CN111077457A - Method and device for evaluating accelerated attenuation of lithium iron phosphate battery by gradient utilization - Google Patents

Abstract

The application relates to an evaluation method and a device for utilizing the accelerated attenuation of a lithium iron phosphate battery in a gradient manner, wherein the evaluation method comprises the following steps: discharging the residual electric quantity of the lithium iron phosphate battery to be detected, and carrying out constant current charging on the lithium battery with the discharged electric quantity to the upper charging limit cut-off voltage U_{On the upper part}Charging at constant voltage until the current is reduced to below 0.1C, standing, and discharging at constant current until the discharge lower limit cut-off voltage U is reached_{Lower part}Stopping discharging; the method comprises the steps of circularly charging and discharging the lithium iron phosphate battery to be tested in the same mode, recording and calculating the voltage and capacity change of the battery in the charging and discharging process within t time after a plurality of voltage points in each circulating process, and calculating the correspondence of a plurality of same voltage points in two circulationsJudging whether the lithium iron phosphate battery to be tested enters an accelerated attenuation stage according to the R value of the deviation R of the voltage capacity change ratio; the evaluation method can be used for quickly and nondestructively evaluating the performance state of the lithium battery, and particularly can be used for accurately judging the accelerated degradation phenomenon of the lithium battery.

Description

Method and device for evaluating accelerated attenuation of lithium iron phosphate battery by gradient utilization

Technical Field

The application belongs to the technical field of lithium batteries, and particularly relates to a method and a device for evaluating the accelerated attenuation of a lithium iron phosphate battery in a gradient manner.

Background

The new generation of energy-saving and environment-friendly automobiles represented by electric automobiles is a necessary trend of the development of the automobile industry, and the popularization and the use of the electric automobiles have continuously huge demands on power batteries. At present, domestic electric automobiles mainly use lithium ion batteries of a lithium iron phosphate system as a power source, but with the increase of the quantity of electric automobiles, lithium iron phosphate batteries which cannot reach the use standard of the electric automobiles will be greatly retired and have an accumulative explosive growth trend. The lithium iron phosphate battery which is replaced from the electric automobile and used in the echelon mode still has higher residual energy, generally 70-80% of the initial energy, and after the batteries are screened and matched again, the batteries can still be applied to other energy storage occasions, so that the echelon utilization of the retired batteries is realized.

In order to screen out the retired batteries which can be continuously used for a power grid and a new energy power generation and energy storage device, the health condition and the attenuation behavior of the retired batteries need to be visually known. Almost all performance degradation of the retired battery under the macro scale is caused by structural or chemical changes of materials under the molecular scale and changes of material morphology or infrared characteristics under the micro-nano scale. The core parameters of the performance of the retired battery are evaluated, so that the safety and quality problems of the battery, such as shell sealing performance, internal micro short circuit condition, falling of active materials and the like, caused in the process of recycling the power battery on an electric vehicle are inspected, and the physicochemical property change characteristics of the battery material under the molecular scale and the micro-nano scale are also inspected. The evaluation method of the health state and the attenuation behavior of the retired lithium iron phosphate battery runs through the whole life cycle of the battery, and relates to collection and analysis of historical operating data of a power battery, disassembly of a battery pack, external characteristic evaluation and analysis, physicochemical characteristics, safety evaluation, life prediction and the like. Based on the research foundation and the model method, the technology and the characterization method for the attenuation performance acceleration and mutation evaluation in the recycling process of the retired power battery can be mastered, and the method for analyzing and predicting the attenuation acceleration and mutation of the battery is established.

In recent years, retired lithium battery life attenuation models are widely applied to the industrial field, various attenuation models for different commercial lithium ion batteries such as lithium iron phosphate batteries are evolved, and at present, certain achievements are obtained for research on retired lithium battery life attenuation models, but it is difficult to accurately analyze the internal state of a battery, and to obtain high-precision battery parameters, so that the precision and reliability of an evaluation result are insufficient. Therefore, the parameter capable of accurately reflecting the internal change of the lithium battery is found, the rapid attenuation behavior of the lithium retired battery is accurately judged, and the method has great practical requirements.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: in order to solve the technical problem that the precision and reliability of the evaluation result of the existing lithium battery life attenuation model are not enough, the application provides an evaluation method and a device for utilizing the accelerated attenuation of the lithium iron phosphate battery in a gradient manner aiming at the lithium iron phosphate battery widely applied to the market.

The technical scheme adopted by the invention for solving the technical problems is as follows:

a method for evaluating the accelerated attenuation of a lithium iron phosphate battery in a gradient manner comprises the following specific steps:

discharging the residual electric quantity of the lithium iron phosphate battery to be detected, carrying out constant-current charging on the lithium battery with the discharged electric quantity to the upper limit cut-off voltage, then carrying out constant-voltage charging until the current is reduced to be lower than the lower limit cut-off current, standing, and then discharging in a constant-current discharging mode until the current is discharged to the lower limit cut-off voltage; the method comprises the steps of circularly charging and discharging the lithium iron phosphate battery to be detected in the same mode, calculating voltage and capacity change within t time after a plurality of voltage points in the battery charging and discharging process in each circulating process, calculating the deviation of corresponding voltage-capacity change ratios after a plurality of same voltage points in two circulations, and judging whether the lithium iron phosphate battery to be detected enters an accelerated attenuation stage or not according to the deviation of the voltage-capacity change ratios.

