A lithium ion battery internal short circuit fault diagnosis method based on relaxation process

Abstract

The application provides a lithium ion battery internal short circuit fault diagnosis method based on a relaxation process, which is an internal short circuit fault early warning method based on a relaxation voltage curve. The method explores the correlation between the internal short circuit resistance and the voltage drop in the relaxation process. By comparing the voltage of the internal short circuit fault battery and the normal battery in the relaxation process, whether the measured battery has an internal short circuit fault is determined, and the size of the internal short circuit resistance can be calculated to determine the severity of the fault. The method has low calculation complexity and is convenient to realize in an embedded battery management system, and provides a new idea for simple and efficient internal short circuit fault early warning.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of lithium ion battery fault diagnosis, and particularly relates to a lithium ion battery internal short circuit fault diagnosis method based on a relaxation process. BACKGROUND

[0002] Due to high power density, long cycle life and other advantages, lithium ion batteries have been widely used in new energy vehicles, consumer electronic products, power grids and user-side energy storage and other scenarios. However, severe use scenarios and complex use conditions may cause battery failures, and even cause safety problems, for example, internal short circuit faults caused by battery abuse may cause battery thermal runaway, and further cause fire and explosion and other serious safety accidents. Therefore, in order to prevent the battery from generating thermal runaway during use, early warning of the internal short circuit of the battery is indispensable.

[0003] Due to the complex mechanism of internal short circuit faults, the generation of internal short circuit faults is the result of multi-factor coupling, and it is difficult to design experiments to analyze the internal short circuit faults of commercial batteries. In order to explore the fault mechanism of internal short circuit and realize the early warning of internal short circuit, researchers have established a thermoelectric coupling model of lithium battery, and the battery voltage and temperature data obtained by detection are substituted into the model to calculate the electrochemical characteristics that can reflect the internal short circuit state, so as to realize the early warning of internal short circuit. However, the model-based method first needs to determine the model parameters, and the complex electrochemical model contains a large number of parameters, and parameter identification is required before the battery is evaluated for internal short circuit fault each time. In addition, the complex electrochemical model is difficult to realize online evaluation, and the electrochemical model is composed of a large number of partial differential equations, which is difficult to realize online calculation in an embedded system. In order to simplify the calculation, another scholar compares the discharge capacity and the charge capacity to evaluate the internal short circuit fault, which first needs to evaluate the state of charge in the charging and discharging process, and calculate the maximum charge and discharge capacity based on the state of charge. If internal short circuit fault occurs, the energy charged into the battery will be greater than the energy that can be discharged, but the state of charge evaluation still needs to establish a circuit model and introduce Kalman filtering to update the model parameters. So far, there is still a lack of simple and effective internal short circuit fault evaluation and early warning method. SUMMARY

[0004] To solve the above technical problems, the application provides a lithium ion battery internal short circuit fault diagnosis method based on a relaxation process, which is an internal short circuit fault early warning method based on a relaxation voltage curve. The method explores the correlation between internal short circuit resistance and voltage drop in the relaxation process. By comparing the voltage of the internal short circuit fault battery and the normal battery in the relaxation process, it is determined whether the measured battery has an internal short circuit fault, and the size of the internal short circuit resistance can be calculated to determine the severity of the fault. The method has low calculation complexity and is easy to realize in an embedded battery management system, providing a new idea for simple and efficient internal short circuit fault early warning.

[0005] To achieve the above object, the technical scheme adopted by the present application is:

[0006] A lithium ion battery internal short circuit fault diagnosis method based on relaxation process, by comparing the voltage of the internal short circuit fault battery and the normal battery in the relaxation process, to determine whether the measured battery has an internal short circuit fault, and to calculate the short circuit resistance to determine the severity of the fault, specifically including the following steps:

[0007] Step 1, establish the association between polarization and relaxation:

[0008] The size of the polarization is represented as overpotential, which increases with the increase of current intensity; the relaxation is the reverse process of polarization, that is, the process of the battery returning to the equilibrium potential after the end of charging and discharging; once the internal short circuit occurs, the short circuit current consumes electrical energy, the state of charge SOC of the battery decreases, causing the battery voltage to drop; the ISC fault is identified by analyzing the voltage response after the battery is discharged;

[0009] Step 2, analyze the relaxation process of the internal short circuit fault battery:

[0010] The relaxation process and the loss process of the internal short circuit fault battery both affect the voltage of the battery after stopping work, and the influence of the relaxation process and the loss process is independent of each other; after the internal short circuit fault battery is discharged, the relaxation voltage V FR and the loss voltage V D of the internal short circuit fault battery are summed to obtain the voltage response V SC of the internal short circuit fault battery, as shown in formula (1):

[0011] V SC = V FR + V D (1)

[0012] The loss voltage V D in formula (1) is related to the short circuit resistance, and is used to evaluate the internal short circuit fault level of the battery; the short circuit resistance value decreases, the short circuit current increases, and the state of charge further decreases, so that the loss voltage V D increases;

[0013] Based on formula (1), the loss voltage V D is calculated from the voltage response V SC of the internal short circuit fault battery and the relaxation voltage V FR , wherein the voltage response V SC of the internal short circuit fault battery is directly obtained by measuring the voltage of the internal short circuit fault battery; the relaxation voltage V FR of formula (1) is replaced by the relaxation voltage V NR of the normal battery to evaluate the internal short circuit fault level, as shown in formula (2):

[0014] V SC =V NR +V D (2)

[0015] Step 3, obtain the relaxation voltage V of normal battery NR :

[0016] In certain current range, the overpotential of battery is linearly related to current, i.e. the relaxation voltage is linearly related to current;

[0017] Step 4, calculate the short-circuit resistance:

[0018] In the process of loss, the electric quantity lost by short-circuit resistance is calculated by formula (3):

[0019] W=UIt (3)

[0020] Wherein, W is the electric energy lost, U and I are the battery voltage and the current flowing through the short-circuit resistance respectively, and t is the standing time after the battery stops working; in order to evaluate the ISC degree of the battery, the discharge cut-off voltage before the battery stops working is set as a constant value, i.e. the battery voltage U is a constant value; the standing time t after the battery stops working is set as a constant value, and the electric energy lost in the same time after the battery stops working is calculated by formula (3);

[0021] For short-circuit resistances r1 and r2 with different internal short-circuit degrees, the current flowing through the short-circuit resistance loop and the electric energy lost are expressed as follows:

[0022]

[0023] Wherein, r2=kr1, the subscripts 1 and 2 represent different internal short-circuit degrees, k is a constant used to represent the numerical relationship of internal short-circuit resistances under different ISC degrees, I1 and I2 represent the current flowing through the short-circuit internal resistance under different ISC degrees respectively, and W1 and W2 represent the electric quantity consumed by the short-circuit internal resistance under different ISC degrees respectively;

[0024] It is obtained from formula (4) that the product of the electric energy lost and the short-circuit resistance is a constant value, which is defined as C0 here, i.e.

[0025] W2r2=W1r1=C0 (5)

[0026] Based on formula (5), the short-circuit resistance of the battery is calculated according to formula (6):

[0027]

[0028] Wherein W is the electric energy lost by the short-circuit resistance, since the relationship between the loss voltage and the loss electric energy is constant, W in formula (6) is replaced by the loss voltage V Dto replace; the constant C in formula (7) represents the product of the loss voltage and the short-circuit resistance:

[0029]

[0030] Finally, the short-circuit resistance is calculated based on formula (7) and the voltage response of the loss process, and the severity of the internal short-circuit fault is determined by the size of the short-circuit resistance.

[0031] Advantages:

[0032] (1) The present application does not require additional hardware devices: the existing internal short-circuit fault detection technology relies more on the temperature and impedance value of the battery. The detection of temperature and impedance requires additional hardware devices, such as thermocouples and additional circuit boards. This increases the cost of the battery management system and requires more space. The method proposed in the present application can diagnose internal short-circuit faults by analyzing the change of the electric signal, i.e. the change of the battery voltage.

