Method and system for predicting maximum safe charging current of lithium-ion battery

Abstract

This invention belongs to the field of lithium battery charging technology, specifically relating to a method for predicting the maximum safe charging current of a lithium-ion battery, comprising the following steps: Step S1: Perform a capacity test on the battery under test to obtain the actual capacity Q₀ in the current state; Step S2: At a preset temperature, test the maximum safe charging current of the battery at at least three different preset states of charge (SOCs); Step S3: Plot SOC as the x-axis and the maximum safe charging current measured in Step S2 as the y-axis, and fit a logarithmic relationship between the maximum safe charging current and SOC; Step S4: Calculate the maximum safe charging current of the battery at other SOCs based on the logarithmic relationship. This method is fast, efficient, and non-destructive, and has good reference value in analyzing the maximum safe charging current of lithium-ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery charging technology, specifically relating to a method and system for predicting the maximum safe charging current of a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, due to their high energy density and long cycle life, have been widely used in new energy vehicles, energy storage systems, and consumer electronics. However, under harsh conditions such as fast charging, lithium plating can easily occur on the negative electrode surface of lithium-ion batteries, accelerating capacity decay and potentially causing internal short circuits and other safety hazards. Therefore, accurately assessing the maximum safe charging current of a battery under different conditions is crucial for developing lossless fast charging strategies and optimizing battery management system (BMS) algorithms.

[0003] Currently, the industry commonly uses the Hybrid Pulse Power Characteristic (HPPC) testing method to evaluate battery power capabilities. This method applies charge and discharge pulses at a specific rate, measures the battery's DC internal resistance, and then estimates the battery's usable power. However, the HPPC method typically tests based on only a single or limited number of current rates, making it difficult to comprehensively reflect the battery's true current handling capability under different states of charge (SOC), and even more so, it cannot predict the maximum safe charging current under any SOC or temperature condition.

[0004] To address the aforementioned issues, several patents have proposed improvement solutions. For example, patent CN 108572321 B discloses a method for testing the safe current of lithium-ion batteries. This method establishes a fitting relationship between current and voltage by performing charge and discharge cycles at different temperatures and SOCs using various current rates, thereby obtaining the battery's maximum charge and discharge current. Patent CN112379289 B proposes a maximum current testing method, which involves adjusting the battery to a target SOC and then testing it using a constant voltage pulse method. Patent CN 118795346 A further proposes an analysis method based on coulombic efficiency, determining the battery's lithium plating window through charge and discharge cycles at different SOCs.

[0005] While the aforementioned existing technologies can obtain the battery's safe current or lithium plating boundary from different perspectives, they still have certain limitations: on the one hand, some methods rely on complex charge-discharge cycles or pulse tests, which are time-consuming; on the other hand, most existing methods can only provide the maximum current value under specific test conditions, lacking the exploration and utilization of the inherent relationship between the maximum safe charging current and SOC and temperature, making it difficult to achieve efficient prediction of the safe current under any operating condition. Summary of the Invention

[0006] The purpose of this invention is to provide a method for predicting the maximum safe charging current of a lithium-ion battery, which can quickly and non-destructively predict the maximum safe charging current of a lithium-ion battery at any state of charge (SOC) and any temperature, so as to support the battery management system to achieve more accurate and safe charging control.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for predicting the maximum safe charging current of a lithium-ion battery, comprising the following steps: Step S1: Perform a capacity test on the battery under test and obtain the actual capacity Q0 in the current state; Step S2: At a preset temperature, test the maximum safe charging current of the battery at at least three different preset states of charge (SOC). Step S3: Using SOC as the x-axis and the maximum safe charging current measured in step S2 as the y-axis, fit the logarithmic relationship between the maximum safe charging current and SOC. Step S4: Calculate the maximum safe charging current of the battery under other SOC conditions based on the logarithmic relationship; Wherein, step S2, testing the maximum safe charging current of the battery at any state of charge (SOC), further includes: Sub-step S21: Charge the battery to the target SOC using a preset charging current and record the time, voltage, current and capacity data during the charging process; Sub-step S22: Plot a curve with dV / dt as the ordinate and voltage as the abscissa; Sub-step S23: Adjust the charging current according to the shape of the tail end of the curve until the curve only shows a downward inflection point. Determine the charging current when the curve is about to reach a downward inflection point but has not yet started to decline as the maximum safe charging current under this SOC.

