Retired Lithium Battery Charging Safety Evaluation Method and Evaluation System

Abstract

The present invention discloses a method and a system for evaluating the charging safety of retired lithium batteries. The method for evaluating the charging safety of retired lithium batteries includes the following steps: obtaining the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under several charging current conditions; extracting the characteristic values of the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under all charging current conditions; calculating the single safety risk coefficient of the retired lithium battery for a specific charging current condition; calculating the comprehensive safety risk coefficient of the retired lithium battery for all charging current conditions; dividing different safety levels and correspondingly giving the charging current limit values. The present invention is beneficial to reorganizing multiple batteries with the same safety level, thus facilitating the control and management of the reorganized battery pack, further improving the consistency and safety of the reorganized battery pack, so as to extend the service life and reduce the probability of thermal runaway.

Description

Technical Field

[0001] The present invention belongs to the technical field of the secondary utilization of retired lithium batteries, and particularly relates to a method and a system for evaluating the charging safety of 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. However, newly retired power batteries still have a remaining capacity of up to 80% and relatively high utilization value. Retired power batteries can also be used in related fields with relatively low battery performance requirements, such as low-speed electric vehicles or energy storage systems. Therefore, in the future, a large number of retired power batteries will enter the secondary utilization link.

[0003] Among them, a power battery system is usually composed of several battery cells connected in series and parallel, and a battery management system performs monitoring work such as information collection, charge and discharge control, and thermal management on it. However, since the occurrence time of lithium battery thermal runaway is very short, if corresponding treatment measures are taken only after detecting the occurrence of thermal runaway, it is extremely easy to cause serious economic losses or personal injuries. 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. Moreover, there are obvious differences in the heat generated by different retired lithium batteries during the charging process. Therefore, there is an urgent need for a method for evaluating the safety of 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 method and a system for evaluating the charging safety of retired lithium batteries, which can help improve the consistency and safety of the reorganized battery pack.

[0005] The object of the present invention is achieved by the following technical solutions:

[0006] In a first aspect, a method for evaluating the charging safety of retired lithium batteries is provided, which includes the following steps:

[0007] Step S1: Obtain the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under several charging current conditions;

[0008] Step S2: Extract the characteristic values of the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under all charging current conditions;

[0009] Step S3: Calculate the single safety risk coefficient of the retired lithium battery for a specific charging current condition;

[0010] Step S4: Calculate the comprehensive safety risk coefficient of the retired lithium battery for all charging current conditions;

[0011] Step S5: Divide different safety levels and correspondingly give the charging current limiting values.

[0012] Further, in step S1, the acquisition period is t , and several charging current conditions include 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C;

[0013] The surface temperature data of the retired lithium battery under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C are respectively represented in sequence 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 ), ……};

[0014] The surface temperature data of the brand - new lithium battery under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C are respectively represented in 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 ), ……};

[0015] The surface temperature data of the end - of - life lithium batteries under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C are respectively represented in 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 ), ……};

[0016] Among them, T a is the surface temperature of the retired lithium battery, T n is the surface temperature of the brand - new lithium battery, T e is the surface temperature of the end - of - life lithium battery; Ta (0.25, 0) is the surface temperature of the retired lithium battery before charging under the charging current condition of 0.25C. T a (0.25, t ) is the surface temperature of the retired lithium battery during charging with a charging current condition of 0.25C t for a certain duration.

[0017] Further, in step S1, the remaining power of the retired lithium battery, brand-new lithium battery, and end-of-life lithium battery before charging SOC = 0.

