A method for estimating SOP of lithium-ion batteries

Abstract

The present invention provides a method for estimating the SOP of a lithium-ion battery, comprising the following steps: obtaining a maximum specified rate of a battery at a specified temperature and duration; selecting a measurement rate that is not higher than the maximum specified rate; obtaining the OCV of the battery after standing at a specified temperature and a specified SOC; obtaining different terminal voltages of the battery at the last moment of a specified temperature, a specified SOC, different measurement rates and the same duration; obtaining different overpotentials; obtaining a maximum allowable overpotential; S7, selecting an overpotential that is less than the maximum allowable overpotential and closest to its value as x1, selecting a measurement rate corresponding to it as y1, selecting an overpotential that is greater than the maximum allowable overpotential and closest to its value as x2, selecting a measurement rate corresponding to it as y2; solving a linear equation through x1, y1, x2 and y2; substituting the maximum allowable overpotential into the linear equation for calculation to obtain the maximum allowable rate.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, in particular to a method for estimating the SOP of a lithium ion battery. Background Art

[0002] Accurate estimation of battery SOP can maximize battery utilization efficiency. For example, when braking, it can absorb as much feedback energy as possible without damaging the battery. When accelerating, it can provide greater power to obtain greater acceleration without damaging the battery. At the same time, it can also ensure that the car will not lose power due to undervoltage or overcurrent protection during driving even when the SOC (state of charge) is very low.

[0003] The FreedomCAR Power-Assisted Electric Vehicle Battery Test Manual and the USABC Battery Test Manual both estimate SOP based on DC internal resistance. The disadvantage is that the battery is not a pure internal resistance, and the DC internal resistance measured at different measurement rates is different. The inconstancy of DC internal resistance seriously affects the estimation accuracy of SOP.

[0004] The estimation method of SOP in the national standard 38661 is to continuously adjust the charge and discharge rate, fit the relationship curve between the peak charge and discharge power and the charge and discharge time of the lithium-ion battery at a specified temperature and a specified SOC, and finally calculate the maximum allowable charge and discharge capacity of the lithium-ion battery. The disadvantage is that when the voltage threshold of the battery needs to be changed according to the actual application project, the estimation method needs to be operated from the beginning, and the estimation efficiency is low. Summary of the invention

[0005] The object of the present invention is to provide a method for estimating the SOP of a lithium-ion battery to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for estimating the SOP of a lithium-ion battery comprises the following steps:

[0008] S1. Obtain the maximum specified rate of the battery at a specified temperature and duration;

[0009] S2. Select a measurement magnification that is not higher than the maximum specified magnification;

[0010] S3, obtaining the OCV of the battery after standing at a specified temperature and a specified SOC;

[0011] S4, obtaining different terminal voltages of the battery at the last moment of a specified temperature, a specified SOC, different measurement rates and the same duration;

[0012] S5, subtracting different terminal voltages from OCV to obtain different overpotentials;

[0013] S6, subtracting the voltage threshold corresponding to the specified temperature from the OCV to obtain the maximum allowable overpotential;

[0014] S7. Compare the maximum allowable overpotential with different overpotentials, select the overpotential that is smaller than the maximum allowable overpotential and closest to it as x1, select the corresponding measurement magnification as y1, and obtain (x1, y1) as point one, select the overpotential that is larger than the maximum allowable overpotential and closest to it as x2, select the corresponding measurement magnification as y2, and obtain (x2, y2) as point two;

[0015] S8. Solve the linear equation by substituting (x1, y1) and (x2, y2) into the linear function respectively;

[0016] S9. Substitute the maximum allowable overpotential into the linear equation for calculation to obtain the maximum allowable rate.

[0017] Furthermore, in step S1, the temperature range of discharge is -20°C to 55°C, and the temperature range of charge is 0°C to 55°C.

[0018] Furthermore, in step S1, the duration is 10 seconds, 30 seconds or 60 seconds.

[0019] Furthermore, in step S2, the measurement magnification is not less than 0.1C.

[0020] Furthermore, in step S3, the SOC range of discharge is 5% to 100%, wherein when the SOC is higher than 30%, the discharge is performed every 10%, and when the SOC is lower than 30%, the discharge is performed every 5%, and each discharge is allowed to stand before the discharge;

[0021] Furthermore, in step S3, the charging SOC range is 0% to 95%, wherein when the SOC is higher than 80%, charging is performed at intervals of 5%, and when the SOC is lower than 80%, charging is performed at intervals of 10%, and the battery is left to stand before each charging.

[0022] Furthermore, in step 3, the range of the standing time is 30 min to 60 min.

[0023] Furthermore, in step S5, the overpotential of discharge is equal to OCV minus the terminal voltage, and the overpotential of charge is equal to the terminal voltage minus OCV.

[0024] Furthermore, in step S6, the voltage threshold for discharging is the lowest voltage, and the voltage threshold for charging is the highest voltage.

