A battery cell measurement method and device

Abstract

The present invention discloses a battery cell measurement method and device, which belongs to the technical field of electrical variable measurement, including applying a DC high voltage to both ends of a battery cell that is not injected with liquid; superimposing a low-voltage AC power supply on the DC high voltage to control the battery cell to charge and discharge repeatedly: detecting the voltage at both ends of the battery cell and the current flowing through the battery cell; if the voltage at both ends of the battery cell drops rapidly to a lower limit value, it is considered that there is a micro-short circuit problem caused by metal impurities inside the battery cell; calculating the AC impedance and equivalent capacitance value of the battery cell; if the equivalent capacitance value is greater than the set capacitance value, it is considered that the thickness of the diaphragm inside the battery cell is too thin; if the AC impedance is greater than the set resistance value, it is considered that there is agglomeration between the positive and negative plates of the battery cell due to uneven dispersion of slurry. The present invention uses AC and DC superposition technology to test the battery cell that is not injected with liquid, which can not only accurately screen out problematic batteries, but also identify the types of defects existing in the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of electric variable measurement, and in particular, relates to a battery cell measurement method and device. Background Art

[0002] With the promotion of new energy, lithium batteries are being used more and more widely. Whether the quality of lithium batteries is qualified is directly related to the power safety of electrical equipment. Therefore, in the production and manufacturing process of lithium batteries, lithium batteries need to be quality tested.

[0003] The production process of lithium batteries includes the production and process of pole pieces. The processes that have a huge impact on the microstructure mainly include mixing, coating drying and rolling. The rolling process includes rolling the diaphragm between the positive and negative pole pieces of the battery; after the diaphragm rolling is completed, the positive and negative pole pieces and the diaphragm are stacked and rolled; after the stacking is completed, they are wound. During the entire rolling process, if the electrode slurry is unevenly dispersed, serious agglomeration will occur, affecting the electrochemical performance of the battery. During the diaphragm rolling process, the diaphragm thickness should meet certain requirements; when stacking and rolling, impurities cannot be introduced between the positive and negative pole pieces and the diaphragm, and impurities cannot be introduced during the winding process. However, during the production process, due to mechanical equipment, personnel and production processes, problems such as thin diaphragm thickness and introduction of metal impurities are prone to occur, which in turn shortens the service life of lithium batteries. If the metal impurity particles are large, it may also cause a short circuit inside the battery, causing a spontaneous combustion accident of the lithium battery.

[0004] In order to identify batteries with quality problems during the manufacturing process, there are three main battery cell measurement methods widely used in the industry:

[0005] The first is the insulation test method. That is, use a DC high voltage source to charge the unfilled battery cell to a set voltage and keep it for a period of time, test the DC resistance of the battery cell, and judge the quality of the battery cell based on the resistance. The problem with this test method is that the data of the battery cell during the charging stage may be the same as that of a normal battery cell, but the discharge speed may suddenly increase during the discharge stage, which indicates that the battery cell has a micro short circuit problem, which cannot be detected by the insulation test method.

[0006] The second is the pulse test method. That is, use a DC high-voltage source to charge the unfilled battery cell to the set voltage and then stop, and test whether the battery voltage will drop rapidly. If it drops rapidly, it means that the battery cell has a micro short circuit or low insulation resistance problem; after the test time is reached, the battery cell is discharged to end the test process. The problem with this test method is that if the metal impurities introduced into the battery cell are relatively small and the short circuit is not obvious, this test method cannot detect problematic batteries.

[0007] The third method is the low-voltage AC test method, which is only applicable to battery cells after injection. That is, inject electrolyte into the battery cell, then apply a low-voltage AC source to both ends of the cell, measure the AC impedance of the cell, and judge the quality of the cell based on the impedance. The problem with this test method is that if the diaphragm between the positive and negative plates of the cell is too thin, the lithium ion crystals produced by the positive electrode of the cell after multiple charge and discharge will more easily pierce the diaphragm, thereby causing the lithium battery to spontaneously combust. Moreover, if a problem is detected in the cell after injection, the cell can no longer be used, which is a huge waste of electrolyte.

[0008] In addition, the above three existing testing methods still have common problems: they cannot detect whether the thickness of the diaphragm between the positive and negative electrodes of the battery cell meets the requirements, and they cannot test the agglomeration phenomenon caused by uneven dispersion of slurry between the electrodes. Summary of the invention

[0009] The object of the present invention is to provide a battery cell measurement method and device to solve the problem that the existing test methods only identify a single type of battery defects.

