CN114951042B - Screening method for improving consistency of echelon batteries - Google Patents
Screening method for improving consistency of echelon batteries Download PDFInfo
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B07—SEPARATING SOLIDS FROM SOLIDS; SORTING
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B07—SEPARATING SOLIDS FROM SOLIDS; SORTING
- B07C—POSTAL SORTING; SORTING INDIVIDUAL ARTICLES, OR BULK MATERIAL FIT TO BE SORTED PIECE-MEAL, e.g. BY PICKING
- B07C5/00—Sorting according to a characteristic or feature of the articles or material being sorted, e.g. by control effected by devices which detect or measure such characteristic or feature; Sorting by manually actuated devices, e.g. switches
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Abstract
The invention relates to a screening method for improving consistency of gradient batteries, which comprises primary screening, primary grouping, secondary grouping, tertiary grouping and final grouping, wherein the primary screening is performed according to appearance, the primary grouping is performed according to a self-discharge K value at high temperature, the secondary grouping is performed according to discharge energy and a charging constant current ratio, the tertiary grouping is performed according to a direct current internal resistance DCIR and a voltage value difference value in a full-charge state, and the final grouping is performed according to a direct current internal resistance DCIR and a voltage value difference value in a 10% charge state. According to the invention, DCIR and self-discharge K values are participated in the matching group through pulse charging, three-time capacity division, different charge states, high-temperature aging and four-time grouping, so that the influence of internal resistance on the gradient battery is reduced, the charge and discharge efficiency under different charge states is improved, the influence of voltage difference expansion under the influence of the increase of the service time of the gradient battery and different multiplying factors is further reduced, the consistency is ensured, and the service life is prolonged.
Description
Technical Field
The invention relates to a screening method for improving consistency of echelon batteries, and belongs to the technical field of battery matching.
Background
The current common matching method is to match or combine the parameters of the static open-circuit voltage, the static discharge capacity and the static alternating current internal resistance, and for a new battery system, the performance and the energy of the battery are more redundant in terms of the applied working condition, so that the new battery is screened and matched at present, and the current common matching method is mainly referred.
However, for the battery which is returned after the service life of the electric automobile is over, the performance difference between the batteries is large due to the overlong service time, the aging degree of the batteries is different, the failure reason is complex, and the traditional screening method for the new battery cannot accurately reflect the aging characteristic and the subsequent service life of the battery. Meanwhile, the influence of various factors on the battery in the use process is considered, and the performance of the battery is reduced continuously along with the use time, such as the single voltage, the internal resistance and the like of the battery, so that the energy of the battery is reduced. The self-discharge, the charge efficiency and the discharge energy of each gradient battery are different from each other, the consistency of the batteries becomes worse, and the service life and even the safety of the batteries are greatly influenced.
Therefore, screening methods suitable for improving uniformity in echelon use must be studied for characteristics of retired batteries.
Disclosure of Invention
In order to solve the technical problems, the invention provides a screening method for improving the consistency of echelon batteries, which comprises the following specific technical scheme:
A screening method for improving the consistency of echelon batteries comprises the following steps:
s1: primary screening: screening according to the requirements of the same manufacturer, the same material system, the same batch and the same appearance, and eliminating the unqualified echelon batteries;
S2: primary grouping: uniformly fully charging the primary screened echelon batteries according to a set pulse charging process, placing the batteries under a high-temperature condition, and carrying out primary grouping according to a set standard;
S3: and (3) secondary grouping: under normal temperature, measuring the discharge energy and the charge constant current ratio of the echelon battery, and carrying out secondary grouping according to a set standard;
S4: three times of grouping: in a normal temperature environment, measuring the direct current internal resistance and the voltage difference value of the same group of gradient batteries in a full state after secondary grouping, and performing tertiary grouping according to a set standard;
S5: final grouping: and under the normal temperature condition, measuring the direct current internal resistance and the voltage difference value of the same group of gradient batteries in the 10% charge state after three times of grouping, and carrying out final grouping according to the set standard.
