Online identification method of internal short circuit induced by overdischarge in lithium batteries based on impedance characteristics

Abstract

The present invention provides an online identification method for lithium battery overdischarge-induced internal short circuits based on impedance characteristics. First, the static electrochemical impedance spectra of the lithium-ion battery are measured at different temperatures and different states of charge. Then, a frequency range in which the overlap rate of the static electrochemical impedance spectra at different temperatures and states of charge is greater than or equal to 90% is selected as the frequency range in which the impedance is least affected by temperature and state of charge, and the average value of this frequency range is used as the characteristic frequency. Finally, through a lithium battery overdischarge-induced internal short circuit experiment, the dynamic impedance of the lithium-ion battery during discharge at the characteristic frequency is obtained. When the dynamic impedance exhibits three characteristics, namely, a half-sine change, a needle-like change, and a significant rebound, it indicates that the lithium battery has overdischarge-induced internal short circuits, thereby achieving online identification of lithium battery overdischarge-induced internal short circuits. This method introduces online impedance measurement technology into lithium-ion battery status assessment and fault diagnosis. The half-sine change characteristic of the dynamic impedance at the end of discharge can provide a warning of lithium-ion battery overdischarge, the needle-like change characteristic of the dynamic impedance during overdischarge can provide a warning of internal short circuit faults, and the significant rebound characteristic of the dynamic impedance can serve as a sign of internal short circuit occurrence.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to an online identification method for internal short circuit induced by overdischarge of a lithium-ion battery based on impedance characteristics. Background Art

[0002] With the advancement of automotive technology, the market share of electric vehicles has gradually increased, but electric vehicle fires are also increasing, causing significant economic losses. Lithium-ion batteries, with their advantages such as high energy density, excellent cycle performance, and high charge and discharge efficiency, have become a key power component for electric and hybrid vehicles. Most current electric vehicle fires are caused by thermal runaway failures of lithium-ion batteries, with internal short circuit failures being a major contributing factor. Lithium-ion battery packs in electric vehicles are typically constructed from numerous cells connected in series. When thermal runaway occurs in one cell, it can quickly spread to other cells within minutes, causing serious safety incidents. Limited by current battery production technology, variations in the consistency of lithium-ion battery cells are unavoidable, typically manifesting as variations in cell capacity. The balancing function of the battery management system (BMS) can address this problem to a certain extent, but its balancing capabilities are limited and cannot completely eliminate capacity variations. During the charging and discharging process of the battery pack, due to the difference in cell capacity, when the smaller cell is fully charged or discharged, the larger cell still has some capacity left. At this time, if the BMS recognizes that charging or discharging continues, it will cause overcharging or over-discharging of the smaller cell, and over-discharging is one of the main causes of internal short circuit.

[0003] Online identification and diagnosis of lithium-ion battery internal short-circuit faults is an effective measure to prevent accidents. Based on the detection principle, online identification of lithium-ion battery internal short-circuit faults can be categorized into parameter-based and model-based methods. Parameter-based methods, based on the abnormal power consumption characteristics of internal short-circuited batteries, monitor parameters such as voltage and capacity and compare them with set thresholds. This method has the advantage of simple hardware implementation. However, due to the variability of cell consistency, the voltage or capacity parameter thresholds used to indicate internal short-circuit faults may vary, resulting in uncertainty in the reliability of parameter-based fault diagnosis. Model-based methods for online diagnosis of lithium-ion battery internal short-circuit faults, based on the characteristics of internal short-circuit faults causing changes in the battery's internal structure, establish a modified pseudo-two-dimensional model, a three-dimensional electrochemical-thermal-internal short-circuit coupled model, and an equivalent circuit model that simulates internal short-circuit with an external parallel resistor. These methods study the correlation between parameters such as voltage, current, and temperature and the internal short-circuit fault, transforming the internal short-circuit problem into a parameter estimation problem. Algorithms such as the recursive least squares method with a forgetting factor are applied to the parameterized model. This method has the advantage of high accuracy, but the complexity of the model and algorithm limits its application in battery management systems.

[0004] Impedance can reflect the internal state information of the battery and has important application potential in lithium-ion battery state estimation and fault diagnosis. Therefore, the present invention proposes an online identification method for internal short circuit induced by overdischarge of lithium batteries based on impedance characteristics. The method can be integrated into the BMS, solving the problem of poor reliability of internal short circuit identification due to consistency differences of single cells. Summary of the Invention

[0005] In view of the deficiencies in the prior art, the technical problem to be solved by the present invention is to provide an online identification method for internal short circuit induced by over-discharge of lithium batteries based on impedance characteristics.