Preferably, the method for calculating the voltage and capacity change of the battery charging process comprises the following steps: when the voltage reaches U in each charging process of the lithium iron phosphate battery to be tested₁To U₂When a plurality of voltage values are obtained, respectively calculating t after the lithium iron phosphate battery to be tested reaches each voltage value₁The voltage differences in minutes are respectively recorded as Δ U_iAnd calculating the lithium iron phosphate battery to be tested at each t₁Change in capacity in minutes, recorded as Q_iSaid Q is_i═ i (t) dt, where i (t) is each t in the charging process of the lithium iron phosphate battery to be tested₁The current values at different time points in time, i is a positive integer;

the method for recording and calculating the voltage and capacity change of the battery in the discharging process comprises the following steps: when the voltage reaches U in each discharging process of the lithium iron phosphate battery to be tested₃To U₄When a plurality of voltage values are obtained, respectively calculating t after the lithium iron phosphate battery to be tested reaches each voltage value₂The voltage differences in minutes are respectively recorded as Δ U_iAnd calculating the lithium iron phosphate battery to be tested at each t₂Change in capacity in minutes, recorded as Q_iSaid Q is_i═ i (t) dt, where i (t) is each t in the discharge process of the lithium iron phosphate battery to be tested₂The current values at different time points in time, i is a positive integer;

the method for judging whether the lithium iron phosphate battery to be detected enters an accelerated attenuation stage comprises the following steps: starting point t with the same voltage₁Or t₂Voltage change of battery delta U in minutes_iDivided by capacity change Q_iRespectively obtain Delta U_i/Q_i(ii) a Charging or discharging the n +1 th cycleMeasured Δ U_i(n+1)/Q_i(n+1)Divided by Δ U measured after the same voltage point of the corresponding charging or discharging process of the nth cycle_i(n)/Q_i(n)Obtaining the deviation (delta U) of the voltage capacity change ratio_i(n+1)/Q_i(n+1))/(ΔU_i(n)/Q_i(n)) (ii) a When the deviation of the voltage capacity change ratio in the charging process and/or the discharging process is larger than the X value, judging that the lithium iron phosphate battery to be tested enters an accelerated attenuation stage, and recording the cycle number at the moment; the value of X is 1.05 to 1.10, preferably 1.05.

Preferably, when determining whether the lithium iron phosphate battery to be measured enters the accelerated attenuation stage, the Δ U measured in the (n + 2) th cycle needs to be measured_i(n+2)/Q_i(n+2)And/or Δ U measured at cycle n +3_i(n+3)/Q_i(n+3)Then will be Δ U_i(n+2)/Q_i(n+2)And/or Δ U_i(n+3)/Q_i(n+3)Divided by the measured Δ U of the nth cycle_i(n)/Q_i(n)If the obtained deviation (Delta U) of the voltage capacity change ratio is obtained_n+2/Q_n+2)/(ΔU_n/Q_n) And/or (Δ U)_n+3/Q_n+3)/(ΔU_n/Q_n) And if the values are all larger than the X value, judging that the lithium iron phosphate battery to be tested enters an accelerated attenuation stage.

Preferably, the upper cut-off voltage is 3.55 to 3.65V, and the lower cut-off voltage is 2.5 to 2.8V.

Preferably, the U is₁Is 3.0 to 3.2V, U₂Is 3.3 to 3.5V.

Preferably, the U is₃Is 3.2 to 3.4V, U₄The voltage is 2.9-3.2V.

Preferably, the current for constant current charging is 0.2-0.4C, the lower limit cut-off current is 0.05C, and the current for constant current discharging is 0.4-0.6C.

Preferably, the lithium iron phosphate battery to be tested needs to stand for 10-20 minutes after the residual electric quantity is discharged, and the standing time after charging is 20-40 minutes.

Preferably, said t₁Or t₂Is 1-3 minutes.

Preferably, the U is₁To U₂The number of the voltage values is 4-8, and the voltage value difference between the adjacent voltage values is 0.03V-0.08V; the U is₃To U₄The number of the voltage values between the two adjacent voltage values is 4-8, and the voltage value difference between the adjacent voltage values is 0.03V-0.08V.

The invention also provides an evaluation device for the accelerated attenuation of the lithium iron phosphate battery in echelon utilization, which comprises the following steps:

the voltage and capacity acquisition unit is used for acquiring voltage and capacity changes of the lithium iron phosphate battery to be detected within t time after a plurality of voltage points in each charging process;

the deviation of the voltage capacity change ratio is used for calculating the deviation of the corresponding voltage capacity change ratio after a plurality of same voltage points of two cycles;

and the accelerated degradation evaluation unit is used for judging whether the lithium iron phosphate battery to be tested enters an accelerated degradation stage according to the deviation of the voltage capacity change ratio.