[0010] In order to solve the above technical problems, the present invention adopts the following technical solutions to achieve the above problems:

[0011] In one aspect, the present invention provides a battery cell measurement method, which is applied to a battery cell without liquid injection, comprising:

[0012] Applying a DC high voltage across the unfilled battery cell;

[0013] A low-voltage AC power supply is superimposed on the DC high voltage to control the battery cells to charge and discharge repeatedly, entering the AC / DC superposition test phase:

[0014] Detecting the voltage across the battery cell and the current flowing through the battery cell;

[0015] When the voltage at both ends of the battery cell drops to the lower limit, it is determined that there is a micro short circuit problem caused by metal impurities inside the battery cell;

[0016] Calculate the AC impedance and equivalent capacitance of the battery cell;

[0017] When the equivalent capacitance value is greater than the set capacitance value, it is determined that the thickness of the diaphragm inside the battery cell is too thin;

[0018] When the AC impedance is greater than the set resistance, it is determined that there is agglomeration between the positive and negative electrodes of the battery cell due to uneven dispersion of the slurry.

[0019] In some embodiments of the present application, the AC impedance and equivalent capacitance of the battery cell may be calculated using the following method:

[0020] In the AC / DC superposition test stage, the voltage across the battery cell and the current flowing through the battery cell are detected, the AC component is separated, and two other AC components with a phase difference of 90° with the AC component are formed. The AC impedance and equivalent capacitance value of the battery cell are calculated using the four AC components.

[0021] Specifically, the voltage at both ends of the battery cell is collected and separated into AC and DC, the AC voltage component U1 is extracted, and another AC voltage component U2 is formed which is 90° out of phase with U1;

[0022] The current flowing through the battery cell is collected and separated into AC and DC, the AC current component I1 is extracted, and another AC current component I2 is formed which is 90° out of phase with I1;

[0023] Use the following formula to calculate the maximum cell voltage U z And the phase angle β:

[0024] U1=U z sin(ωt+β)

[0025] U2=U z sin(ωt+β+90°);

[0026] Use the following formula to calculate the maximum current I flowing through the battery cell: z And the phase angle γ:

[0027] I1=I z sin(ωt+γ)

[0028] I2=I z sin(ωt+γ+90°);

[0029] Calculate the equivalent impedance Z and corresponding phase angle θ of the battery cell:

[0030] Z=U z / I z

[0031] θ = β - γ;

[0032] Calculate the AC impedance R and equivalent capacitance C of the battery cell:

[0033] R=Zcosθ

[0034] C=Zsinθ.

[0035] In some embodiments of the present application, in order to improve the timeliness and accuracy of battery defect detection, two test phases may be configured before the AC / DC superposition test phase, namely:

[0036] Voltage rising stage: applying a DC voltage to both ends of the battery cell, and controlling the DC voltage to gradually rise to a high voltage setting value within a set time; detecting the current flowing through the cell and the voltage at both ends of the cell during the voltage rising process. If the current change or the voltage change exceeds the set threshold, it can be considered that there is a micro short circuit problem caused by metal impurities inside the cell;

[0037] High-voltage holding stage: the DC voltage is maintained at the high-voltage setting value, the current flowing through the battery cell and the voltage across the battery cell are collected in real time, and the resistance change of the battery cell is calculated. If the resistance change exceeds the normal threshold, it can be considered that the battery cell has a micro-short circuit problem or the diaphragm inside the battery cell is thin or punctured by insulating material, causing a low insulation resistance problem.

[0038] In some embodiments of the present application, after the AC / DC superposition test phase is completed, a self-discharge test phase can be further configured, that is: stop applying voltage to the battery cell to allow the cell to enter a self-discharge process; during this period, detect the voltage across the cell. If the voltage across the cell drops by more than a set threshold within a set time, it can be considered that there is a micro-short circuit problem caused by metal impurities inside the cell or that the diaphragm inside the cell is thin or punctured by insulating material, resulting in a low insulation resistance problem.

