A method for accurately determining the capacity of lithium-ion batteries in batches

Abstract

The present invention discloses a method for accurately determining the capacity of lithium-ion batteries in batches, comprising: performing a constant capacity test at a set temperature T1 to measure the capacity C1 of the lithium-ion battery; converting the capacitance C1 at temperature T1 into the capacity C2 at a standard temperature T2, and using the capacity C2 as the constant capacity of the battery. The present invention has the advantage of using a temperature with low capacity sensitivity for constant capacity testing and then converting the tested capacity into the capacity at the standard capacity. This method provides more accurate and reliable constant capacity values, avoiding the drawback of large measurement errors caused by temperature fluctuations during constant capacity testing.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion battery production grading and capacity determination, and in particular to a method for accurate capacity determination of lithium-ion batteries in batches. Background Art

[0002] As lithium-ion batteries become increasingly widely used, the requirements for battery manufacturing are also increasing. Furthermore, in most applications, varying numbers of lithium-ion batteries are connected in series or parallel. This requires consistent capacity within the individual cells within the group. Otherwise, the "barrel effect" can easily occur, causing the battery pack to exhibit low capacity. Therefore, during lithium-ion battery production, battery capacity determination and grading are particularly important for improving grouping efficiency.

[0003] Existing technology typically performs capacity determination at room temperature (generally 25°C). However, due to varying heat dissipation conditions at different locations in the capacity determination cabinet and the inherent heat generation during battery charging and discharging, accurate temperature control is extremely difficult. More importantly, battery capacity is highly sensitive to temperature fluctuations near room temperature. If the battery is determined at room temperature, inaccurate temperature control will cause capacity fluctuations, ultimately resulting in a significant deviation between the determined capacity and the actual capacity, making accurate capacity determination impossible. Therefore, accurate capacity determination based on existing technology is quite challenging. Summary of the Invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for accurately determining the capacity of lithium-ion batteries in batches, so as to solve the defects of the prior art in determining the capacity at room temperature, determine the capacity at a temperature that is not highly sensitive to capacity, and reduce the defect of the prior art in determining the capacity at room temperature that is prone to large capacity fluctuations.

[0005] To achieve the above-mentioned object, the technical solution adopted by the present invention is: a method for accurately determining the capacity of lithium-ion batteries in batches, comprising: performing a constant capacity test at a set temperature T1 to measure the capacity C1 of the lithium-ion battery; converting the capacitance C1 at temperature T1 into the capacity C2 at a standard temperature T2, and using the capacity C2 as the constant capacity of the battery.

[0006] Temperature T1 is the temperature corresponding to when the battery capacity is less sensitive to temperature changes.

[0007] The temperature T1 is a temperature corresponding to when the battery capacity is less sensitive to temperature changes at high temperatures.

[0008] The temperature T1 is a temperature corresponding to a capacity change rate with temperature less than a set threshold value between 35° C. and 50° C.

[0009] The temperature conversion coefficient is used to convert the capacitance C1 at temperature T1 into the capacitance C2 at standard temperature T2.

[0010] The temperature conversion coefficient corresponding to temperature T1 is obtained through a pre-calibration test.

[0011] The pre-calibration test includes: placing lithium-ion batteries in a constant temperature box for constant capacity, and performing constant capacity tests at temperature T1 to obtain C 测1 , perform constant volume test at temperature T2 to obtain capacity C 测2 , C 测2 / C 测1 As the temperature conversion coefficient corresponding to the test temperature T1.

[0012] In the pre-calibration test, multiple lithium-ion batteries are selected for testing to obtain multiple temperature conversion coefficients, and the average value thereof is taken as the final temperature conversion coefficient.

[0013] Multiplying the capacitance C1 at temperature T1 by the temperature conversion coefficient can give the capacitance C2 at the corresponding standard temperature T2.

[0014] The standard temperature T2 is room temperature.

[0015] The advantages of the present invention are that: a constant capacity test is performed at a temperature with low capacity sensitivity, and then the tested capacity is converted into the capacity at a standard temperature. In this way, the constant capacity value is more accurate and reliable, and the defect of large measurement error caused by temperature fluctuation during the constant capacity test is avoided; the constant capacity is converted into the capacity value at the standard temperature by the measured value at high temperature, so as to obtain a more accurate room temperature capacity, provide a basic capacity parameter for capacity grouping, and improve the consistency of grouped battery cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The following is a brief description of the contents and symbols in the drawings of the present invention:

[0017] Figure 1 This is a graph showing the results of a temperature capacity sensitivity test performed on a battery according to the present invention;

[0018] Figure 2 This is a schematic diagram comparing the constant volume capacity calculated by the constant volume method of the present invention with the capacity results of the prior art. DETAILED DESCRIPTION

[0019] The specific implementation of the present invention will be further explained in detail below by describing the best embodiment with reference to the accompanying drawings.

