A method for identifying the inflection point of capacity decay based on multi-stage constant current and constant voltage charging conditions

Abstract

The present invention provides a method for identifying a capacity decay inflection point based on a multi-stage constant current and constant voltage charging condition, specifically a method for identifying a capacity decay inflection point of a lithium ion battery monomer based on a multi-stage constant current and constant voltage charging condition by using the charging capacity in the constant voltage stage, and the steps are as follows: 1) the charging condition of the lithium ion battery is a multi-stage constant current and then constant voltage condition; 2) the capacity in the constant voltage stage of the first charging process is calculated by using the ampere-hour integral, recorded as Q1; 3) Q1 is multiplied by a coefficient to obtain a threshold value Q lim ; 4) Calculate the constant voltage stage charge capacity Q in subsequent cycles n ; 5) When Q n Greater than the threshold Q lim When the capacity of the lithium-ion battery cell reaches the attenuation inflection point, the present invention can quickly and accurately identify the capacity attenuation inflection point, which is of great significance to the capacity prediction after the battery capacity attenuation inflection point and the manufacture of the battery cell.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery management systems, and in particular relates to a method for identifying a capacity decay inflection point based on a multi-stage constant current and constant voltage charging condition. Background Art

[0002] In order to save energy and reduce emissions, the development of electric vehicles (EVs) has become a clear trend in response to the energy crisis and increasingly prominent environmental issues. Electric vehicle manufacturers and customers are most concerned about mileage and service life, which mainly depends on the energy density and remaining life of the power battery. In recent years, with the increase in the demand for mileage of electric vehicles, the requirements for the energy density of lithium-ion batteries have become more and more urgent. At the same time, the performance stability of high-energy-density batteries tends to deteriorate, leading to accelerated aging and safety issues. Therefore, it is crucial to explore the causes of accelerated capacity decay and identify the capacity decay inflection point of lithium-ion batteries commonly used in electric vehicles. This will help promote the sustainable and stable development of electric vehicles and improve the level of advanced applied energy technology.

[0003] Lithium-ion batteries exhibit a two-stage capacity decay behavior: the capacity initially degrades at a lower rate, and then from a certain starting point, the capacity undergoes accelerated degradation until the end of the battery life. This starting point is the so-called "inflection point". IEEE Standard 485TM-2010 defines the "inflection point" as the transition point between the slow capacity decay stage and the rapid capacity decay stage. Therefore, identifying and predicting the occurrence of the inflection point is a key factor in the cycle life of battery cells. Timely identification and prediction of capacity decay inflection points during the cycle aging process of battery cells is very valuable for batteries and battery manufacturers, who can make corresponding adjustments to battery cells to extend their service life and arrange battery maintenance in a cost-effective manner.

[0004] Under the multi-stage constant current and constant voltage charging conditions, as the lithium-ion battery ages, the available capacity of the battery gradually decreases, while the constant current charging time also gradually decreases, and the constant voltage charging time increases. Our research found that as the battery ages, if the constant voltage charging capacity is based on the first constant voltage charging capacity, the constant voltage charging capacity in the slow aging stage will fluctuate within a very small range. However, after the capacity attenuation inflection point appears, the constant voltage charging capacity will increase rapidly, and the constant voltage charging capacity will gradually increase in the proportion of the capacity in the entire charging process. Summary of the invention

[0005] The object of the present invention is to provide a method for identifying a capacity decay inflection point based on a multi-stage constant current and constant voltage charging condition, which is characterized by comprising the following steps:

[0006] S1: Complete the cycle aging condition of lithium-ion batteries by setting the C1-C2-CV charging strategy;

[0007] S2: Extract the constant voltage charging stage data of the aging cycle, and calculate the constant voltage charging capacity of the first cycle using the ampere-hour integration method, which is recorded as Q1;

[0008] S3: Multiply Q1 by an adjustable parameter k to obtain the constant voltage charging capacity threshold Q lim ;

[0009] S4: Calculate the constant voltage charging capacity Q during subsequent cycles using the ampere-hour integration method n ;

[0010] S5: Compare Q n and Q lim , and make a judgment, when Q n >Q lim， Determine that the monomer capacity attenuation reaches the inflection point, record the current cycle number n, and complete the inflection point identification; when Q n ≤Q lim When the monomer capacity attenuation is determined to have not reached the inflection point, n=n+1 is set, and the process returns to step S4, and steps S4-S5 are repeated until Q n >Q lim .

[0011] Furthermore, in S1, the aging cycle condition of the lithium-ion battery is a multi-stage constant current and constant voltage charging condition, C1 is the first constant current step; C2 is the second constant current step, the current of the first constant current step is greater than the current of the second constant current step, and when the constant current charging reaches the upper limit cut-off voltage, the constant voltage charging starts until the current drops to the lower limit cut-off current; the lithium-ion battery is aged and cycled until the battery capacity decays to 80% of the initial capacity.