[0008] Preferably, the method further includes step S5: Repeat steps S2 to S4 at different preset temperatures to obtain the logarithmic relationship between the maximum safe charging current and the state of charge (SOC) at each temperature. Under constant SOC, with temperature as the x-axis and maximum safe charging current as the y-axis, an exponential relationship between maximum safe charging current and temperature is obtained by fitting the equation. The maximum safe charging current corresponding to other temperatures under this SOC is calculated based on the exponential relationship.

[0009] Preferably, adjusting the charging current according to the shape of the curve tail end in step S23 specifically includes: When the curve shows only an upward trend at the end, it indicates that the battery has not undergone lithium plating. The charging current is increased by step size m and sub-steps S21 to S22 are repeated. When the curve shows a downward inflection point at the end and then starts to grow rapidly upward from another position, it indicates that irreversible lithium plating has occurred in the battery. The charging current is reduced by step size m and sub-steps S21 to S22 are repeated. When the curve is about to reach a downward inflection point, the current charging current is determined as the maximum safe charging current.

[0010] Preferably, the logarithmic relationship is y=aln(x)+b, where x is the battery charging state of charge (SOC), y is the maximum safe charging current, and a and b are constants.

[0011] Preferably, the exponential relationship is y=ae. bx , where x is the temperature environment for battery charging, y is the maximum safe charging current, and a and b are constants.

[0012] Preferably, the step size m ranges from 0.01C to 0.05C.

[0013] Preferably, the capacity test in step S1 includes: Discharge the battery under test with a small current until it is empty, and let it stand for a preset time. Perform at least three capacity tests on the battery, and take the result of the last test as the actual capacity Q0 of the current state.

[0014] Preferably, the current of the small current discharge is 0.05C, and the resting time is 30 minutes.

[0015] Preferably, the at least three different preset SOCs include 100% SOC, 90% SOC, and 80% SOC.

[0016] Preferably, the method is used in a battery management system to control the actual charging current from not exceeding the predicted maximum safe charging current.

[0017] The beneficial effects of this invention are as follows: This invention determines the maximum safe charging current of a battery under different states of charge (SOC) based on voltage changes during the constant current charging phase. Analysis of extensive test data and pattern summarization reveals that the relationship between the maximum safe charging current and SOC conforms to a logarithmic relationship, while the relationship between the maximum safe charging current and temperature conforms to an exponential relationship. Therefore, boundary maps of "maximum safe charging current and SOC" under different SOC states at a constant temperature, and boundary maps of "maximum safe charging current and temperature" under different temperatures at a constant SOC, can be plotted. This allows for the prediction of the maximum safe charging current at any temperature and any SOC, ensuring the battery operates within the safe zone of the "lithium plating boundary" at all times, thereby guaranteeing battery life and safety. This method is fast, efficient, and non-destructive, and has good reference value in analyzing the maximum safe charging current of lithium-ion batteries. Attached Figure Description

[0018] Figure 1 The dV / dt-t curves for the charging stage at 100% SOC are shown in the example.

[0019] Figure 2 The dV / dt-t curves for the charging stage at 90% SOC are shown in the example.

[0020] Figure 3 The dV / dt-t curves for the charging stage at 80% SOC are shown in the example.

[0021] Figure 4 The graph shows the relationship between the maximum safe charging current of the battery and its state of charge (SOC) at 10°C, as an example.

[0022] Figure 5 The graph shows the relationship between the maximum safe charging current and the state of charge (SOC) under different temperature conditions, as an example.