[0006] The technical solution adopted by the present invention to solve the technical problem is as follows:

[0007] An online identification method for internal short circuit induced by overdischarge of a lithium battery based on impedance characteristics, characterized in that the method comprises the following steps:

[0008] S1. Measure the static electrochemical impedance spectroscopy of lithium-ion batteries at different temperatures and different states of charge (SOC);

[0009] S2. Select the frequency range in which the overlap rate of static electrochemical impedance spectroscopy at different temperatures and SOCs is greater than or equal to 90% as the frequency range in which the impedance is least affected by temperature and SOC, and use the average value of this frequency range as the characteristic frequency;

[0010] S3. Through a lithium battery overdischarge-induced internal short circuit experiment, the dynamic impedance of the lithium-ion battery during discharge at a characteristic frequency is obtained. When the dynamic impedance exhibits three characteristics, namely, a half-sine change, a needle-like change, and a significant rebound, it indicates that the lithium battery has overdischarged and induced an internal short circuit, thus enabling online identification of lithium battery overdischarge-induced internal short circuits.

[0011] The above-mentioned semi-sinusoidal change means that the change trend of the dynamic impedance curve is similar to the change trend of the sine function in the range of 0 to 180 degrees.

[0012] Furthermore, the system used in the above method includes a single lithium-ion battery, a constant temperature box, a host computer, an impedance online measurement device, a battery testing system and a lithium-ion battery pack;

[0013] The host computer is connected to the impedance online measurement device and the battery testing system, and the single lithium-ion battery and the lithium-ion battery pack are both connected to the impedance online measurement device and the battery testing system; the single lithium-ion battery is placed in a constant temperature box for measuring the static electrochemical impedance spectrum of the lithium battery; the lithium-ion battery pack is composed of a single lithium-ion battery to be over-discharged and at least one normally discharged single lithium-ion battery connected in series, and is used to perform a lithium battery over-discharge-induced internal short circuit experiment;

[0014] The impedance online measurement device includes a main control, a DDS AC small signal generator, an operational amplifier, a voltage-current conversion circuit, a sampling resistor, a signal conditioning circuit, and a dual-channel ADC signal synchronization acquisition circuit; the main control is respectively connected to the host computer and the input end of the DDS AC small signal generator, the output end of the DDS AC small signal generator is connected to the input end of the operational amplifier, the output end of the operational amplifier is connected to the input end of the voltage-current conversion circuit, the output end of the voltage-current conversion circuit is connected to one end of the sampling resistor, the other end of the sampling resistor is connected to the positive electrode of the single lithium-ion battery to be over-discharged, and the negative electrode of the single lithium-ion battery to be over-discharged is grounded; the sampling resistor and the single lithium-ion battery to be over-discharged are each connected to a signal conditioning circuit, both signal conditioning circuits are connected to the dual-channel ADC signal synchronization acquisition circuit, and the dual-channel ADC signal synchronization acquisition circuit is simultaneously connected to the main control.

[0015] Furthermore, the signal conditioning circuit includes a DC removal circuit, an instrumentation amplifier, a programmable gain amplifier and a filter; one end of the DC removal circuit is respectively connected to the two ends of the sampling resistor or the single lithium-ion battery to be over-discharged, the other end of the DC removal circuit is respectively connected to the non-inverting input and the inverting input of the instrumentation amplifier, the output of the instrumentation amplifier is connected to the input of the programmable gain amplifier, the output of the programmable gain amplifier is connected to the input of the source filter, and the output of the filter is connected to the dual-channel ADC signal synchronization acquisition circuit.

[0016] Furthermore, the specific process of step S1 is as follows: setting the temperature of the constant temperature box, the battery testing system controls the single lithium-ion battery to be at a certain SOC, and the single lithium-ion battery is left to stand in the constant temperature box for a period of time until it is in electrochemical and thermal equilibrium; then, measuring the static electrochemical impedance spectrum of the single lithium-ion battery at the current temperature and SOC by an impedance online measurement device; setting different temperatures and SOCs, and repeating the above operations to measure the static electrochemical impedance spectrum of the single lithium-ion battery at different temperatures and different SOCs.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. Based on the characteristic that impedance can reflect the internal state of the battery, the impedance online measurement technology is introduced into the lithium-ion battery state assessment and fault diagnosis. First, the characteristic frequency at which impedance is least affected by temperature and SOC is determined, and the dynamic impedance of the lithium-ion battery during discharge is measured at the characteristic frequency. Based on the dynamic impedance and discharge state, the correlation between impedance and battery faults is clarified. An online internal short circuit identification method based on impedance characteristics is proposed and the reliability of the method is verified. The semi-sinusoidal change characteristic of the dynamic impedance of the lithium-ion battery at the end of discharge can realize the lithium-ion battery over-discharge warning. The needle-shaped change characteristic of the dynamic impedance during over-discharge can realize the internal short circuit fault warning. The obvious rebound characteristic of the dynamic impedance can be used as a sign of the occurrence of internal short circuit. In addition, the impedance change rate characteristic helps to realize the fault identification and early warning of lithium-ion batteries. The pseudo-sinusoidal change characteristic of the impedance change rate can be used as a sign of over-discharge warning. The pseudo-sinusoidal change characteristic with a more regular impedance change rate can be used as a sign of internal short-circuit fault warning. The convex change characteristic with a small impedance change rate can be used as a sign of internal short-circuit fault occurrence. Compared with conventional physical parameters such as voltage and temperature, it has certain advantages, and effectively solves the current lithium-ion battery internal short-circuit fault online diagnosis technology. Due to the consistency differences of single cells in the battery pack, the fault diagnosis cannot meet the reliability and real-time requirements.