[0033] (2) The present application has low computational complexity: the current detection method that relies only on the electric signal usually relies on the battery model, such as the equivalent circuit model. The model-based algorithm introduces high-complexity algorithms, such as Kalman filtering. The method proposed in the present application only needs to fit the unique coefficient in the inverse function of the internal short-circuit resistance and the loss voltage obtained by calibration, which can realize internal short-circuit fault diagnosis and is beneficial to the implementation in embedded battery management systems.

[0034] (3) The present application has high prediction accuracy: the method proposed in the present application has been verified by experiments and can achieve high-precision early warning on 5Ω, 10Ω, 20Ω, 30Ω, 50Ω, 100Ω internal short-circuit fault levels. Its prediction accuracy is consistent with that of high-complexity algorithms. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 is a schematic diagram of the relationship between polarization and relaxation;

[0036] Figure 2 is an equivalent circuit model diagram; wherein, figure (a) is a normal battery; figure (b) is an ISC battery;

[0037] Figure 3 is a schematic diagram of the relaxation voltage of a normal battery and an ISC battery after stopping working;

[0038] Figure 4 is a voltage diagram; wherein, figure (a) is a discharge voltage diagram; figure (b) is a loss process and loss voltage diagram;

[0039] Figure 5 is a battery test platform diagram;

[0040] Figure 6Fig. 1 is a schematic diagram of the relationship between the relaxation voltage and the current of a normal battery; wherein Fig. (a) is the relaxation voltage of the normal battery under different discharge currents; Fig. (b) is a schematic diagram of the linear relationship between the relaxation voltage and the current of the normal battery;

[0041] Figure 7 Fig. 3 is a schematic diagram of the voltage response of the ISC fault battery; wherein Fig. (a) is a schematic diagram of the relaxation voltage of the normal battery and the voltage response of the battery under different ISC fault levels; Fig. (b) is a schematic diagram of the loss voltage of the battery under different ISC levels;

[0042] Figure 8 Fig. 4 is a schematic diagram of the relaxation voltage results obtained in different ways; wherein Fig. (a) is a schematic diagram of the relaxation voltage of the fault battery obtained by direct measurement and calculation according to formula (2); Fig. (b) is a schematic diagram of the difference between the relaxation voltages obtained in two different ways;

[0043] Figure 9 Fig. 5 is a schematic diagram of the loss voltage under different short-circuit resistances obtained by formula (2); wherein Fig. (a) is the resistance of 5 Ω; Fig. (b) is the resistance of 10 Ω; Fig. (c) is the resistance of 20 Ω; Fig. (d) is the resistance of 30 Ω; Fig. (e) is the resistance of 50 Ω;

[0044] Fig. (f) is the resistance of 100 Ω; Fig. (g) is the resistance of 200 Ω;

[0045] Figure 10 Fig. 6 is a schematic diagram of the fitting results of the loss voltage under different short-circuit resistances; wherein Fig. (a) is the resistance of 5 Ω; Fig. (b) is the resistance of 10 Ω; Fig. (c) is the resistance of 20 Ω; Fig. (d) is the resistance of 30 Ω; Fig. (e) is the resistance of 50 Ω; Fig. (f) is the resistance of 100 Ω; Fig. (g) is the resistance of 200 Ω;

[0046] Figure 11 Fig. 7 is a schematic diagram of the relationship between the loss voltage and the short-circuit resistance;

[0047] Figure 12 Fig. 8 is a schematic diagram of the inverse proportional functions obtained by 7 pairs of data and the fitting results of all the data; wherein Fig. (a) is a 3D diagram; Fig. (b) is a front view of the 3D diagram. DETAILED DESCRIPTION

[0048] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0049] The present application will introduce the theoretical basis of the lithium-ion battery internal short-circuit fault diagnosis method based on relaxation process and establish the correlation between internal short-circuit resistance and the pressure drop caused by it in the relaxation process, and illustrate the feasibility of the method. In addition, the influence of different discharge conditions on the present application will be discussed to show its applicability. Finally, the effectiveness of the method is verified by experiment, which highlights the advantages of the present application.

