A method for evaluating and predicting the cycle life of lithium-ion batteries

Abstract

The present invention discloses a method for evaluating and predicting the cycle life of a lithium-ion battery, comprising the following steps: (1) calculating the number of cycles reaching the discharge end point of the battery cycle life according to the operation reliability theory; (2) measuring the restrictiveness of the battery charge and discharge cycle state in a temperature environment based on the restrictive indicators of the discharge capacity variance and the discharge capacity entropy; (3) adopting a classification sorting method in terms of battery charge and discharge test sorting: in order to balance the requirements of calculation time and calculation accuracy, only for batteries that reach the cycle discharge end point in advance due to normal and abnormal conditions, the number of cycles cut off by the charge and discharge cycle of these batteries is searched, and the number of cycles is sorted. Then, for batteries with fewer cycle numbers, a comprehensive restrictive indicator obtained by weighting the discharge capacity restrictive indicator is used for quick sorting, so as to realize the battery cycle number feature extraction and achieve accurate evaluation and prediction of the cycle life, thereby meeting the requirements of different battery cycle life tests.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a method for evaluating and predicting the cycle life of lithium ion batteries. Background Art

[0002] Lithium-ion batteries are widely used in mobile digital products and, in recent years, have also been incorporated into electric vehicles as energy storage systems. Service life is a key indicator of battery performance. Cycling tests under specific operating conditions are the most widely used life testing method in the development, inspection, and selection of lithium-ion batteries. Current standards for testing the cycle life of lithium-ion batteries generally refer to the provisions of two current Chinese standards: QC / T743-2006 Lithium-ion Batteries for Electric Vehicles and QB / T2502-2000 General Specification for Lithium-ion Batteries. Both testing processes are time-consuming.

[0003] In response to this, Chinese invention patent CN106124997A discloses a high-temperature life test method for lithium iron phosphate batteries. The method comprises the following steps: taking a fully formed lithium iron phosphate battery, first performing a small current full charge at a set temperature, then performing a small current float charge at a high temperature for a specific time, and then discharging at a set temperature after the float charge is completed. The above steps constitute one cycle. After several cycles, the capacity is taken three times at the set temperature and compared with 75% of the nominal capacity to determine whether it is qualified. The high-temperature life test method for lithium iron phosphate batteries of this invention has the characteristics of high test accuracy, short test cycle, convenient test process and strong reference value.

[0004] However, this method is only applicable to lithium iron phosphate batteries, and its testing process is based on statistical methods, and there are certain limitations on test accuracy and time consumption. Summary of the Invention

[0005] The technology of the present invention proposes a method for evaluating and predicting the cycle life of lithium-ion batteries. By combining the open circuit voltage and internal resistance of the battery and measuring the discharge capacity of the battery under temperature environment, the cycle life of the battery is quickly and accurately evaluated and predicted.

[0006] The present invention can be achieved through the following technical solutions:

[0007] The present invention discloses a method for evaluating and predicting the cycle life of a lithium-ion battery, comprising the following steps:

[0008] (1) According to the operational reliability theory, the temperature environment, open circuit voltage and internal resistance of the battery are taken into account, and the battery life reliability model that takes into account the kurtosis statistical factors and charge and discharge is adopted to calculate the number of cycles required to reach the discharge end point of the battery cycle life;

[0009] (2) Based on the restrictive indicators of discharge capacity variance and discharge capacity entropy, the restrictiveness of the battery charge and discharge cycle state in the temperature environment is measured;

[0010] (3) In terms of battery charge and discharge test sorting, a classification sorting method is adopted: in order to balance the requirements of calculation time and calculation accuracy, only the normal and abnormal batteries that reach the end of the cycle discharge in advance are searched for the number of cycles ended by these batteries, and the number of cycles is sorted. Then, for batteries with fewer cycles, the comprehensive restriction index obtained by weighting the discharge capacity and internal resistance restriction index is used for quick sorting to realize the battery cycle number feature extraction.

[0011] Furthermore, in step (1), after comprehensively considering the temperature environment, open circuit voltage and internal resistance of the battery, the reliability model of the battery is shown as follows:

[0012]

[0013] Furthermore, in step (2), the restrictive index includes a battery charge and discharge cycle restrictive index based on discharge capacity variance and a battery charge and discharge cycle restrictive index based on discharge capacity entropy.

