A method for screening consistency of lithium ion batteries

Abstract

The application discloses a lithium ion battery consistency screening method, comprising the following steps: step one, providing lithium ion batteries to be screened; step two, charging each lithium ion battery to voltage V1 at current I1, and then voltage V1 is kept constant until current drops to I2, and the constant voltage time t1 is recorded; step three, keeping the lithium ion battery; step four, discharging the lithium ion battery after keeping to voltage V2 at current I3, and then voltage V2 is kept constant until current drops to I4, and the constant voltage time t2 is recorded; and step five, calculating the ratio t of the constant voltage time t1 and the constant voltage time t2, and grading the lithium ion battery according to the ratio t. The method simultaneously considers charging polarization and discharging polarization, obtains a battery with consistent charging polarization and discharging polarization, and can limit avoiding that a battery pack is controlled to charge or discharge in advance due to excessive polarization of a single battery in a long-term charging and discharging cycle.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, and more specifically to a method for screening the consistency of lithium-ion batteries. Background Technology

[0002] Lithium-ion battery packs are typically composed of multiple lithium-ion battery cells connected in series and parallel. The consistency of lithium-ion batteries is a key factor affecting the performance of the battery pack.

[0003] Currently, there are two main methods for screening the consistency of lithium-ion batteries: static and dynamic. The static method uses statistical analysis of parameters such as battery capacity, charge retention rate, and internal resistance to eliminate anomalies. This method is convenient for battery grouping and effectively utilizes battery performance parameters, but it has a large margin of error and cannot reflect the dynamic characteristics of lithium-ion batteries during actual operation. The dynamic method matches discharge curves under specified operating conditions. This method has high screening accuracy, but it requires a large amount of data, is overly complex, and is unsuitable for large-scale engineering applications, severely impacting the efficiency of consistency screening. Summary of the Invention

[0004] The purpose of this invention is to provide a highly efficient, accurate, and simple method for screening lithium-ion batteries, thereby effectively improving the efficiency of battery consistency screening.

[0005] To achieve the above objectives, the present invention provides a lithium-ion battery consistency screening method, comprising:

[0006] Step 1: Provide the lithium-ion batteries to be screened;

[0007] Step 2: Charge each lithium-ion battery with a constant current of I1 to a voltage of V1, maintain the voltage at V1 until the current drops to I2, and record the constant voltage time as t1.

[0008] Step 3: Let the lithium-ion battery stand still;

[0009] Step 4: Discharge the lithium-ion battery with a constant current of I3 to a voltage of V2 after it has been left to stand. Then, maintain the voltage at V2 until the current drops to I4. Record the constant voltage time as t2.

[0010] Step 5: Calculate the ratio t of constant pressure time t1 and constant pressure time t2, and classify the lithium-ion batteries according to the ratio t.

[0011] Optionally, steps two through four are alternately repeated N times, and the ratio t is calculated based on t1 and t2 of the Nth cycle for each of the lithium-ion batteries, where N≥2.

[0012] Alternatively, t = max(t1, t2) / min(t1, t2).

[0013] Optionally, in step five, the comparison value t is divided into levels with an interval of 0.01-0.2.

[0014] Optionally, it also includes: Step six, in which lithium-ion batteries in the same category are further classified according to capacity, charge retention rate and AC internal resistance.

[0015] Optionally, in step two, the range of I1 is 0.2C-1C, the range of V1 is 3.9V-4.2V, and the range of I2 is 0.01C-0.05C.

[0016] Optionally, in step four, the range of I3 is 0.2C-1C, the range of V2 is 3.0V-3.8V, and the range of I4 is 0.01C-0.05C.

[0017] Optionally, it also includes the step of connecting lithium-ion batteries of the same grade in series and parallel to form a lithium-ion battery pack.

[0018] The beneficial effects of this invention are as follows:

[0019] Compared with existing technologies, the lithium-ion battery consistency screening method provided by this invention couples the constant voltage time t1 of the constant current and constant voltage charging stage and the constant voltage time t2 of the constant current and constant voltage discharging stage through a ratio. It simultaneously considers charging polarization and discharging polarization, obtaining batteries with consistent charging and discharging polarization. This effectively avoids premature charging or discharging start-up of the battery pack due to excessive polarization of a single cell during long-term charge-discharge cycles. Furthermore, this invention combines static and dynamic methods, fully utilizing the time parameters in the dynamic curve and the battery indicators in the static method. This method is simple, fast, and highly practical, improving the accuracy of consistency screening without adding new testing procedures. Attached Figure Description

[0020] Figure 1 The flowchart of the lithium-ion battery consistency screening method provided by the present invention is shown.

[0021] Figure 2 This is a distribution diagram of the number of lithium-ion batteries after screening based on the ratio t in Embodiment 1 of the present invention.