[0019] 2. The impedance-based online identification method and hardware device for lithium-ion battery over-discharge-induced internal short circuits proposed in the present invention can be easily integrated into a BMS. They are low-cost and compact, and have significant application potential in the online rapid diagnosis of lithium-ion battery faults.

[0020] 3. The impedance online measurement device uses the all-phase fast Fourier transform (apFFT) algorithm to perform impedance measurement, which effectively solves the problem of incorrect impedance calculation results caused by spectrum leakage and fence effect generated by the current impedance online measurement device based on the traditional FFT algorithm.

[0021] Figures in the specification

[0022] Figure 1 is a structural diagram of the identification system of the present invention;

[0023] Figure 2 It is a structural diagram of the impedance online measurement device of the present invention;

[0024] Figure 3 A dynamic impedance curve diagram of a single lithium-ion battery to be over-discharged at a characteristic frequency of the present invention;

[0025] Figure 4 A curve diagram of the dynamic impedance change rate of a single lithium-ion battery to be over-discharged at a characteristic frequency of the present invention;

[0026] Figure 5 This is a dynamic impedance curve diagram of a single lithium-ion battery of the present invention during normal charging;

[0027] Figure 6 The figure is a dynamic impedance curve diagram of a single lithium-ion battery of the present invention during normal discharge;

[0028] Figure 7 The terminal voltage curve diagram of the single lithium-ion battery of the present invention during over-discharge process;

[0029] Figure 8 A graph showing the average surface temperature of a single lithium-ion battery of the present invention during overdischarge;

[0030] Figure 9 The electrochemical impedance spectra of the single lithium-ion battery of the present invention at different temperatures and SOC conditions, wherein (a) to (e) are electrochemical impedance spectra at different SOC conditions at 5°C, 15°C, 25°C, 35°C, and 45°C, respectively;

[0031] In the figure, 1- single lithium-ion battery; 2- constant temperature box; 3- host computer; 4- impedance online measurement device; 5- battery test system; 6- lithium-ion battery pack; 7- explosion-proof box; 8- voltage sensor; 9- temperature sensor. DETAILED DESCRIPTION

[0032] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and specific implementation methods, but this does not limit the scope of protection of the present application.

[0033] Figure 1 This is a structural diagram of the identification system used in the present invention. The identification system includes a single lithium-ion battery 1, a constant temperature box 2, a host computer 3, an impedance online measurement device 4, a battery testing system 5, a lithium-ion battery pack 6, an explosion-proof box 7, a voltage sensor 8, and a temperature sensor 9;

[0034] Among them, the single lithium-ion battery 1 is placed in the constant temperature box 2, and the single lithium-ion battery 1 is connected to the impedance online measurement device 4 and the battery testing system 5 respectively. The host computer 3 is connected to the impedance online measurement device 4 and the battery testing system 5 respectively to realize control and data storage; the lithium-ion battery pack 6 is composed of a single lithium-ion battery to be over-discharged 11 and at least one normally discharged single lithium-ion battery in series. The lithium-ion battery pack 6 of this embodiment is composed of a normally discharged single lithium-ion battery 10, a single lithium-ion battery to be over-discharged 11 and a normally discharged single lithium-ion battery 12 in series; the lithium-ion battery pack 6 is placed Inside the explosion-proof box 7, the lithium-ion battery pack 6 is connected to the battery testing system 5, and the voltage sensor 8 and the temperature sensor 9 are connected to the single lithium-ion battery 11 to be over-discharged in the lithium-ion battery pack 6, for measuring the terminal voltage and surface temperature of the single lithium-ion battery 11 to be over-discharged; the constant temperature box 2 provides a constant temperature environment for measuring the static electrochemical impedance spectrum, the impedance online measuring device 4 is used to measure the static electrochemical impedance spectrum of the single lithium-ion battery 1 and the dynamic impedance of the single lithium-ion battery 11 to be over-discharged during the charging and discharging process, and the battery testing system 5 is used to control the single lithium-ion battery 1 and the lithium-ion battery pack 6 to complete the charging and discharging.