[0023] Figure 6 The graph shows the relationship between the maximum safe charging current and temperature under different SOC states, as an example. Detailed Implementation

[0024] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0025] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0026] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," "fixed connection," and "fixed connection" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0027] Example 1 This embodiment uses a certain type of lithium-ion battery as the test object to explain in detail the specific implementation process of the maximum safe charging current prediction method of the present invention.

[0028] I. Battery Capacity Test First, a capacity test is performed on the battery under test to obtain its actual capacity in its current state.

[0029] The specific steps are as follows: 1. Prepare a battery, discharge it at 0.05C to reduce it to a fully charged state, and let it stand for 30 minutes; 2. Perform three capacity tests on the battery, and take the last capacity as the current capacity of the battery, Q0, which is 1.935Ah; II. Testing of the maximum safe charging current of each SOC at 10℃ 1. Test the maximum safe charging current at 100% SOC under 10℃ conditions. At 10℃, the battery was charged at a constant current of 0.08C with a preset current C1 to a cutoff capacity of 1.935Ah. Information such as time, voltage, current and capacity was collected, and the battery was considered to be at 100% SOC at this time. The charging process data was analyzed, and a graph was plotted with dV / dt as the vertical axis and voltage as the horizontal axis. When charged to 1.935Ah at a constant current of 0.08C, the dV / dt voltage curve only shows an upward trend at the tail end, indicating that lithium plating has not occurred in the battery. The current is then increased in increments of 0.02C to 0.10C and 0.12C, and the adjusted curves are shown below. Figure 1 Repeat steps 1 and 2 until the obtained curve shows only a downward inflection point, thus obtaining the maximum safe charging current; when the constant current of 0.12C is used to charge to 1.935Ah, when the dV / dt voltage curve shows a downward inflection point Q1 / V1 at the end, it indicates that lithium plating has begun in the battery; at this time, the charging current of 0.1C, which is about to reach the downward inflection point but has not yet started to decrease, is taken as the maximum safe charging current of the battery at 100% SOC.

[0030] 2. Test the maximum safe charging current at 90% SOC under 10℃ conditions. Under 10℃ conditions, the battery was charged at a constant current of 0.2C with a preset current C91, and the cutoff condition was set at 90% SOC capacity, i.e., 1.548Ah. Information such as time, voltage, current and capacity was collected. The charging process data was analyzed, and a graph was plotted with dV / dt as the vertical axis and voltage as the horizontal axis. When the battery is charged to 1.548Ah at a constant current of 0.2C, and the dV / dt voltage curve shows a decreasing inflection point Q91 / V91 at the tail end, followed by a rapid upward increase from Q92 / V92, this indicates that lithium plating has occurred. Furthermore, the deposited lithium undergoes irreversible lithium plating due to side reactions with the electrolyte. At this point, the current is reduced in increments of 0.02C to 0.14C and then 0.16C. The adjusted curve is shown below. Figure 2 Repeat steps 1 and 2 until the curve is about to show a downward inflection point, obtaining the maximum safe charging current; when charging at a constant current of 0.16C to 1.548Ah, the dV / dt voltage curve shows a downward inflection point Q1 / V1 at the end, indicating that lithium plating has begun in the battery; when charging at a constant current of 0.14C to 1.548Ah, the dV / dt voltage curve only shows an upward increasing trend at the end, indicating that lithium plating has not occurred in the battery; at this time, the corresponding charging current of 0.14C is the maximum safe charging current when charging to 90% SOC; 3. Test the maximum safe charging current at 80% SOC under 10℃ conditions. Under 10℃ conditions, the battery was charged with constant current at preset currents C81 of 0.19C, 0.21C, and 0.23C, with 80% SOC capacity as the cutoff condition. Information such as time, voltage, current, and capacity was collected. The data was analyzed and the test current was adjusted according to steps 2-3 above to obtain the inflection point where the dV / dt curve only shows a decrease. The corresponding charging current of 0.19C is the maximum safe charging current at 80% SOC. III. Prediction of the maximum safe charging current for each SOC at 10℃ The maximum safe currents C1, C91, and C81, measured at SOC of 100%, 90%, and 80% respectively, are 0.1C, 0.14C, and 0.19C. Based on the analysis and summarization of a large amount of test data, the inventors found that the relationship between the maximum safe charging current and SOC fits a logarithmic relationship. Therefore, by plotting SOC on the x-axis and the maximum safe current on the y-axis, the fitting relationship is obtained as y = -0.404ln(x) + 0.0991, R² = 0.999 (…). Figure 4Let x be the battery's state of charge (SOC) and y be the maximum safe charging current. By substituting the states of charge (x) of 70%, 60%, 50%, 40%, etc. into the above fitting formula, the maximum safe charging current under the corresponding SOC can be predicted.