[0018] Further, in step S2, the characteristic values of the surface temperature data of the retired lithium battery, brand-new lithium battery, and end-of-life lithium battery under all charging current conditions are represented in 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)};

[0019] Among them, T am (0.25), T nm (0.25), T em(0.25) are the characteristic values of retired lithium batteries, brand-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 )、……}。

[0020] Furthermore, 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);

[0021] C s (0.25) = ( T am (0.25) - T nm (0.25)) / ( T em (0.25) - T nm (0.25));

[0022] C s (0.5) = ( T am (0.5) - T nm (0.5)) / ( T em (0.5) - T nm (0.5));

[0023] C s (0.75) = ( T am (0.75) - T nm (0.75)) / ( T em (0.75) -T nm (0.75));

[0024] C s (1) = ( T am (1) - T nm (1)) / ( T em (1) - T nm (1));

[0025] C s (2) = ( T am (2) - T nm (2)) / ( T em (2) - T nm (2));

[0026] C s (3) = ( T am (3) - T nm (3)) / ( T em (3) - T nm (3)).

[0027] Further, several charging current conditions include several slow charging current conditions and several fast charging current conditions;

[0028] Calculating the comprehensive safety risk coefficient of retired lithium batteries for all charging current conditions includes:

[0029] Calculating the multi - condition safety risk coefficient of retired lithium batteries for all slow charging current conditions and the multi - condition safety risk coefficient for all fast charging current conditions;

[0030] According to the multi - condition safety risk coefficient for all slow charging current conditions and the multi - condition safety risk coefficient for all fast charging current conditions, calculate the comprehensive safety risk coefficient of retired lithium batteries for all charging current conditions.

[0031] Further, in step S4, the comprehensive safety risk coefficient of retired lithium batteries for all charging current conditions is C m ; C m =C m ( s ) * 0.8 + C m ( f ) * 0.2;

[0032] Among them, C m ( s ) is the multi - condition safety risk coefficient of retired lithium - ion batteries under all slow - charge current conditions, C m ( s ) = ( C s (0.25) + C s (0.5) + C s (0.75)) / 3;

[0033] C m ( f ) is the multi - condition safety risk coefficient of retired lithium - ion batteries under all fast - charge current conditions, C m ( f ) = ([{END]] C s (1) + C s (2) + C s (3)) / 3.

[0034] Furthermore, different safety levels are divided, and the corresponding charging current limit values are given as follows:

[0035] If 0 < C m ≤0.2, the thermal stability of the retired lithium - ion battery during charging is good, and the upper limit value of the charging current is 1.5C;

[0036] If 0.2 < C m ≤0.4, the thermal stability of the retired lithium - ion battery during charging is average, and the upper limit value of the charging current is 1C;

[0037] If 0.4 < C m < 1, the thermal stability of the retired lithium - ion battery during charging is poor, and the upper limit value of the charging current is 0.5C.

[0038] Furthermore, in steps S1 - S5, the number of retired lithium - ion batteries is at least two;

[0039] The method for evaluating the charging safety of retired lithium batteries further includes the following steps: Recombining retired lithium batteries with the same safety level.

[0040] In a second aspect, a system for evaluating the charging safety of retired lithium batteries is provided, which includes:

[0041] A charging data acquisition module, which is used to obtain the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under several charging current conditions;

[0042] A temperature data preprocessing module, which is used to extract the characteristic values of the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under all charging current conditions;

[0043] A single risk coefficient calculation module, which is used to calculate the single safety risk coefficient of retired lithium batteries for a specific charging current condition;

[0044] A comprehensive risk coefficient calculation module, which is used to calculate the comprehensive safety risk coefficient of retired lithium batteries for all charging current conditions;

[0045] A charging safety level classification module, which is used to classify different safety levels and correspondingly give the charging current limit values.

[0046] The beneficial effects of the present invention are as follows:

[0047] The present invention provides a new method and system for evaluating the charging safety of retired lithium batteries, which can classify the safety levels of retired lithium batteries and correspondingly give the charging current limit values, so as to use this as an important basis for screening and recombining retired lithium batteries and formulating charging strategies, which is beneficial to recombining multiple batteries with the same safety level, thus facilitating the control and management of the recombined battery pack, further improving the consistency and safety of the recombined battery pack, and prolonging the service life and reducing the probability of thermal runaway. Description of the Drawings

[0048] In the following, the present invention will be described in more detail based on embodiments and with reference to the drawings. Among them:

[0049] Figure 1 Shows the flow chart of the evaluation method in the present invention;

[0050] Figure 2 Shows the structural schematic diagram of the evaluation system in the present invention;

[0051] Figure 3 Shows the circuit connection schematic diagram of the charging data acquisition module in the present invention;

[0052] In the drawings, like parts are designated by like reference numerals. The drawings are not drawn to scale.