Further, the step S2 of the primary grouping specifically includes the following steps:
s21: uniformly performing constant-current discharge on the N gradient batteries subjected to primary screening under the normal temperature condition until the voltage is cut off, and encoding the N gradient batteries into 1,2 and 3.
S22: at normal temperature, the N echelon batteries are charged in a pulse mode by utilizing the capacity-dividing cabinet until the echelon batteries are fully charged;
S23: measuring the voltages of N echelon batteries by using a universal meter to obtain voltages V Front part 1 and a measurement time node t Front part 1, respectively {V1 Front part 1、V2 Front part 1、V3 Front part 1...VN Front part 1}、{t1 Front part 1、t2 Front part 1、t3 Front part 1...tN Front part 1},, then placing the N echelon batteries at a high temperature for standing for 7 days, measuring the voltages of the N echelon batteries after standing by using the universal meter to obtain voltages V Rear part (S) 1 and a measurement time node t Rear part (S) 1, respectively {V1 Rear part (S) 1、V2 Rear part (S) 1、V3 Rear part (S) 1...VN Rear part (S) 1}、{t1 Rear part (S) 1、t2 Rear part (S) 1、t3 Rear part (S) 1...tN Rear part (S) 1},, and respectively calculating the self-discharge K values of the N echelon batteries according to a formula K= (V Front part 1-V Rear part (S) 1)/(t Front part 1-t Rear part (S) 1);
S24, eliminating unqualified echelon batteries according to the self-discharge K value obtained in the step S23 and the set standard, and performing primary grouping.
Further, the step of the secondary grouping in the step S3 specifically includes the following steps:
S31: standing the L groups of echelon batteries obtained after the primary grouping for 8 hours at normal temperature, uniformly performing discharging treatment, and coding the L groups of echelon batteries into 1,2 and 3.
S32: the method comprises the steps of encoding M echelon batteries of a 1 st group into 1,2 and 3..M, under normal temperature conditions, carrying out constant-current constant-voltage charging on the M echelon batteries by utilizing a capacity division cabinet, then carrying out constant-current discharging, sequentially carrying out three charge-discharge cycle tests, and representing the charge-discharge cycle tests as A, B, C, recording charge constant-current ratios and discharge energies corresponding to the M echelon batteries after each charge-discharge test is finished, wherein the three charge constant-current ratios are CC％A＝{CC％1A、CC％2A、CC％3A...CC％MA}、CC％B＝{CC％1B、CC％2B、CC％3B...CC％MB} and CC% C＝{CC％1C、CC％2C、CC％3C...CC％MC respectively, taking an average value of the three charge constant-current ratios, namely the average charge constant-current ratio is CC% 0＝{CC％10、CC％20、CC％30...CC％M0, the three discharge energies are QA＝{Q1A、Q2A、Q3A...QMA}、QB＝{Q1B、Q2B、Q3B...QMB} and Q C＝{Q1C、Q2C、Q3C...QMC respectively, and taking an average value of the three discharge energies, namely the average discharge energy is Q 0＝{Q10、Q20、Q30...QM0;
S33: group L repeat step S32;
s34: and (3) according to the average charging constant current ratio and the average discharging energy of the L groups of echelon batteries obtained in the steps S32 and S33, rejecting the unqualified echelon batteries according to the set qualification standard and performing secondary grouping.
Further, the third grouping in the step S4 includes the following specific steps:
S41: uniformly charging the P groups of echelon batteries obtained after the secondary grouping to a full-charge state at normal temperature, standing for 24 hours, and encoding the P groups of echelon batteries into 1, 2 and 3.