[0022] Figure 3 The image shows the individual cell pressure difference curves after 5800 cycles of the lithium-ion batteries selected in Example 1 of this invention. Detailed Implementation

[0023] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Because the polarization resistance of lithium-ion batteries significantly affects the upper and lower limits of charging during actual use, it should be a factor to consider when screening and grouping lithium-ion batteries. Polarization resistance includes ohmic resistance, concentration resistance, and electrochemical resistance. In actual charging and discharging processes, ohmic polarization is often the same, but concentration polarization and electrochemical polarization differ due to variations in charging and discharging mechanisms and electrochemical behavior. Therefore, polarization resistance is a complex dynamic resistance that is difficult to evaluate using fixed values.

[0025] For constant current and constant voltage charging and discharging, the constant voltage stage is mainly a process of eliminating polarization. Different constant voltage times correspond to different polarization internal resistances. Using constant voltage time as a screening index for polarization internal resistance is simple, quick, and highly representative.

[0026] The positive and negative electrode reaction mechanisms and electrochemical behaviors during battery charging and discharging are different, resulting in different polarizations during charging and discharging. Conventional screening methods only screen for polarization during charging while ignoring polarization during discharging. Therefore, to screen for batteries with excellent consistency, this invention couples charging and discharging polarizations, taking both polarizations into account.

[0027] Based on this, such as Figure 1 As shown, the present invention provides a lithium-ion battery consistency screening method, comprising:

[0028] Step 1: Provide the lithium-ion batteries to be screened.

[0029] Step 2: Charge each lithium-ion battery with a constant current of I1 to a voltage of V1, maintain the voltage at V1 until the current drops to I2, and record the constant voltage time as t1.

[0030] In some embodiments, I1 ranges from 0.2C to 1C, V1 ranges from 3.9V to 4.2V, and I2 ranges from 0.01C to 0.05C.

[0031] Step 3: Let the lithium-ion battery stand still.

[0032] Step 4: Discharge the lithium-ion battery with a constant current of I3 to a voltage of V2 after it has been left to stand. Then, maintain the voltage at V2 until the current drops to I4. Record the constant voltage time as t2.

[0033] In some embodiments, I3 ranges from 0.2C to 1C, V2 ranges from 3.0V to 3.8V, and I4 ranges from 0.01C to 0.05C.

[0034] Step 5: Calculate the ratio t of constant pressure time t1 and constant pressure time t2, and classify the lithium-ion batteries according to the ratio t.

[0035] This invention couples the constant voltage time t1 of the constant current and constant voltage charging stage and the constant voltage time t2 of the constant current and constant voltage discharging stage through a ratio, where t is approximately equivalent to [(ohmic polarization + concentration polarization + electrochemical polarization)]. 充电或放电 / (Ohmic polarization + concentration polarization + electrochemical polarization) 放电或充电 The ratio t is a standard for simultaneously evaluating charge and discharge polarization; it is a coupling parameter. Unlike dynamic matching methods, which require significant time for data matching, this ratio t allows for batteries with good consistency in charging and discharging behavior. By selecting batteries with consistent charging and discharging polarization through t-selection, batteries with uniform charging and discharging polarization can be obtained, which can help prevent premature charge or discharge activation of the battery pack during long-term charge-discharge cycles due to excessive polarization of a single cell.

[0036] In some embodiments, steps two through four are alternately cycled N times, and the ratio t is calculated based on t1 and t2 of the Nth cycle for each lithium-ion battery, where N≥2. That is, t1 and t2 are taken for the same number of cycles for each lithium-ion battery, and the t value is calculated. In actual charging and discharging processes, different values ​​are selected for the charging constant voltage and the discharging constant voltage, resulting in different constant voltage times. It is possible that the charging constant voltage time t1 is greater than the discharging constant voltage time t2, and it is also possible that the charging constant voltage time t1 is less than the discharging constant voltage time t2. From the perspective of actual production, in the same batch of qualified cells, as long as the screening charging and discharging voltages are determined, the relationship between t1 and t2 can be determined. To facilitate the division of screening levels, it is always stipulated that the maximum value is compared with the minimum value. Therefore, in some embodiments, t = max(t1, t2) / min(t1, t2). That is, under the same number of cycles, the charging constant voltage time t1 and the discharging constant voltage time t2 are recorded, and the maximum value of t1 and t2 is used as the numerator and the minimum value as the denominator to calculate t.

[0037] In some embodiments, the comparison value t is divided into tiers with an interval of 0.01-0.2. In actual screening, the tiering interval of the ratio t can be flexibly determined according to the number of cells to be screened, and is not limited to 0.01-0.2. When the number of battery cells to be screened is small, the tiering interval can be appropriately increased; for example, the tiering interval can be 0.4.

[0038] Step 6: Among lithium-ion batteries in the same category, the lithium-ion batteries are further classified according to their capacity, charge retention rate, and AC internal resistance.