[0050] As shown in Figures 1-4 , the lithium-ion battery internal short-circuit fault diagnosis method based on relaxation process of the present application specifically includes the following steps:

[0051] Step 1, establish the correlation between polarization and relaxation:

[0052] The phenomenon that the electrode potential (or electrochemical cell potential) deviates from the equilibrium potential due to the Faraday current passing through the electrochemical system is called polarization, and the size of polarization is expressed as overpotential, which increases with the increase of current intensity. Relaxation can be regarded as the reverse process of polarization, that is, the process of the battery returning to the equilibrium potential after the end of charging and discharging, which can also be called depolarization. Once the internal short circuit (ISC) occurs in the battery, the short-circuit current consumes electric energy, the state of charge of the battery decreases, and the battery voltage drops. The present application takes this idea as the core, and identifies the ISC fault by analyzing the voltage response after the battery is discharged. Figure 1 The relationship between polarization and relaxation is shown.

[0053] Step 2, analyze the relaxation process of the ISC fault battery:

[0054] For a normal battery without ISC fault, the diaphragm can prevent the formation of current loop inside the battery. However, the abuse of the battery can reduce the performance of the diaphragm, leading to short circuit inside the battery. In terms of electrical characteristics, the equivalent circuit model (ECM) as shown in Figure 2 can be used to explain the internal short circuit. Unlike the ECM of the normal battery, the ECM of the ISC battery contains a closed current loop, which can be presented as a parallel short-circuit resistance. In this closed loop, after the battery stops working, there is still a short-circuit current flowing through the battery, and the intensity of the current depends on the short-circuit resistance value, while the short-circuit resistance value is related to the degree of diaphragm degradation. In the initial stage of ISC, the short-circuit resistance is large, and the short-circuit current intensity is low. As shown in step 1 (overpotential increases with the increase of current intensity), the overpotential caused by the smaller short-circuit current is lower, which is much smaller than the overpotential generated by the current during the normal operation of the battery. Therefore, it can be considered that the overpotential caused by the short-circuit current can be ignored. On this basis, when the ISC battery stops discharging, the ISC battery approximately returns to the equilibrium potential, as shown in Figure 3As shown. Furthermore, as part of the overpotential, the ohmic overpotential disappears immediately after discharge, exhibiting a very fast response speed, and is unaffected by short-circuit current. Therefore, the relaxation voltage change trend of the ISC battery after discharge is the same as that of a normal battery, as shown... Figure 3 As shown above, the trend and final value of the relaxation process in ISC batteries are similar to those in normal batteries. Based on this, it can be inferred that ISC batteries have a relaxation process that is highly consistent with that of normal batteries.

[0055] Although a short circuit does not significantly affect the relaxation process, the energy loss caused by the short circuit has a noticeable impact on the voltage. Unlike normal batteries, in an ISC battery, the short-circuit current continues to flow through the battery after it stops operating. Even a small short-circuit current, i.e., in the initial ISC stage when the short-circuit resistance is relatively high, can lead to a significant voltage drop, especially at a large depth of discharge (DOD). Figure 4 As shown in Figure (a). In this invention, the effect of voltage drop caused by short-circuit current is defined as a loss process, and this voltage drop is defined as the loss voltage, as... Figure 4 As shown in Figure (b).