[0035] like Figure 2 As shown, the impedance online measurement device 4 includes a main control, a DDS AC small signal generator, an operational amplifier, a voltage-current conversion circuit, a sampling resistor, a signal conditioning circuit and a dual-channel ADC signal synchronization acquisition circuit; the main control is connected to the host computer and the input end of the DDS AC small signal generator respectively, the output end of the DDS AC small signal generator is connected to the input end of the operational amplifier, the output end of the operational amplifier is connected to the input end of the voltage-current conversion circuit, the output end of the voltage-current conversion circuit is connected to one end of the sampling resistor, the other end of the sampling resistor is connected to the positive electrode of the single lithium-ion battery to be over-discharged, and the negative electrode of the single lithium-ion battery to be over-discharged is grounded; the sampling resistor and the single lithium-ion battery to be over-discharged are each connected to a signal conditioning circuit, and both signal conditioning circuits are connected to the dual-channel ADC signal synchronization acquisition circuit, and the dual-channel ADC signal synchronization acquisition circuit is also connected to the main control. The main controller controls the DDS AC small signal generator to generate an AC voltage small signal of the target frequency. The AC voltage small signal is amplified by the operational amplifier and then converted into an AC current signal by the voltage-current conversion circuit. The AC current signal is used as the excitation of the single lithium-ion battery to be over-discharged. Under the disturbance of the AC current signal, the impedance of the single lithium-ion battery to be over-discharged during the discharge process is measured. The voltage of the sampling resistor and the voltage of the single lithium-ion battery to be measured are passed through their respective signal conditioning circuits and then acquired by the dual-channel ADC signal synchronous acquisition circuit. The dual-channel ADC signal synchronous acquisition circuit converts the analog signal into a digital signal and then transmits it to the main controller. The main controller converts the time domain signal into the frequency domain through the apFFT algorithm and then transmits it to the host computer 3, thereby realizing the online measurement of the lithium-ion battery impedance.

[0036] The signal conditioning circuit includes a DC removal circuit, an instrumentation amplifier, a programmable gain amplifier, and a fourth-order Butterworth active filter; one end of the DC removal circuit is respectively connected to the two ends of a sampling resistor or a single lithium-ion battery to be over-discharged, the other end of the DC removal circuit is respectively connected to the non-inverting input and the inverting input of the instrumentation amplifier, the output of the instrumentation amplifier is connected to the input of the programmable gain amplifier, the output of the programmable gain amplifier is connected to the input of the fourth-order Butterworth active filter, and the output of the fourth-order Butterworth active filter is connected to a dual-channel ADC signal synchronization acquisition circuit.

[0037] The online identification method of an internal short circuit induced by overdischarge of a lithium battery based on impedance characteristics of the present invention comprises the following steps:

[0038] S1. The temperature is set by the thermostat 2. The host computer 3 controls the battery test system 5 to keep the single lithium-ion battery 1 at a certain SOC. The single lithium-ion battery 1 is left in the thermostat 2 for 3 hours at the target temperature and SOC to ensure that the single lithium-ion battery 1 is in electrochemical and thermal equilibrium. The static electrochemical impedance spectrum of the single lithium-ion battery 1 at the current temperature and SOC is measured by the impedance online measurement device 4. Different temperatures and SOCs are set, and the above operation is repeated to measure the static electrochemical impedance spectrum of the single lithium-ion battery 1 at different temperatures and different SOCs. The set temperature and SOC should cover the normal working environment of the lithium-ion battery as much as possible, so that the characteristic frequency is fully decoupled from the SOC and temperature as much as possible, and the influence of temperature and SOC on the impedance measurement is eliminated as much as possible.

[0039] S2. Analyze the static electrochemical impedance spectrum obtained in step S1, select the frequency range in which the overlap rate of the static electrochemical impedance spectrum at different temperatures and SOCs is greater than or equal to 90% as the frequency range in which the impedance is least affected by temperature and SOC, use the average value of this frequency range as the characteristic frequency, and use the dynamic impedance at the characteristic frequency as the characteristic impedance for identifying internal short circuits in lithium batteries;

[0040] S3. Through the lithium battery over-discharge induced internal short circuit experiment, the dynamic impedance of the lithium-ion battery during the discharge process at the characteristic frequency is obtained. When the dynamic impedance shows three characteristics in sequence: half-sine change, needle-shaped change and obvious rebound, it indicates that the lithium battery over-discharge induced internal short circuit, and the online identification of lithium battery over-discharge induced internal short circuit is realized; among them, the half-sine change characteristic of the dynamic impedance can be used as a warning sign of lithium battery over-discharge, the needle-shaped change characteristic can be used as a warning sign of lithium battery internal short circuit, and the obvious rebound characteristic can be used as a sign of the occurrence of lithium battery internal short circuit.