[0012] Furthermore, in S2, Q1 is calculated by the following formula:

[0013]

[0014] In the calculation formula of Q1, Q1 is the constant voltage charging capacity during the first charging process, t1 is the start time of constant voltage charging, and t2 is the end time of constant voltage charging.

[0015] Furthermore, in S3, the adjustable parameter k is an empirical value, Q lim It is expressed by the following formula:

[0016] Q lim =Q1*k.

[0017] Further, in S4, Q is calculated by the following formula: n :

[0018]

[0019] Q n In the calculation formula, Q n is the constant voltage charging capacity during the nth charging process, t n is the starting time of constant voltage charging, t n+1 This is the time when constant voltage charging ends.

[0020] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in: the present invention proposes a new capacity decay inflection point identification method: that is, by setting a constant voltage charging capacity threshold, when the constant voltage charging capacity during the aging cycle exceeds the threshold, it is considered that the monomer capacity has reached the decay inflection point. The method is simple and effective, with less calculation and less time consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a flow chart of the method for identifying the capacity decay inflection point based on the multi-stage constant current and constant voltage charging condition in the embodiment of the present invention;

[0022] Figure 2 This is a capacity decay inflection point identification diagram of the batch 1 monomer of the "2018-04-12" batch in the Stanford public data in an embodiment of the present invention;

[0023] Figure 3 This is a capacity decay inflection point identification diagram of batch 2 monomers of the "2018-04-12" batch in the Stanford public data in an embodiment of the present invention;

[0024] Figure 4 This is a capacity decay inflection point identification diagram of batch 3 monomers based on the "2018-04-12" batch in Stanford public data in an embodiment of the present invention. DETAILED DESCRIPTION

[0025] The following is a more detailed description of a method for identifying a capacity decay inflection point based on a multi-stage constant current and constant voltage charging condition of the present invention in conjunction with a schematic diagram, wherein a preferred embodiment of the present invention is shown. It should be understood that those skilled in the art can modify the present invention described herein while still achieving the beneficial effects of the present invention. Therefore, the following description should be understood as being widely known to those skilled in the art and not as a limitation to the present invention.

[0026] like Figure 1 As shown, a method for identifying a capacity decay inflection point based on a multi-stage constant current and constant voltage charging condition includes the following steps:

[0027] Step 1: Aging cycle conditions of lithium-ion batteries Multi-stage constant current and constant voltage charging conditions, namely "C1-C2-CV", where C1 and C2 are the first and second constant current steps respectively, the current in the C1 stage is greater than the current in the C2 stage, and constant voltage charging begins when the upper cut-off voltage is reached until the current drops to the cut-off current. The aging cycle ends when the capacity decays to 80% of the initial capacity.

[0028] Step 2: Extract the constant voltage charging phase data of the aging cycle, and use the ampere-hour integration method to calculate the constant voltage charging capacity of the first cycle, recorded as Q1.

[0029]

[0030] In the calculation formula of Q1, Q1 is the constant voltage charging capacity during the first charging process, t1 is the start time of constant voltage charging, and t2 is the end time of constant voltage charging.

[0031] Step 3: Multiply Q1 by an adjustable parameter k to obtain the constant voltage charging capacity threshold Q lim The adjustable parameter k is an empirical value. Initially, a value is given according to the battery type and experience, and then debugging is performed according to the obtained lithium-ion battery cell inflection point identification results to determine the optimal parameter.

[0032] Q lim =Q1*k, (k is an adjustable parameter)#

[0033] Step 4: Calculate the constant voltage charging capacity Q during subsequent cycles using the ampere-hour integration method n .

[0034]

[0035] In the calculation formula of Qn, Q n is the constant voltage charging capacity during the nth charging process, t n is the starting time of constant voltage charging, t n+1 This is the time when constant voltage charging ends.

[0036] Step 5: Determine whether Qn is greater than the threshold Q lim If it exceeds the threshold, the monomer capacity decay reaches the inflection point, and the number of cycles at this time is recorded; if it does not exceed, the monomer capacity decay has not reached the inflection point, n=n+1, return to S4, and repeat S4-S5 until the monomer capacity decay reaches the inflection point.

[0037] Example

[0038] In order to make the present invention more convincing, we used the data of three batches of 144 lithium ion phosphate (LFP) / graphite batteries (APR18650M1A) disclosed by Stanford for verification. The data was cycled in a forced convection temperature chamber at 30°C. The nominal capacity of the battery is 1.1Ah and the nominal voltage is 3.3V. For fast charging, the format of the strategy is "C1-C2", where C1 and C2 are the first and second constant current steps, respectively. The second current step ends at 80% SOC, after which the battery is charged at 1C CC-CV. The upper and lower cutoff potentials are 3.6V and 2.0V, respectively, and all batteries are discharged at 4C. For both charging and discharging, the cutoff current of the constant voltage step is C / 20.