[0039] In some embodiments, lithium-ion batteries of the same grade are connected in series and parallel to form a lithium-ion battery pack.

[0040] Example

[0041] This embodiment provides a method for screening the consistency of lithium-ion batteries, including:

[0042] Step 1: Provide a batch of 30Ah lithium-ion batteries to be screened. Let the individual lithium-ion battery cells in this batch stand at room temperature (20℃) for 1 hour.

[0043] Step 2: Charge each lithium-ion battery at a constant current of 0.5C to a voltage of 4.2V, and maintain the voltage at 4.2V until the current drops to 0.05C. Record the constant voltage time as t1.

[0044] Step 3: Let the lithium-ion battery stand for 1 hour.

[0045] Step 4: Discharge the lithium-ion battery at a constant current of 0.5C to a voltage of 3.7V after it has been left to stand. Then, maintain the voltage at 3.7V until the current drops to 0.05C. Record the constant voltage time as t2.

[0046] Step 5: Alternate steps 2 to 4 three times. Calculate the ratio t based on the constant pressure time t1 and constant pressure time t2 of the third cycle, t = max(t1, t2) / min(t1, t2).

[0047] like Figure 2 As shown, in this embodiment, the range of t is 1.72-1.92, and it is divided into 5 levels according to a increment of 0.04.

[0048] Individual cells with t-values ​​between 1.80 and 1.84 (i.e., the third group in this embodiment) are selected for secondary grading. The 0.5C capacity, charge retention rate, and AC internal resistance of each lithium-ion battery cell are statistically analyzed. Lithium-ion batteries whose 0.5C capacity, charge retention rate, and AC internal resistance exceed three standard deviations σ of the normal distribution are removed from this group, leaving 22 lithium-ion battery cells with good consistency. These 22 cells are then combined into a battery pack.

[0049] To further verify the consistency of the batteries, a cycle test was conducted on the assembled lithium-ion battery pack. Figure 3 As shown, after 5800 cycles of specific on-orbit charge-discharge cycles, the voltage difference between the 22 cells remained almost constant at around 10mV, indicating that the screened lithium-ion batteries have excellent consistency.

[0050] In summary, the lithium-ion battery consistency screening method provided by this invention couples the constant voltage time t1 of the constant current constant voltage charging stage and the constant voltage time t2 of the constant current constant voltage discharging stage through a ratio. It simultaneously considers charging polarization and discharging polarization, resulting in batteries with consistent charging and discharging polarization. This effectively avoids premature charging or discharging start-up of the battery pack due to excessive polarization of a single cell during long-term charge-discharge cycles. Furthermore, this invention combines static and dynamic methods, fully utilizing the time parameters in the dynamic curve and the battery indicators in the static method. This method is simple, fast, and highly practical, improving the accuracy of consistency screening without adding new testing procedures.

[0051] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A method of screening for consistency of lithium-ion batteries, characterized by, include: Step 1: Provide the lithium-ion batteries to be screened; Step 2: Charge each lithium-ion battery with a constant current of I1 to a voltage of V1, and maintain the voltage at V1 until the current drops to I2. Record the constant voltage time as t1. The range of I1 is 0.2C-1C, the range of V1 is 3.9V-4.2V, and the range of I2 is 0.01C-0.05C. Step 3: Let the lithium-ion battery stand still; Step 4: Discharge the lithium-ion battery with a constant current of I3 to a voltage of V2 after it has been left to stand. Then, maintain the voltage at V2 until the current drops to I4. Record the constant voltage time as t2. The range of I3 is 0.2C-1C, the range of V2 is 3.0V-3.8V, and the range of I4 is 0.01C-0.05C. Step 5: Calculate the ratio t of constant pressure time t1 and constant pressure time t2, and classify the lithium-ion batteries according to the ratio t.

2. The lithium-ion battery consistency screening method as described in claim 1, characterized in that, The alternating cycles of steps two through four are repeated N times. Based on t1 and t2 of the Nth cycle for each lithium-ion battery, the ratio t is calculated, where N ≥ 2.

3. The method of claim 1, wherein the step of determining the consistency of the lithium ion battery is performed by a method comprising: t=max(t1,t2) / min(t1,t2). ​ 4. The method of claim 1, wherein the step of determining the consistency of the lithium ion battery is performed by a method comprising: In step five, the comparison value t is divided into levels with an interval of 0.01-0.

2. ​ 5. The method of claim 1, wherein the step of determining the consistency of the lithium ion battery is performed by a method comprising: Also includes: ​ Step 6: Among lithium-ion batteries in the same category, the lithium-ion batteries are further classified according to their capacity, charge retention rate, and AC internal resistance.

6. The method of claim 1-5, wherein the method is performed on a plurality of lithium ion batteries. It also includes the step of connecting lithium-ion batteries of the same grade in series and parallel to form a lithium-ion battery pack.

Images