[0041] The lithium battery over-discharge-induced internal short circuit test process is as follows: the three single lithium-ion batteries of the lithium-ion battery pack 6 are activated using the 1C rate standard charge and discharge conditions. The specific process is as follows: the single lithium-ion battery is charged at a 1C rate constant current to an upper cut-off voltage of 3.6V, and then charged at a constant voltage of 3.6V and a 1C rate current until the current is equal to 0.05C, which is considered charging complete. The battery is then left for 10 minutes; then, it is discharged at a 1C rate constant current to a lower cut-off voltage, which is considered discharging complete. The battery is then left for 10 minutes, completing the 1C rate standard charge and discharge cycle. The above operation is repeated for a total of 5 cycles.

[0042] The initial charge of the single lithium-ion battery to be over-discharged 11 is set to 50% SOC, and the initial charge of the single lithium-ion battery to be normally discharged 10 and the single lithium-ion battery to be normally discharged 12 is set to 100% SOC. Under the condition of 1C constant current, the lithium-ion battery pack 6 is discharged to 70% SOC to achieve an over-discharge of 20% SOC of the single lithium-ion battery to be over-discharged. During the process of over-discharge-induced internal short circuit, the dynamic impedance of the single lithium-ion battery to be over-discharged 11 is measured at the characteristic frequency, see Figure 3 ;from Figure 3It can be seen that the dynamic impedance changes smoothly from 0 to 1566s and decreases from 1566 to 1603s. At 1603s, the dynamic impedance increases significantly, and the battery discharges about 94.5% SOC. As the discharge progresses, the internal state of the battery gradually stabilizes. At 1651s, the dynamic impedance falls back to a stable state, and the battery discharges about 95.9% SOC. The dynamic impedance shows a semi-sine change from 1603 to 1651s. During this period, the dynamic impedance amplitude changes greatly and lasts for 48s, which is easy to capture. The semi-sine change feature can realize lithium-ion battery over-discharge warning about 144s in advance. The dynamic impedance changes smoothly from 1651 to 2033s, and increases sharply at 2033s, when the battery discharges about 106.5% SOC. At 2044s, the dynamic impedance falls back to a stable state, and the battery discharges about 106.8%. During the SOC period of 2033-2044s, the dynamic impedance exhibits a needle-like change. This needle-like change can provide an early warning of an internal short circuit in the lithium-ion battery approximately 152s in advance. The dynamic impedance significantly rebounds at 2187s, when the battery is discharged to approximately 110.7% SOC. At 2196s, when the battery is discharged to approximately 111% SOC, the dynamic impedance begins to slowly increase as the battery is over-discharged, indicating an internal short circuit. This significant rebound can be used as a sign of internal short circuit occurrence in lithium-ion batteries. Therefore, during the discharge process of a lithium-ion battery at a characteristic frequency, when the dynamic impedance exhibits three characteristics, namely, a half-sine change, a needle-like change, and a significant rebound, it indicates that the battery has been over-discharged and induced an internal short circuit. The half-sine change in the dynamic impedance can serve as an early warning sign of over-discharge, the needle-like change can serve as an early warning sign of an internal short circuit, and the significant rebound can serve as an early warning sign of an internal short circuit. The half-sine change refers to the dynamic impedance curve changing in a manner similar to that of a sine function within the range of 0-180 degrees.

[0043] The impedance change rate curve is drawn based on the dynamic impedance during the over-discharge induced internal short circuit process. Figure 4 ;from Figure 4 It can be seen that a symmetrical pseudo-sinusoidal change feature begins to appear at about 1600s at the end of normal discharge, when the battery is discharged to about 94.4% SOC, and the impedance change rate gradually returns to a stable state at 1653s. The pseudo-sinusoidal change feature of the impedance change rate is used as an early warning sign of battery overdischarge, which can be about 3s earlier than the semi-sinusoidal change feature of the dynamic impedance at the end of normal discharge; the impedance change rate shows a more regular pseudo-sinusoidal curve at 2028s, which is about 5s earlier than the needle-shaped change feature of the dynamic impedance during the overdischarge process; there is a significant bump in the impedance change rate curve at 2185s, which is about 2s earlier than the obvious rebound feature of the dynamic impedance during the overdischarge process; in addition to the above features, the impedance change rate curve is a smooth straight line during the entire overdischarge process, making the above features easier to identify. Therefore, the impedance change rate can be further used as a feature for identifying short circuits within lithium batteries.