[0039] Step 1: Collect the battery current, voltage, and charging time data. Use the ampere-hour integration method to obtain the single cell capacity during the aging process.

[0040] Step 2: Extract the constant voltage charging phase data of the aging cycle, and use the ampere-hour integration method to calculate the constant voltage charging capacity of the first cycle, recorded as Q1.

[0041] Step 3: Based on Q1, the optimal parameter k is determined to be 1.1 according to the debugging, and the constant voltage charging capacity threshold is further calculated to be Q lim =Q1*1.1.

[0042] Step 4: Calculate the constant voltage charging capacity of the nth (n>1) cycle using the ampere-hour integration method and determine whether it is greater than the threshold value Q lim , if the constant voltage charging capacity of the nth cycle is less than Q lim , the constant voltage charging capacity of the n+1th cycle is calculated to continue the judgment. If it exceeds the threshold, the monomer capacity decay reaches the inflection point, and the cycle number n at this time is recorded.

[0043] According to the above steps, the verification was carried out in combination with 144 lithium iron phosphate batteries published by Stanford. Figure 2 The capacity decay inflection point of the batch 1 monomer of the "2018-04-12" batch is the 798th cycle. The monomer capacity decays by 4.31% in the 798 cycles before the inflection point; and by 13.06% in the 209 cycles after the inflection point. Figure 3 The capacity decay inflection point of the batch 2 monomer of the "2018-04-12" batch is the 875th cycle. The monomer capacity decays by 5.26% in the 875 cycles before the inflection point; and by 12.59% in the 209 cycles after the inflection point. Figure 4The capacity decay inflection point of the batch 3 monomer of the "2018-04-12" batch is the 869th cycle. The monomer capacity decays by 3.78% in the 869 cycles before the inflection point, and by 13.97% in the 396 cycles after the inflection point. Therefore, the monomer capacity decay inflection point identified by the capacity decay inflection point identification method based on multi-stage constant current and constant voltage charging conditions of the present invention meets the requirements.

[0044] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any technician in the relevant technical field, without departing from the scope of the technical solution of the present invention, makes any form of equivalent replacement or modification to the technical solution and technical content disclosed in the present invention, which does not depart from the content of the technical solution of the present invention and still falls within the protection scope of the present invention.

Claims

1. A method for identifying the capacity attenuation inflection point based on a multi-stage constant current and constant voltage charging condition, characterized in that: The steps include: S1: Complete the cycle aging condition of lithium-ion batteries by setting the C1-C2-CV charging strategy; S2: Extract the constant voltage charging stage data of the aging cycle, and calculate the constant voltage charging capacity of the first cycle using the ampere-hour integration method, which is recorded as Q1; S3: Multiply Q1 by an adjustable parameter k to obtain the constant voltage charging capacity threshold Q lim ; S4: Calculate the constant voltage charging capacity Q during subsequent cycles using the ampere-hour integration method n ; S5: Compare Q n and Q lim , and make a judgment, when Q n >Q lim， Determine that the monomer capacity attenuation reaches the inflection point, record the current cycle number n, and complete the inflection point identification; when Q n ≤Q lim When the monomer capacity attenuation is determined to have not reached the inflection point, n=n+1 is set, and the process returns to step S4, and steps S4-S5 are repeated until Q n >Q lim ; In S1, the aging cycle condition of the lithium-ion battery is a multi-stage constant current and constant voltage charging condition, wherein C1 is the first constant current step; C2 is the second constant current step, the current of the first constant current step is greater than the current of the second constant current step, and when the constant current charging reaches the upper limit cut-off voltage, the constant voltage charging starts until the current drops to the lower limit cut-off current; the lithium-ion battery is aged and cycled until the battery capacity decays to 80% of the initial capacity.

2. The method for identifying the capacity attenuation inflection point based on a multi-stage constant current and constant voltage charging condition according to claim 1, characterized in that: In S2, Calculate using the following formula: ； In the calculation formula, is the constant voltage charging capacity during the first charging process, is the starting time of constant voltage charging, This is the time when constant voltage charging ends.

3. The method for identifying the capacity attenuation inflection point based on multi-stage constant current and constant voltage charging conditions according to claim 1, characterized in that: In S3, the adjustable parameter k is an empirical value, Q lim It is expressed by the following formula: 。 4. The method for identifying the capacity attenuation inflection point based on multi-stage constant current and constant voltage charging conditions according to claim 1, characterized in that: In S4, Q is calculated by the following formula n : ； Q n In the calculation formula, is the constant voltage charging capacity during the nth charging process, is the starting time of constant voltage charging, This is the time when constant voltage charging ends.

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