S42: the W ladder cells of group 1 were coded as 1,2, 3..w., the voltage of the W echelon cells was measured using a multimeter, the voltage V Front part 2＝{V1 Front part 2、V2 Front part 2、V3 Front part 2...VW Front part 2 is obtained, W echelon batteries are discharged for 10S by pulse 1C by utilizing the capacity division cabinet, measuring the voltage of the W echelon batteries to obtain voltage V Rear part (S) 2＝{V1 Rear part (S) 2、V2 Rear part (S) 2、V3 Rear part (S) 2...VW Rear part (S) 2, and respectively calculating the direct current internal resistances of the W echelon batteries according to the formula R DCIR＝(V Front part 2-V Rear part (S) 2)/10;
s43: measuring the voltages of the W gradient batteries in the step S42 by using a universal meter to obtain a voltage V Front part 3＝{V1 Front part 3、V2 Front part 3、V3 Front part 3...VW Front part 3, standing the gradient batteries at normal temperature for 7 days, measuring the voltage after standing to obtain a voltage V Rear part (S) 3＝{V1 Rear part (S) 3、V2 Rear part (S) 3、V3 Rear part (S) 3...VW Rear part (S) 3, and respectively calculating the pressure differences of the W gradient batteries according to a formula DeltaV= (V Front part 3-V Rear part (S) 3);
s44: group P repeat steps S42 and S43;
s45: and (3) removing the unqualified echelon batteries and grouping for three times according to the direct current internal resistance and the voltage difference obtained in the steps S42, S43 and S44 and the set standard.
Further, the final grouping in the step S5 includes the following specific steps:
S51: uniformly discharging the H groups of gradient batteries subjected to three times of grouping to 10% of the charged quantity at normal temperature, standing for 24 hours, coding the H groups of gradient batteries into 1,2 and 3.
S52: and (3) removing the unqualified echelon batteries according to the set standard and performing final grouping according to the direct current internal resistance and the voltage difference obtained in the step S51.
Further, in the steps S21 and S22, the discharge rate is 0.5C, the discharge cut-off voltage ranges from 2.0V to 2.7V, the charge rate ranges from 0.2C to 0.5C, the charge cut-off current ranges from 0.02C to 0.05C, and the charge cut-off voltage ranges from 3.65V to 4.2V, and the pulse charging in the step S22 includes three stages: pre-charging, constant-current charging and pulse charging, wherein the pre-charging current is 0.1C, the constant-current charging current is 0.5C, the pulse charging current is 1C, the pulse charging time is 9min, the rest time is 3min, the discharging current is 1C, and the discharging time is 7min.
Further, the high temperature in the step S23 is 40±2 ℃, and the primary grouping standard in the step S24 is: the range of the difference between the maximum value and the minimum value of the K values of the same group of gradient batteries is less than or equal to 0.15mV/h.
Further, the qualification standard in the step S34 is that the average charging constant current ratio is more than or equal to 93%, the average discharging energy is more than or equal to rated energy, the secondary grouping standard is that the charging constant current ratio range of the same group of echelon batteries is 0-3%, and the discharging energy range is 0-1%o or 0-2%o.
Further, the setting criteria in the step S45 are as follows: and if the direct current internal resistances are the same, the three grouping standards in the step S45 are that the differential pressure of the echelon batteries in the same group is less than or equal to 5mV or 10mV, and the direct current internal resistance difference is less than or equal to 2mΩ.
Further, the setting criteria in the step S52 are as follows: and if the direct current internal resistances are the same, the direct current internal resistances are orderly sequenced from small to large, and the final grouping standard in the step S52 is that the differential pressure of the echelon batteries in the same group is less than or equal to 10mV or 20mV, and the direct current internal resistance difference is less than or equal to 2mΩ.
Further, the multimeter models used above are fluke-287C, with a voltage accuracy of 0.025%.
The beneficial effects of the invention are as follows:
according to the invention, DCIR and self-discharge K values are participated in the matching group through pulse charging, three-time capacity division, different charge states, high-temperature aging and four-time grouping, so that the performance indexes of the echelon batteries in the same group after final grouping tend to be the same, the influence of internal resistance on the echelon batteries is reduced, the charge and discharge efficiency under different charge states is improved, the influence of voltage difference expansion under the influence of the increase of the service time of the echelon batteries and different multiplying powers is further reduced, the consistency of the echelon batteries is ensured, and the service life of the echelon batteries is prolonged.
