Method for improving test precision of capacity retention ratio of battery cell

Abstract

The invention provides a method for improving the test precision of the capacity retention ratio of a battery cell, and relates to the technical field of batteries, and the method comprises the following steps: when the capacity of the battery cell at a series of initial temperatures is tested, intermittently discharging the battery cell at a certain initial temperature T at a specific rate, discharging is stopped when the temperature of the battery cell reaches T + 5 DEG C or above after each time of discharging, then discharging is started again when the temperature of the battery cell is reduced to T-1 DEG C to T + 2 DEG C, intermittent discharging is conducted in this way till the final voltage is reached, and then the battery cell is discharged; the capacity retention ratio of the battery cell at a certain initial temperature T is (the capacity of the battery cell measured at the initial temperature T / the capacity of the battery cell measured at the initial temperature of 25 DEG C) * 100%. According to the invention, the test precision of the battery cell capacity retention ratio is improved by improving the test method, so that the SOC estimation precision is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a method for improving the test accuracy of the capacity retention rate of battery cells. Background Art

[0002] Currently, when testing the capacity retention rate of lithium iron phosphate battery cells, a method of continuously discharging at a specific rate is often adopted to obtain the capacity retention rate at a certain temperature relative to room temperature, that is, each charge-discharge cycle is carried out under the condition of continuous charging or discharging at a specific rate. However, due to the difference between the test conditions and the actual operating conditions of the battery cells in the vehicle, there is a deviation between the test results and the actual capacity retention rate of the battery cells during actual use, which may further lead to inaccurate SOC estimation and customer complaints.

[0003] In view of this, the present invention is specifically proposed. Summary of the Invention

[0004] One of the objectives of the present invention is to provide a method for improving the test accuracy of the capacity retention rate of battery cells, which improves the test accuracy of the capacity retention rate of battery cells by improving the test method, and further improves the SOC estimation accuracy.

[0005] Another objective of the present invention is to provide a test method for the capacity retention rate of battery cells, and the capacity retention rate of the battery cells obtained by this method is closer to the actual capacity retention rate during the use of the battery cells, with higher accuracy.

[0006] To achieve the above objectives, the present invention adopts the following technical solutions:

[0007] In the first aspect of the present invention, there is provided a method for improving the test accuracy of the capacity retention rate of battery cells, the method comprising: when testing the capacity of the battery cells at the starting temperature of a test series, the discharging method is to intermittently discharge a battery cell with an initial temperature of T at a specific rate, and stop discharging each time when the temperature of the battery cell reaches above T + 5°C after discharging, then wait until the temperature of the battery cell drops to T - 1°C to T + 2°C, and then start discharging again, and so on for intermittent discharging until the termination voltage is reached. The capacity retention rate of the battery cell at a certain initial temperature T is (the capacity of the battery cell measured at the initial temperature T / the capacity of the battery cell measured at the initial temperature of 25°C) * 100%.

[0008] Further, the battery cell with an initial temperature of T is obtained by placing the battery cell in a constant temperature environment with a temperature of T and allowing it to stand until the temperature of the battery cell is T ± 1°C;

[0009] Further, stop discharging each time when the temperature of the battery cell reaches T + 5°C during each discharging process.

[0010] Further, the certain initial temperature T is any temperature in the range of -30°C to 45°C;

[0011] Further, the starting temperature T of the series is selected from the temperatures ranging from -30°C to 45°C.

[0012] Further, the specific rate is 0.33C or / and 1C.

[0013] Further, the method for selecting the starting temperature of the series is as follows: when the temperature is below 10°C, the starting temperature of the series is selected at intervals of 10°C; when the temperature is above 10°C, the starting temperature of the series is selected at intervals of 15°C.

[0014] Further, the battery cell is a lithium iron phosphate battery cell or a ternary battery cell.

[0015] In the second aspect of the present invention, a method for testing the capacity retention rate of a battery cell at a series of starting temperatures is provided, including the following steps:

[0016] Step S1: Select a certain starting temperature T from the series of starting temperatures, and then place the battery cell in a constant temperature environment at temperature T and let it stand until the temperature of the battery cell reaches T.

[0017] Step S2: Charge the battery cell in step S1 at a constant current at a specific rate to the highest voltage, let it stand for a specific time until the temperature of the battery cell reaches the starting temperature T selected in step S1, and then discharge it in the intermittent discharge mode described in the first aspect to obtain the discharge capacity of the battery cell at the selected starting temperature T.