[0044] Figure 5 、 6 is the dynamic impedance change curve of a single lithium-ion battery during normal charge and discharge at the characteristic frequency; Figure 5 It can be seen that the dynamic impedance decreases slowly during constant current charging, but the overall change is small; at the end of constant current charging and the beginning of constant voltage charging, the dynamic impedance drops sharply due to the change in charging properties. As the charging current gradually decreases during constant voltage charging, the internal temperature of the battery drops, and the battery is prone to polarization at a higher SOC, which causes the dynamic impedance to rise significantly. Figure 6 It can be seen that the overall change of dynamic impedance during normal discharge of the battery is stable. At the end of discharge, the dynamic impedance drops significantly and then increases sharply, and returns to a stable state as the discharge ends. During normal charging and discharging, the half-sinusoidal change characteristic of the dynamic impedance of the lithium battery only appears at the end of normal discharge, and the two characteristics of the dynamic impedance needle-shaped change and obvious rebound do not appear. Therefore, the effectiveness of using the three characteristics of half-sinusoidal change, needle-shaped change and obvious rebound of dynamic impedance as internal short circuit identification in the process of over-discharge induced internal short circuit of lithium-ion batteries is verified.

[0045] In order to verify the effectiveness of using dynamic impedance as a method for identifying internal short circuits, the terminal voltage and surface temperature of the lithium-ion battery 11 to be over-discharged during the internal short circuit induced by over-discharge were also measured. Figure 7 、 8 ;from Figure 7 It can be seen that the terminal voltage begins to drop sharply at 1782s and discharges to the lower limit cut-off voltage at 1795s, that is, it enters the over-discharge process at 1795s. However, the sharp change of the terminal voltage only occurs 13s before the lower limit cut-off voltage, which is not easy to capture. The terminal voltage rebounds significantly at 2033s, but during the over-discharge process, the terminal voltage does not have obvious characteristics to indicate the occurrence of an internal short circuit fault. Therefore, the identification of internal short circuits based on terminal voltage changes has certain limitations. Figure 8 It can be seen that after 1840s, the battery surface temperature rises sharply, but at this time it has been over-discharged for about 45s. During the entire over-discharge process, the battery surface temperature is always below 32°C, and there is no obvious feature indicating the occurrence of an internal short circuit. Therefore, the identification of internal short circuits by surface temperature also has certain limitations. Figure 3 It can be seen that during the process of over-discharge-induced internal short circuit, the dynamic impedance of the lithium battery has obvious characteristics, and these characteristics are easy to capture.

[0046] In order to further verify the feasibility of identifying internal short circuits induced by over-discharge in lithium batteries based on impedance characteristics, the identification and verification of internal short circuits induced by over-discharge in lithium batteries were carried out according to the irreversible capacity loss and self-discharge characteristics of internal short circuit batteries and battery disassembly, including:

[0047] 1) Mark four single lithium-ion batteries as B1 to B4 and activate them; perform capacity tests on the four activated batteries under standard charge and discharge conditions at a 1C rate and record the discharge capacity;

[0048] 2) Performing a 1C standard charge on the four batteries that completed step 1), leaving the fully charged batteries at room temperature for 72 hours, and then discharging them at a 1C constant current rate to the lower cutoff voltage, recording the discharge capacity of the four batteries;

[0049] 3) The four batteries that completed step 2) were charged at a standard rate of 1C until fully charged; the four batteries were discharged and their dynamic impedances were measured. Battery B1 was discharged at a constant current rate of 1C to the lower cutoff voltage, battery B2 was discharged until the dynamic impedance completely exhibited a semi-sinusoidal change characteristic, battery B3 was discharged until the dynamic impedance completely exhibited a needle-like change characteristic, and battery B4 was discharged until the dynamic impedance showed a significant recovery characteristic during the overdischarge process; then, the four batteries were fully charged and allowed to stand at room temperature for 72 hours, and then discharged at a constant current rate of 1C to the lower cutoff voltage, and the discharge capacity of the four batteries was recorded;

[0050] 4) The four batteries that completed step 3) were capacity tested under standard charge and discharge conditions at a 1C rate and cycled three times, and the average discharge capacity of the four batteries during the three capacity tests was recorded; the difference between the discharge capacity of the battery in step 1) and step 2) was taken as the normal battery self-discharge capacity loss and recorded as capacity loss 1, the difference between the discharge capacity of the battery in step 1) and step 4) was taken as the discharge capacity loss under different discharge termination conditions and recorded as capacity loss 2, and the difference between the discharge capacity of the battery in step 4) and step 3) was taken as the self-discharge capacity loss after different degrees of discharge and recorded as capacity loss 3. The experimental results are shown in Table 1;

