SOC adaptive correction and estimation method for lithium iron phosphate cell

Abstract

An SOC adaptive correction and estimation method for a lithium iron phosphate cell, which method relates to the field of lithium batteries. The specific implementation solution involves: establishing an offline open-circuit voltage (OCV) curve, and obtaining state parameters of a lithium iron phosphate cell by means of an HPPC test; calculating an average electromotive force VEMF of a battery model on the basis of a third-order RC model and the state parameters of the lithium iron phosphate cell; and acquiring a voltage deviation dv on the basis of the average electromotive force VEMF of the battery model, and a standard electromotive force VEMF_lut of the battery model that is obtained by using a current SOC to perform table lookup on an average EMF curve, setting an adaptive correction starting threshold dvthreshold, and determining whether SOC correction needs to be performed. On the basis of a method combining a third-order equivalent circuit model and ampere-hour integration, real-time high-precision estimation can be performed for the SOC of a battery pack, SOC adaptive correction is performed under a relatively large error, and an SOC correction threshold and an SOC correction speed are adaptively adjusted on the basis of different working conditions and battery states.

Description

A SOC adaptive correction estimation method for lithium iron phosphate battery cells Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a method for adaptively correcting and estimating the SOC of a lithium iron phosphate battery cell. Background Art

[0002] The state of charge (SOC) of a power battery reflects the remaining charge and is a key parameter for measuring the battery's discharge capacity, cell balancing, and charge / discharge cutoff conditions. Currently, the current battery SOC is calculated using the ampere-hour integration method. This value is then substituted into the battery's second-order RC equivalent model to calculate the current battery terminal voltage (Uestimated). This difference is then calculated from the actual sampled terminal voltage (UReal). Existing solutions do not consider the impact of hysteresis voltage on SOC calculation and only use sensor measurement error as a fixed threshold to determine whether to enable SOC correction. This can easily lead to frequent and erroneous corrections. Summary of the Invention

[0003] Based on this, the present invention provides a method for adaptively correcting and estimating the SOC of a lithium iron phosphate battery cell in order to solve the problem that the prior art easily causes frequent corrections and erroneous corrections.

[0004] The present invention provides a method for adaptively correcting and estimating the SOC of a lithium iron phosphate battery cell, comprising:

[0005] Establish an offline open circuit voltage (OCV) curve and obtain the status parameters of lithium iron phosphate cells through HPPC testing;

[0006] Calculation of the average electromotive force of the battery model based on the third-order RC model and the state parameters of the lithium iron phosphate battery cell ;

[0007] The average electromotive force of the battery model The standard electromotive force of the battery model obtained by looking up the average EMF curve at the current SOC Get the voltage deviation dv and set the adaptive correction start threshold , and determine whether SOC correction is needed.

[0008] The open circuit voltage OCV curve includes the charging open circuit voltage Curve and discharge open circuit voltage curve.

[0009] The state parameters of the lithium iron phosphate battery include the third-order polarization resistance R p1 、 R p2 、 R p3 , third-order polarized capacitors C1, C2, C3 and DC resistance R0.

[0010] The average electromotive force of the battery model The calculation includes,

[0011] Calculate the average electromotive force of the battery model based on the third-order equivalent circuit model , specifically including:

[0012]

[0013] in, Indicates the single-unit measurement voltage. represents the voltage drop corresponding to the DC impedance, 、 、 represents the third-order polarization voltage, Indicates the hysteresis voltage.

[0014] Third-order polarization voltage 、 、 The calculation includes,

[0015] = * + I* *(1 - )

[0016] Where dt represents the sampling time, , I represents the current, and the subscript j takes values ​​1-3, representing three-order different polarization processes respectively.

[0017] Hysteresis voltage The calculation includes,

[0018] = * + sign(I)* (t)*(1 - )

[0019] in, = - ) / 2, Indicates charging OCV, Indicates discharge OCV, represents the hysteresis time constant;

[0020]

[0021] in, represents the variable coefficient, Indicates the change in the cell SOC caused by the current per unit time.