[0020] The present invention adopts high-temperature constant capacity, which can effectively eliminate the influence of diffusion impedance difference on capacity testing, and achieve the purpose of measuring more accurate capacity under the same temperature difference conditions. Secondly, the high-temperature capacity and 25°C capacity are accurately calibrated in a constant temperature box, and the ratio of 25°C capacity to high-temperature capacity is used as the capacity calibration coefficient, also called temperature coefficient. It is generally believed that the temperature coefficient is only related to the material system and electrode design. The capacity conversion temperature coefficient of mass-produced batches of battery cells is relatively fixed. Therefore, the calibrated temperature coefficient can be used to batch calibrate the high-temperature constant capacity of the production line, so as to obtain more accurate room temperature capacity and classification, so as to improve the consistency of grouped battery cells and improve the grouping efficiency of the Pack.

[0021] like Figure 1 As shown, this application observes the effect of temperature on test capacity through actual measurement.

[0022] Randomly select two qualified battery cells from the production line and measure their capacities at a series of temperatures (Table 1) in a constant temperature box (temperature control accuracy ±1°C). The temperatures are 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, and 50°C. The capacities at different temperatures (based on the capacity at 25°C) are converted into capacity retention rates and plotted against temperature ( Figure 1 ).

[0023] Table 1: Test capacity of a certain type of battery at different temperatures

[0024]

[0025]

[0026] Converted from Table 1 Figure 1 It can be seen from the curve shown that: as the temperature gradually increases, the capacity decreases in sensitivity to temperature, and when the temperature is above 40°C, the capacity is basically stable. Assuming that the constant volume is selected at 45±5°C, the capacity deviation is about 0.07%, while if the constant volume is selected at 25±5°C, the capacity deviation reaches 8.9%. Therefore, when the constant volume temperature is selected ≥40°C, the capacity test is much more accurate than that at 25°C. Even if the temperature fluctuates during the constant volume process, the capacity fluctuation caused by high temperature is very small, and can be basically ignored compared to the capacity fluctuation at 25°C. Therefore, it is possible to consider performing constant volume testing at high temperature and then converting it into the capacity at standard room temperature to provide basic capacity data for capacity division and grouping.

[0027] To this end, the present application provides a method for accurately determining the capacity of lithium-ion batteries in batches, including: performing a constant capacity test at a set temperature T1 to measure the capacity C1 of the lithium-ion battery; converting the capacitance C1 at temperature T1 into the capacity C2 at a standard temperature T2, and using the capacity C2 as the constant capacity of the battery.

[0028] The temperature T1 here is the temperature corresponding to when the battery capacity is less sensitive to temperature changes. Since lithium-ion batteries are insensitive to both high and low temperatures, although the capacity is insensitive at low temperatures (Comments: the battery capacity is more sensitive to temperature changes at low temperatures), charging and discharging at constant capacity at low temperatures may cause lithium precipitation. This phenomenon is irreversible and will damage the battery. Therefore, this application chooses to perform constant capacity testing at high temperatures. However, the temperature of the lithium-ion battery cannot be too high. Too high a temperature will cause the SE I film of the lithium-ion battery negative electrode to decompose, thereby affecting the electrical performance. Therefore, how to determine the temperature of the high-temperature constant capacity test needs to be considered, such as Figure 1 As shown, this application has found through testing that the capacity is insensitive to temperature changes within the temperature range of 35-50°C. When the rate of change of capacity with temperature between 35°C and 50°C is less than the set threshold, it is considered insensitive to temperature changes. The temperature in the insensitive range is selected as the constant capacity test temperature T1. The general temperature is between 35-50°C, and the specific temperature will vary depending on the model of the lithium battery and needs to be confirmed based on actual conditions.

[0029] After selecting the constant capacity test temperature T1, the constant capacity test of charge and discharge can be performed at T1 to measure the capacity C1. The obtained capacity C1 is converted into the capacity C2 at the standard temperature T2 through the temperature conversion coefficient K. The standard temperature T2 is generally room temperature 25°C. The capacity at this temperature generally provides basic data for grouping and grading, so the capacity measured at high temperature also needs to be converted into the capacity at the standard temperature. Temperature T1 uses the temperature conversion coefficient to convert the capacity C1 at temperature T1 into the capacity C2 at standard temperature T2. Multiplying the capacitance C1 at temperature T1 by the temperature conversion coefficient will give the corresponding capacity C2 at standard temperature T2.