The invention has the beneficial effects that:

the invention relates to a method and a device for evaluating the accelerated attenuation of a lithium iron phosphate battery in a echelon manner, which mainly calculate the deviation of the corresponding voltage-capacity change ratio after a plurality of same voltage points of two cycles by testing and analyzing the change of voltage, current and capacity in the charging and discharging process of the battery, so as to carry out rapid nondestructive evaluation on the performance state of the lithium battery, and particularly can accurately judge the accelerated attenuation phenomenon of the lithium battery.

Drawings

The technical solution of the present application is further explained below with reference to the drawings and the embodiments.

Fig. 1 is a graph of capacity fading characteristics of a lithium iron phosphate battery (8#) used in a gradient manner according to example 1 of the present application, as a function of cycle number;

fig. 2 is a graph showing a capacity fading rate of a lithium iron phosphate battery (8#) according to the number of cycles in accordance with the gradation of example 1 of the present application.

Detailed Description

It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict.

The technical solutions of the present application will be described in detail below with reference to the accompanying drawings in conjunction with embodiments.

Example 1

In this embodiment, 12 randomly selected retired lithium iron phosphate single batteries are used as objects, and an evaluation method for the accelerated degradation of a lithium iron phosphate battery by gradient utilization is provided, where the number of each battery group is 1# to 12#, the rated capacity is 20Ah, and the remaining capacity during retirement is about 80%; the evaluation method specifically includes:

(1) the method for circularly charging and discharging the 1# to 12# retired lithium iron phosphate batteries comprises the following steps: discharging the residual electricity of the battery at room temperature (20 +/-5 ℃), standing for 15 minutes, performing constant current charging at 0.3 ℃ until the voltage is reduced to 3.65V, converting the constant current charging into constant voltage charging until the charging current is reduced to 0.05C, standing for 30 minutes, and then performing constant current discharging at 0.5C until the voltage is reduced to 2.8V;

(2) when the voltage reaches 3.20V, 3.25V, 3.30V, 3.35V and 3.40V in each charging process of the battery, the voltage change of the battery in the following 2 minutes is respectively calculated and is respectively recorded as delta U₁、ΔU₂、ΔU₃、ΔU₄And Δ U₅(ii) a The change in capacity of the cell was calculated for each 2 minutes and recorded as Q₁、Q₂、Q₃、Q₄And Q₅The capacity calculation method comprises the following steps: q ═ i (t) dt; i (t) is the current value of each lithium iron phosphate battery to be tested at different time points within 2 minutes in the charging process;

voltage change deltau of the cell within 2 minutes from the same voltage starting point_iDivided by capacity change Q_iRespectively obtain Delta U₁/Q₁、ΔU₂/Q₂、ΔU₃/Q₃、ΔU₄/Q₄And Δ U₅/Q₅；

Δ U measured for n +1 cycles_1(n+1)/Q_1(n+1)、ΔU_2(n+1)/Q_2(n+1)、ΔU_3(n+1)/Q_3(n+1)、ΔU_4(n+1)/Q_4(n+1)And Δ U_5(n+1)/Q_5(n+1)(denoted as. DELTA.U_i(n+1)/Q_i(n+1)) Δ U measured after dividing by the same voltage point of the nth cycle_1(n)/Q_1(n)、ΔU_2(n)/Q_2(n)、ΔU_3(n)/Q_3(n)、ΔU_4(n)/Q_4(n)And Δ U_5(n)/Q_5(n)(denoted as. DELTA.U_i(n)/Q_i(n)) Obtaining the deviation (delta U) of the voltage capacity change ratio_i(n+1)/Q_i(n+1))/(ΔU_i(n)/Q_i(n)) When the deviation of the voltage capacity change ratio is more than 1.05, the battery is focused on, and then the measured delta U of the n +2 th cycle is measured_i(n+2)/Q_i(n+2)And Δ U measured at cycle n +3_i(n+3)/Q_i(n+3)And calculating (Δ U)_i(n+2)/Q_i(n+2))/(ΔU_i(n)/Q_i(n)) And (Δ U)_i(n+3)/Q_i(n+3))/(ΔU_i(n)/Q_i(n))；

(3) When the voltage reaches 3.30V, 3.25V, 3.20V, 3.15V and 3.10V in each discharging process of the battery, the voltage change of the battery in the following 2 minutes is calculated and recorded as delta U respectively₁、ΔU₂、ΔU₃、ΔU₄And Δ U₅(ii) a The change in capacity of the cell was calculated for each 2 minutes and recorded as Q₁、Q₂、Q₃、Q₄And Q₅The capacity calculation method comprises the following steps: q ═ i (t) dt;

i (t) is the current value of each lithium iron phosphate battery to be tested at different time points within 2 minutes in the charging process;

voltage change deltau of the cell within 2 minutes from the same voltage starting point_iDivided by capacity change Q_iRespectively obtain Delta U₁/Q₁、ΔU₂/Q₂、ΔU₃/Q₃、ΔU₄/Q₄And Δ U₅/Q₅；