[0056] In summary, for ISC batteries, both the relaxation and degradation processes affect the voltage after the battery stops operating, but their mechanisms differ. During degradation, the insertion and extraction of lithium ions induces a Faradaic current due to short-circuit resistance, leading to a decrease in the state of charge (SOC) and ultimately a voltage drop. The relaxation process, on the other hand, affects the voltage through the redistribution of lithium ions within the electrodes, and no Faradaic current exists in the electrodes. Under these different mechanisms, the effects of relaxation and degradation are independent of each other. Therefore, after the ISC battery discharges, the relaxation voltage V of the ISC battery... FR and loss voltage V D By summing the results, we can obtain the voltage response V of the ISC battery. SC As shown in equation (1):

[0057] V SC =V FR +V D (8)

[0058] It is worth noting that the loss voltage V in equation (1) D It is related to short-circuit resistance. A decrease in short-circuit resistance leads to an increase in short-circuit current, further reducing the state of charge (SOC) and thus increasing the loss voltage. Since the loss voltage is related to short-circuit resistance, it can be used to assess the battery's independent charge (ISC) failure level.

[0059] Based on equation (1), the loss voltage V D The voltage response V of the ISC battery is required. SC and relaxation voltage V FRThe voltage response V of the ISC battery is calculated, where V is the relaxation voltage of the normal battery SC The relaxation voltage V of the normal battery can be directly obtained by measuring the voltage of the ISC battery. Since the relaxation process of the ISC battery is similar to that of the normal battery, and the relaxation voltage V of the normal battery is NR easy to measure, the relaxation voltage V of formula (1) is replaced by the relaxation voltage V of the normal battery FR NR to evaluate the ISC failure level, as shown in formula (2):

[0060] V SC = V NR + V D (9)

[0061] The relaxation voltage of the normal battery is used to evaluate the degree of ISC of the lithium battery.

[0062] Step 3, obtain the relaxation voltage of the normal battery:

[0063] As described in step 1, relaxation is the reverse process of polarization, indicating that the overpotential is related to the amplitude of the relaxation voltage. Therefore, by studying the relationship between overpotential and current, the relationship between the size of the relaxation voltage and the current can be determined.

[0064] The overpotential of the electrochemical cell is mainly composed of three parts, including ohmic overpotential, concentration overpotential and activation overpotential. Among them, when the battery is discharged at a medium rate (about 1C), the ohmic overpotential occupies a large proportion, and the relationship between the ohmic overpotential and the current conforms to Ohm's law, that is, they are linearly related. Therefore, within a certain current range, the overpotential of the battery is linearly related to the current, that is, the size of the relaxation voltage is linearly related to the current.

[0065] Step 4, calculate the short-circuit resistance:

[0066] In the loss process, the amount of electricity lost by the short-circuit resistance is calculated by formula (3):

[0067] W=UIt (10)

[0068] where W is the electrical energy lost, U and I are the battery voltage and the current flowing through the short-circuit resistance, respectively, and t is the standing time after the battery stops working. In order to evaluate the degree of ISC of the battery, the discharge cutoff voltage before the battery stops working is set as a constant value, that is, the battery voltage U is a constant value. In addition, the standing time t after the battery stops working is set as a constant value, so formula (3) can calculate the electrical energy lost within the same time after the battery stops working.

[0069] Based on the above settings, for different ISC degrees of the short-circuit resistance r1, r2, the current flowing through the short-circuit resistance circuit (as shown in figure (b) of Figure 2 ) and the lost electrical energy can be expressed as follows: ​

[0070]

[0071] where r2=kr1, subscript 1 and 2 represent different levels of ISC, k is a constant to represent the numerical relationship of internal short-circuit resistance under different levels of ISC, I1 and I2 represent the current flowing through the short-circuit internal resistance under different levels of ISC, and W1 and W2 represent the electric quantity consumed by the short-circuit internal resistance under different levels of ISC.

[0072] As can be seen from equation (4), the product of the loss electric energy and the short-circuit resistance is a constant value, which is defined as C0 here, i.e.,

[0073] W2r2=W1r1=C0 (12)

[0074] Based on equation (5), the battery short-circuit resistance can be calculated according to equation (6):

[0075]

[0076] where W is the electric energy consumed by the short-circuit resistance; in addition, since the discharge cutoff voltage is constant, it means that the loss process starts at the same voltage, so the relationship between the loss voltage and the loss electric energy is also constant, and therefore, based on this constant relationship, W in equation (6) is replaced by the loss voltage V D . Accordingly, the constant C in equation (7) is used to represent the product of the loss voltage and the short-circuit resistance:

[0077]

[0078] Finally, based on equation (7) and the voltage response of the loss process, the short-circuit resistance can be calculated, and the severity of the fault can be judged by the size of the short-circuit resistance.