The capacity-division matching method is complex in operation, high in practicality, accurate in data and obvious in effect, improves matching accuracy, reduces influences caused by internal resistance to a great extent, reduces influences of voltage difference, internal resistance difference, capacity difference and charging efficiency of the matched battery on the decrease of time, guarantees consistency of the gradient batteries, prolongs service life of the gradient batteries, and improves electrical performance and safety of the gradient batteries to the greatest extent. Meanwhile, the consistency of the voltage obtained by the charging terminal and the discharging terminal can be ensured, the charging efficiency and the discharging efficiency of the battery are greatly improved, and the discharging capability of the same group of batteries is ensured to be consistent to a great extent.
The invention adopts pulse charging, and comprises three stages: the method comprises the steps of pre-charging, constant-current charging and pulse charging, wherein the pulse charging is performed by charging a section, placing a section, discharging a section, and circulating. In the pulse charging stage, the influence of polarization voltage accumulation can be reduced, the high Wen Gezhi is added in the charging process, ohmic polarization and electrochemical polarization can be eliminated to a certain extent, and meanwhile, the concentration polarization is eliminated by discharging for a period of time. Compared with the conventional charging, the pulse charging can charge with larger current, concentration polarization and ohmic polarization of the battery can be eliminated in the period of stopping charging, so that the next round of charging is carried out more smoothly, the charging speed is high, the temperature change is small, and the influence on the battery life is small. The pre-charging process is added in the early stage of pulse charging, so that the process can activate active substances of the battery to a great extent and optimize the performance of the battery.
Compared with the conventional steps, the method has the advantages that the process step of measuring the self-discharge K value of the battery cell at high temperature is advanced, the self-discharge condition of the echelon battery can be reflected, the screened battery cell is more remarkable and excellent, and meanwhile, the active substances of the echelon battery are further activated and the performance is improved by being matched with the high-temperature aging after the pulse charging in the previous step.
And (3) screening the battery cells with high self-discharge rate at a high temperature, and carrying out capacity division on the qualified battery cells for three times to obtain an average value of constant current ratio and an average value of discharge energy. Compared with the prior art, the average value of discharge capacity is not taken, but the average value of discharge energy is taken, and the principle of energy conservation is mainly followed according to the echelon batteries with the same specification, so that the energy trend of the echelon batteries in the same group can be ensured to be the same. The discharge energy relates to parameters such as voltage, current, internal resistance, time and the like, while the discharge capacity relates to current and time, so that the discharge energy is very one-sided, and the discharge energy is not comprehensive. The constant current ratio is a sign for displaying the charging energy of the batteries, the magnitude of the constant current ratio determines key indexes of the charging efficiency and the discharging energy, and the constant current ratio is a dynamic parameter, so that the parameter is added during battery screening, the charging efficiency and the discharging energy of the batteries in the same group tend to be consistent, and the consistency of the batteries can be improved.
Compared with the traditional battery screening, the method has the advantages that the alternating-current internal resistance is abandoned, the direct-current internal resistance DCIR is selected, and the direct-current internal resistance is calculated after a certain time of high-current pulse discharge and is dynamic. Compared with the traditional alternating-current internal resistance ACIR, the direct-current internal resistance DCIR can more intuitively reflect the working condition of the battery, and meanwhile, direct-current internal resistance measurement in a full-charge state and a 10% charge state is added, so that the consistency of the direct-current internal resistances of the battery in the two extreme conditions of the full-charge state and the empty-charge state can be ensured, and the consistency of the working states of the battery at the charging end and the discharging end is ensured.
And finally, through screening of the voltage after standing for a period of time under the full-charge state and the 10% charge state, compared with the traditional screening, the method can ensure the consistency of the voltage of the charging terminal and ensure that the voltage of the discharging terminal cannot diverge at the same time, so that both the charging efficiency and the discharging efficiency of the battery can be considered. Compared with the battery cells screened in a single state, the battery cell has the advantages that the charge and discharge efficiency and the cycle life are greatly improved, and the consistency of the batteries in the same group is ensured.