[0022] The voltage deviation The calculation includes,

[0023]

[0024] in, represents the average electromotive force of the battery model, Indicates the standard electromotive force of the battery model obtained by looking up the average EMF curve at the current SOC.

[0025] The judgment on whether SOC correction is needed includes:

[0026] Setting the voltage deviation threshold , if and only if the absolute value of dv is greater than , only then will SOC correction be performed, otherwise no correction will be performed;

[0027] set up To correct the voltage, correct the voltage The calculation includes,

[0028]

[0029] .

[0030] Based on the actual battery capacity and corrected voltage Deriving the current correction value ,

[0031] =

[0032] Among them, Q represents the actual capacity of the battery, Kp represents the variable correction coefficient, and I represents the current current.

[0033] Adaptive correction start threshold The value is affected by temperature and current , a two-dimensional table lookup can be used to query the function of temperature T and related current ;

[0034]

[0035]

[0036] in,

[0037] Defined as the current With the reference voltage drop current The ratio of Indicates the maximum polarization voltage drop.

[0038] Beneficial effects: Based on a method combining a third-order equivalent circuit model with ampere-hour integration, the SOC of the battery pack can be estimated in real time with high precision, and the SOC can be adaptively corrected under large errors. The SOC correction threshold and correction speed are adaptively adjusted according to different operating conditions and battery status. The SOC adaptive correction estimation method provided in this application effectively adapts to the platform area characteristics of lithium iron phosphate batteries, takes into account hysteresis and polarization effects, improves the SOC estimation accuracy of lithium iron phosphate batteries, and improves the reliability and durability of the algorithm.

[0039] It should be understood that the content described in this section is not intended to identify the key or important features of the embodiments of the present invention, nor is it intended to limit the scope of the present invention. Other features of the present invention will become readily understood through the following description.

[0040] BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The accompanying drawings are provided for a better understanding of the present invention and do not constitute a limitation of the present invention.

[0042] FIG1 is a schematic diagram of overall feedback provided according to the present invention;

[0043] FIG2 is a flow chart provided according to the present invention;

[0044] FIG3 is a schematic diagram of a third-order RC model provided according to the present invention;

[0045] FIG4 is a SOC line graph without considering the hysteresis voltage according to the present invention;

[0046] FIG5-1 is a schematic diagram of the current operating condition provided by the present invention;

[0047] FIG5-2 is a line graph showing changes in actual SOC and expected SOC according to the present invention;

[0048] FIG5-3 is a schematic diagram of the error between the actual SOC and the expected SOC provided by the present invention. DETAILED DESCRIPTION

[0049] The following description of exemplary embodiments of the present invention is made in conjunction with the accompanying drawings, in which various details of the embodiments of the present invention are included to facilitate understanding. These details should be considered as merely exemplary. Therefore, those skilled in the art will recognize that various changes and modifications may be made to the embodiments described herein without departing from the scope and spirit of the present invention. Similarly, for the sake of clarity and conciseness, descriptions of well-known functions and structures are omitted in the following description.

[0050] The present invention provides a method for adaptively correcting and estimating the SOC of a lithium iron phosphate battery cell, the feedback schematic diagram of which is shown in FIG1 and the flow chart of which is shown in FIG2 , including:

[0051] S1: Establish an offline open circuit voltage (OCV) curve and obtain the state parameters of the lithium iron phosphate battery cell through the HPPC (hybrid power pulse capability characteristic) test.

[0052] The open circuit voltage OCV curve includes the charging open circuit voltage Curve and discharge open circuit voltage curve.

[0053] The state parameters of lithium iron phosphate batteries include the third-order polarization resistance R p1 、 R p2 、 R p3 , third-order polarized capacitors C1, C2, C3 and DC resistance R0.

[0054] S2: Calculate the average electromotive force of the battery model based on the third-order RC model (as shown in Figure 3) and the state parameters of the lithium iron phosphate battery cell It should be noted that:

[0055] Average electromotive force of battery model The calculation includes,

[0056] Calculate the average electromotive force of the battery model based on the third-order equivalent circuit model , specifically including:

[0057]

[0058] in, Indicates the single-unit measurement voltage. represents the voltage drop corresponding to the DC impedance, 、 、 represents the third-order polarization voltage, Indicates the hysteresis voltage.