[0079] As Figure 5 shown, the battery test platform used in the present application includes the following components: a battery charging and discharging device, with a voltage and current range of 20V and 10A respectively, and a measurement accuracy of 0.1% FS for both voltage and current; a thermostat, which maintains a constant ambient temperature during the charging and discharging process, with an adjustable temperature range of 15-60℃ and a temperature control accuracy of 2℃; a host computer for collecting data and controlling the battery charging and discharging device; a lithium iron phosphate battery for testing, with specific parameters as shown in Table 1.

[0080] As mentioned above, the relaxation voltage of a normal battery is needed to calculate the short-circuit resistance. In order to obtain and verify the linear relationship with the current, the battery is discharged to 2.5V at different currents and then relaxed, and the voltage during the relaxation process is recorded as V NRThe ISC fault at different levels is simulated by connecting external resistors of 5 Ω, 10 Ω, 20 Ω, 30 Ω, 50 Ω, 100 Ω, 200 Ω in parallel to the battery under test, respectively. After the battery is discharged to 2.5 V and then relaxed, the voltage V SC The specific steps are as follows:

[0081] Step 1: Discharge the battery to 2.5 V and stand for 2 hours;

[0082] Step 2: Charge to the upper limit cut-off voltage at 1C;

[0083] Step 3: Stand for 10 minutes;

[0084] Step 4: Discharge to 2.5 V at 0.5C, 0.75C, 1C, 1.25C, 1.5C, 1.75C, 2C, respectively;

[0085] Step 5: Stand for 10 minutes.

[0086] To avoid the influence of internal resistance and capacity difference, steps 1-5 are measured with the same battery.

[0087] Table 1

[0088]

[0089] It should be noted that the effectiveness of formula (2) is the basis for short-circuit resistance calculation, so in addition to the relaxation voltage of normal battery and ISC battery, the loss voltage also needs to be measured to verify formula (2). Since the loss voltage V D is caused by the decrease of SOC, during the measurement process, the same battery SOC decrease as in the loss process needs to be achieved. For this purpose, the battery is discharged to 2.5 V, and then discharged under constant resistance (5 Ω, 10 Ω, 20 Ω, 30 Ω, 50 Ω, 100 Ω, 200 Ω) to simulate the power consumption under different ISC levels in the consumption process. The specific measurement scheme is shown in steps 6-7:

[0090] Step 6: Discharge at constant resistance 5 Ω, 10 Ω, 20 Ω, 30 Ω, 50 Ω, 100 Ω, 200 Ω;

[0091] Step 7: Stand for 10 minutes.

[0092] The relaxation voltage curve of the normal battery at different rates is shown in Figure 6 The voltage curve in figure (a) of Figure 6 is the relaxation voltage curve of the battery after being discharged to 2.5 V at 2C to 0.5C from top to bottom. The end value of each curve in figure (a) of Figure 6 is selected, and the relationship between the fitting result and the current rate is fitted, as shown in Figure 6As shown in Figure (b), the battery relaxation voltage exhibits a linear relationship with the discharge rate. Based on this linear relationship, the resting voltage after the battery is discharged at different rates can be calculated, providing a possibility for ISC fault assessment under different discharge scenarios. To facilitate verification of the effectiveness of the method proposed in this invention, the following analysis will focus on the experimental results obtained at a 1C discharge rate.

[0093] V SC V NR and V D The experimental results are as follows Figure 7 As shown. In Figure 7 In Figure (a), the voltage curves from top to bottom correspond to the normal relaxation voltage V. NR And V under short-circuit resistance of 200Ω to 5Ω SC . Figure 7 In Figure (b), the voltage curves from top to bottom correspond to V under short-circuit resistances of 200Ω to 5Ω, respectively. D In addition, by Figure 7 As shown in Figure (a), the voltage response of batteries with different ISC fault levels can be clearly distinguished within 300 seconds, which makes rapid ISC testing possible. For this purpose, in this invention, data within 300 seconds after discharge is stopped are used for ISC fault level assessment.