Drawings
Figure 1 is a general flow chart of the present invention,
Figure 2 is a flow chart of the primary grouping of the present invention,
Figure 3 is a flow chart of the secondary grouping of the present invention,
Figure 4 is a three-pass packet flow diagram of the present invention,
Fig. 5 is a final packet flow diagram of the present invention.
Detailed Description
The invention will now be described in further detail with reference to the accompanying drawings. The drawings are simplified schematic representations which merely illustrate the basic structure of the invention and therefore show only the structures which are relevant to the invention.
Taking the battery with the original model number 11140160-3.2V20AH as an example (the echelon battery is based on the 16AH capacity), as shown in fig. 1, a screening method for improving the consistency of the echelon battery comprises S1 primary screening, S2 primary grouping, S3 secondary grouping, S4 tertiary grouping and S5 final grouping, and the universal meters used in the embodiment are fluke-287C in model number and have the voltage accuracy of 0.025%.
Firstly, screening in the step S1 for one time, and screening according to the requirements of the same manufacturer, the same material system, the same batch and the appearance, and eliminating unqualified echelon batteries.
Next, as shown in fig. 2, the first grouping is performed in step S2, and the specific flow steps are as follows:
s21: 100 gradient batteries with model numbers of 11140160-3.2V20AH are randomly selected and coded into TC11140160LFP-21110001, TC11140160LFP-21110002 and TC11140160LFP-21110003.
S22: at normal temperature, 100 echelon batteries are charged by utilizing the capacity-dividing cabinet until the echelon batteries are fully charged, and the pulse charging process is as follows: first, pre-charging: the charging current is 1.6A, and the charging cut-off voltage is 3.0V; and then constant current charging is carried out: the charging current is 8A, and the charging cut-off voltage is 3.2V; and finally, pulse charging: the constant current charging current is 16A, the charging time is 9min, the rest time is 3min, the discharging current is 16A, the discharging time is 7min, the charging cut-off voltage is 3.65V, and the cut-off current is 0.32A.
S23: and measuring the voltages of 100 echelon batteries by using a universal meter to obtain a voltage V Front part 1 and a measurement time node t Front part 1, standing the 100 echelon batteries at a high temperature of 40+/-2 ℃ for 7 days, measuring the voltages of the 100 echelon batteries after standing by using the universal meter to obtain a voltage V Rear part (S) 1 and a measurement time node t Rear part (S) 1, and respectively calculating the self-discharge K values of the 100 echelon batteries according to a formula K= (V Front part 1-V Rear part (S) 1)/(t Front part 1-t Rear part (S) 1). Wherein, the model number of the universal meter is fluke-287C, and the voltage accuracy is 0.025%.
S24, removing unqualified echelon batteries according to the self-discharge K value obtained in the step S23, and performing primary grouping, wherein the primary grouping standard is that the difference range between the maximum value and the minimum value of the K values of the same group of echelon batteries is less than or equal to 0.15mV/h, removing unqualified echelon batteries exceeding the range, and the primary grouping obtains the following 6 groups of data:
as shown in fig. 3, the step S3 is performed for secondary grouping, and the specific flow steps are as follows:
S31: and standing the 6 groups of echelon batteries obtained after the primary grouping for 8 hours at normal temperature, and uniformly performing emptying treatment.
S32: under normal temperature, each group of gradient batteries are charged by constant current and constant voltage by using the capacity-dividing cabinet, then discharged by constant current, the discharge current is 8A, the discharge cut-off voltage is 2.0V, the charge current is 8A, the charge cut-off current range is 0.32A, and the charge cut-off voltage is 3.65V. And sequentially performing three charge and discharge cycle tests, recording charge constant current ratios CC% A、CC％B、CC％C and discharge energy Q A、QB、QC corresponding to each echelon battery after each charge and discharge test is finished, taking the average value of the three charge constant current ratios to obtain an average charge constant current ratio CC% 0, and taking the average value of the three discharge energy to obtain an average discharge energy Q 0.