[0059] Third-order polarization voltage 、 、 The calculation includes,

[0060] = * + I* *(1 - )

[0061] Where dt represents the sampling time, , I represents the current, and the subscript j takes values ​​1-3, representing three-order different polarization processes respectively.

[0062] There is voltage hysteresis in the discharge characteristics of lithium iron phosphate. At the same SOC, due to the different directions of charge and discharge currents, a maximum deviation of about 40mV will occur. As shown in Figure 4, if the hysteresis voltage is not considered and the SOC is calculated using the discharge OCV and charge OCV respectively, the SOC will have an error of about 40% in the voltage plateau area (about 3300mV). Therefore, it is necessary to introduce a hysteresis voltage. .

[0063] Hysteresis voltage The calculation includes,

[0064] = * + sign(I)* (t)*(1 - )

[0065] in, = - ) / 2, Indicates charging OCV, Indicates discharge OCV, represents the hysteresis time constant;

[0066]

[0067] in, represents the variable coefficient, Indicates the change in the cell SOC caused by the current per unit time.

[0068] S3: Average electromotive force of the battery model The standard electromotive force of the battery model obtained by looking up the average EMF curve at the current SOC Get the voltage deviation dv and set the adaptive correction start threshold , and determine whether SOC correction is needed. It should be noted that:

[0069] Voltage deviation Used to correct the current SOC calculation.

[0070] Voltage deviation The calculation includes,

[0071]

[0072] in, represents the average electromotive force of the battery model, Indicates the standard electromotive force of the battery model obtained by looking up the average EMF curve at the current SOC.

[0073] The judgment on whether SOC correction is needed includes:

[0074] Setting the voltage deviation threshold , if and only if the absolute value of dv is greater than , only then will SOC correction be performed, otherwise no correction will be performed;

[0075] set up To correct the voltage, correct the voltage The calculation includes,

[0076]

[0077] .

[0078] Based on the actual battery capacity and corrected voltage Deriving the current correction value ,

[0079] =

[0080] Among them, Q represents the actual capacity of the battery, Kp represents the variable correction coefficient, which is determined by the model calculation error, temperature, and lithium iron phosphate discharge characteristics, and I represents the current.

[0081] Adaptive correction start threshold The value is affected by temperature and current , a two-dimensional table lookup can be used to query the function of temperature T and related current ;

[0082]

[0083]

[0084] in, Defined as the current With the reference voltage drop current The ratio of Indicates the maximum polarization voltage drop, which is determined by the battery operating conditions and battery polarization characteristics. The two-dimensional lookup table form is shown in Table 1.

[0085] Table 1: The specific two-dimensional lookup table form of .

[0086]

[0087] in, Indicates that when the temperature is ,and exist Time ;

[0088] Indicates that when the temperature is ,and exist Time ;

[0089] Indicates that when the temperature is ,and exist Time ;

[0090] Indicates that when the temperature is ,and exist Time ;

[0091] The design principle is negatively correlated with temperature. Positive correlation, the range should be selected as 4-10 times the voltage sensor measurement noise. If the current temperature is and between, or exist and Between, then The table is looked up using the linear interpolation method.

[0092] Follow the above steps to build a lithium iron phosphate battery cell model. Using a cell capacity Q of 37AH, set the variable correction coefficient Kp to a value range of 0~5e-3 based on customer needs, battery operating conditions, and lithium iron phosphate cell characteristics. .

[0093] Set the SOC correction start threshold It is a two-dimensional table lookup method, index one is temperature T, and the table lookup interval is (-10, 10); index two is , the table lookup interval is (0.5,1), and the two-dimensional table is:

[0094] 0.51-104080102030

[0095] As shown in Figures 5-1 to 5-3, Figure 5-1 illustrates the current operating condition, Figure 5-2 illustrates the variation between the actual SOC and the desired SOC, and Figure 5-3 illustrates the error between the actual SOC and the desired SOC. Under the current operating condition shown in Figure 5-1, with an initial SOC deviation of approximately 20% set at the beginning of the experiment (as shown in Figure 5-2), the inventive method gradually corrected the actual SOC to the desired SOC during battery operation. After reaching a steady state, the error between the actual SOC and the desired SOC was less than 3% (as shown in Figure 5-3), demonstrating the effectiveness of the inventive method.