[0051] Table 1 Over-discharge induced internal short circuit verification test results

[0052]

[0053]

[0054] As shown in Table 1, battery B1 discharges to the lower cutoff voltage, indicating normal discharge. Battery B2 discharges until its dynamic impedance exhibits a complete half-sinusoidal variation. There is no significant capacity loss for batteries B1 and B2, and the three types of capacity loss exhibit essentially the same trend. This validates the feasibility of using the half-sinusoidal variation of dynamic impedance as a warning sign for battery overdischarge. Battery B3 discharges until its dynamic impedance exhibits a complete needle-shaped variation, indicating a significant increase in actual usable capacity and minimal capacity loss due to self-discharge. Based on this, normal charge and discharge cycles of battery B3 reveal that this does not affect normal battery use but accelerates aging. Battery B4 discharges until its dynamic impedance exhibits a significant rebound during overdischarge, indicating a significant increase in irreversible capacity loss and self-discharge capacity loss. Based on this, battery B4 undergoes two normal charge and discharge cycles, failing to fully charge. Its terminal voltage gradually decreases as charging progresses, indicating an internal short circuit. This validates the feasibility of using the needle-shaped variation and significant rebound characteristics of dynamic impedance as warning signs and occurrence indicators for internal short circuits.

[0055] Batteries B1 and B4, which were discharged to the lower limit cut-off voltage and had a significant rebound in dynamic impedance in step 3), were disassembled. It was found that a layer of yellow sediment appeared on the surface of the positive electrode current collector aluminum foil of battery B4, while the surface of the aluminum foil of battery B1 was a clean carbon layer, verifying that the DOD corresponding to the significant rebound in dynamic impedance caused the formation of a copper layer and metal dendrites on the positive electrode current collector of the battery; the copper current collector of the negative electrode current collector copper foil of battery B4 was corroded and the surface carbon layer was damaged, while the surface of the negative electrode current collector copper foil of battery B1 was a complete carbon layer with no obvious signs of corrosion; the diaphragm of battery B4 was relatively sparse and showed signs of damage, and yellow sediment adhered to the surface of the diaphragm, while the diaphragm of battery B1 was relatively dense, indicating that battery B4 induced an internal short circuit, verifying the feasibility of using the significant rebound in dynamic impedance during overdischarge as a sign of internal short circuit in the battery.

[0056] In this embodiment, all single lithium-ion batteries are lithium iron phosphate 18650 cylindrical batteries, and their parameters are shown in Table 2.

[0057] Table 2 Parameters of lithium iron phosphate 18650 cylindrical battery

[0058]

[0059] In step S1 of this embodiment, the static electrochemical impedance spectroscopy is measured at 5°C, 15°C, 25°C, 35°C, 45°C and 10% SOC, 30% SOC, 50% SOC, 70% SOC, and 90% SOC, respectively, with a frequency range of 0.01 to 10 kHz. Figure 9 (a)~(e); from Figure 9As shown in Figures (a) to (e), in the low-frequency range (0.01–0.3 Hz), the impedance changes exhibit a symmetrical trend with 25°C as the critical point. In the 5–25°C environment, the impedance increases with increasing SOC, and this difference gradually decreases with rising temperature. In the 25–45°C environment, the impedance decreases with increasing SOC, and the difference becomes increasingly pronounced with increasing temperature. This characteristic variation is primarily due to the fact that the low-frequency range is dominated by diffusion impedance. In the mid-frequency range (0.3–40 Hz), the impedance gradually decreases with increasing SOC, and the difference disappears with increasing temperature. This is primarily due to the fact that this frequency range is dominated by charge transfer impedance. In the high-frequency range (40–1 kHz), the electrochemical impedance spectra are essentially overlapping when the temperature is below 25°C. This is primarily due to the fact that this frequency range is dominated by the SEI layer impedance, which is affected only by temperature and has little to do with SOC. In the ultra-high frequency range (1kHz to 10kHz), EIS curves overlap well in relatively low-temperature environments. However, as temperature increases, the differences become increasingly pronounced. This is primarily due to the fact that this frequency range is primarily contributed by components such as ohmic resistance, SEI layer impedance, and wire inductance, and is also related to the connection method between the impedance measurement device and the battery electrodes. Based on the aforementioned analysis, the frequency range in which battery impedance is least affected by SOC is primarily 40-100Hz. While the battery generates relatively little heat during normal discharge, significant heat is released during overdischarge due to SEI film decomposition and copper current collector dissolution. The conductivity of the battery's solid-phase materials is virtually unaffected by temperature. Therefore, the effect of temperature on online impedance measurement during overdischarge-induced internal short-circuit experiments can be attributed to the occurrence of battery side reactions triggered by overdischarge. For dynamic impedance measurement, measurement time increases exponentially as the excitation signal frequency decreases. Since internal short-circuit identification requires real-time performance, the average value of the frequency range least affected by SOC and temperature is selected as the characteristic frequency. In this embodiment, the characteristic frequency is 70Hz.