S33: and (3) removing unqualified echelon batteries according to the average charging constant current ratio and the average discharging energy of each group of echelon batteries obtained in the step (S32) and the set qualification standard, and performing secondary grouping. The standard of qualification is that the average charging constant current ratio is more than or equal to 93%, the average discharging energy is more than or equal to rated energy, the standard of secondary grouping is that the charging constant current ratio range of the same group of echelon batteries is 0-3%, the discharging energy range is 0-1 permillage or 0-2 permillage, and the secondary grouping obtains the following 5 groups of data:
As shown in fig. 4, the step of S4 three times of grouping is performed, and the specific flow steps are as follows:
S41: and uniformly charging 5 groups of echelon batteries obtained after the secondary grouping to a full-charge state at normal temperature, and standing for 24 hours.
S42: and measuring the voltage V Front part 2 of each group of gradient batteries by using a universal meter, discharging each group of gradient batteries by using a capacity division cabinet in a pulse 1C manner for 10S, measuring the voltage V Rear part (S) 2 of each group of gradient batteries, and respectively calculating the direct current internal resistance of each group of gradient batteries according to the formula R DCIR＝(V Front part 2-V Rear part (S) 2)/10.
S43: and (3) measuring the voltage V Front part 3 of the W gradient batteries in the step S42 by using a universal meter, standing the gradient batteries for 7 days at normal temperature, measuring the voltage V Rear part (S) 3 after standing, and respectively calculating the pressure difference of each group of gradient batteries according to a formula DeltaV= (V Front part 3-V Rear part (S) 3).
S44: and (3) removing the unqualified echelon batteries and grouping for three times according to the direct current internal resistance and the voltage difference obtained in the steps S42 and S43 and the set standard. Wherein, the standard is as follows: sequencing from large to small according to the internal resistance of the direct current, sequencing from small to large according to the differential pressure if the internal resistances of the direct current are the same, wherein the three grouping standards in the step S45 are that the differential pressure of the echelon batteries in the same group is less than or equal to 5mV or 10mV, the internal resistance of the direct current is less than or equal to 2mΩ, and the three grouping obtain the following 5 groups of data:
as shown in fig. 5, the final grouping in step S5 is performed, and the specific flow steps are as follows:
s51: and uniformly discharging 5 groups of gradient batteries subjected to three times of grouping to 10% of the electric quantity at normal temperature, standing for 24 hours, and repeating the steps S42 and S43.
S52: and (3) removing the unqualified echelon batteries according to the set standard and performing final grouping according to the direct current internal resistance and the voltage difference obtained in the step S51. Wherein, the setting standard is as follows: sequencing from large to small according to the internal resistance of the direct current, sequencing from small to large according to the differential pressure if the internal resistances of the direct current are the same, wherein the final grouping standard in the step S52 is that the differential pressure of the echelon batteries in the same group is less than or equal to 10mV or 20mV, the internal resistance of the direct current is less than or equal to 2mΩ, and the final grouping obtains the following 4 groups of data:
With the above-described preferred embodiments according to the present invention as an illustration, the above-described descriptions can be used by persons skilled in the relevant art to make various changes and modifications without departing from the scope of the technical idea of the present invention. The technical scope of the present invention is not limited to the description, but must be determined according to the scope of claims.