Δ U measured for n +1 cycles_1(n+1)/Q_1(n+1)、ΔU_2(n+1)/Q_2(n+1)、ΔU_3(n+1)/Q_3(n+1)、ΔU_4(n+1)/Q_4(n+1)And Δ U_5(n+1)/Q_5(n+1)(denoted as. DELTA.U_i(n+1)/Q_i(n+1)) Δ U measured after dividing by the same voltage point of the nth cycle_1(n)/Q_1(n)、ΔU_2(n)/Q_2(n)、ΔU_3(n)/Q_3(n)、ΔU_4(n)/Q_4(n)And Δ U_5(n)/Q_5(n)(denoted as. DELTA.U_i(n)/Q_i(n)) Obtaining the deviation (delta U) of the voltage capacity change ratio_i(n+1)/Q_i(n+1))/(ΔU_i(n)/Q_i(n)) When the deviation of the voltage capacity change ratio is more than 1.05, the battery is focused on, and then the measured delta U of the n +2 th cycle is measured_i(n+2)/Q_i(n+2)And Δ U measured at cycle n +3_i(n+3)/Q_i(n+3)And calculating (Δ U)_i(n+2)/Q_i(n+2))/(ΔU_i(n)/Q_i(n)) And (Δ U)_i(n+3)/Q_i(n+3))/(ΔU_i(n)/Q_i(n))；

As a result: when the cycle is carried out to 1243 times, the deviation of the voltage capacity change ratio in the charging and discharging process of the 8# battery is greatly increased and exceeds the threshold value of 1.05, the discharge capacity is suddenly reduced to 0.12Ah, only 0.6% of the residual capacity is remained compared with the initial battery ratio, and the deviation of the voltage capacity change ratio of 1244 th and 1245 th times is also larger than 1.05, so that the battery is judged to generate the battery fading acceleration phenomenon, the charging and discharging test of the 8# battery is continued, and the fading is continued to be close to 0Ah, and the detailed figure 1 is shown.

In addition, the capacity decay rate curve of the 8# battery is shown in fig. 2, the decay rate is 4.4% on average before the cycle is cycled to 1242 times, and a good linear relation is kept between the capacity decay rate and the cycle number before 1242 times; after the cycle reaches 1243 times and later, the capacity fade rate suddenly increases by nearly 100%, indicating that the battery has a battery fade acceleration phenomenon.

In this embodiment, the charge and discharge tests are continuously performed on the remaining batteries at the same time, and the deviation of the voltage-to-capacity variation ratio does not exceed the threshold of 1.05, and in fact, the capacity fading acceleration phenomenon does not occur in any of the remaining batteries.

Example 2

In this embodiment, 12 randomly selected retired lithium iron phosphate single batteries are used as objects, and an evaluation method for the accelerated degradation of a lithium iron phosphate battery by gradient utilization is provided, where the number of each battery group is 13# to 24#, the rated capacity is 20Ah, and the remaining capacity during retirement is about 80%; the evaluation method specifically includes:

(1) the method for circularly charging and discharging the 13# to 24# retired lithium battery comprises the following steps: discharging the residual electricity of the battery at room temperature (20 +/-5 ℃), standing for 10 minutes, carrying out constant current charging at 0.2 ℃ until the voltage is reduced to 3.55V, converting the constant current charging into constant voltage charging until the charging current is reduced to 0.05C, and after standing for 20 minutes, carrying out constant current discharging at 0.4C until the voltage is reduced to 2.7V;

(2) when the voltage reaches 3.1V, 3.13V, 3.16V, 3.19V, 3.22V, 3.25V, 3.28V and 3.31V in each charging process of the battery, the voltage change of the battery in the following 3 minutes is calculated and recorded as delta U respectively₁、ΔU₂、ΔU₃、ΔU₄、ΔU₅、ΔU₆、ΔU₇And Δ U₈(ii) a The change in capacity of the cell was calculated for each 3 minutes and recorded as Q₁、Q₂、Q₃、Q₄、Q₅、Q₆、Q₇And Q₈The capacity calculation method comprises the following steps: q ═ i (t) dt; i (t) is the current value of each lithium iron phosphate battery to be tested at different time points within 3 minutes in the charging process;

change in cell voltage Δ U within 3 minutes from the same voltage starting point_iDivided by capacity change Q_iRespectively obtain Delta U₁/Q₁、ΔU₂/Q₂、ΔU₃/Q₃、ΔU₄/Q₄、ΔU₅/Q₅、ΔU₆/Q₆、ΔU₇/Q₇And Δ U₈/Q₈；

