State estimation and self-correction method and device for lithium iron phosphate battery pack

Abstract

The present invention relates to the technical field of state estimation of lithium batteries, and in particular to a state estimation and self-correction method and device for a lithium iron phosphate battery pack. The method comprises: performing real-time SOC estimation on a lithium iron phosphate battery pack; determining whether the average temperature T of the battery pack is abnormal; during charging, measuring in real time dU / dt of a battery cell having the highest cell voltage, if dU / dt is greater than or equal to i / 10, correcting an SOCR to 98%, and calculating a correction value e; during discharging, measuring in real time dU / dt of a battery cell having the lowest cell voltage; if dU / dt is less than or equal to i / 10, correcting an SOCR to 5%, and calculating the correction value e; correcting an SOH on the basis of the correction value e; and calculating an SOE of the battery pack on the basis of the SOCR. In the method, a correction point is added on the basis of the characteristic that a voltage suddenly changes in the first and final stages of charging and discharging, the cumulative error in an ampere-hour integral method is reduced, and an SOC, an SOH, and an SOE are corrected.

Description

A state estimation self-correction method and device for a lithium iron phosphate battery pack. TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery state estimation, and particularly relates to a state estimation self-correction method and device for a lithium iron phosphate battery pack. BACKGROUND

[0002] Lithium iron phosphate batteries have the advantages of low cost and long cycle life, and are suitable for use as energy storage power batteries in energy storage scenarios. However, as the energy storage equipment is continuously used, the capacity of the battery pack decreases and the safety decreases, and continued use will increase the risk of accidents. Therefore, it is crucial to evaluate the SOC, SOH and SOE of the battery pack of the energy storage system, so as to ensure the safety, reliability and economy of the energy storage system.

[0003] The SOC and SOE estimation methods based on voltage cannot be applied to lithium iron phosphate batteries due to their long plateau. The data-driven machine learning method requires a large amount of experimental data and has poor robustness. The SOC estimation by ampere-hour integration is the simplest, but the influence of current integration error and SOH needs to be considered.

[0004] The current mainstream SOH estimation methods include full charge and full discharge test method, resistance measurement method, impedance analysis method and data-based machine learning algorithm. These methods either require a large number of aging tests or are difficult to apply to complex working conditions in actual use and produce errors.

[0005] Therefore, it is necessary to design a collaborative estimation self-correction method for lithium iron phosphate battery packs according to the actual working conditions of energy storage. SUMMARY

[0006] The present application aims to provide a state estimation self-correction method and device for a lithium iron phosphate battery pack, which solves the problem of the need for a large number of aging tests in the existing lithium battery estimation method and the difficulty in applying to complex working conditions and producing errors.

[0007] To achieve the above-mentioned purpose, in a first aspect, the present application provides a state estimation self-correction method for a lithium iron phosphate battery pack, comprising the following steps:

[0008] S1: using the ampere-hour integration method to estimate the real-time SOC of the lithium iron phosphate battery pack, and the estimated reference remaining capacity is denoted as SOC', and the displayed true value SOC is SOC'; R

[0009] S2: determining whether the average temperature T of the battery pack is abnormal, if T is not in the normal working temperature range of the battery, then go to step S6, if T is in the normal working temperature range of the battery, then determine the charging and discharging condition again, if it is a charging process, then go to step S3, otherwise go to step S4;​

[0010] S3: In the charging process, that is, the current i ≥ 0, the dU / dt of the single battery voltage highest cell is detected in real time, if dU / dt ≥ i / 10 and the SOC' obtained by the ampere-hour integration method is ≤ 98%, the SOC is corrected to 98%, and the correction value e = |SOC'-98| is calculated. R

[0011] S4: In the discharging process, that is, the current i < 0, the dU / dt of the single battery voltage lowest cell is detected in real time, if dU / dt ≤ i / 10 and the SOC' obtained by the ampere-hour integration method is ≥ 5%, the SOC is corrected to 5%, and the correction value e = |SOC'-5| is calculated. R

[0012] S5: The SOH is corrected according to the correction value e.

[0013] S6: The battery pack SOE is calculated according to the SOC R , and the process returns to step S1 after completion.

[0014] The lithium iron phosphate battery pack is composed of multiple series-connected lithium iron phosphate cells of the same specification.

[0015] The battery normal working temperature range is [20, 45] in Celsius.

[0016] The ampere-hour integration method calculation formula is as follows: Q max = SOH R * Q 出厂

[0017] In the formula, SOC' is the SOC estimation value calculated by the ampere-hour integration method, SOC R is the real SOC value displayed by the system, t is the unit sampling interval, i is the current battery current, positive for charging and negative for discharging, in amperes, Q max is the current maximum available capacity of the battery pack, Q 出厂 is the calibrated maximum capacity of the battery pack at the factory, which is gradually lost as the battery is used, and Q max and SOH are constantly decreasing.