[0039] In another aspect, the present invention further provides a battery cell measuring device for performing quality inspection on a battery cell that is not filled with liquid, comprising:

[0040] An adjustable DC high voltage source, which is used to apply a DC high voltage across the two ends of the unfilled battery cell;

[0041] An adjustable AC voltage source, which is used to superimpose a low-voltage AC power source on the DC high voltage output by the adjustable DC high voltage source, so as to control the battery cell to charge and discharge repeatedly and enter the AC / DC superposition test phase;

[0042] A voltage measurement circuit, which is used to detect the voltage across the battery cell;

[0043] A current measurement circuit for detecting the current flowing through the battery cell;

[0044] an AC / DC separation circuit, which is used to perform AC / DC separation on the voltage detection signal output by the voltage measurement circuit or the current detection signal output by the current measurement circuit, and output an AC voltage component or an AC current component;

[0045] An oscillator circuit, which is used to generate a square wave signal of a set frequency, and use the square wave signal to adjust the frequency of the low-voltage AC power output by the adjustable AC voltage source;

[0046] A phase shift circuit receives the square wave signal output by the oscillation circuit and generates a phase-shifted square wave having a phase difference of 90° with the square wave signal;

[0047] A switch circuit receives the AC voltage component or the AC current component and outputs two voltage signals or current signals with a phase difference of 90° according to the square wave signal and the phase-shifted square wave;

[0048] A control unit receives the voltage detection signal output by the voltage measurement circuit during the AC / DC superposition test phase, and generates a test result indicating that a micro short circuit problem exists inside the battery cell due to metal impurities when the voltage detection signal drops to a lower limit value; the control unit receives the voltage signal and current signal output by the switch circuit, calculates the AC impedance and equivalent capacitance of the battery cell; generates a test result indicating that the thickness of the diaphragm inside the battery cell is too thin when the equivalent capacitance value is greater than a set capacitance value; and generates a test result indicating that agglomeration exists between the positive and negative electrodes of the battery cell due to uneven dispersion of the slurry when the AC impedance is greater than a set resistance value.

[0049] In some embodiments of the present application, the control unit may be configured to calculate the AC impedance and equivalent capacitance of the battery cell in the following manner:

[0050] Generate two voltage components U1 and U2 with a phase difference of 90° and two current components I1 and I2 with a phase difference of 90° according to the voltage signal and the current signal output by the switch circuit;

[0051] Use the following formula to calculate the maximum cell voltage U z And the phase angle β:

[0052] U1=U z sin(ωt+β)

[0053] U2=U z sin(ωt+β+90°);

[0054] Use the following formula to calculate the maximum current I flowing through the battery cell: z And the phase angle γ:

[0055] I1=I z sin(ωt+γ)

[0056] I2=I z sin(ωt+γ+90°);

[0057] Calculate the equivalent impedance Z and corresponding phase angle θ of the battery cell:

[0058] Z=U z / I z

[0059] θ = β - γ;

[0060] Calculate the AC impedance R and equivalent capacitance C of the battery cell:

[0061] R=Zcosθ

[0062] C=Zsinθ.

[0063] In some embodiments of the present application, in order to obtain two voltage signals or current signals with a phase difference of 90°, the following may be set in the switch circuit:

[0064] an inverter, which receives the AC voltage component or the AC current component output by the AC / DC separation circuit and inverts the phase of the AC voltage component or the AC current component;

[0065] A first analog switch, comprising two switch paths, one of which receives the AC voltage component or the AC current component output by the AC / DC separation circuit, and the other receives the inverted component output by the inverter; the first analog switch controls the two switch paths to be alternately connected according to the phase-shifted square wave output by the phase-shifting circuit;

[0066] The second analog switch includes two switch paths, one of which receives the AC voltage component or the AC current component output by the AC / DC separation circuit, and the other receives the inverted component output by the inverter; the second analog switch controls its two switch paths to be alternately connected according to the square wave signal output by the oscillation circuit; in this way, two voltage signals or current signals with a phase difference of 90° can be output through the first analog switch and the second analog switch.

[0067] In some embodiments of the present application, in order to improve the accuracy of signal detection, a first filter and a second filter may be further provided in the battery cell measuring device to respectively receive the voltage signal or the current signal output by the first analog switch and the second analog switch, and filter out the high-frequency components therefrom before sending them to the control unit.

[0068] In some embodiments of the present application, the control unit may be provided with:

[0069] A control chip, which outputs a high voltage setting value to the adjustable DC high voltage source, and controls the adjustable DC high voltage source to gradually increase its output DC voltage to the high voltage setting value and maintain it within a set time;

[0070] a first switching switch connected between the adjustable DC high voltage source and the adjustable AC voltage source, wherein the control chip controls the first switching switch to be turned on during the AC / DC superposition test phase, and controls the adjustable DC high voltage source to transmit a DC bus voltage to the adjustable AC voltage source to generate a low-voltage AC power source;

[0071] A second switching switch receives a control signal output by the control chip during an AC / DC superposition test phase, so as to superimpose the low-voltage AC power output by the adjustable AC voltage source on the DC high voltage output by the adjustable DC high voltage source, thereby achieving AC / DC superposition;