Δ U measured for n +1 cycles_1(n+1)/Q_1(n+1)、ΔU_2(n+1)/Q_2(n+1)、ΔU_3(n+1)/Q_3(n+1)、ΔU_4(n+1)/Q_4(n+1)、ΔU_5(n+1)/Q_5(n+1)、ΔU_6(n+1)/Q_7(n+1)、ΔU_7(n+1)/Q_7(n+1)And ΔU_8(n+1)/Q_8(n+1)(denoted as. DELTA.U_i(n+1)/Q_i(n+1)) Δ U measured after dividing by the same voltage point of the nth cycle_1(n)/Q_1(n)、ΔU_2(n)/Q_2(n)、ΔU_3(n)/Q_3(n)、ΔU_4(n)/Q_4(n)、ΔU_5(n)/Q_5(n)、ΔU_6(n)/Q_6(n)、ΔU_7(n)/Q_7(n)And Δ U_8(n)/Q_8(n)(denoted as. DELTA.U_i(n)/Q_i(n)) Obtaining the deviation (delta U) of the voltage capacity change ratio_i(n+1)/Q_i(n+1))/(ΔU_i(n)/Q_i(n)) When the deviation of the voltage capacity change ratio is more than 1.05, the battery is focused on, and then the measured delta U of the n +2 th cycle is measured_i(n+2)/Q_i(n+2)And Δ U measured at cycle n +3_i(n+3)/Q_i(n+3)And calculating (Δ U)_i(n+2)/Q_i(n+2))/(ΔU_i(n)/Q_i(n)) And (Δ U)_i(n+3)/Q_i(n+3))/(ΔU_i(n)/Q_i(n))；

(3) When the voltage reaches 3.2V, 3.17V, 3.14V, 3.11V, 3.08V, 3.05V, 3.02V and 2.99V in each discharging process of the battery, the voltage change of the battery in the following 3 minutes is calculated and recorded as delta U respectively₁、ΔU₂、ΔU₃、ΔU₄、ΔU₅、ΔU₆、ΔU₇And Δ U₈(ii) a The change in capacity of the cell was calculated for each 3 minutes and recorded as Q₁、Q₂、Q₃、Q₄、Q₅、Q₆、Q₇And Q₈The capacity calculation method comprises the following steps: q ═ i (t) dt; i (t) is the current value of each lithium iron phosphate battery to be tested at different time points within 3 minutes in the charging process;

change in cell voltage Δ U within 3 minutes from the same voltage starting point_iDivided by capacity change Q_iRespectively obtain Delta U₁/Q₁、ΔU₂/Q₂、ΔU₃/Q₃、ΔU₄/Q₄、ΔU₅/Q₅、ΔU₆/Q₆、ΔU₇/Q₇And Δ U₈/Q₈；

Δ U measured for n +1 cycles_1(n+1)/Q_1(n+1)、ΔU_2(n+1)/Q_2(n+1)、ΔU_3(n+1)/Q_3(n+1)、ΔU_4(n+1)/Q_4(n+1)、ΔU_5(n+1)/Q_5(n+1)、ΔU_6(n+1)/Q_7(n+1)、ΔU_7(n+1)/Q_7(n+1)And Δ U_8(n+1)/Q_8(n+1)(denoted as. DELTA.U_i(n+1)/Q_i(n+1)) Δ U measured after dividing by the same voltage point of the nth cycle_1(n)/Q_1(n)、ΔU_2(n)/Q_2(n)、ΔU_3(n)/Q_3(n)、ΔU_4(n)/Q_4(n)、ΔU_5(n)/Q_5(n)、ΔU_6(n)/Q_6(n)、ΔU_7(n)/Q_7(n)And Δ U_8(n)/Q_8(n)(denoted as. DELTA.U_i(n)/Q_i(n)) Obtaining the deviation (delta U) of the voltage capacity change ratio_i(n+1)/Q_i(n+1))/(ΔU_i(n)/Q_i(n)) When the deviation of the voltage capacity change ratio is more than 1.05, the battery is focused on, and then the measured delta U of the n +2 th cycle is measured_i(n+2)/Q_i(n+2)And Δ U measured at cycle n +3_i(n+3)/Q_i(n+3)And calculating (Δ U)_i(n+2)/Q_i(n+2))/(ΔU_i(n)/Q_i(n)) And (Δ U)_i(n+3)/Q_i(n+3))/(ΔU_i(n)/Q_i(n))。

As a result: when the cycle is repeated to 1435 times, the voltage capacity change ratio of the 15# battery in the charging and discharging processes exceeds the threshold value of 1.05, and the deviation of the voltage capacity change ratio of 1436 and 1437 times is larger than 1.05, so that the battery is judged to generate the battery fading acceleration phenomenon, the 15# battery is continuously subjected to the charging and discharging test, and the residual capacity of the 1450 time is only left about 2Ah (10%), and the battery fading acceleration phenomenon is determined to be generated.