[0018] In a second aspect, the application further provides a lithium iron phosphate battery pack estimation self-correction device, comprising a data acquisition module, a processor and a memory, wherein the data acquisition module comprises a voltage acquisition unit, a current acquisition unit, a temperature acquisition unit and a timer.

[0019] ​​The application discloses an estimated self-correction method of a lithium iron phosphate battery pack. R =SOC'; S2: judging whether the average temperature T of the battery pack is abnormal, if T is not in the normal working temperature interval of the battery, going to step S6, if T is in the normal working temperature interval of the battery, judging the charging and discharging condition again, if the charging process, going to step S3, otherwise going to step S4; S3: in the charging process, that is, the current i >= 0, real-time detecting the dU / dt of the highest cell of the single cell voltage, if dU / dt >= i / 10 and the SOC' obtained by the ampere-hour integral method is <= 98%, the SOC R is corrected to 98%, and the correction value e = |SOC'-98| is calculated; S4: in the discharging process, that is, the current i < 0, real-time detecting the dU / dt of the lowest cell of the single cell voltage, if dU / dt <= i / 10 and the SOC' obtained by the ampere-hour integral method is >= 5%, the SOC R is corrected to 5%, and the correction value e = |SOC'-5| is calculated; S5: correcting the SOH according to the correction value e; S6: calculating the SOE of the battery pack according to the SOC R , and returning to step S1 after completion. The method is based on the ampere-hour integral method, the algorithm is simple to realize, the calculation power requirement is low, the SOC correction point is added according to the characteristics of the voltage sudden change of the lithium iron phosphate battery at the beginning and end of the charging and discharging, the cumulative error of the ampere-hour integral method is reduced, the SOH is corrected, the problem that the cycle number is difficult to define in actual use is avoided, the aging test time cost and economic cost are saved, the dU / dt of the single cell is detected in real time according to the actual working condition in the charging and discharging process of the battery pack, and then the SOC and the SOH are corrected, the SOE estimation is associated with the SOC, and the method is simple and easy to operate, solves the problems that a large number of aging tests are needed in the existing lithium battery estimation method, and errors are difficult to be generated in complex working conditions. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only some embodiments of the present application, and for those skilled in the art, other drawings can be obtained without creative labor on the basis of these drawings.

[0021] Fig. 1 is a flow chart of an estimated self-correction method of a lithium iron phosphate battery pack provided by the present application.

[0022] Fig. 2 is an OCV-SOC curve and a slope diagram.

[0023] Figure 3 is a table of test results for different health batteries.

[0024] Figure 4 is a schematic diagram of an estimated self-correcting system for a lithium iron phosphate battery pack provided by the present application. DETAILED DESCRIPTION

[0025] Embodiments of the present application are described in detail below with reference to the accompanying drawings, in which like or similar elements or elements having the same or similar functions are denoted by the same or similar reference numerals throughout the drawings. The embodiments described below by reference to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be understood as limiting the present application.

[0026] Referring to Figures 1 to 4, in a first aspect, the present application provides an estimated self-correcting method for a lithium iron phosphate battery pack, comprising the following steps:

[0027] S1: Real-time SOC estimation of the lithium iron phosphate battery pack using ampere-hour integration method, the estimated reference remaining capacity is denoted as SOC', and the displayed true value SOC is denoted as SOC. R = SOC';

[0028] Specifically, the lithium iron phosphate battery pack should be composed of multiple lithium iron phosphate cells of the same specification in series, and does not contain other types of batteries. For example, BYD, Weilai and other companies use ternary lithium batteries and lithium iron phosphate batteries in series, and estimate the SOC of ternary lithium batteries by voltage value to equivalently represent the SOC of lithium iron phosphate batteries, but due to the different cycle life and temperature characteristics of the two types of batteries, the difference between the cells at the end of the life of the battery pack is large, which may cause greater error. In addition, in order to reduce the difference between the cells in the group, it is recommended that the user use a charging and discharging current less than 0.5C. Smaller current not only improves the available capacity and cycle life of the battery pack, but also reduces the thermal energy consumption of the internal resistance of the battery, thereby reducing the loss difference caused by temperature difference.