[0014] Furthermore, in the battery charge and discharge cycle limiting index based on discharge capacity variance, the charge and discharge capacity variance of the battery under normal temperature environment can be expressed as:

[0015]

[0016] Where n and η i0 are the discharge capacity of the normal temperature environment during charge and discharge and the discharge capacity of the battery for the i-th time,

[0017] The discharge capacity variance of the temperature environment after the battery reaches the end of the cycle discharge k times It can be expressed as:

[0018]

[0019]

[0020] Where η k,i is the discharge capacity of the battery i times after the battery reaches the end of cycle discharge k times,

[0021] In summary, the limiting index of battery k regarding the variance of charge and discharge capacity is defined as follows:

[0022]

[0023] Furthermore, in the battery charge and discharge cycle limiting index based on discharge capacity entropy, the discharge capacity entropy H0 of the battery at each time during normal operation in the temperature environment is defined as follows:

[0024]

[0025]

[0026] Where η i0 (or η j0 ) are the discharge capacity of the normal charge and discharge temperature environment and the discharge capacity of the battery for the i-th (or j-th) time,

[0027] After the battery reaches the end of cycle discharge k times, the discharge capacity of the temperature environment battery times capacity entropy H k The definition is as follows:

[0028]

[0029]

[0030] Where η k,i (or η k,j ) is the discharge capacity of the battery i (or j) time after the battery reaches the end of the cycle discharge k times;

[0031] The limiting indicator of battery k based on the entropy of charge and discharge capacity is defined as follows:

[0032]

[0033] Similarly, the indicators in the following formula can be normalized, and the normalized indicators based on the charge and discharge capacity entropy can be obtained as shown in the following formula.

[0034]

[0035] Where,

[0036] Furthermore, the end point of the cycle life discharge is set to 80% of the initial discharge capacity, the internal resistance is the internal resistance of the battery in a fully charged state, and the open circuit voltage is the battery voltage after being left stable in the fully charged state.

[0037] The method for evaluating and predicting the cycle life of a lithium-ion battery of the present invention has the following beneficial technical effects:

[0038] The method of the present invention can be used to calculate the battery cycle life under the temperature environment charge and discharge cycle state in combination with the battery's open circuit voltage and internal resistance. It can effectively extract the weak influencing factors of the battery cycle life, accurately predict and evaluate the battery cycle life, with high accuracy and short time consumption, and meet the needs of cycle life prediction and evaluation of different types of lithium-ion batteries. DETAILED DESCRIPTION

[0039] In order to enable those skilled in the art to better understand the technical solution of the present invention, the following is a further detailed description of the product of the present invention in conjunction with embodiments.

[0040] The present invention discloses a method for evaluating and predicting the cycle life of a lithium-ion battery, comprising the following steps:

[0041] (1) According to the operational reliability theory, the temperature environment, open circuit voltage and internal resistance of the battery are taken into account, and the battery life reliability model that takes into account the kurtosis statistical factors and charge and discharge is adopted to calculate the number of cycles required to reach the discharge end point of the battery cycle life;

[0042] (2) Based on the restrictive indicators of discharge capacity variance and discharge capacity entropy, the restrictiveness of the battery charge and discharge cycle state in the temperature environment is measured;

[0043] (3) In terms of battery charge and discharge test sorting, a classification sorting method is adopted: in order to balance the requirements of calculation time and calculation accuracy, only the normal and abnormal batteries that reach the end of the cycle discharge in advance are searched for the number of cycles ended by these batteries, and the number of cycles is sorted. Then, for batteries with fewer cycles, the comprehensive restriction index obtained by weighting the discharge capacity and internal resistance restriction index is used for quick sorting to realize the battery cycle number feature extraction.

[0044] Furthermore, in step (1), after comprehensively considering the temperature environment, open circuit voltage and internal resistance of the battery, the reliability model of the battery is shown as follows:

[0045]

[0046] Furthermore, in step (2), the restrictive index includes a battery charge and discharge cycle restrictive index based on discharge capacity variance and a battery charge and discharge cycle restrictive index based on discharge capacity entropy.

[0047] Furthermore, in the battery charge and discharge cycle limiting index based on discharge capacity variance, the charge and discharge capacity variance of the battery under normal temperature environment can be expressed as:

[0048]

[0049] Where n and η i0 are the discharge capacity of the normal temperature environment during charge and discharge and the discharge capacity of the battery for the i-th time,

[0050] The discharge capacity variance of the temperature environment after the battery reaches the end of the cycle discharge k times It can be expressed as:

[0051]

[0052]

[0053] Where η k,i is the discharge capacity of the battery i times after the battery reaches the end of cycle discharge k times,

[0054] In summary, the limiting index of battery k regarding the variance of charge and discharge capacity is defined as follows:

[0055]

[0056] Furthermore, in the battery charge and discharge cycle limiting index based on discharge capacity entropy, the discharge capacity entropy H0 of the battery at each time during normal operation in the temperature environment is defined as follows:

[0057]

[0058]

[0059] Where η i0 (or η j0 ) are the discharge capacity of the normal charge and discharge temperature environment and the discharge capacity of the battery for the i-th (or j-th) time,

[0060] After the battery reaches the end of cycle discharge k times, the discharge capacity of the temperature environment battery times capacity entropy H k The definition is as follows:

[0061]

[0062]

[0063] Where η k,i (or η k,j ) is the discharge capacity of the battery i (or j) time after the battery reaches the end of the cycle discharge k times;

[0064] The limiting indicator of battery k based on the entropy of charge and discharge capacity is defined as follows:

[0065]

[0066] Similarly, the indicators in the following formula can be normalized, and the normalized indicators based on the charge and discharge capacity entropy can be obtained as shown in the following formula.