[0072] a third switching switch, which receives the voltage detection signal and the current detection signal output by the voltage measurement circuit and the current measurement circuit respectively, and under the control of the control chip, selects one of the detection signals to be transmitted to the AC / DC separation circuit;

[0073] An ADC chip communicates with the control chip, receives different analog signals in different test phases, converts them into digital signals, and sends them to the control chip; wherein,

[0074] When the DC voltage output by the adjustable DC high-voltage source is in the rising stage and the high-voltage holding stage, the control chip controls the ADC chip to perform analog-to-digital conversion on the voltage detection signal and the current detection signal output by the voltage measurement circuit and the current measurement circuit; in the voltage rising stage, the control chip calculates the current change and the voltage change according to the received voltage data and current data. If the current change or the voltage change exceeds the set threshold, a test result indicating that there is a micro-short circuit problem caused by metal impurities inside the battery cell can be generated; in the high-voltage holding stage, the control chip calculates the resistance change of the battery cell according to the received voltage data and current data. If the resistance change exceeds the normal threshold, a test result indicating that there is a micro-short circuit problem in the battery cell or that the diaphragm inside the battery cell is thin or punctured by insulating material, causing a low insulation resistance problem can be generated;

[0075] During the AC / DC superposition test phase, the control chip controls the ADC chip to perform analog-to-digital conversion on the voltage signal and the current signal output by the switch circuit, thereby identifying the battery defect type according to the received voltage data and current data.

[0076] In some embodiments of the present application, in order to achieve comprehensive detection of the battery cells, the control chip can be configured to enter the self-discharge test phase after the AC / DC superposition test phase ends, that is, the adjustable DC high-voltage source is controlled to stop running to make the battery cells self-discharge; in the self-discharge phase, the control chip is configured to control the ADC chip to perform analog-to-digital conversion on the voltage detection signal output by the voltage measurement circuit. If the decrease amplitude of the received voltage data exceeds a set threshold within a set time, a test result can be generated that indicates that there is a micro-short circuit problem caused by metal impurities inside the battery cell or that the insulation resistance of the diaphragm inside the battery cell is low due to the thin thickness or puncture by insulating material.

[0077] Compared with the prior art, the advantages and positive effects of the present invention are mainly reflected in:

[0078] 1. The present invention designs a measurement method for battery cells that have not been filled with liquid. Before the battery cell is filled with liquid, applying DC high voltage at both ends cannot detect whether the thickness of the diaphragm between the positive and negative plates of the battery cell meets the requirements, and cannot test the agglomeration phenomenon caused by uneven dispersion of slurry between the plates. After the battery cell is filled with liquid, DC high voltage cannot be applied at both ends. The DC high voltage will cause the battery cell after liquid injection to be broken down by high voltage and cause battery explosion. This leads to certain types of battery defects that cannot be identified by only applying low-voltage AC power supply at both ends of the liquid-filled battery cell. To this end, the present invention adopts the method of superimposing a low-voltage AC power supply on the DC high voltage to form an AC voltage with a high-voltage bias, which is applied to both ends of the battery cell that has not been filled with liquid. This can not only meet the power supply requirements of the battery cell and ensure the safety of the battery cell test, but also detect the parameter characteristics of the battery cell under AC power supply, and then identify certain defects that may appear in the battery cell under AC power supply.

[0079] 2. In order to achieve comprehensive detection of possible quality problems in battery cells, the present invention designs four test stages: voltage rise, high voltage maintenance, AC / DC superposition, and self-discharge, and designs different strategies in each test stage to perform multi-faceted detection and identification of different types of defects that may exist in battery cells. This can promptly detect problem batteries, end the test process as soon as possible, and avoid further damage to the problem batteries.

[0080] 3. The present invention can not only detect batteries with quality problems, but also identify the defect types of the problem batteries, thereby guiding personnel to carry out targeted repair work and improving battery production efficiency.

[0081] 4. The present invention designs a measurement method for battery cells that are not filled with liquid, which solves the problem of serious waste of electrolyte caused by the problem batteries being unusable after testing the filled batteries with traditional low-voltage AC testing methods. It can greatly reduce the manufacturing cost of the battery and save resources.

[0082] 5. The battery cell measuring device designed by the present invention can fully automatically detect the battery cells that are not filled with liquid, and automatically generate the test results without human intervention, which is convenient and fast.