[0053] Reference numerals:

[0054] 1 - Charging data acquisition module, 2 - Temperature data pre - processing module, 3 - Single risk coefficient calculation module, 4 - Comprehensive risk coefficient calculation module, 5 - Charging safety level division module, 6 - Retired lithium battery, 7 - Brand - new lithium battery, 8 - Lithium battery at the end of its life. Detailed implementation manners

[0055] The present invention will be further described below in conjunction with the drawings.

[0056] Embodiment 1

[0057] This embodiment provides a method for evaluating the charging safety of retired lithium batteries, as Figure 1 shown, which includes the following steps:

[0058] Step S1: Obtain the surface temperature data of the retired lithium battery 6, the brand - new lithium battery 7, and the lithium battery 8 at the end of its life under several charging current conditions;

[0059] Step S2: Extract the characteristic values of the surface temperature data of the retired lithium battery 6, the brand - new lithium battery 7, and the lithium battery 8 at the end of its life under all charging current conditions;

[0060] Step S3: Calculate the single safety risk coefficient of the retired lithium battery 6 for a specific charging current condition;

[0061] Step S4: Calculate the comprehensive safety risk coefficient of the retired lithium battery 6 for all charging current conditions;

[0062] Step S5: Divide different safety levels and correspondingly give the charging current limit values.

[0063] Specifically, in step S1, the acquisition period is 30 s, and the total charging time is not less than 10 min; 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 brand - new lithium battery 7, and the lithium battery 8 before charging SOC = 0;

[0064] 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 represented in 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), ……};

[0065] The surface temperature data of the brand-new lithium battery 7 under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C are respectively represented in sequence as { T n (0.25, 0), T n (0.25, 30), ……}, { T n (0.5, 0), T n (0.5, 30), ……}, { T n (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), ……};

[0066] The surface temperature data of the lithium battery 8 at the end of its life under the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C are respectively represented in 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), ……;

[0067] wherein, T a is the surface temperature of a retired lithium - ion battery, T n is the surface temperature of a brand - new lithium - ion battery, T e is the surface temperature of a lithium - ion battery at the end of its life;

[0068] T a (0.25, 0)is the surface temperature of a retired lithium - ion battery before charging under the charging current condition of 0.25C, T a (0.25, t )is the surface temperature of a retired lithium - ion battery during the charging duration t under the charging current condition of 0.25C; correspondingly, T n (0.25, 0)is the surface temperature of a brand - new lithium - ion battery before charging under the charging current condition of 0.25C, T n (0.25, 30)is the surface temperature of a brand - new lithium - ion battery after charging for 30s under the charging current condition of 0.25C, T e (0.25, 0)is the surface temperature of a lithium - ion battery at the end of its life before charging under the charging current condition of 0.25C, T e(0.25, 30) is the surface temperature of the end-of-life lithium battery when charged at a charging current condition of 0.25C for 30s. Similarly, the surface temperature data of the remaining retired lithium batteries 6, brand-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.

[0069] In step S2, the characteristic values of the surface temperature data of the retired lithium battery 6, brand-new lithium battery 7, and end-of-life lithium battery 8 under all charging current conditions are represented in 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)};

[0070] Among them, T am (0.25), T nm (0.25), T em (0.25) are the characteristic values of the retired lithium battery 6, brand-new lithium battery 7, and end-of-life lithium battery 8 under the charging current condition of 0.25C respectively, T am (0.25) = max{ Ta (0.25, 0), T a (0.25, t ), ……}; Similarly, the characteristic values of the retired lithium battery 6, the brand-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.

