Evaluation method and evaluation system for charging safety of retired lithium battery

Abstract

The invention discloses an evaluation method and an evaluation system for the charging safety of a decommissioned lithium battery. The evaluation method for the charging safety of the decommissioned lithium battery comprises the following steps: acquiring surface temperature data of the decommissioned lithium battery, a brand new lithium battery and an end-of-life lithium battery under a plurality of charging current working conditions; extracting characteristic values of the surface temperature data of the retired lithium battery, the brand new lithium battery and the end-of-life lithium battery under all charging current working conditions; calculating a single safety risk coefficient of the retired lithium battery under a specific charging current working condition; calculating a comprehensive safety risk coefficient of the retired lithium battery under all charging current working conditions; and different safety levels are divided, and charging current limiting values are correspondingly given. According to the invention, a plurality of batteries with the same safety level can be recombined, so that the control management of the recombined battery pack is facilitated, the consistency and safety of the recombined battery pack are improved, the service life is prolonged, and the probability of thermal runaway is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of cascade utilization of retired lithium batteries, and in particular relates to a charging safety assessment method and assessment system for retired lithium batteries. Background Art

[0002] With the rapid development of the electric vehicle industry, a large number of retired power batteries will be generated. The newly retired power batteries still have up to 80% remaining capacity and a high utilization value. Retired power batteries can also be used in low-speed electric vehicles or energy storage systems and other related fields with lower requirements on battery performance; therefore, in the future there will be a large number of retired power batteries entering the cascade utilization link.

[0003] Among them, the power battery system is usually composed of several battery cells connected in series and parallel, and the battery management system monitors it through information collection, charge and discharge control, and thermal management. However, since the occurrence time of thermal runaway of lithium batteries is very short, if the corresponding treatment measures are not taken until the thermal runaway is detected, it is very easy to cause serious economic losses or personal injury. At the same time, the internal resistance of retired lithium batteries increases due to aging, and more heat will be generated during their operation, making them more prone to thermal runaway problems. There are also obvious differences in the heat generated by different retired lithium batteries during the charging process. Therefore, there is an urgent need for a safety assessment method for retired lithium batteries during the charging process. Summary of the invention

[0004] In order to overcome the defects of the prior art, the present invention provides a charging safety assessment method and assessment system for retired lithium batteries, which can be beneficial to improving the consistency and safety of reconstituted battery packs.

[0005] The object of the present invention is achieved through the following technical solutions: In a first aspect, a method for evaluating charging safety of retired lithium batteries is provided, which comprises the following steps: Step S1: obtaining surface temperature data of retired lithium batteries, new lithium batteries and end-of-life lithium batteries under several charging current conditions; Step S2: extracting characteristic values ​​of surface temperature data of retired lithium batteries, new lithium batteries and end-of-life lithium batteries under all charging current conditions; Step S3: Calculating a single safety risk factor of the retired lithium battery under a specific charging current condition; Step S4: Calculate the comprehensive safety risk factor of the retired lithium battery under all charging current conditions; Step S5: Different safety levels are divided and corresponding charging current limit values ​​are given.

[0006] Further, in step S1, the acquisition period is t, several charging current conditions include 0.25C, 0.5C, 0.75C, 1C, 2C and 3C; The surface temperature data of retired lithium batteries under 0.25C, 0.5C, 0.75C, 1C, 2C and 3C charging current conditions are expressed in sequence form as { T a (0.25, 0), T a (0.25, t )、……}、{ T a (0.5, 0), T a (0.5, t )、……}、{ T a (0.75, 0), T a (0.75, t )、……}、{ T a (1,0), T a (1, t )、……}、{ T a (2,0), T a (2, t )、……}、{ T a (3,0), T a (3, t ),...}; The surface temperature data of the new lithium battery under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are respectively expressed in the form of a sequence as { T n (0.25, 0), T n (0.25, t )、……}、{ T n (0.5, 0), T n (0.5, t )、……}、{ T n (0.75, 0), T n (0.75, t )、……}、{ T n (1,0), T n (1,t )、……}、{ T n (2,0), T n (2, t )、……}、{ T n (3,0), T n (3, t ),...}; The surface temperature data of the end-of-life lithium battery under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are respectively expressed in the form of a sequence as { T e (0.25, 0), T e (0.25, t )、……}、{ T e (0.5, 0), T e (0.5, t )、……}、{ T e (0.75, 0), T e (0.75, t )、……}、{ T e (1,0), T e (1, t )、……}、{ T e (2,0), T e (2, t )、……}、{ T e (3,0), T e (3, t ),...}; in, T a is the surface temperature of retired lithium batteries, T n is the surface temperature of a new lithium battery, T e The surface temperature of lithium battery at the end of life; T a (0.25, 0) is the surface temperature of the retired lithium battery before charging at a charging current of 0.25C. T a (0.25, t) is charged at a charging current of 0.25C t Surface temperature of retired lithium batteries over time.