[0031] The battery was compared and verified using other SOC tests: Based on the relationship between SOC and maximum safe charging current at 10℃, y=-0.404ln(x)+0.0991, the maximum safe charging current when the battery is charged to 50% SOC at 10℃ under constant current is calculated to be 0.38C. The actual test result was 0.35C. The maximum safe charging current when the battery is charged to 40% SOC at 10℃ under constant current is calculated to be 0.47C. The actual test result was 0.43C. The error was less than 100mA.

[0032] IV. Prediction of maximum safe charging current under different temperatures and SOC states 1. Logarithmic relationship between SOC and maximum safe charging current at different temperatures Change the temperature to 0℃ and 25℃, and test the maximum safe current at at least three different SOCs following the aforementioned steps. Plot the SOC on the x-axis and the maximum safe current on the y-axis to obtain the relationship between battery SOC and maximum safe charging current at different temperatures of 0℃ and 25℃, as shown in the figure. Figure 5 Therefore, the relationship between battery SOC and maximum safe charging current at 0℃ is y = -0.095ln(x) + 0.0592, R 2 = 0.9928; Under 25℃ conditions, the relationship between battery SOC and maximum safe charging current is y = -2.158ln(x) + 0.4475, R 2 = 0.9985.

[0033] 2. The exponential relationship between temperature and maximum safe charging current under constant SOC Based on the maximum safe charging current obtained from the aforementioned tests at 0℃, 10℃, and 25℃ at 90% SOC, 80% SOC, and 50% SOC, a graph showing the relationship between the maximum safe charging current and temperature under constant SOC conditions is obtained, with the maximum safe charging current as the ordinate and temperature as the abscissa. Figure 6 Therefore, the relationship between the battery charging temperature and the maximum safe charging current under 90% SOC conditions can be derived as y = 0.064e 0.0933x R 2= 0.9971; The relationship between battery temperature and maximum safe charging current under 80% SOC conditions is y = 0.0762e 0.0996x R 2 = 0.9996; The relationship between battery temperature and maximum safe charging current under 50% SOC conditions is y = 0.1157e 0.111x R 2 = 0.9997.

[0034] Where x is temperature (°C) and y is the maximum safe charging current (C). Figure 6 The figure shown is a graph showing the relationship between the maximum safe charging current and temperature under different SOCs.

[0035] Based on the above fitting relationship, the maximum safe charging current of the battery at any temperature and any SOC can be predicted, thereby achieving precise control of the battery charging process.

[0036] The maximum safe charging current prediction method for lithium-ion batteries provided by this invention can be integrated into a battery management system (BMS). In practical applications, the BMS can calculate the maximum safe charging current of the battery in the current state in real time based on the battery's current SOC and temperature, using pre-established fitting formulas (logarithmic and exponential formulas), and control the actual charging current within the range that does not exceed the predicted value. This ensures that the battery always operates within the safe zone of the lithium plating boundary, effectively extending battery life and ensuring charging safety.