[0094] In equation (2), V NR With V D The sum is obtained through superposition. Figure 7 The normal relaxation voltage curve in Figure (a) and Figure 7 The results are obtained from each curve in Figure (b), and the superimposed results are... Figure 7 The curves corresponding to the resistance levels in Figure (a) are compared, and the comparison results are as follows: Figure 8 As shown. Figure 8 In Figure (a), the voltage curves from top to bottom correspond to V under short-circuit resistances of 200Ω to 5Ω, respectively. SC The dashed line is obtained by superimposing equation (2), while the solid line is obtained by direct experimental measurement. Figure 8 Figure (b) shows the V obtained by experimental measurement and by superimposing equation (2). SC The error between them shows that the V obtained by direct experimental measurement is different from that obtained by equation (2). SC The results are highly consistent; except for certain intervals when the short-circuit resistance is 5Ω and 200Ω, the error between the two is less than 5 millivolts. These results verify the correctness of equation (2), therefore V D This can be calculated, thereby enabling a quantitative assessment of the ISC fault level. Under different short-circuit resistance conditions, V is obtained based on equation (2). D likeFigure 9 As shown in the figure, it can be observed that as the short-circuit resistance increases, such as Figure 9 As shown in Figures (a), (b), (c), (d), (e), (f), and (g), the noise in the voltage values ​​is more significant, which directly affects the evaluation of ISC. Therefore, a function fitting method was used to obtain a smooth V... D Curve. According to V D The curve characteristics, V in equation (8) D (t) is used to fit V D Where a, b, c, p1, and p2 are the coefficients to be fitted, and t is the time after the discharge stops. The fitting result is as follows: Figure 10 Figures (a), (b), (c), (d), (e), (f), and (g) are shown.

[0095]

[0096] From the coefficient of determination R 2 It can be seen that a better fitting result can be obtained when the short-circuit resistance is less than 100Ω. When the short-circuit resistance reaches 100Ω, V D The amplitude of the signal decreases significantly, leading to a drop in the signal-to-noise ratio, which will directly affect R. 2 Therefore, the relatively low R 2 That's acceptable. From Figure 10 As can be seen from Figures (f) and (g), the fitting results still accurately captured the changing trend.

[0097] Based on the above fitting results, V at a specific time... D The value can be obtained and used to investigate its relationship with the short-circuit resistance, as shown in equation (7). In this invention, V is the value at 300 seconds after the battery stops discharging. D The values ​​were used to explore the relationship in equation (7). Seven different short-circuit resistance values ​​(5Ω, 10Ω, 20Ω, 30Ω, 50Ω, 100Ω, 200Ω) and their corresponding V values ​​were used. D It was used to fit an inverse proportional function, such as Figure 11 As shown in the figure, the results demonstrate the relationship between short-circuit resistance and V. D They are inversely proportional.

[0098] As shown in equation (7), the inverse proportional function has only one coefficient C0. Therefore, after proving the inverse proportional relationship, only a pair of short-circuit resistors and the corresponding V are needed. D This allows us to determine the relationship between the two. Based on Figure 11 The seven sets of data (5Ω, 10Ω, 20Ω, 30Ω, 50Ω, 100Ω, 200Ω and their corresponding V) D Seven inverse proportional functions in Figure 12 They were drawn separately.Figure 9 The inverse proportional function obtained by fitting all seven sets of data was also plotted. Figure 12 For comparison (data number 8) Figure 12 The coordinates of point 8 in Figure (a). Figure 12 As can be seen in Figure (b), besides 200Ω and the corresponding V D In addition, the other seven function curves in the figure show a high degree of consistency, which indicates that a relationship can be effectively established with a pair of data, meaning that ISC fault assessment can be achieved with very little data.