[0007] Furthermore, in step S1, the remaining power of the retired lithium battery, the new lithium battery and the lithium battery at the end of life before charging is SOC =0.

[0008] Further, in step S2, the characteristic values ​​of the surface temperature data of the retired lithium battery, the new lithium battery and the end-of-life lithium battery under all charging current conditions are expressed in the form of a sequence as { T am (0.25), T nm (0.25), T em (0.25)}, { T am (0.5), T nm (0.5), T em (0.5)}, { T am (0.75), T nm (0.75), T em (0.75)}, { T am (1) T nm (1) T em (1)}、{ T am (2) T nm (2) T em (2)}、{ T am (3) T nm (3) T em (3)}; in, T am (0.25), T nm (0.25), T em (0.25) are the characteristic values ​​of retired lithium batteries, new lithium batteries and end-of-life lithium batteries under the charging current condition of 0.25C, T am (0.25) = max{ T a(0.25, 0), T a (0.25, t ),……}.

[0009] Further, in step S3, the single safety risk coefficients of the retired lithium battery under six specific charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are respectively C s (0.25), C s (0.5), C s (0.75), C s (1) C s (2) and C s (3); C s (0.25) = ( T am (0.25) - T nm (0.25)) / ( T em (0.25) - T nm (0.25)); C s (0.5) = ( T am (0.5) - T nm (0.5)) / ( T em (0.5) - T nm (0.5)); C s (0.75) = ( T am (0.75) - T nm (0.75)) / ( T em (0.75) - T nm (0.75)); C s (1) = ( T am (1)- T nm (1)) / ( Tem (1)- T nm (1)); C s (2) = T am (2)- T nm (2)) / ( T em (2)- T nm (2)); C s (3) = ( T am (3)- T nm (3)) / ( T em (3)- T nm (3)).

[0010] Further, the several charging current operating conditions include several slow charging current operating conditions and several fast charging current operating conditions; Calculation of the comprehensive safety risk factor of retired lithium batteries under all charging current conditions includes: Calculate the multi-condition safety risk factor of retired lithium batteries under all slow charging current conditions and the multi-condition safety risk factor under all fast charging current conditions; Based on the multi-condition safety risk factor under all slow charging current conditions and the multi-condition safety risk factor under all fast charging current conditions, the comprehensive safety risk factor of retired lithium batteries under all charging current conditions is calculated.

[0011] Further, in step S4, the comprehensive safety risk factor of the retired lithium battery under all charging current conditions is C m ; C m = C m ( s )*0.8+ C m ( f )*0.2; in, C m ( s ) is the multi-condition safety risk factor of retired lithium batteries under all slow charging current conditions, C m ( s )=( Cs (0.25)+ C s (0.5)+ C s (0.75) / 3; C m ( f ) is the multi-condition safety risk factor of retired lithium batteries under all fast charging current conditions, C m ( f )=( C s (1)+ C s (2) + C s (3)) / 3.

[0012] Furthermore, different safety levels are divided and corresponding charging current limit values ​​are given, including: If 0< C m ≤0.2, the thermal stability of the retired lithium battery is good, and the upper limit of the charging current is 1.5C; If 0.2< C m ≤0.4, the thermal stability of the retired lithium battery is average, and the upper limit of the charging current is 1C; If 0.4< C m <1, the thermal stability of the retired lithium battery is poor, and the upper limit of the charging current is 0.5C.

