Cell capacity grading method

Abstract

The invention provides a battery core capacity grading method, which comprises the following steps of: performing deep charging and shallow discharging on a battery which is formed and prepared for capacity grading at a high SOC (State of Charge), performing deep discharging and shallow charging at a low SOC, and calculating the sum of the capacity after the first deep discharging and the climbing capacity; respectively calculating the difference value between the capacity at each temperature and the capacity at the temperature of 25 DEG C of the activated battery cells, fitting a relation curve of the column of capacity difference values and the corresponding temperatures, and performing temperature correction compensation on all the battery cells according to the relation curve fitted by the corresponding temperatures, so that the compensated capacity is the capacity of the battery cells; according to the method, deep charging and shallow discharging are carried out on the battery cell under the high SOC and deep discharging and shallow charging are carried out under the low SOC, the activation degree is judged by calculating the ratio of the capacity difference of each time of charging and discharging to the rated capacity, and the activated battery cell capacity is calculated for matching and grading; according to the method, the accurate capacity of the activated battery cell can be obtained, the time is short, the cost is low, the battery cells are grouped in a grading manner, the real capacity difference is small, and the voltage difference after module assembling is smaller.

Description

Technical Field

[0001] The present invention relates to the field of lithium ion batteries, and in particular to a method for dividing the capacity of a battery cell. Background Art

[0002] Lithium-ion batteries are increasingly used in the fields of energy storage and power, and cell consistency is crucial to the life and safety of cells in applications. When cells are grouped, a matching method is usually adopted to classify the capacity and internal resistance of the cells, and the cells of the same level are assembled into modules to improve the consistency of the cells in the module. Among them, the capacity of the cell is usually taken from the capacity division during the cell production and manufacturing process, so the accuracy of the capacity division directly affects the consistency of the cell grouping and grading.

[0003] At present, the common method of cell capacity grading is to obtain the capacity by subjecting the cell to a charge-discharge cycle after secondary liquid injection. However, due to the activation characteristics of the positive and negative electrode materials, the capacity of the cell will "climb" during the subsequent charge and discharge cycles, and the capacity will increase to varying degrees, and the capacity increase of each cell will be different. This results in the cell in the module assembled according to the capacity grading still having capacity inconsistency after 1 to 2 cycles, resulting in abnormal pressure difference.

[0004] The usual method is to charge and discharge the battery cell 2 to 4 times when dividing the capacity, and take the last capacity as the capacity for grading. However, the disadvantage of this method is that it greatly increases the time of the capacity grading process and increases the manufacturing cost of the battery cell.

[0005] To this end, we propose a method for cell capacity grading to improve the accuracy of cell capacity grading, improve the consistency of cells in the module after capacity grading, and reduce costs. Summary of the invention

[0006] The purpose of the present invention is to solve the shortcomings existing in the prior art. In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A method for cell capacity division, the steps are as follows:

[0008] Step 1: Charge the battery that has been formed and is ready for capacity division with a current I 1 Constant current and constant voltage charging to the upper cut-off voltage, the cut-off current is I 2 , after the battery is fully charged, it is set to be set to T;

[0009] Step 2: With rated current I e Constant current discharge 5% ~ 20% SOC power, set the time T, and then discharge with current I 1 Charge at constant current until the upper cut-off voltage is reached, and record the charge capacity as C 1 ;

[0010] Step 3: Repeat step 2 and record the charging capacity in step 2 as C. 2 , C 3 ~C i , until C i With C i-1 The ratio of the capacity difference to the rated capacity of the battery cell is less than X%, and the battery cell is deeply charged and shallowly discharged at high SOC to achieve the purpose of activating the positive and negative electrode materials;

[0011] Step 4: With rated current I e Discharge at a constant current until the lower cut-off voltage is reached, and the discharge capacity is recorded as D m ;

[0012] Step 5: With current I 1 Constant current charging 5% ~ 20% SOC power, set the time to T, and then charge with rated current I e Discharge to the lower cut-off voltage and record the discharge capacity D 1 ;

[0013] Step 6: Repeat step 5 and record the discharge capacities in step 5 as D 2 , D 3 ~D i , until D i With D i-1 The ratio of the capacity difference to the rated capacity of the battery cell is less than Y%;

[0014] Step 7: Calculate D i With D 1 The difference is △D, calculate D m The sum of the capacity of △D is D n ;

[0015] Step 8: Take Z cells that have completed step 6 and place them in a constant temperature box at a temperature ranging from 20 to 30°C, with a rated current I e Perform a charge and discharge, record the corresponding discharge capacity, calculate the difference between the capacity at each temperature and the capacity at 25°C, and fit the relationship curve F=f(t) between the difference in capacity and the corresponding temperature;

[0016] Step 9: Use the temperature-capacity relationship curve in step 8 to compensate Dn and obtain the compensated capacity Dn', Dn'=Dn+F, which is the capacity of the battery cell at 25°C;

[0017] Step 10: Group the capacity into different grades according to the compensated capacity Dn' and assemble them into modules.