[0099] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for diagnosing internal short circuit fault in a lithium-ion battery based on relaxation process, characterized in that: The internal short circuit fault of the battery is determined by comparing the voltage of the battery in the relaxation process with the voltage of a normal battery, and the size of the short circuit resistance is calculated to determine the severity of the fault, and the method comprises the following steps: Step 1, establish the association between polarization and relaxation: the size of the polarization is represented as overpotential, and the overpotential increases with the increase of the current intensity; the relaxation is the reverse process of the polarization, that is, the process of the battery returning to the equilibrium potential after the battery finishes charging and discharging; Once the internal short circuit occurs in the battery, the short circuit current consumes electric energy, the state of charge SOC of the battery decreases, and the voltage of the battery decreases; the ISC fault is identified by analyzing the voltage response of the battery after discharging; Step 2, analyze the relaxation process of the internal short circuit fault battery: The relaxation process and the loss process of the internal short circuit fault battery both affect the voltage of the battery after it stops working, and the effects of the relaxation process and the loss process are independent of each other; after the internal short circuit fault battery is discharged, the relaxation voltage V FR and the loss voltage V D of the internal short circuit fault battery are summed to obtain the voltage response V SC of the internal short circuit fault battery, as shown in formula (1): V SC = V FR + V D (1) the loss voltage V in formula (1) D related to the short-circuit resistance, for assessing the internal short-circuit failure level of the battery; the short-circuit resistance value decreases, the short-circuit current increases, and the state of charge further decreases, so that the loss voltage V D increases; Based on equation (1), the loss voltage V D The voltage response V of the internal short circuit fault battery SC and the relaxation voltage V FR is calculated, where the voltage response V of the internal short circuit fault battery SC is obtained directly by measuring the internal short circuit fault battery voltage; the relaxation voltage V of equation (1) FR is replaced by the relaxation voltage V of the normal battery NR to assess the internal short circuit fault level, as shown in equation (2): V SC = V NR + V D (2) Step 3, obtain the relaxation voltage V of the normal battery NR : In a certain current range, the overpotential of the battery is linearly related to the current, that is, the relaxation voltage is linearly related to the current Step 4, calculate the short circuit resistance: In the loss process, the electric energy consumed by the short circuit resistance loss is calculated by formula (3): W=UIt (3) Wherein, W is the electric energy consumed by the loss, U and I are the battery voltage and the current flowing through the short circuit resistance respectively, and t is the static time after the battery stops working; in order to evaluate the ISC degree of the battery, the discharge cutoff voltage before the battery stops working is set as a constant value, that is, the battery voltage U is a constant value; the static time t after the battery stops working is set as a constant value, and the electric energy consumed in the same time after the battery stops working is calculated by formula (3); For short circuit resistances r1 and r2 of different internal short circuit degrees, the current flowing through the short circuit resistance loop and the electric energy consumed are expressed as follows: Wherein, r2=kr1, the subscripts 1 and 2 represent different internal short circuit degrees, k is a constant, which is used to represent the numerical relationship of the internal short circuit resistance under different ISC degrees, I1 and I2 represent the current flowing through the short circuit internal resistance under different ISC degrees respectively, and W1 and W2 represent the electric energy consumed by the short circuit internal resistance under different ISC degrees respectively; It is obtained from formula (4) that the product of the consumed electric energy and the short circuit resistance is a constant value, which is defined as C0 here, that is: W2r2=W1r1=C0 (5) Based on formula (5), the short circuit resistance of the battery is calculated according to formula (6): where W is the electrical energy lost due to the short-circuit resistance, and since the relationship between the loss voltage and the loss electrical energy is constant, W in equation (6) is replaced by the loss voltage V D ; the constant C in equation (7) represents the product of the loss voltage and the short-circuit resistance: Finally, the short circuit resistance is calculated based on formula (7) and the voltage response in the loss process, and the severity of the internal short circuit fault is determined by the size of the short circuit resistance.

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