In addition, the decay rate of the 15# battery is 4.6% on average before the battery is cycled to 1434 times, and a good linear relation is kept between the capacity decay rate and the cycle number before the battery is cycled to 1434 times; after the cycle reaches 1435 times, the capacity fading rate suddenly increases to nearly 100%, which indicates that the cell fading acceleration phenomenon occurs in the cell.

In this embodiment, the charge and discharge tests are continuously performed on the remaining batteries at the same time, and the deviation of the voltage-to-capacity variation ratio does not exceed the threshold of 1.05, and in fact, the capacity fading acceleration phenomenon does not occur in any of the remaining batteries.

Example 3

In this embodiment, 12 randomly selected retired lithium iron phosphate single batteries are used as an object, and an evaluation method for accelerated degradation of a lithium iron phosphate battery by gradient utilization is provided, where the number of each battery group is 25# to 36#, the rated capacity of the battery is 80Ah, and the remaining capacity during retirement is about 85%, where the evaluation method specifically includes:

(1) the method for circularly charging and discharging 25# to 36# retired lithium batteries comprises the following steps: discharging the residual electricity of the battery at room temperature (20 +/-5 ℃), standing for 20 minutes, carrying out constant current charging at 0.4 ℃ until the voltage is reduced to 3.6V, converting the constant current charging into constant voltage charging until the charging current is reduced to 0.05C, and after standing for 40 minutes, carrying out constant current discharging at 0.6C until the voltage is reduced to 2.6V;

(2) when the voltage reaches 3.2V, 3.25V, 3.3V, 3.35V and 3.4V in each charging process of the battery, the voltage change of the battery in the subsequent 1.5 minutes is calculated and recorded as delta U respectively₁、ΔU₂、ΔU₃、ΔU₄And Δ U₅(ii) a The change in capacity of the cell was calculated at each 1.5 minute and recorded as Q₁、Q₂、Q₃、Q₄And Q₅The capacity calculation method comprises the following steps: q ═ i (t) dt; i (t) is the current value of each lithium iron phosphate battery to be tested at different time points within 1.5 minutes in the charging process;

voltage change deltau of the cell within 1.5 minutes using the same voltage starting point_iDivided by capacity change Q_iRespectively obtain Delta U₁/Q₁、ΔU₂/Q₂、ΔU₃/Q₃、ΔU₄/Q₄And Δ U₅/Q₅；

Δ U measured for n +1 cycles_1(n+1)/Q_1(n+1)、ΔU_2(n+1)/Q_2(n+1)、ΔU_3(n+1)/Q_3(n+1)、ΔU_4(n+1)/Q_4(n+1)And Δ U_5(n+1)/Q_5(n+1)(denoted as. DELTA.U_i(n+1)/Q_i(n+1)) Δ U measured after dividing by the same voltage point of the nth cycle_1(n)/Q_1(n)、ΔU_2(n)/Q_2(n)、ΔU_3(n)/Q_3(n)、ΔU_4(n)/Q_4(n)And Δ U_5(n)/Q_5(n)(denoted as. DELTA.U_i(n)/Q_i(n)) Obtaining the deviation (delta U) of the voltage capacity change ratio_i(n+1)/Q_i(n+1))/(ΔU_i(n)/Q_i(n)) When the deviation of the voltage capacity change ratio is more than 1.05, the battery is focused on, and then the measured delta U of the n +2 th cycle is measured_i(n+2)/Q_i(n+2)And Δ U measured at cycle n +3_i(n+3)/Q_i(n+3)And calculating (Δ U)_i(n+2)/Q_i(n+2))/(ΔU_i(n)/Q_i(n)) And (Δ U)_i(n+3)/Q_i(n+3))/(ΔU_i(n)/Q_i(n))；

(3) When the voltage reaches 3.4V, 3.35V, 3.3V, 3.25V and 3.2V during each discharge of the battery, the voltage change of the battery in the following 1.5 minutes is calculated and recorded as delta U respectively₁、ΔU₂、ΔU₃、ΔU₄And Δ U₅(ii) a The change in capacity of the cell was calculated at each 1.5 minute and recorded as Q₁、Q₂、Q₃、Q₄And Q₅The capacity calculation method comprises the following steps: q ═ i (t) dt; i (t) is the current value of each lithium iron phosphate battery to be tested at different time points within 1.5 minutes in the charging process;

voltage change deltau of the cell within 1.5 minutes using the same voltage starting point_iDivided by capacity change Q_iRespectively obtain Delta U₁/Q₁、ΔU₂/Q₂、ΔU₃/Q₃、ΔU₄/Q₄And Δ U₅/Q₅；