[0067]

[0068] Where,

[0069] Furthermore, the end point of the cycle life discharge is set to 80% of the initial discharge capacity, the internal resistance is the internal resistance of the battery in a fully charged state, and the open circuit voltage is the battery voltage after being left stable in the fully charged state.

[0070] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any ordinary technician in this industry can smoothly implement the present invention as shown in the specification and described above. However, any technician familiar with this profession can make some changes, modifications and equivalent changes made by using the technical content disclosed above without departing from the scope of the technical solution of the present invention, which are all equivalent embodiments of the present invention. At the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the scope of protection of the technical solution of the present invention.

Claims

1. A method for evaluating and predicting the cycle life of a lithium-ion battery, characterized in that The following steps are involved: (1) According to the operational reliability theory, the temperature environment, open circuit voltage and internal resistance of the battery are taken into account, and the battery life reliability model that takes into account the kurtosis statistical factors and charge and discharge is adopted to calculate the number of cycles required to reach the discharge end point of the battery cycle life; (2) Based on the restrictive indicators of discharge capacity variance and discharge capacity entropy, the restrictiveness of the battery charge and discharge cycle state in the temperature environment is measured; (3) In terms of battery charge and discharge test sorting, a classification sorting method is adopted: in order to balance the requirements of calculation time and calculation accuracy, only the normal and abnormal batteries that reach the end of the cycle discharge in advance are searched for the number of cycles ended by these batteries, and the number of cycles is sorted. Then, for batteries with fewer cycles, the comprehensive restriction index obtained by weighting the discharge capacity and internal resistance restriction index is used for quick sorting to realize the battery cycle number feature extraction.

2. The method for evaluating and predicting the cycle life of a lithium-ion battery according to claim 1, wherein: In step (1), after comprehensively considering the temperature environment, open circuit voltage and internal resistance of the battery, the battery reliability model is shown as follows:

3. The method for evaluating and predicting the cycle life of a lithium-ion battery according to claim 2, wherein: In step (2), the restriction index includes a battery charge-discharge cycle restriction index based on discharge capacity variance and a battery charge-discharge cycle restriction index based on discharge capacity entropy.

4. The method for evaluating and predicting the cycle life of a lithium-ion battery according to claim 3, wherein: In the battery charge and discharge cycle limiting index based on discharge capacity variance, the charge and discharge capacity variance of the battery under normal temperature environment can be expressed as: Where n is the total number of normal charge and discharge cycles, η i0 is the discharge capacity of the i-th cycle during normal charge and discharge; The discharge capacity variance of the temperature environment after the battery reaches the end of the cycle discharge k times It can be expressed as: Where η k,i is the discharge capacity of the battery i times after the battery reaches the end of cycle discharge k times, The limiting indicator for battery k regarding the variance of charge and discharge capacity is defined as follows:

5. The method for evaluating and predicting the cycle life of a lithium-ion battery according to claim 3, wherein: In the battery charge and discharge cycle limiting index based on discharge capacity entropy, the discharge capacity entropy H0 of the battery at each time during normal operation in the temperature environment is defined as follows: Where η i0 or η j0 are the discharge capacity under normal temperature environment during charge and discharge and the discharge capacity of the battery for the i or j time, After the battery reaches the end of cycle discharge k times, the discharge capacity of the temperature environment battery times capacity entropy H k The definition is as follows: Where η k,i or η k,j is the discharge capacity of the battery i or j times after the battery reaches the end of cycle discharge k times; The limiting indicator of battery k based on the entropy of charge and discharge capacity is defined as follows: Similarly, the indicators in the following formula can be normalized, and the normalized indicators based on the charge and discharge capacity entropy can be obtained as shown in the following formula. Where, 6. The method for evaluating and predicting the cycle life of a lithium-ion battery according to claim 3, wherein: The end point of the cycle life discharge is set to 80% of the initial discharge capacity, the internal resistance is the internal resistance of the battery in the fully charged state, and the open circuit voltage is the battery voltage after the fully charged state is left to stabilize.

Images