[0018] The present invention performs deep charging and shallow discharging of battery cells at high SOC and deep discharging and shallow charging at low SOC, calculates the ratio of the capacity difference between each charge and discharge and the rated capacity, determines the degree of activation, and calculates the capacity of the battery cells after activation for grouping and grading.

[0019] Further preferably, the current I 1 is 0.1~1C, and the cut-off current is I 2 It is 0.01~0.05C.

[0020] Further preferably, the set shelving time T is 5 minutes to 30 minutes to balance the charge distribution inside the battery and eliminate the internal stress of the battery cell.

[0021] More preferably, the value of X in step three is 0.5-2.

[0022] Further preferably, the value of Y in step six is ​​0.1-1.

[0023] Further preferably, the Z value in step eight is not less than 5.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention performs deep charging and shallow discharging of battery cells at high SOC and deep discharging and shallow charging at low SOC, calculates the ratio of the capacity difference between each charge and discharge and the rated capacity, determines the degree of activation, and calculates the capacity of the battery cells after activation for grouping and grading. The present invention can obtain the precise capacity of the battery cells after activation, and the time is short and the cost is low. The battery cells are grouped in grades, the actual capacity difference is small, and the pressure difference is smaller after the module is assembled. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0027] A method for cell capacity division, characterized in that the steps are as follows:

[0028] Step 1: Charge the battery that has been formed and is ready for capacity division with a current I 1 Constant current and constant voltage charging to the upper cut-off voltage, the cut-off current is I 2 After the battery is fully charged, it is set aside for a set time of T to balance the charge distribution inside the battery and eliminate the internal stress of the battery cell; the current I 1 is 0.1~1C, and the cut-off current is I 2 The temperature is 0.01-0.05°C; the set time T for the standby is 5-30 minutes;

[0029] Step 2: With rated current Ie The constant current discharge is 5% to 20% SOC, and the set time is T, and the set time T is 5min to 30min; to balance the charge distribution inside the battery and eliminate the internal stress of the battery cell, and then the current I 1 Charge at constant current until the upper cut-off voltage is reached, and record the charge capacity as C 1 ;

[0030] Step 3: Repeat step 2 and record the charging capacity in step 2 as C. 2 , C 3 ~C i , until C i With C i-1 The ratio of the capacity difference to the rated capacity of the battery cell is less than X%; the value of X is 0.5 to 2;

[0031] The present invention first fully charges the battery cell to be divided, then discharges a certain amount of electricity, and then charges to the cut-off voltage, and repeatedly charges and discharges several times to achieve the purpose of activating the positive and negative electrode materials. Because the positive and negative electrode materials have various defects inside the materials before they are fully activated, the lithium ion deintercalation channel is blocked. Under high SOC conditions, especially above >90% SOC, the difficulty of positive electrode delithiation and negative electrode lithium intercalation gradually increases, which is manifested as a large DCR of the battery cell under high SOC. Through repeated charging and discharging under high SOC, some defects can be eliminated and the channels for lithium ions to enter and exit the material can be opened. By calculating the ratio of the capacity difference of each charge to the rated capacity, the degree of activation under high SOC can be analyzed.

[0032] Step 4: With rated current I e Discharge at constant current until the lower cut-off voltage is reached, and record the discharge capacity D m ;

[0033] After activation is completed under high SOC state, discharge is performed to the lower cut-off voltage to obtain the initial capacity.

[0034] Step 5: With current I 1 Constant current charging 5% to 20% SOC, set the shelf time to T, and the shelf time T is 5min to 30min; to balance the charge distribution inside the battery and eliminate the internal stress of the battery cell; then charge the battery at the rated current I e Discharge to the lower cut-off voltage and record the discharge capacity D 1 ;

[0035] Step 6: Repeat step 5 and record the discharge capacities in step 5 as D 2 , D 3 ~D i , until D i With D i-1The ratio of the capacity difference to the rated capacity of the battery cell is less than Y%; the value of Y is 0.1 to 1;

[0036] The battery cell is charged to a certain amount of electricity, then discharged to the cut-off voltage, and charged and discharged repeatedly several times to achieve the purpose of activating the positive and negative electrode materials. When the battery cell is in a low SOC state, especially below 20% SOC, the difficulty of positive electrode lithium removal and negative electrode lithium insertion gradually increases, which is also manifested in the large DCR of the battery cell at low SOC. Through repeated charging and discharging at low SOC, some defects can be eliminated and the lithium ion channel can be opened. By calculating the ratio of the capacity difference of each discharge to the rated capacity, the degree of activation at low SOC can be analyzed.