[0029] The ampere-hour integration formula is as follows: Q max = SOH R * Q 出厂 (2)

[0030] In the formula, SOC' is the SOC estimate value calculated by ampere-hour integration, SOC is the true value of SOC displayed by the system, t is the unit sampling interval, i is the current of the battery, positive for charging and negative for discharging, unit is ampere (A), Q is the current maximum available capacity of the battery pack, Q is the calibrated maximum capacity of the battery pack at the time of factory shipment, which is continuously consumed and reduced as the battery is used, and Q and SOH are continuously reduced. R max 出厂 max

[0031] ​​​​S2: judging whether the battery pack average temperature T is abnormal, if T is not in the battery normal working temperature interval, going to step S6, if T is in the battery normal working temperature interval, judging the charging and discharging condition again, if the charging process, going to step S3, otherwise going to step S4;

[0032] Specifically, the battery normal working temperature interval is [20, 45], unit: degree Celsius.

[0033] S3: in the charging process, i.e. current i≥0, real-time detecting the dU / dt of the single battery voltage highest cell, if dU / dt≥i / 10 and the SOC' obtained by ampere-hour integration method is ≤98%, the SOC is corrected to 98%, and the correction value e=|SOC'-98| is calculated; R

[0034] S4: in the discharging process, i.e. current i<0, real-time detecting the dU / dt of the single battery voltage lowest cell, if dU / dt≤i / 10 and the SOC' obtained by ampere-hour integration method is ≥5%, the SOC is corrected to 5%, and the correction value e=|SOC'-5| is calculated; R

[0035] Specifically, S3 or S4 real-time detects the dU / dt of the single battery voltage highest (lowest) cell, the specific steps are as follows: taking 10 seconds as the unit time, recording the single battery voltage highest (lowest) cell voltage U n every 1 second, the formula is as follows: dU / dt=U n -U n-10 (3)

[0036] In the formula, U n is the cell voltage recorded this time, U n-10 is the cell voltage recorded 10 seconds ago, unit: mV.

[0037] Due to the series connection of multiple lithium iron phosphate cells, the electric quantity passing through them in unit time is the same, but due to the different temperatures and individual differences of the cells, different degrees of loss will occur during use, which is usually called "barrel effect". In order to protect all cells, the single battery voltage maximum cell is detected during charging to prevent single battery overvoltage; the single battery voltage minimum cell is detected during discharging to prevent single battery under-voltage;

[0038] Fig. 2 shows the OCV-SOC curve and the slope graph. The OCV-SOC curve changes little under different aging states. As can be seen from the figure, the platform voltage of the lithium iron phosphate battery changes little, while there is obvious voltage change in the first and last stages, and the corresponding relationship between the battery OCV-SOC changes little in the whole life cycle, so the voltage change rate can be used as a characteristic value for analysis in the first and last stages. dU / dt is the voltage change rate, which can be rewritten as follows:​​

[0039] In the formula, is the derivative of the terminal voltage with respect to the electric quantity Q, that is, the derivative of the electric quantity with respect to time is also the current i, in amperes, positive for charging and negative for discharging. When the battery is in a charging and discharging state for a long time, at this time the polarization effect of the battery can be ignored, and the change of the terminal voltage is consistent with the change of the OCV. Its SOC represents the state of charge of the battery, that is, one unit of SOC is equivalent to a certain unit of electric quantity, so Under certain conditions, it can be replaced by .

[0040] In this embodiment, different health degrees of batteries are tested to obtain Fig. 3 by 0.4C constant current charging and discharging. The true SOC is determined by the total amount of charging and discharging and the record of the charging and discharging machine, and the test is carried out using the uncorrected error SOH = 100%. When the voltage change rate is 4mV for 10s, the SOC corresponding to the time point is about 5%, and the error is not more than 0.5%, which meets the accuracy requirements of the national standard “GB / T 34131—2023 Battery Management System for Electric Power Energy Storage”.

[0041] S5: correcting SOH according to the correction value e;

[0042] Specifically, the specific formula for correcting SOH according to the correction value e is as follows: SOH R = SOH R -[e] (4)

[0043] In the formula, SOH R refers to the health state of the lithium iron phosphate battery pack, specifically the SOH display value of the current system, and [e] represents rounding off the estimation error. Assuming that the current accuracy in the ampere-hour integral formula meets the standard, the source of the error is the capacity after attenuation, that is, the battery health state SOH. After the battery is attenuated, the total capacity will decrease, and if the SOH of the initial full capacity is still calculated, it will cause the SOC' obtained by the ampere-hour integration to be smaller than the actual SOC R . If the error is large, it means that the SOH has decreased, so the SOH is corrected according to the correction value e. This method can correct the SOH in real time based on the first and last segments of charging and discharging under actual working conditions, and the estimation error is less than 3%.

[0044] S6: calculating the battery pack SOE according to SOC R , and returning to step S1 after completion.