[0037] For those skilled in the art, various improvements and modifications can be made without departing from the principles of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention.

Claims

1. A method for predicting the maximum safe charging current of a lithium-ion battery, characterized in that, Includes the following steps: Step S1: Perform a capacity test on the battery under test and obtain the actual capacity Q0 in the current state; Step S2: At a preset temperature, test the maximum safe charging current of the battery at at least three different preset states of charge (SOC). Step S3: Using SOC as the x-axis and the maximum safe charging current measured in step S2 as the y-axis, fit the logarithmic relationship between the maximum safe charging current and SOC. Step S4: Calculate the maximum safe charging current of the battery under other SOC conditions based on the logarithmic relationship; Wherein, step S2, testing the maximum safe charging current of the battery at any state of charge (SOC), further includes: Sub-step S21: Charge the battery to the target SOC using a preset charging current and record the time, voltage, current and capacity data during the charging process; Sub-step S22: Plot a curve with dV / dt as the ordinate and voltage as the abscissa; Sub-step S23: Adjust the charging current according to the shape of the tail end of the curve until the curve only shows a downward inflection point. Determine the charging current when the curve is about to reach a downward inflection point but has not yet started to decline as the maximum safe charging current under this SOC.

2. The method for predicting the maximum safe charging current of a lithium-ion battery according to claim 1, characterized in that, It also includes step S5: Repeat steps S2 to S4 at different preset temperatures to obtain the logarithmic relationship between the maximum safe charging current and the state of charge (SOC) at each temperature. Under constant SOC, with temperature as the x-axis and maximum safe charging current as the y-axis, an exponential relationship between maximum safe charging current and temperature is obtained by fitting the equation. The maximum safe charging current corresponding to other temperatures under this SOC is calculated based on the exponential relationship.

3. The method for predicting the maximum safe charging current of a lithium-ion battery according to claim 1, characterized in that, The step S23, which involves adjusting the charging current according to the shape of the curve's tail end, specifically includes: When the curve shows only an upward trend at the end, it indicates that the battery has not undergone lithium plating. The charging current is increased by step size m and sub-steps S21 to S22 are repeated. When the curve shows a downward inflection point at the end and then starts to grow rapidly upward from another position, it indicates that irreversible lithium plating has occurred in the battery. The charging current is reduced by step size m and sub-steps S21 to S22 are repeated. When the curve is about to reach a downward inflection point, the current charging current is determined as the maximum safe charging current.

4. The method for predicting the maximum safe charging current of a lithium-ion battery according to claim 1, characterized in that, The logarithmic relationship is y=aln(x)+b, where x is the battery's state of charge (SOC), y is the maximum safe charging current, and a and b are constants.

5. The method for predicting the maximum safe charging current of a lithium-ion battery according to claim 2, characterized in that, The exponential relationship is y=ae bx , where x is the temperature environment for battery charging, y is the maximum safe charging current, and a and b are constants.

6. The method for predicting the maximum safe charging current of a lithium-ion battery according to claim 3, characterized in that, The step size m ranges from 0.01C to 0.05C.

7. The method for predicting the maximum safe charging current of a lithium-ion battery according to claim 1, characterized in that, The capacity test described in step S1 includes: Discharge the battery under test with a small current until it is empty, and let it stand for a preset time. Perform at least three capacity tests on the battery, and take the result of the last test as the actual capacity Q0 of the current state.

8. The method for predicting the maximum safe charging current of a lithium-ion battery according to claim 7, characterized in that, The discharge current is 0.05C, and the settling time is 30 minutes.

9. The method for predicting the maximum safe charging current of a lithium-ion battery according to claim 1, characterized in that, The at least three different preset SOCs include 100% SOC, 90% SOC, and 80% SOC.

10. The method for predicting the maximum safe charging current of a lithium-ion battery according to claim 1, characterized in that, The method is used in a battery management system to control the actual charging current from not exceeding the predicted maximum safe charging current.

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