Δ U measured for n +1 cycles_1(n+1)/Q_1(n+1)、ΔU_2(n+1)/Q_2(n+1)、ΔU_3(n+1)/Q_3(n+1)、ΔU_4(n+1)/Q_4(n+1)And Δ U_5(n+1)/Q_5(n+1)(denoted as. DELTA.U_i(n+1)/Q_i(n+1)) Δ U measured after dividing by the same voltage point of the nth cycle_1(n)/Q_1(n)、ΔU_2(n)/Q_2(n)、ΔU_3(n)/Q_3(n)、ΔU_4(n)/Q_4(n)And Δ U_5(n)/Q_5(n)(denoted as. DELTA.U_i(n)/Q_i(n)) Obtaining the deviation (delta U) of the voltage capacity change ratio_i(n+1)/Q_i(n+1))/(ΔU_i(n)/Q_i(n)) When the deviation of the voltage capacity change ratio is more than 1.05, the battery is focused on, and then the measured delta U of the n +2 th cycle is measured_i(n+2)/Q_i(n+2)And Δ U measured at cycle n +3_i(n+3)/Q_i(n+3)And calculating (Δ U)_i(n+2)/Q_i(n+2))/(ΔU_i(n)/Q_i(n)) And (Δ U)_i(n+3)/Q_i(n+3))/(ΔU_i(n)/Q_i(n))；

As a result: when the cycle is 1315 times, the deviation of the voltage capacity change ratio in the charging and discharging process of the 31# battery is greatly increased and exceeds the threshold value of 1.05, the discharge capacity is suddenly reduced to 0.2Ah, only 1% of the residual capacity is remained compared with the initial battery ratio, and the deviation of the voltage capacity change ratio of 1316 th and 1317 th times is also larger than 1.05, so that the battery fading acceleration phenomenon of the battery is judged to occur, the charging and discharging test of the 31# battery is continued, and the fading is continued to be close to 0 Ah.

In addition, the decay rate of the 31# battery is 4.1% on average before the battery is cycled for 1314, and a good linear relation is kept between the capacity decay rate and the cycle number before the battery is cycled for 1314; after the cycle reaches 1315 times and later, the capacity fade rate suddenly increases by nearly 100%, indicating that the battery has a battery fade acceleration phenomenon.

From the experimental results of the embodiment 1 to the embodiment 3, it can be known that the method for calculating the deviation of the voltage-capacity change ratio successfully predicts the fading acceleration of the batteries of # 8, # 15 and # 31, has high reliability, and can successfully guide the retired battery with capacity fading mutation to timely quit the operation at the first time.

In this embodiment, the charge and discharge tests are continuously performed on the remaining batteries at the same time, and the deviation of the voltage-to-capacity variation ratio does not exceed the threshold of 1.05, and in fact, the capacity fading acceleration phenomenon does not occur in any of the remaining batteries.

Example 4

The invention also provides an evaluation device for the accelerated attenuation of the lithium iron phosphate battery in echelon utilization, which comprises the following steps:

the voltage and capacity acquisition unit is used for acquiring voltage and capacity changes of the lithium iron phosphate battery to be detected within t time after a plurality of voltage points in each charging process;

the deviation of the voltage capacity change ratio is used for calculating the deviation of the corresponding voltage capacity change ratio after a plurality of same voltage points of two cycles;

and the accelerated degradation evaluation unit is used for judging whether the lithium iron phosphate battery to be tested enters an accelerated degradation stage according to the deviation of the voltage capacity change ratio.

In light of the foregoing description of the preferred embodiments according to the present application, it is to be understood that various changes and modifications may be made without departing from the spirit and scope of the invention. The technical scope of the present application is not limited to the contents of the specification, and must be determined according to the scope of the claims. As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.

Claims (10)

1. A method for evaluating the accelerated attenuation of a lithium iron phosphate battery in a gradient manner is characterized by comprising the following steps: discharging the residual electric quantity of the lithium iron phosphate battery to be detected, carrying out constant-current charging on the lithium battery with the discharged electric quantity to the upper limit cut-off voltage, then carrying out constant-voltage charging until the current is reduced to be lower than the lower limit cut-off current, standing, and then discharging in a constant-current discharging mode until the current is discharged to the lower limit cut-off voltage; the method comprises the steps of circularly charging and discharging the lithium iron phosphate battery to be detected in the same mode, calculating voltage and capacity change within t time after a plurality of voltage points in the battery charging and discharging process in each circulating process, calculating the deviation of corresponding voltage-capacity change ratios after a plurality of same voltage points in two circulations, and judging whether the lithium iron phosphate battery to be detected enters an accelerated attenuation stage or not according to the deviation of the voltage-capacity change ratios.