[0013] Further, in steps S1 to S5, the number of retired lithium batteries is at least two; The method for assessing charging safety of retired lithium batteries also includes the following steps: reorganizing retired lithium batteries with the same safety level. In a second aspect, a retired lithium battery charging safety assessment system is provided, which includes: A charging data acquisition module, which is used to obtain surface temperature data of retired lithium batteries, new lithium batteries and end-of-life lithium batteries under several charging current conditions; Temperature data preprocessing module, which is used to extract characteristic values ​​of surface temperature data of retired lithium batteries, new lithium batteries and end-of-life lithium batteries under all charging current conditions; A single risk factor calculation module is used to calculate a single safety risk factor of retired lithium batteries under specific charging current conditions; A comprehensive risk factor calculation module is used to calculate the comprehensive safety risk factor of retired lithium batteries under all charging current conditions; The charging safety level classification module is used to classify different safety levels and give corresponding charging current limit values.

[0014] The beneficial effects of the present invention are: The present invention provides a new charging safety assessment method and assessment system for retired lithium batteries, which can classify retired lithium batteries into safety levels and give corresponding charging current limit values, so as to use this as an important basis for screening, reorganizing and formulating charging strategies for retired lithium batteries, so as to facilitate the reorganization of multiple batteries with the same safety level, thereby facilitating the control and management of the reorganized battery group, and further facilitating the improvement of the consistency and safety of the reorganized battery group, so as to facilitate the extension of service life and reduce the probability of thermal runaway. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will be described in more detail below based on embodiments and with reference to the accompanying drawings, wherein: Figure 1 A flow chart showing the evaluation method of the present invention; Figure 2 A schematic diagram of the structure of the evaluation system in the present invention is shown; Figure 3 The circuit connection diagram of the charging data acquisition module in the present invention is shown; In the drawings, like reference numerals are used for like parts. The drawings are not necessarily to scale.

[0016] Reference numerals: 1- Charging data acquisition module, 2- Temperature data preprocessing module, 3- Single risk factor calculation module, 4- Comprehensive risk factor calculation module, 5- Charging safety level classification module, 6- Retired lithium battery, 7- New lithium battery, 8- End of life lithium battery. DETAILED DESCRIPTION

[0017] The present invention will be further described below in conjunction with the accompanying drawings.

[0018] Example 1 This embodiment provides a method for evaluating charging safety of retired lithium batteries. Figure 1 As shown, it includes the following steps: Step S1: obtaining surface temperature data of retired lithium batteries 6, new lithium batteries 7 and end-of-life lithium batteries 8 under several charging current conditions; Step S2: extracting characteristic values ​​of surface temperature data of the retired lithium battery 6, the new lithium battery 7 and the end-of-life lithium battery 8 under all charging current conditions; Step S3: Calculating a single safety risk factor of the retired lithium battery 6 under a specific charging current condition; Step S4: Calculate the comprehensive safety risk factor of the retired lithium battery 6 under all charging current conditions; Step S5: Different safety levels are divided and corresponding charging current limit values ​​are given.