[0018] Step S3: Select another starting temperature T from the series of starting temperatures, repeat steps S1 and S2 to obtain the discharge capacity of the battery cell at another selected starting temperature T; and measure the discharge capacity of the battery cell at different starting temperatures in this way.

[0019] Step S4: Calculate the capacity retention rate of the battery cell at a certain starting temperature T by using the following formula: (the capacity of the battery cell measured at this starting temperature T / the capacity of the battery cell measured at the starting temperature of 25°C) * 100%.

[0020] Further, in step S2, the specific rate is 0.33C or / and 1C.

[0021] The method for improving the test accuracy of the capacity retention rate of a battery cell and the method for testing the capacity retention rate of a battery cell provided by the present invention have the beneficial effects that:

[0022] By improving the method for testing the capacity retention rate of a battery cell at different temperatures, changing continuous discharge to intermittent discharge, reducing problems such as large errors in battery cell parameters caused by temperature rise, improving the test accuracy of the capacity retention rate, and further improving the accuracy of SOC estimation.

[0023] The denser the selection of the starting temperatures of the series, the higher the data accuracy. However, for efficiency considerations, the method for selecting the starting temperatures of the series can be as follows: when the temperature is below 10°C, select the starting temperatures of the series at intervals of 10°C; when the temperature is above 10°C, select the starting temperatures of the series at intervals of 15°C. The interval between adjacent starting temperatures in the low-temperature region is 10°C, and the adjacent starting temperatures are set at intervals of 15°C at high temperatures. This method of selecting the starting temperatures of the series not only improves efficiency but also does not have too much impact on the accuracy of SOC estimation. Brief Description of the Drawings

[0024] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0025] Figure 1 It is a flowchart of the method for improving the test accuracy of the cell capacity retention rate provided by the present invention. Detailed Embodiments

[0026] To make the objectives, technical solutions, and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in combination with the embodiments of the present invention. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. The process parameters without specific conditions noted in the following embodiments are usually in accordance with conventional conditions.

[0027] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0028] The inventor found that the existing test method for the capacity retention rate of battery cells did not consider the issue of the temperature rise of the battery cells during the test. Based on the characteristics of lithium iron phosphate batteries that like heat and fear cold, during low-temperature charge and discharge tests, due to the airtight and windless test chamber and the fixed discharge rate, the actual temperature of the battery cells will be higher than the ambient temperature. However, the battery packs on the vehicle are affected by the external environment, and the temperature rise is much lower than that in the test environment. Therefore, the test conditions will be better than the vehicle usage conditions, and the measured value of the capacity retention rate will also be higher than the actual capacity retention rate of the battery cells during actual use. Thus, the present invention is proposed.

[0029] According to the first aspect of the present invention, the present invention provides a method for improving the test accuracy of the capacity retention rate of battery cells. The method includes: when measuring the capacity of the battery cells at the starting temperature of a test series, the discharge method is to intermittently discharge a battery cell with a certain starting temperature of T at a specific rate. After each discharge, when the temperature of the battery cell reaches above T + 5°C, stop discharging. Then, when the temperature of the battery cell drops to T - 1°C to T + 2°C, start discharging again. Discharge intermittently in this way until the termination voltage is reached. The capacity retention rate of the battery cell at a certain starting temperature T is (the capacity of the battery cell measured at the starting temperature T / the capacity of the battery cell measured at the starting temperature of 25°C) * 100%.

[0030] The present invention changes continuous discharge to intermittent discharge, reduces the problem of large errors between the parameters of the battery cells caused by temperature rise and the parameters of the battery cells during actual use, improves the test accuracy of the capacity retention rate, and further improves the accuracy of SOC estimation.

[0031] As an optional implementation manner of the method for improving the test accuracy of the capacity retention rate of battery cells according to the present invention, the battery cell with the starting temperature of T is placed in a constant temperature environment with a temperature of T and left to stand until the temperature of the battery cell is T ± 1°C; of course, other conventional methods can also be used to ensure that the internal and external temperatures of the battery cell are the same and the starting temperature is T°C.

[0032] As an optional implementation manner of the method for improving the test accuracy of the capacity retention rate of battery cells according to the present invention, stop discharging immediately when the temperature of the battery cell reaches T + 5°C during each discharge process. In actual operation, it can also stop discharging slightly higher than T + 5°C. By reducing the temperature rise, the test temperature is closer to the actual use environment temperature, thereby reducing the error of the test result and improving the test accuracy.