[0025] Furthermore, the maximum allowable overpotential for discharge is equal to OCV minus the lowest voltage, and the maximum allowable overpotential for charge is equal to the highest voltage minus OCV.

[0026] Beneficial effects of the present invention:

[0027] The present invention can comprehensively calibrate the SOP of the battery at different temperatures, measurement rates, SOCs and durations. Compared with the method of estimating the SOP based on the DC internal resistance, the SOP of the present invention is not affected by the DC internal resistance, thereby improving the estimation accuracy of the SOP. Compared with the method of estimating the SOP according to the national standard 38661, when the voltage threshold needs to be changed, the data of the previous comprehensive calibration of the battery can be used, and it is only necessary to repeat steps S6 to S9 without having to execute the steps from the beginning, thereby achieving high estimation efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 : Flowchart of the present invention.

[0029] Figure 2 : Example table of SOP estimation for 30Ah lithium titanate battery at 25℃ and 30 seconds discharge.

[0030] Figure 3 : Example table of SOP estimation for 30Ah lithium titanate battery at 25℃ and 30 seconds charging. DETAILED DESCRIPTION

[0031] The present invention is further described below: Embodiment 1:

[0032] Taking the estimation of the SOP of a 30Ah lithium titanate battery at 25°C and 50% SOC for 30 seconds as an example, the specific steps include:

[0033] S1. Obtain the maximum discharge rate of the battery at 25°C and 30 seconds from the battery specification sheet, which is 10C;

[0034] S2, the selected measurement magnification is 3C, 4C and 5C;

[0035] S3. The instrument measured the voltage of the battery at 50% SOC at 25°C for 60 minutes, that is, the OCV is 2.206V;

[0036] S4. The instrument measured that at 25°C and 50% SOC, the voltage of the battery discharged at 3C to the 30th second, that is, the terminal voltage 1, is 2.115V, the voltage of the battery discharged at 4C to the 30th second, that is, the terminal voltage 2, is 2.084V, and the voltage of the battery discharged at 5C to the 30th second, that is, the terminal voltage 3, is 2.025V;

[0037] S5, overpotential 1 is equal to OCV minus terminal voltage 1, that is, 2.206-2.115=0.091V, overpotential 2 is equal to OCV minus terminal voltage 2, that is, 2.206-2.084=0.122V, overpotential 3 is equal to OCV minus terminal voltage 3, that is, 2.206-2.025=0.181V;

[0038] S6. In this embodiment, the minimum voltage required for the battery at 25°C is set to 2.1V, and the maximum allowable overpotential is equal to OCV minus the minimum voltage, that is, 2.206-2.1=0.106V.

[0039] S7. By comparing the magnitudes of overpotential 1, overpotential 2, overpotential 3 and the maximum allowable overpotential, the value of the maximum allowable overpotential is between the values ​​of overpotential 1 and overpotential 2. Overpotential 1 and 3C are recorded as x1 and y1, respectively, and (x1, y1) is obtained, that is, (0.091, 3). Overpotential 2 and 4C are recorded as x2 and y2, respectively, and (x2, y2) is obtained, that is, (0.122, 4).

[0040] S8. Substitute (0.091, 3) and (0.122, 4) into y=ax+b respectively to solve the linear equation: y=32.258x+0.0645;

[0041] S9. Substitute 0.106V as x into y=32.258x+0.0645 for calculation, and obtain y, which is the maximum allowable rate of 3.5C, which is the SOP of the battery when it is discharged for 30 seconds at 25°C and 50% SOC.

[0042] To verify the accuracy of the SOP, it was measured that at 25°C and 50% SOC, the voltage of the battery discharged at the maximum allowable rate to the 30th second was 2.098V. The error was 2mV obtained by subtracting 2.098V from the minimum voltage, i.e. 2.1V. The smaller the value, the more accurate the estimation result.

[0043] Figure 2 The figure shows the charging SOP of the battery at 25°C, calibrated by different SOCs and measurement rates. Embodiment 2:

[0044] Taking the estimation of the SOP of a 30Ah lithium titanate battery at 25°C and 50% SOC for 30 seconds as an example, the specific steps include:

[0045] S1. Obtain the maximum specified charging rate of the battery at 25°C and charging for 30 seconds from the battery specification sheet, which is 10C;

[0046] S2, the selected measurement magnification is 3C, 4C and 5C;

[0047] S3. The instrument measured the voltage of the battery at 50% SOC at 25°C for 60 minutes, that is, the OCV is 2.206V;

[0048] S4. The instrument measured that at 25°C and 50% SOC, the voltage of the battery at 3C charging to the 30th second, that is, the terminal voltage one, is 2.286V, the voltage of the battery at 4C charging to the 30th second, that is, the terminal voltage two, is 2.312V, and the voltage of the battery at 5C charging to the 30th second, that is, the terminal voltage three, is 2.337V.