[0060] Any matters not described in the present invention are applicable to the prior art.

Claims

1. An online identification method for internal short circuit induced by over-discharge of lithium battery based on impedance characteristics, characterized in that: The method comprises the following steps: S1. Measure the static electrochemical impedance spectroscopy of lithium-ion batteries at different temperatures and states of charge; S2. Select the frequency range in which the overlap rate of static electrochemical impedance spectra at different temperatures and charge states is greater than or equal to 90% as the frequency range in which the impedance is least affected by temperature and charge state, and use the average value of this frequency range as the characteristic frequency; S3. Through a lithium battery overdischarge-induced internal short circuit experiment, the dynamic impedance of the lithium-ion battery during discharge at a characteristic frequency is obtained. When the dynamic impedance exhibits three characteristics, namely, a half-sine change, a needle-like change, and a significant rebound, it indicates that the lithium battery has overdischarged and induced an internal short circuit, thus enabling online identification of lithium battery overdischarge-induced internal short circuits. The above-mentioned semi-sinusoidal change means that the change trend of the dynamic impedance curve is similar to the change trend of the sine function in the range of 0 to 180 degrees.

2. The online identification method for internal short circuit induced by overdischarge of lithium battery based on impedance characteristics according to claim 1, characterized in that: The system used in the method includes a single lithium-ion battery, a constant temperature box, a host computer, an impedance online measurement device, a battery testing system and a lithium-ion battery pack; The host computer is connected to the impedance online measurement device and the battery testing system, and the single lithium-ion battery and the lithium-ion battery pack are both connected to the impedance online measurement device and the battery testing system; the single lithium-ion battery is placed in a constant temperature box for measuring the static electrochemical impedance spectrum of the lithium battery; the lithium-ion battery pack is composed of a single lithium-ion battery to be over-discharged and at least one normally discharged single lithium-ion battery connected in series, and is used to perform a lithium battery over-discharge-induced internal short circuit experiment; The impedance online measurement device includes a main control, a DDS AC small signal generator, an operational amplifier, a voltage-current conversion circuit, a sampling resistor, a signal conditioning circuit, and a dual-channel ADC signal synchronization acquisition circuit; the main control is respectively connected to the host computer and the input end of the DDS AC small signal generator, the output end of the DDS AC small signal generator is connected to the input end of the operational amplifier, the output end of the operational amplifier is connected to the input end of the voltage-current conversion circuit, the output end of the voltage-current conversion circuit is connected to one end of the sampling resistor, the other end of the sampling resistor is connected to the positive electrode of the single lithium-ion battery to be over-discharged, and the negative electrode of the single lithium-ion battery to be over-discharged is grounded; the sampling resistor and the single lithium-ion battery to be over-discharged are each connected to a signal conditioning circuit, both signal conditioning circuits are connected to the dual-channel ADC signal synchronization acquisition circuit, and the dual-channel ADC signal synchronization acquisition circuit is simultaneously connected to the main control.

3. The online identification method for internal short circuit induced by overdischarge of lithium battery based on impedance characteristics according to claim 2, characterized in that: The signal conditioning circuit includes a DC removal circuit, an instrumentation amplifier, a programmable gain amplifier, and a filter; one end of the DC removal circuit is respectively connected to the two ends of a sampling resistor or a single lithium-ion battery to be over-discharged, the other end of the DC removal circuit is respectively connected to the non-inverting input and the inverting input of the instrumentation amplifier, the output of the instrumentation amplifier is connected to the input of the programmable gain amplifier, the output of the programmable gain amplifier is connected to the input of the source filter, and the output of the filter is connected to a dual-channel ADC signal synchronization acquisition circuit.

4. The online identification method for internal short circuit induced by overdischarge of lithium battery based on impedance characteristics according to claim 2, characterized in that: The specific process of step S1 is as follows: the temperature of the constant temperature chamber is set, the battery testing system controls the single lithium-ion battery to be in a certain state of charge, and the single lithium-ion battery is left to stand in the constant temperature chamber for a period of time until it reaches an electrochemical and thermal equilibrium state; then, the static electrochemical impedance spectrum of the single lithium-ion battery at the current temperature and state of charge is measured using an impedance online measurement device; different temperatures and states of charge are set, and the aforementioned operation is repeated to measure the static electrochemical impedance spectrum of the single lithium-ion battery at different temperatures and different states of charge.

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