[0033] As an optional implementation manner of the method for improving the test accuracy of the capacity retention rate of battery cells according to the present invention, the certain starting temperature T is any temperature from -30°C to 45°C;

[0034] As an alternative implementation of the method for improving the test accuracy of the cell capacity retention rate of the present invention, the series starting temperature T is selected from the temperatures between -30°C and 45°C. For example, the series temperatures are -30°C, -25°C, -20°C, -15°C, -10°C, -5°C, 0°C, 5°C, 10°C, 20°C, 30°C, 40°C, 45°C.

[0035] The specific rate described in the first aspect of the present invention can be a conventional rate used in the art. As an alternative implementation of the method for improving the test accuracy of the cell capacity retention rate of the present invention, the specific rate is 0.33 or / and 1C.

[0036] As an alternative implementation of the method for improving the test accuracy of the cell capacity retention rate of the present invention, the method for selecting the series starting temperature is as follows: When the temperature is below 10°C, the series starting temperature T is selected at intervals of 10°C; when the temperature is above 10°C, the series starting temperature T is selected at intervals of 15°C. This setting method not only has high efficiency but also has little impact on the SOC estimation accuracy.

[0037] As an alternative implementation of the method for improving the test accuracy of the cell capacity retention rate of the present invention, the cell is a lithium iron phosphate cell or a ternary cell.

[0038] According to the second aspect of the present invention, the present invention provides a method for testing the cell capacity retention rate at a series of starting temperatures, including the following steps:

[0039] Step S1: Select a starting temperature T from the series of starting temperatures, and then place the cell in a constant-temperature environment at temperature T and let it stand until the cell temperature reaches T;

[0040] Step S2: Charge the cell in step S1 at a constant current at a specific rate to the highest voltage, and let it stand for a specific time until the cell temperature reaches the starting temperature T selected in step S1, and then discharge it in the intermittent discharge mode described in the first aspect to obtain the discharge capacity of the cell at the selected starting temperature T;

[0041] Step S3: Select another starting temperature T from the series of starting temperatures, repeat steps S1 and S2 to obtain the discharge capacity of the cell at the other selected starting temperature T; and thus measure the discharge capacities of the cell at different starting temperatures T;

[0042] Step S4: Calculate the cell capacity retention rate at a certain starting temperature T using the following formula: (the capacity of the cell measured at this starting temperature T / the capacity of the cell measured at the starting temperature of 25°C) * 100%.

[0043] The specific rate described in the second aspect of the present invention may be a conventional rate used in the art. As an alternative embodiment of the test method in the second aspect of the present invention, in step S2, the specific rate is 0.33C or / and 1C.

[0044] The present invention will be further described in detail below with reference to specific examples and comparative examples.

[0045] The battery cells used in the examples and comparative examples are lithium iron phosphate battery cells of the same batch with the same initial capacity, and the initial capacity is 166Ah.

[0046] Example

[0047] This example provides a test method for capacity retention rate, which specifically includes the following steps:

[0048] (1). Set the target temperature T of the thermostat to -30°C, -20°C, -10°C, 0°C, 10°C, 25°C, and 45°C respectively, and place the battery cell in the thermostat and let it stand until the temperature of the battery cell is consistent with the target temperature;

[0049] (2). Constantly charge the battery cell at 1C to the maximum voltage of 3.65V at a certain target temperature T°C in step (1), then let it stand for 24h to keep the temperature of the battery cell at T°C and then discharge it. The specific discharge is as follows: discharge at 0.33C and 1C respectively. When the temperature rise of the battery cell is about 5°C, immediately stop discharging; when the temperature of the battery cell drops to T - T + 2°C, start discharging again. When the temperature of the battery cell rises to about T + 5°C again, stop discharging again and let it cool down. Repeat this cycle until the discharge reaches the cut-off voltage of 2.5V, thereby obtaining the discharge capacity of the battery cell at this target temperature;

[0050] (3). Repeat step S2 to perform the charge and discharge operations of the battery cell at other target temperatures, and obtain the capacities of the battery cell at each target temperature in the range of -30 to 45°C. Among them, the set target temperature (i.e., the ambient temperature) is shown in Tables 1 and 2.

[0051] (4). Calculate the capacity retention rate of the battery cell at the starting temperature T. The calculation formula for the capacity retention rate is (the capacity of the battery cell measured at this starting temperature T / the capacity of the battery cell measured at the starting temperature of 25°C) * 100%.

[0052] Comparative Example

[0053] The difference between this comparative example and the example is only that the discharge method is continuous discharge without controlling the temperature rise of the battery cell. The set target temperatures (i.e., the ambient temperatures) are shown in Table 1.