[0019] Specifically, in step S1, the acquisition cycle is 30s, and the total charging time is not less than 10min; several charging current conditions include 0.25C, 0.5C, 0.75C, 1C, 2C and 3C; the remaining power of the retired lithium battery 6, the new lithium battery 7 and the end-of-life lithium battery 8 before charging SOC =0; The surface temperature data of the retired lithium battery 6 under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are respectively expressed in the form of a sequence as { T a (0.25, 0), T a (0.25, 30), ...}, { T a (0.5, 0), T a (0.5, 30), ...}, { T a (0.75, 0), T a (0.75, 30), ...}, { T a (1,0), T a (1, 30), ...}, { T a (2,0), T a (2, 30), ...}, { T a (3,0), T a (3, 30), ...}; The surface temperature data of the new lithium battery 7 under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are respectively expressed in the form of a sequence as { T n (0.25, 0), T n (0.25, 30), ...}, { T n (0.5, 0), T n (0.5, 30), ...}, { Tn (0.75, 0), T n (0.75, 30), ...}, { T n (1,0), T n (1, 30), ...}, { T n (2,0), T n (2, 30), ...}, { T n (3,0), T n (3, 30), ...}; The surface temperature data of the end-of-life lithium battery 8 under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are respectively expressed in the form of a sequence as { T e (0.25, 0), T e (0.25, 30), ...}, { T e (0.5, 0), T e (0.5, 30), ...}, { T e (0.75, 0), T e (0.75, 30), ...}, { T e (1,0), T e (1, 30), ...}, { T e (2,0), T e (2, 30), ...}, { T e (3,0), T e (3, 30), ...}; in, T a is the surface temperature of retired lithium batteries, T n is the surface temperature of a new lithium battery, T e The surface temperature of lithium battery at the end of life; T a (0.25, 0) is the surface temperature of the retired lithium battery before charging at a charging current of 0.25C. Ta (0.25, t ) is the charging time at 0.25C charging current t The surface temperature of retired lithium batteries; accordingly, T n (0.25, 0) is the surface temperature of a new lithium battery before charging at a charging current of 0.25C. T n (0.25, 30) is the surface temperature of a new lithium battery charged at 0.25C for 30 seconds. T e (0.25, 0) is the surface temperature of the end-of-life lithium battery before charging at a charging current of 0.25C. T e (0.25, 30) is the surface temperature of the end-of-life lithium battery charged for 30 seconds at a charging current of 0.25C; similarly, the surface temperature data of the remaining retired lithium batteries 6, new lithium batteries 7 and end-of-life lithium batteries 8 under charging current conditions of 0.5C, 0.75C, 1C, 2C and 3C can also be understood in the same way and will not be explained one by one here.

[0020] In step S2, the characteristic values ​​of the surface temperature data of the retired lithium battery 6, the new lithium battery 7 and the end-of-life lithium battery 8 under all charging current conditions are expressed in the form of a sequence as { T am (0.25), T nm (0.25), T em (0.25)}, { T am (0.5), T nm (0.5), T em (0.5)}, { T am (0.75), T nm (0.75), T em (0.75)}, { T am (1) T nm (1) T em (1)}、{ T am (2) T nm (2) T em (2)}、{ Tam (3) T nm (3) T em (3)}; in, T am (0.25), T nm (0.25), T em (0.25) are the characteristic values ​​of retired lithium battery 6, new lithium battery 7 and end-of-life lithium battery 8 under the charging current condition of 0.25C, T am (0.25) = max{ T a (0.25, 0), T a (0.25, t ),...}; Similarly, the characteristic values ​​of the retired lithium battery 6, the new lithium battery 7 and the end-of-life lithium battery 8 under the charging current conditions of 0.5C, 0.75C, 1C, 2C and 3C can also be understood in the same way and will not be explained one by one here.

[0021] In step S3, the charging safety risk coefficient of the new lithium battery 7 is set to 0, and the charging safety risk coefficient of the end-of-life lithium battery 8 is set to 1, that is, the charging safety risk coefficient of the retired lithium battery 6 ranges from 0 to 1, and the single safety risk coefficients of the retired lithium battery 6 under the six specific charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are respectively C s (0.25), C s (0.5), C s (0.75), C s (1) C s (2) and C s (3); C s (0.25) = ( T am (0.25) - T nm (0.25)) / ( T em (0.25) - T nm (0.25)); C s (0.5) = (T am (0.5) - T nm (0.5)) / ( T em (0.5) - T nm (0.5)); C s (0.75) = ( T am (0.75) - T nm (0.75)) / ( T em (0.75) - T nm (0.75)); C s (1) = ( T am (1)- T nm (1)) / ( T em (1)- T nm (1)); C s (2) = T am (2)- T nm (2)) / ( T em (2)- T nm (2)); C s (3) = ( T am (3)- T nm (3)) / ( T em (3)- T nm (3)).