[0045] Specifically, the battery pack SOE is calculated according to SOC R , and the specific calculation formula is as follows:

[0046] Formula (5) is a battery energy definition formula, E n E represents the remaining energy at the current n moment, E0 represents the total energy that the battery can store, and U(SOC) represents a function relationship polynomial of the open circuit voltage and SOC.

[0047] The function relationship polynomial of the open circuit voltage and SOC can be obtained by MATLAB fitting: U(SOC) = a0 + a1SOC 1 +a2SOC 2 +…+a n SOC n (8)

[0048] Wherein, a n is the coefficient of the n order term.

[0049] In a second aspect, the application further provides an estimated self-correction device for a lithium iron phosphate battery pack, comprising a data acquisition module, a processor and a memory, wherein the data acquisition module comprises a voltage acquisition unit, a current acquisition unit, a temperature acquisition unit and a timer. The voltage acquisition unit monitors the single cell voltage of all cells and the total voltage of the battery pack through a voltmeter, the current acquisition unit is used to detect the total current passing through the battery pack, the temperature acquisition unit judges whether the working temperature of the battery pack is abnormal through a temperature sensor, and the timer is used to judge and update the data statistical time and is used for algorithm correction judgment.

[0050] In the embodiment, the memory stores a computer readable program that can be executed by the processor, and the processor executing the computer readable program can realize the steps in the estimated self-correction method for the lithium iron phosphate battery pack provided by the application. The processor can be a microprocessor MCU, a programmable logic device FPGA, etc. The memory can be various types of memories that store information by using electrical energy, such as RAM, ROM, etc.

[0051] The above only discloses a preferred embodiment of the estimated self-correction method and device for the lithium iron phosphate battery pack, and of course cannot limit the scope of the application. Those skilled in the art can understand that all or part of the above-mentioned embodiments can be implemented, and equivalent changes made according to the claims of the application still fall within the scope of the application.

Claims

1. A method of estimating self-correction of a lithium iron phosphate battery pack, characterized by, Comprising the following steps: S1: real-time SOC estimation of lithium iron phosphate battery pack using ampere-hour integration method, the estimated reference remaining capacity is denoted as SOC', and the displayed true value SOC R = SOC'; S2: judging whether the battery pack average temperature T is abnormal, if T is not in the battery normal working temperature interval, then go to step S6, if T is in the battery normal working temperature interval, then judge the charging and discharging condition again, if it is charging process, then go to step S3, otherwise go to step S4; S3: During charging, i.e. current i≥0, real-time detection of dU / dt of the highest cell of the single battery voltage, if dU / dt≥i / 10 and SOC' obtained by ampere-hour integration method is ≤98%, then SOC is corrected to 98%, and the correction value e=|SOC'-98| is calculated. R corrected to 98%, and the correction value e=|SOC'-98| is calculated. S4: During discharging, i.e. current i < 0, the dU / dt of the cell with the lowest voltage is detected in real time, if dU / dt < i / 10 and SOC' obtained by ampere-hour integration is ≥ 5%, then the SOC is corrected to 5%, and the correction value e = |SOC' - 5| is calculated. R corrected to 5%, and the correction value e = |SOC' - 5| is calculated. S5: correcting the SOH according to the correction value e; S6: According to the SOC R The battery pack SOE is calculated, and after completion, the process returns to step S1. 2.The self-correcting method for estimating a lithium iron phosphate battery pack according to claim 1, wherein the lithium iron phosphate battery pack is composed of multiple same-specification lithium iron phosphate battery cells connected in series. 3.The self-correcting method for estimating a lithium iron phosphate battery pack according to claim 1, wherein the battery normal working temperature interval is [20, 45] degrees Celsius. 4.The self-correcting method for estimating a lithium iron phosphate battery pack according to claim 1, wherein the SOH is corrected according to the correction value e. 5.A self-correcting device for estimating a lithium iron phosphate battery pack, applied to the self-correcting method for estimating a lithium iron phosphate battery pack according to any one of claims 1-4, comprising a data acquisition module, a processor and a memory, wherein the data acquisition module comprises a voltage acquisition unit, a current acquisition unit, a temperature acquisition unit and a timer. ​ The ampere-hour integration method calculation formula is shown as follows: Q max = SOH R * Q 出厂 In the formula, SOC' is the estimated value of SOC obtained by ampere-hour integration, and SOC R The system displays the actual SOC value, t is the unit sampling interval, i is the current battery current (positive for charging, negative for discharging), and the unit is amperes. Q max Q represents the current maximum available capacity of the battery pack. 出厂 Q represents the maximum capacity calibrated at the time of manufacture of the battery pack. This capacity decreases as the batteries are used. max The SOH content continues to decrease. ​ ​

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