Claims (8)
1. A screening method for improving consistency of echelon batteries is characterized by comprising the following steps:
s1: primary screening: screening according to the requirements of the same manufacturer, the same material system, the same batch and the same appearance, and eliminating the unqualified echelon batteries;
S2: primary grouping: uniformly fully charging the primary screened echelon batteries according to a set pulse charging process, placing the batteries under a high-temperature condition, and carrying out primary grouping according to a set standard;
S3: and (3) secondary grouping: under normal temperature, measuring the discharge energy and the charge constant current ratio of the echelon battery, and carrying out secondary grouping according to a set standard;
S4: three times of grouping: in a normal temperature environment, measuring the direct current internal resistance and the voltage difference value of the same group of gradient batteries in a full state after secondary grouping, and performing tertiary grouping according to a set standard;
S5: final grouping: under normal temperature, measuring the direct current internal resistance and voltage difference value of the same group of gradient batteries in 10% charge state after three times of grouping, and carrying out final grouping according to a set standard;
the third grouping in the step S4 specifically includes the following steps:
s41: uniformly charging the P groups of echelon batteries obtained after the secondary grouping to a full-charge state at normal temperature, standing for 24 hours, and encoding the P groups of echelon batteries into 1,2 and 3.
S42: the method comprises the steps of encoding W echelon batteries of a1 st group into 1,2 and 3..W, measuring the voltages of the W echelon batteries by using a universal meter to obtain voltages V front 2= { V1 front 2, V2 front 2 and V3 front 2..VW front 2}, discharging the W echelon batteries in pulse 1C for 10S by using a capacity division cabinet, measuring the voltages of the W echelon batteries to obtain voltages V rear 2= { V1 rear 2, V2 rear 2 and V3 rear 2..VW rear 2}, and respectively calculating the direct current internal resistances of the W echelon batteries according to a formula RDCIR = (V front 2-V rear 2)/10;
s43: measuring the voltages of the W gradient batteries in the step S42 by using a universal meter to obtain voltages V front 3= { V1 front 3, V2 front 3, V3 front 3..VW front 3}, standing the gradient batteries at normal temperature for 7 days, measuring the voltage after standing to obtain voltages V rear 3= { V1 rear 3, V2 rear 3, V3 rear 3..VW rear 3}, and respectively calculating the pressure difference of the W gradient batteries according to a formula DeltaV= (V front 3-V rear 3);
s44: group P repeat steps S42 and S43;
S45: removing unqualified echelon batteries and grouping for three times according to the direct current internal resistance and the pressure difference obtained in the steps S42, S43 and S44 and the set standard;
the final grouping in the step S5 is specifically as follows:
s51: uniformly discharging the H groups of gradient batteries subjected to three times of grouping to 10% of the electric quantity at normal temperature, standing for 24 hours, coding the H groups of gradient batteries into 1, 2 and 3.
S52: and (3) removing the unqualified echelon batteries according to the set standard and performing final grouping according to the direct current internal resistance and the voltage difference obtained in the step S51.
2. The screening method for improving the uniformity of a gradient battery according to claim 1, wherein: the step S2 of the primary grouping is as follows:
s21: uniformly performing constant-current discharge on the N gradient batteries subjected to primary screening under the normal temperature condition until the voltage is cut off, and encoding the N gradient batteries into 1,2 and 3.
S22: at normal temperature, the N echelon batteries are charged in a pulse mode by utilizing the capacity-dividing cabinet until the echelon batteries are fully charged;
S23: measuring the voltages of N gradient batteries by using a universal meter to obtain voltages V front 1 and measuring time nodes t front 1, wherein the voltages V front 1= { V1 front 1, V2 front 1, V3 front 1..VN front 1}, t front 1= { t1 front 1, t2 front 1, t3 front 1..tN front 1}, then placing the N gradient batteries at a high temperature for standing for 7 days, measuring the voltages of the N gradient batteries after standing by using the universal meter to obtain voltages V back 1 and measuring time nodes t back 1, which are V back 1= { V1 back 1, V2 back 1, V3 back 1..VN back 1}, t back 1= { t1 back 1, t2 back 1, t3 back 1..tN back 1}, and respectively calculating self-K values of the N gradient batteries according to a formula K= (V front 1-V back 1)/(t front 1-t back 1 };
S24, eliminating unqualified echelon batteries according to the self-discharge K value obtained in the step S23 and the set standard, and performing primary grouping.