[0022] In addition, the several charging current operating conditions include several slow charging current operating conditions and several fast charging current operating conditions; Calculation of the comprehensive safety risk factor of retired lithium battery 6 under all charging current conditions includes: Calculate the multi-operating condition safety risk coefficient of retired lithium battery 6 under all slow charging current conditions and the multi-operating condition safety risk coefficient under all fast charging current conditions; Based on the multi-condition safety risk factor under all slow charging current conditions and the multi-condition safety risk factor under all fast charging current conditions, the comprehensive safety risk factor of retired lithium battery 6 under all charging current conditions is calculated.

[0023] In step S4, the comprehensive safety risk factor of the retired lithium battery 6 under all charging current conditions is C m ; C m = C m ( s )*0.8+ C m ( f )*0.2; in, C m ( s ) is the multi-condition safety risk factor of retired lithium battery 6 under all slow charging current conditions, C m ( s )=( C s (0.25)+ C s (0.5)+ C s (0.75) / 3; C m ( f ) is the multi-condition safety risk factor of retired lithium battery 6 under all fast charging current conditions, C m ( f )=( C s (1)+ C s (2) + C s (3)) / 3.

[0024] In step S5, different safety levels are divided and corresponding charging current limit values ​​are given, including: If 0< C m ≤0.2, the thermal stability of the retired lithium battery 6 is good, and the upper limit of the charging current is 1.5C; If 0.2< C m ≤0.4, the thermal stability of the retired lithium battery 6 is average, and the upper limit of the charging current is 1C; If 0.4< C m<1, the thermal stability of the retired lithium battery 6 is poor, and the upper limit of the charging current is 0.5C.

[0025] By adopting the retired lithium battery charging safety assessment method provided in this embodiment, the retired lithium batteries 6 are classified into safety levels and corresponding charging current limit values ​​are given, so as to use this as an important basis for screening, reorganizing and formulating charging strategies for retired lithium batteries 6, so as to facilitate the reorganization of multiple batteries with the same safety level, thereby facilitating the control and management of the reorganized battery pack, and further facilitating the improvement of the consistency and safety of the reorganized battery pack, so as to facilitate the extension of the service life and reduce the probability of thermal runaway. Example 2 Based on Example 1, this embodiment evaluates the charging safety of multiple retired lithium batteries 6 so as to classify the multiple retired lithium batteries 6 into safety levels, thereby facilitating the reorganization of multiple retired lithium batteries 6 with the same safety level.

[0026] Example 3 This embodiment provides a retired lithium battery charging safety assessment system, which can perform the retired lithium battery charging safety assessment method in Example 1, such as Figure 2 As shown, it specifically includes: A charging data acquisition module 1, the charging data acquisition module 1 is used to obtain surface temperature data of retired lithium batteries 6, new lithium batteries 7 and end-of-life lithium batteries 8 under several charging current conditions; A temperature data preprocessing module 2 is used to extract characteristic values ​​of surface temperature data of retired lithium batteries 6, new lithium batteries 7 and end-of-life lithium batteries 8 under all charging current conditions; A single risk factor calculation module 3, which is used to calculate a single safety risk factor of the retired lithium battery 6 under a specific charging current condition; A comprehensive risk factor calculation module 4, which is used to calculate the comprehensive safety risk factor of the retired lithium battery 6 under all charging current conditions; The charging safety level classification module 5 is used to classify different safety levels and provide corresponding charging current limit values.

[0027] like Figure 3 As shown, all retired lithium batteries 6, new lithium batteries 7 and end-of-life lithium batteries 8 are attached with surface temperature sensors, and the charging data acquisition module 1 acquires the data through these surface temperature sensors.

[0028] Example 4 This embodiment evaluates the charging safety of a retired lithium battery 6 based on the embodiment 1.