[0037] Step 7: Calculate D i With D 1 The difference is △D, calculate D m The sum of the capacity of △D is D n ;

[0038] Step 8: Take Z cells that have completed step 6, where the value of Z is not less than 5; place them in a constant temperature box at a temperature ranging from 20 to 30°C, and heat them at a rated current I every 1°C. e Perform a charge and discharge, record the corresponding discharge capacity, calculate the difference between the capacity at each temperature and the capacity at 25°C, and fit the relationship curve F=f(t) between the difference in capacity and the corresponding temperature;

[0039] Step 9: Use the temperature-capacity relationship curve in step 8 to calculate D n Perform compensation correction to obtain the compensated capacity D n ', D n '=D n +F, which is the capacity of the battery at 25℃;

[0040] Calculate the capacity in step 4 and the difference between the last discharge and the first discharge in step 6; take the activated cells and calculate the capacity at each temperature and the capacity difference at 25°C, and fit the capacity difference in this column to the corresponding temperature to form a relationship curve. All cells can be compensated for temperature correction by fitting the relationship curve to the corresponding temperature, and the capacity after compensation is the capacity of the cell.

[0041] Step 10: Group the capacity into different grades according to the compensated capacity Dn' and assemble them into modules.

[0042] The present invention performs deep charging and shallow discharging of battery cells at high SOC and deep discharging and shallow charging at low SOC, calculates the ratio of the capacity difference between each charge and discharge and the rated capacity, determines the degree of activation, and calculates the capacity of the battery cells after activation for grouping and grading. The present invention can obtain the precise capacity of the battery cells after activation, and the time is short and the cost is low. The battery cells are grouped in grades, the actual capacity difference is small, and the pressure difference is smaller after the module is assembled.

[0043] Example 1

[0044] Take 200 lithium iron phosphate batteries with a rated current of 0.5C and a rated current of 100Ah, and implement the following steps:

[0045] Step 1: Charge at a constant current and constant voltage of 0.2C to an upper cut-off voltage of 3.65V, and a cut-off current of 0.05C (the external power supply first charges the battery at a constant current of 0.2C, and after the battery voltage rises to 3.65V, the external power supply charges the battery at a constant voltage, and the battery current drops to 0.05C and is cut off), and then leave it for 5 minutes;

[0046] Step 2: Discharge 10% SOC at a rated current of 0.5C, leave for 10 minutes, then charge at a current of 0.2C to an upper cut-off voltage of 3.65V, record the charge capacity as C 1 ;

[0047] Step 3: Repeat step 2 and record the charging capacity in step 2 as C. 2 , C 3 ~C i , until C i With C i-1 The ratio of the capacity difference to the rated capacity of the battery cell is less than 0.5%, and the first activation is considered to be completed, and the next step is entered;

[0048] Step 4: Discharge at a constant current of 0.5C to a lower cut-off voltage of 2.5V, and record the discharge capacity as D m ;

[0049] Step 5: Charge 10% at a rated current of 0.5C, leave for 10 minutes, then discharge at a rated current of 0.5C to a lower cut-off voltage of 2.5V, and record the discharge capacity D 1 ;

[0050] Step 6: Repeat step 5 and record the discharge capacities in step 5 as D 2 , D 3 ~D i , until D i With D i-1 The ratio of the capacity difference of Di to the rated capacity of the battery cell is less than 0.5%, and the second activation is determined to be completed. At the same time, the difference between Di and D1 is △D, and D is calculated. m The sum of the capacity of △D is D n ;

[0051] Step 7: Take 10 cells that have completed step 6, and charge and discharge them at different temperatures t (t is 20, 25, 30, and 35°C) in a constant temperature box at a rated current of 0.5C. Record the difference between the discharge capacity and the capacity at 25°C as F, and fit the capacity difference in this column with the corresponding temperature to form a relationship curve F=f(t);

[0052] Step 8: D n Make corrections and use the temperature-capacity relationship curve in step 7 to adjust D n Perform compensation correction to obtain the compensated capacity D n ', D n '=D n +F, which is the capacity of the battery cell at 25℃; finally, the capacity is divided into groups according to the compensated capacity Dn' and assembled into a 1P16S module;

[0053] The module is charged and discharged at a rated current of 0.5C to obtain its charging voltage difference and discharging voltage difference.