[0096] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed by the present invention shall be covered by the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A method for adaptively correcting and estimating the SOC of a lithium iron phosphate battery cell, characterized in that: include: Establish an offline open circuit voltage OCV curve and obtain the status parameters of the lithium iron phosphate battery cell through HPPC test; Calculation of average electromotive force of battery model based on third-order RC model and state parameters of lithium iron phosphate battery cells ； The average electromotive force of the battery model The standard electromotive force of the battery model obtained by looking up the average EMF curve with the current SOC Get the voltage deviation dv and set the adaptive correction start threshold , and determine whether SOC correction is needed.

2. The SOC adaptive correction estimation method of a lithium iron phosphate battery cell according to claim 1, characterized in that: The open circuit voltage OCV curve includes a charging open circuit voltage Curve and discharge open circuit voltage curve.

3. The SOC adaptive correction estimation method of a lithium iron phosphate battery cell according to claim 1 or 2, characterized in that: The state parameters of the lithium iron phosphate battery cell include the third-order polarization resistance R p1 , R p2 , R p3 , third-order polarized capacitors C1, C2, C3 and DC resistance R0.

4. The SOC adaptive correction estimation method of a lithium iron phosphate battery cell according to claim 3 is characterized in that: The average electromotive force of the battery model The calculation includes, Calculate the average electromotive force of the battery model based on the third-order equivalent circuit model , specifically including: in, Indicates the single-cell measurement voltage. represents the voltage drop corresponding to the DC impedance, 、 、 represents the third-order polarization voltage, Indicates the hysteresis voltage.

5. The method for adaptively correcting and estimating SOC of a lithium iron phosphate battery cell according to claim 4, characterized in that: Third-order polarization voltage , , The calculation includes, = * + I* *(1 - ) where dt represents the sampling time, , I represents the current, and the subscript j takes values ​​of 1-3, representing three-order different polarization processes respectively.

6. The method for adaptively correcting and estimating the SOC of a lithium iron phosphate battery cell according to claim 5, characterized in that: Hysteresis voltage The calculation includes, = * + sign(I)* (t)*(1 - ) in, = - ) / 2， Indicates charging OCV, Indicates discharge OCV, represents the hysteresis time constant; in, represents the variable coefficient, Indicates the change in the cell SOC caused by the current per unit time.

7. The method for adaptively correcting and estimating SOC of a lithium iron phosphate battery cell according to claim 6, characterized in that: The voltage deviation The calculation includes, in, represents the average electromotive force of the battery model, It indicates the standard electromotive force of the battery model obtained by looking up the average EMF curve at the current SOC.

8. The method for adaptively correcting and estimating SOC of a lithium iron phosphate battery cell according to claim 7, characterized in that: The judgment on whether SOC correction is needed includes: Setting the voltage deviation threshold , if and only if the absolute value of dv is greater than , only then SOC correction is performed, otherwise no correction is performed; set up To correct the voltage, correct the voltage The calculation includes, ， 。 9. The method for adaptively correcting and estimating SOC of a lithium iron phosphate battery cell according to claim 8, characterized in that: Based on the actual battery capacity and corrected voltage Derive the current correction value , = Among them, Q represents the actual capacity of the battery, Kp represents the variable correction coefficient, and I represents the current current.

10. The method for adaptively correcting and estimating SOC of a lithium iron phosphate battery cell according to any one of claims 5 to 9, characterized in that: Adaptive correction start threshold The value is affected by temperature and current , the function of temperature T and related current can be queried by two-dimensional table lookup ; ， in, Defined as the current With reference voltage drop current The ratio of Represents the maximum polarization voltage drop.

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