[0029] Specifically, through step S1, the surface temperature data of the retired lithium battery 6, the new lithium battery 7 and the end-of-life lithium battery 8 under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are collected; Through step S2, characteristic values ​​of surface temperature data of retired lithium battery 6, new lithium battery 7 and end-of-life lithium battery 8 under all charging current conditions are extracted and expressed in sequence form as {28.7°C, 27.1°C, 33.7°C}, {31.4°C, 29.6°C, 35.8°C}, {34.5°C, 32.6°C, 39.3°C}, {37.7°C, 35.6°C, 42.1°C}, {42.2°C, 39.3°C, 47.4°C}, {46.1°C, 42.7°C, 51.6°C}; Calculated by the calculation formula in step S3: C s (0.25) ≈ 0.24, C s (0.5)≈0.29, C s (0.75)≈0.28, C s (1)≈0.32, C s (2) ≈ 0.36, C s (3) ≈ 0.38; Calculated by the calculation formula in step S4: C m ( s ) = 0.27, C m ( f )≈0.35; Then calculate: C m ≈0.29; Therefore, the thermal stability of the retired lithium battery 6 during charging is general, and the upper limit of the charging current is 1C.

[0030] In the description of the present invention, it is necessary to understand that the terms "upper", "lower", "bottom", "top", "front", "back", "inside", "outside", "left", "right", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0031] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be used in other described embodiments.

Claims

1. A method for evaluating charging safety of retired lithium batteries, characterized in that: The following steps are involved: Step S1: obtaining surface temperature data of retired lithium batteries, new lithium batteries and end-of-life lithium batteries under several charging current conditions; Step S2: extracting characteristic values ​​of surface temperature data of retired lithium batteries, new lithium batteries and end-of-life lithium batteries under all charging current conditions; Step S3: Calculating a single safety risk factor of the retired lithium battery under a specific charging current condition; Step S4: Calculate the comprehensive safety risk factor of the retired lithium battery under all charging current conditions; Step S5: Different safety levels are divided and corresponding charging current limit values ​​are given.

2. The method for evaluating charging safety of retired lithium batteries according to claim 1, characterized in that: In step S1, the acquisition cycle is t , the several charging current conditions include 0.25C, 0.5C, 0.75C, 1C, 2C and 3C; The surface temperature data of retired lithium batteries under 0.25C, 0.5C, 0.75C, 1C, 2C and 3C charging current conditions are expressed in sequence form as { T a (0.25, 0), T a (0.25, t )、……}、{ T a (0.5, 0), T a (0.5, t )、……}、{ T a (0.75, 0), T a (0.75, t )、……}、{ T a (1,0), T a (1, t )、……}、{ T a (2,0), T a (2, t )、……}、{ T a (3,0), T a (3, t ),...}; The surface temperature data of the new lithium battery under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are respectively expressed in the form of a sequence as { T n (0.25, 0), T n (0.25, t )、……}、{ T n (0.5, 0), T n (0.5, t )、……}、{ T n (0.75, 0), T n (0.75, t )、……}、{ T n (1,0), T n (1, t )、……}、{ T n (2,0), T n (2, t )、……}、{ T n (3,0), T n (3, t ),...}; The surface temperature data of the end-of-life lithium battery under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are respectively expressed in the form of a sequence as { T e (0.25, 0), T e (0.25, t )、……}、{ T e (0.5, 0), T e (0.5, t )、……}、{ T e (0.75, 0), T e (0.75, t )、……}、{ T e (1,0), T e (1, t )、……}、{ T e (2,0), T e (2, t )、……}、{ T e (3,0), T e (3, t ),...}; in, T a is the surface temperature of retired lithium batteries, T n is the surface temperature of a new lithium battery, T e The surface temperature of lithium battery at the end of life; T a (0.25, 0) is the surface temperature of the retired lithium battery before charging at a charging current of 0.25C. T a (0.25, t ) is charged at a charging current of 0.25C t Surface temperature of retired lithium batteries over time.

3. The method for evaluating charging safety of retired lithium batteries according to claim 2, characterized in that: In step S1, the remaining power of the retired lithium battery, the new lithium battery and the lithium battery at the end of life before charging SOC =0.