[0071] In step S3, let the charging safety risk coefficient of the brand-new lithium battery 7 be 0, and the charging safety risk coefficient of the end-of-life lithium battery 8 be 1. That is, the charging safety risk coefficient range of the retired lithium battery 6 is 0 to 1, and the single safety risk coefficients of the retired lithium battery 6 for 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);

[0072] C s (0.25) = ( T am (0.25) - T nm (0.25)) / ( T em (0.25) - T nm (0.25));

[0073] C s (0.5) = ( T am (0.5) - T nm (0.5)) / ( T em (0.5) - T nm (0.5));

[0074] C s (0.75) = ( T am (0.75) - T nm(0.75)) / ( T em (0.75)- T nm (0.75));

[0075] C s (1)=( T am (1)- T nm (1)) / ( T em (1)- T nm (1));

[0076] C s (2)=( T am (2)- T nm (2)) / ( T em (2)- T nm (2));

[0077] C s (3)=( T am (3)- T nm (3)) / ( T em (3)- T nm (3))。

[0078] In addition, several charging current conditions include several slow charging current conditions and several fast charging current conditions;

[0079] Calculating the comprehensive safety risk coefficient of the retired lithium battery 6 under all charging current conditions includes:

[0080] Calculating the multi-condition safety risk coefficient of the retired lithium battery 6 under all slow charging current conditions and the multi-condition safety risk coefficient under all fast charging current conditions;

[0081] According to the multi-condition safety risk coefficient under all slow charging current conditions and the multi-condition safety risk coefficient under all fast charging current conditions, calculate the comprehensive safety risk coefficient of the retired lithium battery 6 under all charging current conditions.

[0082] In step S4, the comprehensive safety risk coefficient of the retired lithium battery 6 under all charging current conditions is Cm ; C m = C m ( s ) * 0.8 + C m ( f ) * 0.2;

[0083] Wherein, C m ( s ) is the multi - condition safety risk coefficient of the retired lithium - ion battery 6 for all slow - charge current conditions, C m ( s ) = ( C s (0.25) + C s (0.5) + C s (0.75)) / 3;

[0084] C m ( f ) is the multi - condition safety risk coefficient of the retired lithium - ion battery 6 for all fast - charge current conditions, C m ( f ) = ( C s (1) + C s (2) + C s (3)) / 3.

[0085] In step S5, different safety levels are divided, and the corresponding charging current limit values are given as follows:

[0086] If 0 < C m ≤ 0.2, the thermal stability of the retired lithium - ion battery 6 during charging is good, and the upper limit value of the charging current is 1.5C;

[0087] If 0.2 < C m ≤ 0.4, the thermal stability of the retired lithium - ion battery 6 during charging is average, and the upper limit value of the charging current is 1C;

[0088] If 0.4 < C m < 1, the thermal stability of the retired lithium - ion battery 6 during charging is poor, and the upper limit value of the charging current is 0.5C.

[0089] By using the method for evaluating the charging safety of retired lithium batteries provided in this embodiment, the safety levels of the retired lithium batteries 6 are classified and the corresponding charging current limiting values are given, which are used as an important basis for screening, reorganizing and formulating charging strategies for the retired lithium batteries 6, so as to facilitate the reorganization of multiple batteries with the same safety level, thus 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.

[0090] Embodiment 2

[0091] Based on Embodiment 1, this embodiment evaluates the charging safety of multiple retired lithium batteries 6, so as to classify the safety levels of multiple retired lithium batteries 6, thus facilitating the reorganization of multiple retired lithium batteries 6 with the same safety level.

