3s (or any value between 0 and 3600s), U1 can be solved (t＝3s) .

[0022] Preferably, let dU = U1 (t＝1h)-U1(t＝3s) , then when charging and discharging stops for 3 seconds, the value of the currently measured battery terminal voltage Ut plus or minus dU (minus dU when stopping charging, plus dU when stopping discharging) can be used to look up the OCV-SOC table and correct the current SOC.

[0023] In summary, the present invention includes the following beneficial technical effects:

[0024] 1. A method for realizing the instantaneous correction of SOC of lithium-ion electric vehicles when charging and discharging stops. By further studying the static process of lithium-ion batteries (the instantaneous ohmic internal resistance affects the voltage and the continuous polarization effect makes the voltage tend to a constant value for a long time), and based on the offline and online parameter identification methods of the lithium-ion battery equivalent circuit model, combined with OCV-SOC data, the polarization characteristics of lithium-ion batteries and the parameter identification of the equivalent circuit model are used to realize the effect of correcting the SOC of lithium-ion electric vehicles when charging and discharging stops, without the need for more original offline test data, eliminating the condition that the open circuit voltage method needs to be fully static before correcting the SOC, thereby improving the correction efficiency.

[0025] 2. A method for correcting the SOC of a lithium-ion electric vehicle when charging and discharging stops. The design can realize selective correction at any time point from the end of driving or charging to the BMS sleep period, which is more in line with the actual vehicle application scenario, and can greatly increase the SOC correction opportunities in the actual vehicle scenario, and effectively avoid the large SOC accumulation error caused by long-term non-triggered correction. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the equivalent circuit model diagram of a lithium-ion battery;

[0027] Figure 2The current and voltage diagrams before and after the battery stops charging;

[0028] Figure 3 It is the current and voltage diagram before and after the battery stops discharging;

[0029] Figure 4 It is a zero-input response data diagram that represents the voltage U1 across the polarization capacitor after the battery stops external charging and discharging;

[0030] Figure 5 It is a flow chart of the technical solution. DETAILED DESCRIPTION

[0031] The following is combined with Figure 1-5 The present invention is described in further detail.

[0032] The present invention discloses a method for realizing instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging are stopped. Figure 1-5 , a method for realizing the instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging stops, comprising the following steps:

[0033] S1. Establishment of equivalent circuit model

[0034] Establish a first-order RC equivalent circuit model of lithium-ion batteries, and use the zero-input response characteristics of polarization voltage and polarization resistance to characterize the slow voltage change process after the end of charging and discharging. When building it specifically, Uoc represents the actual open circuit voltage of the battery, Ut represents the real-time voltage between the positive and negative electrodes of the battery, R0 represents the ohmic internal resistance that affects the instantaneous voltage rise and fall of the battery, and R1 and C1 represent the polarization resistance and polarization capacitance that affect the battery after charging and discharging stops;

[0035] S2. Voltage data research

[0036] Study the voltage data of lithium-ion batteries at the end of charging and discharging, and further determine the exponential function expression of zero input response: U1=U0*e -(t / (R1C1)) In the exponential function expression, U1 is the real-time voltage across the R1C1 loop, U0 is the voltage across the R1C1 loop at the moment when the battery stops charging and discharging, e is the natural logarithm, R1 is the polarization resistance, C1 is the polarization capacitance, and t is the time variable;

[0037] S3. Determination of numerical value

[0038] Based on the first-order RC equivalent circuit model established in step S1, the values ​​of R1, C1 and U0 in the first-order RC equivalent circuit model are determined by using an online identification method or an offline identification method. In the offline identification method, a limited number of SOC points are first selected for pulse charge and discharge tests, and the test data are used to determine the R1, C1 and U0 parameters corresponding to the selected SOC points. When used in a real vehicle, the approximate values ​​of R1, C1 and U0 in the current state are confirmed by interpolation or other methods based on the offline parameter values ​​of the limited SOC points;

[0039] S4. Correction of SOC

[0040] According to the general definition of OCV in the BMS industry, we can find U1 at t = 1h. (t＝1h) It can be considered as OCV. When t = 3s (or any value between 0 and 3600s), U1 can be solved (t＝3s) , then let dU=U1 (t＝1h)-U1(t＝3s) , when charging and discharging stops for 3 seconds, the value of the currently measured battery terminal voltage Ut plus or minus dU (minus dU when charging stops, plus dU when discharging stops) can be used to look up the OCV-SOC table and correct the current SOC. The correction accuracy is equivalent to the traditional open circuit voltage correction accuracy that requires standing for more than 1 hour, but there is no need to wait for the standing time;

[0041] When studying the terminal voltage Ut data of lithium-ion batteries after the end of charging and discharging, based on the analysis of the lithium-ion equivalent circuit model, its instantaneous voltage jump is caused by the ohmic internal resistance R0, and its slow change stage is caused by the polarization loop composed of R1 and C1. The slow change process is a quasi-exponential function change trend. In the circuit system analysis, the process of this RC loop circuit after there is no external input is called zero input response. The zero input response part affected by U1 is extracted, and then the data graph is obtained;

[0042] In the offline method, the approximate values ​​of R1, C1 and U0 in the current state are confirmed by interpolation and other methods. The approximation here will inevitably introduce certain parameter errors. The online parameter identification solves the true state of the parameters in real time. In theory, the parameter values ​​are more accurate. The following will take the online parameter identification as an example to obtain R1, C1 and U0. The online parameter identification is implemented using the recursive least squares algorithm. The process is as follows:

[0043] (1) Establish the state space equation of the lithium ion equivalent circuit model:

[0044] (2) Perform Laplace transform on equation (1) and convert it into an s-domain expression:

[0045] (3) Establish the bilinear transformation formula: z is the corresponding operator on the discrete z domain, and the bilinear transformation realizes the conversion between the continuous-time system and the discrete-time system.