4. The method for evaluating charging safety of retired lithium batteries according to claim 2, characterized in that: In step S2, the characteristic values ​​of the surface temperature data of the retired lithium battery, the new lithium battery and the end-of-life lithium battery under all charging current conditions are expressed in the form of a sequence as { T am (0.25), T nm (0.25), T em (0.25)}, { T am (0.5), T nm (0.5), T em (0.5)}, { T am (0.75), T nm (0.75), T em (0.75)}, { T am (1) T nm (1) T em (1)}、{ T am (2) T nm (2) T em (2)}、{ T am (3) T nm (3) T em (3)}; in, T am (0.25), T nm (0.25), T em (0.25) are the characteristic values ​​of retired lithium batteries, new lithium batteries and end-of-life lithium batteries under the charging current condition of 0.25C, T am (0.25) = max{ T a (0.25, 0), T a (0.25, t ),……}.

5. The method for evaluating charging safety of retired lithium batteries according to claim 4, characterized in that: In step S3, the single safety risk factors of retired lithium batteries under six specific charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C and 3C are respectively C s (0.25), C s (0.5), C s (0.75), C s (1) C s (2) and C s (3); C s （0.25）=（ T am （0.25）- T nm （0.25）） / （ T em （0.25）- T nm （0.25））； C s （0.5）=（ T am （0.5）- T nm （0.5）） / （ T em （0.5）- T nm （0.5））； C s （0.75）=（ T am （0.75）- T nm （0.75）） / （ T em （0.75）- T nm （0.75））； C s （1）=（ T am （1）- T nm （1）） / （ T em （1）- T nm （1））； C s （2）=（ T am （2）- T nm （2）） / （ T em （2）- T nm （2））； C s （3）=（ T am （3）- T nm （3）） / （ T em （3）- T nm （3））。 6. The method for evaluating charging safety of retired lithium batteries according to claim 5, characterized in that: The several charging current operating conditions include several slow charging current operating conditions and several fast charging current operating conditions; The calculation of the comprehensive safety risk factor of retired lithium batteries under all charging current conditions includes: Calculate the multi-condition safety risk factor of retired lithium batteries under all slow charging current conditions and the multi-condition safety risk factor under all fast charging current conditions; Based on the multi-condition safety risk factor under all slow charging current conditions and the multi-condition safety risk factor under all fast charging current conditions, the comprehensive safety risk factor of retired lithium batteries under all charging current conditions is calculated.

7. The method for evaluating charging safety of retired lithium batteries according to claim 6, characterized in that: In step S4, the comprehensive safety risk factor of retired lithium batteries under all charging current conditions is C m ; C m = C m ( s )*0.8+ C m ( f )*0.2; in, C m ( s ) is the multi-condition safety risk factor of retired lithium batteries under all slow charging current conditions, C m ( s )=( C s (0.25)+ C s (0.5)+ C s (0.75) / 3; C m ( f ) is the multi-condition safety risk factor of retired lithium batteries under all fast charging current conditions, C m ( f )=( C s (1)+ C s (2) + C s (3)) / 3.

8. The method for evaluating charging safety of retired lithium batteries according to claim 7, characterized in that: The different safety levels and corresponding charging current limit values ​​include: If 0< C m ≤0.2, the thermal stability of the retired lithium battery is good, and the upper limit of the charging current is 1.5C; If 0.2< C m ≤0.4, the thermal stability of the retired lithium battery is average, and the upper limit of the charging current is 1C; If 0.4< C m <1, the thermal stability of the retired lithium battery is poor, and the upper limit of the charging current is 0.5C.

9. The method for evaluating charging safety of retired lithium batteries according to claim 1, characterized in that: In steps S1 to S5, the number of retired lithium batteries is at least two; The following steps are also included: reorganizing retired lithium batteries with the same safety level.

10. A retired lithium battery charging safety assessment system, characterized in that: include: A charging data acquisition module, which is used to obtain surface temperature data of retired lithium batteries, new lithium batteries and end-of-life lithium batteries under several charging current conditions; A temperature data preprocessing module, which is used to extract characteristic values ​​of surface temperature data of retired lithium batteries, new lithium batteries and end-of-life lithium batteries under all charging current conditions; A single risk factor calculation module, which is used to calculate a single safety risk factor of a retired lithium battery under a specific charging current condition; A comprehensive risk factor calculation module, which is used to calculate the comprehensive safety risk factor of retired lithium batteries under all charging current conditions; The charging safety level classification module is used to classify different safety levels and provide corresponding charging current limit values.

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