[0054] Example 2

[0055] The implementation method and steps are the same as those of Example 1, except that the discharge capacity in step 2 is 5% SOC.

[0056] Example 3

[0057] The implementation method and steps are the same as those of Example 1, except that the charging current in step 2 is 0.5C.

[0058] Example 4

[0059] The implementation method and steps are the same as those in Example 1, except that in step 3, C i With C i-1 The ratio of the capacity difference to the rated capacity of the battery cell is less than 1%.

[0060] Example 5

[0061] The implementation method and steps are the same as those of Example 1, except that the charging capacity in step 5 is 5% SOC.

[0062] Example 6

[0063] The implementation method and steps are the same as those in Example 1, except that in step 6, D i With D i-1 The ratio of the capacity difference to the rated capacity of the battery cell is less than 2%.

[0064] 200 cells were taken from each of Examples 1 to 6 for capacity testing, and then the standard deviation σ of the capacity was calculated. After being binned according to the same binning and grouping conditions, they were assembled into a 1P16S module, and charged and discharged at a rated current of 0.5C to obtain the charge and discharge voltage difference. The information is as shown in the following table:

[0065] Group Capacity standard deviation σ value Ah Module charging voltage difference mV Module discharge voltage difference mV Example 1 0.2 26 43 Example 2 0.22 31 52 Example 3 0.24 33 56 Example 4 0.31 42 68 Example 5 0.22 30 49 Example 6 0.29 38 65

[0066] The above table shows the results obtained by setting different boundary conditions in different embodiments. It can be seen that the capacity consistency of the battery cells grouped by the present invention is better, and the smaller the charge and discharge voltage difference of the module, the better the capacity consistency.

[0067] The present invention performs deep charging and shallow discharging of battery cells at high SOC and deep discharging and shallow charging at low SOC, calculates the ratio of the capacity difference between each charge and discharge and the rated capacity, determines the degree of activation, and calculates the capacity of the battery cells after activation for grouping and grading. The present invention can obtain the precise capacity of the battery cells after activation, and the time is short and the cost is low. The battery cells are grouped in grades, the actual capacity difference is small, and the pressure difference is smaller after the module is assembled.

Claims

1. A method for dividing the capacity of a battery cell, characterized in that: Here are the steps: Step 1: Charge the battery that has been formed and is ready for capacity division with a constant current and constant voltage of current I1 to the upper cut-off voltage, the cut-off current is I2, and the battery is set aside for a set time of T after charging is completed; Step 2: With rated current I e Discharge 5%~20% SOC at constant current, set the time to T, then charge at constant current I1 to the upper cut-off voltage, and record the charge capacity as C1; Step 3: Repeat step 2 and record the charging capacities in step 2 as C2, C3~C i , until C i With C i-1 The ratio of the capacity difference to the rated capacity of the battery cell is less than X%; Step 4: With rated current I e Discharge at a constant current until the lower cut-off voltage is reached, and the discharge capacity is recorded as D m ; Step 5: Charge 5%~20% SOC with constant current I1, set the time T, and then charge with rated current I e Discharge to the lower cut-off voltage and record the discharge capacity D1; Step 6: Repeat step 5, and record the discharge capacities in step 5 as D2, D3~D i , until D i With D i-1 The ratio of the capacity difference to the rated capacity of the battery cell is less than Y%; Step 7: Calculate D i The difference from D1 is △D, calculate D m The sum of the capacity of △D is D n ; Step 8: Take Z cells that have completed step 6 and place them in a constant temperature box at a temperature ranging from 20 to 30°C, with a rated current I e Perform a charge and discharge, record the corresponding discharge capacity, calculate the difference between the capacity at each temperature and the capacity at 25°C, and fit the relationship curve F=f(t) between the difference in capacity and the corresponding temperature; Step 9: Use the temperature-capacity relationship curve in step 8 to compensate Dn and obtain the compensated capacity Dn', Dn'=Dn+F, which is the capacity of the battery cell at 25°C; Step 10: Group the capacity into different grades according to the compensated capacity Dn' and assemble them into modules.

2. A method for cell capacity division according to claim 1, characterized in that: The current I1 is 0.1-1C, and the cut-off current I2 is 0.01-0.05C.

3. A method for cell capacity division according to claim 2, characterized in that: The set suspension time T is 5 minutes to 30 minutes.

4. A method for cell capacity division according to claim 1, characterized in that: The value of X in step 3 is 0.5-2.

5. A method for cell capacity division according to claim 1, characterized in that: The value of Y in step six is ​​0.1-1.

6. A method for cell capacity division according to claim 1, characterized in that: In step eight, the Z value is not less than 5.