[0046] (4) Using equation (3), we can map equation (2) from the s domain to the z domain, and obtain the transfer function of the system in the z domain. We can then simplify it and obtain

[0047] (5)T is the sampling period; θ 1,k ,θ 2,k ,θ 3, k is a simplified representation of the moment coefficient, specifically

[0048] The expression is:

[0049]

[0050] The system conforms to the basic form of the least squares method, and the parameter vector of the least squares is θ(k)=[θ 1,k ,θ 2,k ,θ 3,k ] T , the data vector is The parameter vector θ(k) can be identified in real time with the help of the least squares recursive formula. After θ(k) is identified, the model parameters of the equivalent circuit can be inversely calculated from equation (5):

[0051] U 0 =R 1 *I 0 (I 0 Stop for battery

[0052] The current at the moment before the charge and discharge is stopped);

[0053] The formula in (5) can be used to obtain R1, C1 and U0 at any time, and then import them into step S4 to correct the current SOC. The correction accuracy is comparable to that of the traditional open-circuit voltage method that requires a static state of more than 1 hour, but it does not require a static state, which is more in line with the actual vehicle application scenario. It can greatly increase the chance of SOC correction in the actual vehicle scenario and effectively avoid the large SOC accumulation error caused by long-term non-triggered correction.

[0054] The above are all preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention. Therefore, any equivalent changes made based on the structure, shape, and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for realizing the instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging stops, characterized in that: The following steps are involved: S1. Establishment of equivalent circuit model A first-order RC equivalent circuit model of lithium-ion batteries is established, and the zero-input response characteristics of polarization voltage and polarization resistance are used to characterize the slow voltage change process after the end of charging and discharging. S2. Voltage data research Study the voltage data of lithium-ion batteries at the end of charging and discharging, and further determine the exponential function expression of zero input response; S3. Determination of numerical value Based on the first-order RC equivalent circuit model established in step S1, the value of each data is determined by using an online identification method or an offline identification method; S4. Correction of SOC The solution is based on the general definition of OCV in the BMS industry to further correct the current SOC and ensure that its correction accuracy is equivalent to that of the traditional open circuit voltage method that requires a static state of more than 1 hour, but without the need for a static state waiting time.

2. A method for realizing instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging stops according to claim 1, characterized in that: When the first-order RC equivalent circuit model of the lithium-ion battery in step S1 is constructed, Uoc represents the actual open circuit voltage of the battery, Ut represents the real-time voltage between the positive and negative electrodes of the battery, R0 represents the ohmic internal resistance that affects the instantaneous voltage rise and fall of the battery, and R1 and C1 represent the polarization resistance and polarization capacitance that affect the battery after charging and discharging stops.

3. A method for realizing instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging stops according to claim 1, characterized in that: In step S2, the voltage data of the lithium-ion battery after charging and discharging are studied to determine the exponential function expression of the zero input response: U1 = U0*e -(t / (R1C1)) .

4. A method for realizing instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging stops according to claim 3, characterized in that: In the exponential function expression, U1 is the real-time voltage across the R1C1 loop, U0 is the voltage across the R1C1 loop at the moment when the battery stops charging and discharging, e is the natural logarithm, R1 is the polarization resistance, C1 is the polarization capacitance, and t is the time variable.

5. A method for realizing instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging stops according to claim 1, characterized in that: In step S3, the values ​​of R1, C1 and U0 in the first-order RC equivalent circuit model are determined using an online identification method or an offline identification method.

6. A method for realizing instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging stops according to claim 5, characterized in that: The offline identification method is relatively simple, firstly selects a limited number of SOC points for pulse charge and discharge tests, and uses the test data to determine the R1, C1 and U0 parameters corresponding to the selected SOC points. When applied in actual vehicles, based on the offline parameter values ​​of the limited SOC points, the approximate values ​​of R1, C1 and U0 in the current state are confirmed by interpolation and other methods.

7. A method for realizing instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging stops according to claim 1, characterized in that: In step S4, according to the common definition of OCV in the BMS industry, U1 is obtained at t = 1h. (t＝1h) It can be considered as OCV. When t = 3s (or any value between 0 and 3600s), U1 can be solved (t＝3s) .

8. A method for realizing instantaneous correction of SOC of a lithium-ion electric vehicle when charging and discharging stops according to claim 7, characterized in that: Let dU=U1 (t＝1h)-U1(t＝3s) , then when charging and discharging stops for 3 seconds, the value of the currently measured battery terminal voltage Ut plus or minus dU (minus dU when stopping charging, plus dU when stopping discharging) can be used to look up the OCV-SOC table and correct the current SOC.

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