[0092] Embodiment 3

[0093] This embodiment provides a system for evaluating the charging safety of retired lithium batteries, which can perform the method for evaluating the charging safety of retired lithium batteries in Embodiment 1, as Figure 2 shown, and specifically includes:

[0094] The charging data acquisition module 1 is used to obtain the surface temperature data of the retired lithium battery 6, the brand-new lithium battery 7 and the end-of-life lithium battery 8 under several charging current conditions;

[0095] The temperature data preprocessing module 2 is used to extract the characteristic values of the surface temperature data of the retired lithium battery 6, the brand-new lithium battery 7 and the end-of-life lithium battery 8 under all charging current conditions;

[0096] The single risk coefficient calculation module 3 is used to calculate the single safety risk coefficient of the retired lithium battery 6 for a specific charging current condition;

[0097] The comprehensive risk coefficient calculation module 4 is used to calculate the comprehensive safety risk coefficient of the retired lithium battery 6 for all charging current conditions;

[0098] The charging safety level classification module 5 is used to classify different safety levels and give the corresponding charging current limiting values.

[0099] As Figure 3 shown, surface temperature sensors are attached to all the retired lithium batteries 6, brand-new lithium batteries 7 and end-of-life lithium batteries 8, and the charging data acquisition module 1 obtains through these surface temperature sensors.

[0100] Embodiment 4

[0101] Based on Embodiment 1, the charging safety of a retired lithium battery 6 is evaluated in this embodiment.

[0102] Specifically, surface temperature data of the retired lithium battery 6, brand-new lithium battery 7, and end-of-life lithium battery 8 under charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C are collected through step S1;

[0103] Characteristic values of the surface temperature data of the retired lithium battery 6, brand-new lithium battery 7, and end-of-life lithium battery 8 under all charging current conditions are extracted through step S2 and expressed in sequence 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};

[0104] Calculated through the calculation formula in step S3:

[0105] 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;

[0106] Calculated through the calculation formula in step S4:

[0107] C m ( s ) = 0.27, C m ( f ) ≈ 0.35;

[0108] Then calculated:

[0109] C m ≈ 0.29;

[0110] Therefore, the thermal stability of the retired lithium battery 6 during charging is average, and the upper limit value of the charging current is 1C.

[0111] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "bottom", "top", "front", "rear", "inner", "outer", "left", "right", etc. is based on the orientation or positional relationship shown in the drawings, and is 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 should not be construed as a limitation on the present invention.

[0112] Although the present invention has been 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. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed, as long as they do not depart from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the different dependent claims and the features described herein can be combined in a manner different from that described in the original claims. It should also be understood that the features described in connection with a single embodiment can be used in other described embodiments.

Claims

1. A method for evaluating the charging safety of retired lithium batteries, characterized in that, It includes the following steps: Step S1: Obtain the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under several charging current conditions; Step S2: Extract the characteristic values of the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under all charging current conditions; Step S3: Calculate the single safety risk coefficient of a retired lithium battery for a specific charging current condition; Step S4: Calculate the comprehensive safety risk coefficient of a retired lithium battery for all charging current conditions; Step S5: Divide different safety levels and correspondingly give the charging current limit values; In step S1, a period of t is obtained, and 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 the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C are respectively represented in sequence 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 represented in 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 end-of-life lithium batteries under charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C are respectively represented in 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 ), ……}; Among them, T a is the surface temperature of the retired lithium battery, T n is the surface temperature of the brand-new lithium battery, T e is the surface temperature of the lithium battery at the end of its life; T a (0.25, 0) is the surface temperature of the retired lithium battery before charging under the charging current condition of 0.25C, T a (0.25, t ) is the surface temperature of the retired lithium battery during the charging duration under the charging current condition of 0.25C; t ​ In step S2, the characteristic values of the surface temperature data of the retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under all charging current conditions are represented in 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)}; Among them, T am (0.25), T nm (0.25), T em (0.25) are the characteristic values of retired lithium batteries, brand-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 )...}; In step S3, the single safety risk coefficients of the 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））； The several charging current conditions include several slow charging current conditions and several fast charging current conditions; The calculation of the comprehensive safety risk coefficient of a retired lithium battery for all charging current conditions includes: Calculating the multi-condition safety risk coefficient of a retired lithium battery for all slow charging current conditions and the multi-condition safety risk coefficient for all fast charging current conditions; According to the multi-condition safety risk coefficient for all slow charging current conditions and the multi-condition safety risk coefficient for all fast charging current conditions, calculate the comprehensive safety risk coefficient of a retired lithium battery for all charging current conditions; In step S4, the comprehensive safety risk coefficient 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; Among them, C m ( s ) is the multi-condition safety risk coefficient of retired lithium batteries for all slow charging current conditions, C m ( s ) = ([ C C s (0.25) + C s (0.5) + C s (0.75)) / 3; C m ( f ) is the multi-condition safety risk coefficient of retired lithium batteries under all fast charging current conditions, C m ( f ) = ( C s (1) + C s (2) + C s (3)) / 3.