[0049] S5, overpotential 1 is equal to terminal voltage 1 minus OCV, that is, 2.286-2.206=0.08V, overpotential 2 is equal to terminal voltage 2 minus OCV, that is, 2.312-2.206=0.106V, overpotential 3 is equal to terminal voltage 3 minus OCV, that is, 2.337-2.206=0.131V;

[0050] S6. In this embodiment, the maximum voltage required for the battery at 25°C is set to 2.3V, and the maximum allowable overpotential is equal to the maximum voltage minus OCV, that is, 2.3-2.206=0.094V; when the solution of the present invention is actually used, the maximum voltage can be obtained from the battery specification.

[0051] S7. By comparing the magnitudes of overpotential 1, overpotential 2, overpotential 3 and the maximum allowable overpotential, the value of the maximum allowable overpotential is between the values ​​of overpotential 1 and overpotential 2. Overpotential 1 and 3C are recorded as x1 and y1 respectively, and (x1, y1) is obtained, that is, (0.08, 3). Overpotential 2 and 4C are recorded as x2 and y2 respectively, and (x2, y2) is obtained, that is, (0.106, 4).

[0052] S8. Substitute (0.08, 3) and (0.106, 4) into y=ax+b respectively to solve the linear equation: y=38.462x+0.077;

[0053] S9. Substitute 0.094V as x into y=38.462x+0.077 for calculation, and obtain y, which is the maximum allowable rate of 3.7C, which is the SOP of the battery when charged for 30 seconds at 25°C and 50% SOC.

[0054] To verify the accuracy of the SOP, it was measured that at 25°C and 50% SOC, the voltage of the battery when charged at the maximum allowable rate to the 30th second was 2.303V. The error obtained by subtracting 2.303V from the highest voltage, i.e. 2.3V, was -3mV. The smaller the value, the more accurate the estimation result.

[0055] Figure 3 The figure shows the charging SOP of the battery at 25°C, calibrated by different SOCs and measurement rates.

[0056] The present invention can comprehensively calibrate the SOP of the battery at different temperatures, measurement rates, SOCs and durations. Compared with the method of estimating the SOP based on the DC internal resistance, the SOP of the present invention is not affected by the DC internal resistance, thereby improving the estimation accuracy of the SOP. Compared with the method of estimating the SOP according to the national standard 38661, when the minimum voltage or the maximum voltage needs to be changed, in the first or second embodiment, it is only necessary to recalculate the maximum allowable overpotential and execute step S6. Figure 2 or Figure 3 In the table, determine (x1, y1) and (x2, y2) and execute step S7; solve the linear equation and execute step S8; finally, calculate the maximum allowable ratio and execute step S9. There is no need to execute the steps from the beginning, and the estimation efficiency is high.

[0057] The above does not limit the technical scope of the present invention in any way. Any modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for estimating the SOP of a lithium-ion battery, characterized in that: The following steps are involved: S1. Obtain the maximum specified rate of the battery at a specified temperature and duration; S2. Select a measurement magnification that is not higher than the maximum specified magnification; S3, obtaining the OCV of the battery after standing at a specified temperature and a specified SOC; S4, obtaining different terminal voltages of the battery at the last moment of a specified temperature, a specified SOC, different measurement rates and the same duration; S5, subtracting different terminal voltages from OCV to obtain different overpotentials; S6. Subtract the voltage threshold corresponding to the specified temperature from the OCV to obtain the maximum allowable overpotential, where the voltage threshold is the lowest voltage for discharge and the voltage threshold is the highest voltage for charge; S7, compare the maximum allowable overpotential with different overpotentials, select the overpotential that is smaller than the maximum allowable overpotential and closest to it as x1, select the corresponding measurement magnification as y1, select the overpotential that is larger than the maximum allowable overpotential and closest to it as x2, select the corresponding measurement magnification as y2; S8, solve the linear equation using x1, y1, x2 and y2; S9. Substitute the maximum allowable overpotential into the linear equation for calculation to obtain the maximum allowable rate.

2. A method for estimating the SOP of a lithium-ion battery according to claim 1, characterized in that: In the step S1, the temperature range of the discharge is -20°C to 55°C, and the temperature range of the charge is 0°C to 55°C.

3. A method for estimating the SOP of a lithium-ion battery according to claim 1, characterized in that: In step S1, the duration is 10 seconds, 30 seconds or 60 seconds.

4. A method for estimating the SOP of a lithium-ion battery according to claim 1, characterized in that: In step S2, the measurement magnification is not less than 0.1C.

5. A method for estimating the SOP of a lithium-ion battery according to claim 1, characterized in that: In step 3, the range of the standing time is 30 min to 60 min.

6. A method for estimating the SOP of a lithium-ion battery according to claim 1, characterized in that: In step S5, the overpotential of discharge is equal to OCV minus the terminal voltage, and the overpotential of charge is equal to the terminal voltage minus OCV.

7. A method for estimating the SOP of a lithium-ion battery according to claim 1, characterized in that: The maximum allowable overpotential for discharge is equal to OCV minus the minimum voltage, and the maximum allowable overpotential for charge is equal to the maximum voltage minus OCV.

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