[0054] Tables 1 and 2 are the test results of the examples and comparative examples respectively. The test results at each temperature and rate are the averages of three battery cells of the same batch.

[0055] Test Results of Comparative Examples in Table 1

[0056]

[0057] Test Results of Examples in Table 2

[0058]

[0059] It can be seen from the test results of the examples and comparative examples that:

[0060] 1. After controlling the temperature rise range of the battery cell, the capacity retention rate measured in the low-temperature range below 0°C is closer to the true value under low-temperature conditions because the test temperature is closer to the actual temperature, reducing the problem of the vehicle breaking down due to the high virtual SOC during actual vehicle use caused by the large error in the capacity retention rate under low-temperature conditions.

[0061] 2. In the temperature range above 25°C, the capacity of the battery cell during discharge at a rate of 0.33C in the comparative example will slightly decrease as the temperature increases. However, after controlling the temperature rise range of the battery cell in the example, during discharge at a rate of 0.33C, the actual capacity has a small increase; during discharge at a rate of 1C in the example, compared with the battery cell capacities at 25°C and 45°C in the comparative example, there is not much improvement, while in the example, there is a significant increase in the battery cell capacities at 25°C and 45°C.

[0062] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for improving the test accuracy of the cell capacity retention rate, characterized in that, The method includes: when measuring the capacity of the battery cell at the starting temperature of the test series, the discharging method is to intermittently discharge a battery cell with a starting temperature of T at a specific rate. Each time after discharging, when the temperature of the battery cell reaches above T + 5°C, the discharging stops. Then, when the temperature of the battery cell drops to T - 1°C to T + 2°C, the discharging is started again. Such intermittent discharging is carried out until the termination voltage is reached. The capacity retention rate of the battery cell at a certain starting temperature T is (the capacity of the battery cell measured at the starting temperature T / the capacity of the battery cell measured at the starting temperature of 25°C) * 100%.

2. The method for improving the test accuracy of the cell capacity retention rate according to claim 1, wherein The battery cell with the starting temperature of T is obtained by placing the battery cell in a constant temperature environment with a temperature of T and allowing it to stand until the temperature of the battery cell is T ± 1°C.

3. The method for improving the test accuracy of the cell capacity retention rate according to claim 1, wherein, During each discharging process, when the temperature of the battery cell reaches T + 5°C, the discharging stops.

4. The method for improving the test accuracy of the cell capacity retention rate according to claim 1, characterized in that, The certain starting temperature T is any temperature from -30°C to 45°C.

5. The method for improving the test accuracy of the cell capacity retention rate according to claim 1, characterized in that, The series of starting temperatures are selected from the temperatures from -30°C to 45°C.

6. The method for improving the test accuracy of the cell capacity retention rate according to claim 1, wherein The specific rate is 0.33 or / and 1C.

7. The method for improving the test accuracy of the cell capacity retention rate according to claim 1, characterized in that The method for selecting the series of starting temperatures is as follows: when it is below 10°C, the series of starting temperatures are selected at intervals of 10°C; when it is above 10°C, the series of starting temperatures are selected at intervals of 15°C.

8. The method for improving the test accuracy of the cell capacity retention rate according to claim 1, characterized in that, The battery cell is a lithium iron phosphate battery cell or a ternary battery cell.

9. A test method for the capacity retention rate of an electric cell at a series of starting temperatures, characterized in that, It includes the following steps: Step S1: Select a certain starting temperature T from the series of starting temperatures, and then place the battery cell in a constant temperature environment with a temperature of T and allow it to stand until the temperature of the battery cell is T; Step S2: Constantly charge the battery cell in Step S1 at a specific rate to the highest voltage, and after standing for a specific time to make the temperature of the battery cell reach the starting temperature T selected in Step S1, then discharge it in the intermittent discharging manner described in any one of Claims 1 - 8 to obtain the discharging capacity of the battery cell at the selected starting temperature T; Step S3: Select another starting temperature T from the series of starting temperatures, repeat Steps S1 and S2 to obtain the discharging capacity of the battery cell at another selected starting temperature T; and thus measure the discharging capacities of the battery cell at different starting temperatures; Step S4: Calculate the capacity retention rate of the battery cell at a certain starting temperature T using the following formula: (the capacity of the battery cell measured at the starting temperature T / the capacity of the battery cell measured at the starting temperature of 25°C) * 100%.

10. The test method according to claim 9, characterized in that, In Step S2, the specific rate is 0.33C or / and 1C.

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