2. The method for evaluating the charging safety of retired lithium batteries according to claim 1, wherein In step S1, the remaining battery levels of the retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries before charging SOC = 0.

3. The method for evaluating the charging safety of retired lithium batteries according to claim 1, wherein, The division of different safety levels and the corresponding giving of the charging current limit values include: If 0 < C m ≤ 0.2, then the thermal stability of the retired lithium battery during charging is good, and the upper limit value of the charging current is 1.5C; If 0.2 < C m ≤ 0.4, the thermal stability of the retired lithium battery during charging is average, and the upper limit value of the charging current is 1C; If 0.4 < C m < 1, the thermal stability of the retired lithium battery during charging is poor, and the upper limit value of the charging current is 0.5C.

4. The method for evaluating the charging safety of retired lithium batteries according to claim 1, wherein In Steps S1 to S5, the number of retired lithium batteries is at least two; It further includes the following step: Recombine the retired lithium batteries with the same safety level.

5. A charging safety evaluation system for retired lithium batteries, characterized in that It includes: A charging data acquisition module, which is used to obtain the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under several charging current conditions; A temperature data preprocessing module, which is used to extract the characteristic values of the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under all charging current conditions; A single risk coefficient calculation module, which is used to calculate the single safety risk coefficient of a retired lithium battery for a specific charging current condition; A comprehensive risk coefficient calculation module, which is used to calculate the comprehensive safety risk coefficient of a retired lithium battery for all charging current conditions; A charging safety level division module, which is used to divide different safety levels and correspondingly give the charging current limit values; Among them, the acquisition period of the charging data acquisition module is t , and 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 the charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C are respectively represented in sequence 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 represented in 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 end-of-life lithium batteries under charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C are respectively represented in 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 ), ……}; Among them, T a is the surface temperature of the retired lithium battery, T n is the surface temperature of the brand-new lithium battery, T e is the surface temperature of the lithium battery at the end of its life; T a (0.25, 0) is the surface temperature of the retired lithium battery before charging under the charging current condition of 0.25C, T a (0.25, t ) is the surface temperature of the retired lithium battery during the charging duration under the charging current condition of 0.25C; t ​ The temperature data preprocessing module represents the eigenvalues of the surface temperature data of retired lithium batteries, brand-new lithium batteries, and end-of-life lithium batteries under all charging current conditions 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)}; Among them, T am (0.25), T nm (0.25), T em (0.25) are the characteristic values of retired lithium batteries, brand-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 )...}; The single risk coefficient calculation module calculates the single safety risk coefficients of retired lithium batteries under six specific charging current conditions of 0.25C, 0.5C, 0.75C, 1C, 2C, and 3C, respectively, as 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））； The comprehensive risk coefficient calculation module calculates the multi-condition safety risk coefficients of retired lithium batteries for all slow charging current conditions C m ( s ), and the multi-condition safety risk coefficients for all fast charging current conditions C m ( f ), where C m ( s ) = (( C s (0.25) + C s (0.5) + C s (0.75)) / 3, C m ( f ) = (( C s (1) + C s (2) + C s (3)) / 3; According to the multi-condition safety risk coefficients under all slow charge current conditions C m ( s ) and the multi-condition safety risk coefficients for all fast charge current conditions C m ([[]] f ), calculate the comprehensive safety risk coefficient of retired lithium batteries for all charge current conditions C m , where C m = C m ([[]] s ) * 0.8 + C m ([[]] f ). * 0.2

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