2. The method for evaluating the accelerated degradation of lithium iron phosphate batteries in a gradient manner according to claim 1, wherein the method for calculating the voltage and capacity change of the battery in the charging process comprises the following steps: when the voltage reaches U in each charging process of the lithium iron phosphate battery to be tested₁To U₂When a plurality of voltage values are obtained, respectively calculating t after the lithium iron phosphate battery to be tested reaches each voltage value₁The voltage differences in minutes are respectively recorded as Δ U_iAnd calculating the lithium iron phosphate battery to be tested at each t₁Change in capacity in minutes, recorded as Q_iSaid Q is_i═ i (t) dt, where i (t) is each t in the charging process of the lithium iron phosphate battery to be tested₁The current values at different time points in time, i is a positive integer;

the method for recording and calculating the voltage and capacity change of the battery in the discharging process comprises the following steps: when the voltage reaches U in each discharging process of the lithium iron phosphate battery to be tested₃To U₄When a plurality of voltage values are obtained, respectively calculating t after the lithium iron phosphate battery to be tested reaches each voltage value₂The voltage differences in minutes are respectively recorded as Δ U_iAnd calculating the lithium iron phosphate battery to be tested at each t₂Change in capacity in minutes, recorded as Q_iSaid Q is_i═ i (t) dt, where i (t) is each t in the discharge process of the lithium iron phosphate battery to be tested₂The current values i at different points in time are positive integers.

3. The echelon utilization lithium iron phosphate battery as claimed in claim 2The method for evaluating the accelerated degradation is characterized in that the method for judging whether the lithium iron phosphate battery to be tested enters the accelerated degradation stage comprises the following steps: starting point t with the same voltage₁Or t₂Voltage change of battery delta U in minutes_iDivided by capacity change Q_iRespectively obtain Delta U_i/Q_i(ii) a Measured in the n +1 th cycle charging or discharging process_i(n+1)/Q_i(n+1)Divided by Δ U measured after the same voltage point of the corresponding charging or discharging process of the nth cycle_i(n)/Q_i(n)Obtaining the deviation (delta U) of the voltage capacity change ratio_i(n+1)/Q_i(n+1))/(ΔU_i(n)/Q_i(n)) (ii) a When the deviation of the voltage capacity change ratio in the charging process and/or the discharging process is larger than the X value, judging that the lithium iron phosphate battery to be tested enters an accelerated attenuation stage; the value of X is 1.05-1.10.

4. The method for evaluating the accelerated degradation of lithium iron phosphate batteries in a echelon manner according to claim 3, wherein when determining whether the lithium iron phosphate battery to be tested enters an accelerated degradation stage, the delta U measured in the (n + 2) th cycle is measured_i(n+2)/Q_i(n+2)And/or Δ U measured at cycle n +3_i(n+3)/Q_i(n+3)Then will be Δ U_i(n+2)/Q_i(n+2)And/or Δ U_i(n+3)/Q_i(n+3)Divided by the measured Δ U of the nth cycle_i(n)/Q_i(n)If the obtained deviation (Delta U) of the voltage capacity change ratio is obtained_n+2/Q_n+2)/(ΔU_n/Q_n) And/or (Δ U)_n+3/Q_n+3)/(ΔU_n/Q_n) And if the values are all larger than the X value, judging that the lithium iron phosphate battery to be tested enters an accelerated attenuation stage.

5. The method for evaluating the accelerated degradation of lithium iron phosphate batteries in a echelon manner according to claim 1, wherein the upper cut-off voltage is 3.55 to 3.65V, and the lower cut-off voltage is 2.5 to 2.8V.

6. The echelon utilization iron phosphate as claimed in claim 2The method for evaluating the accelerated degradation of the lithium battery is characterized in that the U is₁Is 3.0 to 3.2V, U₂3.3-3.5V; the U is₃Is 3.2 to 3.4V, U₄The voltage is 2.9-3.2V.

7. The method for evaluating the accelerated degradation of the lithium iron phosphate battery in steps according to claim 1, wherein the current for constant current charging is 0.2-0.4C, the lower limit cut-off current is 0.05C, and the current for constant current discharging is 0.4-0.6C.

8. The method for evaluating the accelerated attenuation of the lithium iron phosphate battery in the echelon according to claim 2, wherein the time for standing is 20-40 minutes after the residual electric quantity of the lithium iron phosphate battery to be tested is discharged and is 10-20 minutes after the residual electric quantity of the lithium iron phosphate battery is discharged, and the time t is₁Or t₂Is 1-3 minutes.

9. The method for evaluating the accelerated degradation of lithium iron phosphate batteries in a echelon according to claim 2, wherein the U is₁To U₂The number of the voltage values is 4-8, and the voltage value difference between the adjacent voltage values is 0.03V-0.08V; the U is₃To U₄The number of the voltage values between the two adjacent voltage values is 4-8, and the voltage value difference between the adjacent voltage values is 0.03V-0.08V.

10. The utility model provides an evaluation device that echelon utilized lithium iron phosphate battery to accelerate decay which characterized in that includes:

the voltage and capacity acquisition unit is used for acquiring voltage and capacity changes of the lithium iron phosphate battery to be detected within t time after a plurality of voltage points in each charging process;

the deviation of the voltage capacity change ratio is used for calculating the deviation of the corresponding voltage capacity change ratio after a plurality of same voltage points of two cycles;

and the accelerated degradation evaluation unit is used for judging whether the lithium iron phosphate battery to be tested enters an accelerated degradation stage according to the deviation of the voltage capacity change ratio.

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