[0030] The temperature conversion coefficient corresponding to temperature T1 is obtained through a pre-calibration test. The pre-calibration test includes: placing a lithium-ion battery in a constant temperature box for constant capacity, and performing constant capacity tests at temperature T1 to obtain C 测1 , perform constant volume test at temperature T2 to obtain capacity C 测2 , C 测2 / C 测1 As the temperature conversion coefficient corresponding to the test temperature T1.

[0031] In the pre-calibration test, in order to reduce the impact of errors, the present application selects multiple lithium-ion batteries for testing to obtain multiple temperature conversion coefficients, and takes the average value as the final temperature conversion coefficient.

[0032] In actual operation, this application has been verified through experiments including:

[0033] 32 battery cells of a certain model that passed the production line were randomly selected and their capacities at 25°C and 45°C were tested in a constant temperature chamber (as shown in Table 2). The ratio of the capacities at 25°C to 45°C was calculated to obtain the temperature coefficients of the different battery cells. The average value was taken as the capacity calibration coefficient (temperature coefficient) of the batch of battery cells, which was 95.63%.

[0034] Table 2 Temperature coefficient calibration data table

[0035]

[0036]

[0037] The temperature conversion coefficient of the battery selected in this application is calculated to be 95.63%. The capacity C1*95.63% obtained after the charge and discharge constant capacity test at high temperature (45°C) is the capacity value at the standard temperature, which is used as the final constant capacity for grading, grouping, etc.

[0038] In order to verify the accuracy of the capacity determination method, this application takes 300 cells from the production line and determines the capacity at an ambient temperature of 45°C, and uses the temperature coefficient calibration to obtain the calculated capacity at 25°C; and then conducts a capacity determination test at 25°C to obtain the constant capacity data at 25°C, which is plotted as follows: Figure 2 The cell capacity distribution diagram is based on the data in Table 1 and Figure 2 It can be seen that the capacity distribution obtained by high-temperature conversion in this application is concentrated between 30.3-30.7Ah, and the distribution is more concentrated. When measured directly at 25°C, the battery capacity is more dispersed. The reason for this dispersion is that the temperature fluctuation caused by the inability to accurately control the temperature brings about the fluctuation of battery capacity. Therefore, it can be concluded that the capacity obtained by the capacity determination method of this application is more accurate and reliable.

[0039] Obviously, the specific implementation of the present invention is not limited to the above-mentioned methods. As long as various non-substantial improvements are made using the method concept and technical solution of the present invention, they are all within the scope of protection of the present invention.

Claims

1. A method for accurately determining the capacity of lithium-ion batteries in batches, characterized by: A constant capacity test is performed at a set temperature T1 to measure the capacity C1 of the lithium-ion battery; the battery capacity C1 at temperature T1 is converted into the capacity C2 at a standard temperature T2, and the capacity C2 is used as the constant capacity of the battery; temperature T1 is a temperature corresponding to when the battery capacity is less sensitive to temperature changes; the temperature T1 is a temperature corresponding to when the battery capacity is less sensitive to temperature changes at high temperatures; a temperature conversion coefficient is used to convert the capacitance C1 at temperature T1 into the capacity C2 at a standard temperature T2; The temperature T1 is a temperature corresponding to a capacity change rate with temperature less than a set threshold value between 35° C. and 50° C.

2. The method for accurately determining the capacity of lithium-ion batteries in batches according to claim 1, wherein: The temperature conversion coefficient corresponding to temperature T1 is obtained through a pre-calibration test.

3. The method for accurately determining the capacity of lithium-ion batteries in batches according to claim 2, wherein: The pre-calibration test includes: placing lithium-ion batteries in a constant temperature box for constant capacity, and performing constant capacity tests at temperature T1 to obtain C 测1 , perform constant volume test at temperature T2 to obtain capacity C 测2 , C 测2 / C 测1 As the temperature conversion coefficient corresponding to the test temperature T1.

4. A method for accurately determining the capacity of lithium-ion batteries in batches according to claim 3, characterized in that: In the pre-calibration test, multiple lithium-ion batteries are selected for testing to obtain multiple temperature conversion coefficients, and the average value thereof is taken as the final temperature conversion coefficient.

5. A method for accurately determining the capacity of lithium-ion batteries in batches according to any one of claims 1 to 4, characterized in that: Multiplying the capacitance C1 at temperature T1 by the temperature conversion coefficient can give the capacitance C2 at the corresponding standard temperature T2.

6. The method for accurately determining the capacity of lithium-ion batteries in batches according to claim 1, wherein: The standard temperature T2 is room temperature.

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