3. The screening method for improving the uniformity of a gradient battery according to claim 1, wherein: the step S3 of the secondary grouping comprises the following specific steps:
s31: standing the L groups of echelon batteries obtained after the primary grouping for 8 hours at normal temperature, uniformly performing discharging treatment, and coding the L groups of echelon batteries into 1, 2 and 3.
S32: the M gradient batteries of the 1 st group are coded into 1,2 and 3..M, under the normal temperature condition, the constant-current constant-voltage charge and the constant-current discharge are carried out on the M gradient batteries by utilizing a capacity division cabinet, three charge-discharge cycle tests are sequentially carried out, and the charge-discharge cycle tests are shown as A, B, C, the charge constant-current ratio and the discharge energy corresponding to the M gradient batteries after each charge-discharge test is finished are recorded, the three charge constant-current ratios are respectively CC% A= { CC%1A, CC%2A, CC%3 A..CC% MA }, CC% B= { CC%1B, CC%2B, CC%3 B..CC% MB } and CC% C= { CC%1C, CC%2C, CC%3 C..CC% }, taking the average value of three charging constant current ratios, namely, the average charging constant current ratio is CC% 0= { CC%10, CC%20, CC% 30..CC% M0}, and the three discharging energy is QA= { Q1A, Q A, Q3 A..QMA }, QB= { Q1B, Q B, Q3 B..QMB } and QC= { Q1C, Q C, Q3 C..QMC }, respectively, and taking the average value of three discharging energy, namely, the average discharging energy is Q0= { Q10, Q20, Q30..QM0 };
S33: group L repeat step S32;
s34: and (3) according to the average charging constant current ratio and the average discharging energy of the L groups of echelon batteries obtained in the steps S32 and S33, rejecting the unqualified echelon batteries according to the set qualification standard and performing secondary grouping.
4. The screening method for improving the uniformity of a gradient battery according to claim 2, wherein: the discharge multiplying power is 0.5C in the steps S21 and S22, the discharge cut-off voltage range is 2.0V-2.7V, the charging multiplying power range is 0.2C-0.5C, the charging cut-off current range is 0.02C-0.05C, the charging cut-off voltage range is 3.65V-4.2V, and the pulse charging in the step S22 comprises three stages: pre-charging, constant-current charging and pulse charging, wherein the pre-charging current is 0.1C, the constant-current charging current is 0.5C, the pulse charging current is 1C, the pulse charging time is 9min, the rest time is 3min, the discharging current is 1C, and the discharging time is 7min.
5. The screening method for improving the uniformity of a gradient battery according to claim 2, wherein: the high temperature in the step S23 is 40+/-2 ℃, and the primary grouping standard in the step S24 is as follows: the range of the difference between the maximum value and the minimum value of the K values of the same group of gradient batteries is less than or equal to 0.15mV/h.
6. A screening method for improving the uniformity of a gradient battery according to claim 3, wherein: the qualification standard in the step S34 is that the average charging constant current ratio is more than or equal to 93%, the average discharging energy is more than or equal to rated energy, the standard of secondary grouping is that the charging constant current ratio range of the batteries in the same group in the echelon is 0-3%, and the discharging energy range is 0 to +/-1 mill or 0 to +/-2 mill.
7. The screening method for improving the uniformity of a gradient battery according to claim 1, wherein: the setting criteria in step S45 are as follows: and if the direct current internal resistances are the same, the three grouping standards in the step S45 are that the differential pressure of the echelon batteries in the same group is less than or equal to 5mV or 10mV, and the direct current internal resistance difference is less than or equal to 2mΩ.
8. The screening method for improving the uniformity of a gradient battery according to claim 1, wherein: the setting criteria in the step S52 are as follows: and if the direct current internal resistances are the same, the direct current internal resistances are orderly sequenced from small to large, and the final grouping standard in the step S52 is that the differential pressure of the echelon batteries in the same group is less than or equal to 10mV or 20mV, and the direct current internal resistance difference is less than or equal to 2mΩ.
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