[0083] After reading the specific embodiments of the present invention in conjunction with the accompanying drawings, other features and advantages of the present invention will become more clear. BRIEF DESCRIPTION OF THE DRAWINGS

[0084] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0085] Figure 1 It is a circuit principle block diagram of an embodiment of the battery cell measuring device proposed by the present invention;

[0086] Figure 2 It is an overall flow chart of an embodiment of the battery cell measurement method proposed by the present invention;

[0087] Figure 3 It is the voltage waveform diagram of each test stage. DETAILED DESCRIPTION

[0088] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0089] Since the battery cells without liquid injection cannot be supplied with alternating positive and negative AC power, and the battery cells after liquid injection cannot be supplied with DC high voltage, otherwise it is easy to cause the battery cells to spontaneously combust or explode. Therefore, the existing battery cell measurement methods all apply one type of power supply at both ends of the battery cell, which makes it difficult for certain defects of the battery to be clearly manifested in one power supply mode, resulting in a high rate of missed detection of problematic batteries and difficulty in identifying the defect type.

[0090] In order to achieve comprehensive detection of quality problems of battery cells, this embodiment designs four test stages: voltage rise stage, high voltage maintenance stage, AC / DC superposition stage and self-discharge stage. By adopting different power supply methods in different test stages and coordinating different parameter detection methods, it is possible to capture possible problems inside the battery from all angles and reduce the missed detection rate of problem batteries.

[0091] In order to automatically detect problematic batteries, this embodiment designs a battery cell measuring device, such as Figure 1 As shown, it mainly includes an adjustable DC high voltage source, an adjustable AC voltage source, a voltage measurement circuit, a current measurement circuit, an AC / DC separation circuit, an oscillation circuit, a phase shift circuit, a switch circuit, a control unit and other components.

[0092] Among them, the adjustable DC high-voltage source is used to convert the external AC mains into a DC power supply, and apply DC high voltage to the battery cell under test. In this embodiment, the DC voltage output by the adjustable DC high-voltage source can be freely adjusted between 25V and 2000V, and a control unit is configured to be connected to the adjustable DC high-voltage source to provide a high-voltage set value for the adjustable DC high-voltage source. For example, the high-voltage set value can be set at above 500V to enable the battery cell under test to operate under a high-voltage power supply state. At the same time, the adjustable DC high-voltage source of this embodiment also has the function of adjusting its output voltage rise time according to the user's set time. Therefore, the control unit can be configured to output the set time to the adjustable DC high-voltage source to control the adjustable DC high-voltage source to gradually increase its output voltage to the high-voltage set value within the set time, such as Figure 3 Segment a in is used for testing during the voltage rise phase.

[0093] The adjustable AC voltage source is used to output an AC power source with adjustable frequency and voltage, which is superimposed on the DC voltage output by the adjustable DC high-voltage source to form a positive voltage with sinusoidal variation. In this embodiment, in order to meet the test requirements, the adjustable AC voltage source can be configured to output a low-voltage AC power source (for example, an AC power source below 100V), which is superimposed on the DC high voltage output by the adjustable DC high-voltage source to form an AC voltage with a high-voltage bias, such as Figure 3 The c segment in the figure is applied to the AC / DC superposition test stage.

[0094] In some embodiments, an adjustable DC high voltage source may be used to provide a DC bus voltage for an adjustable AC voltage source, so as to simplify the system circuit design and achieve the purpose of regulating the output voltage of the adjustable AC voltage source.

[0095] In order to obtain the DC high voltage and low voltage AC power required for the battery cell test, and realize the superposition of DC high voltage and low voltage AC power in the AC / DC superposition test stage, the control chip MCU, the first switch and the second switch are provided in the control unit of this embodiment, such as Figure 1 As shown. The control chip MCU generates a high voltage setting value and a DC bus voltage value, which are transmitted to the adjustable DC high voltage source to adjust the DC high voltage applied by the adjustable DC high voltage source to both ends of the battery cell and the DC bus voltage output to the adjustable AC voltage source. At the same time, the control chip MCU generates a switch control signal to control the on and off of the first switch and the second switch.

[0096] In this embodiment, the first switch is connected between the adjustable DC high voltage source and the adjustable AC voltage source, and the second switch is connected between the adjustable AC voltage source and the power supply circuit of the battery cell. In the AC / DC superposition test phase, the control chip MCU controls the first switch and the second switch to be turned on, and the DC bus voltage output by the adjustable DC high voltage source is transmitted to the adjustable AC voltage source, so that the adjustable AC voltage source is powered on and operates, and the inverter outputs a low-voltage AC power supply, which is transmitted to the power supply circuit of the battery cell via the second switch, thereby realizing AC / DC superposition.

[0097] Since the frequency of the low-voltage AC power output by the adjustable AC voltage source determines the path switching frequency of the switch circuit, a unified frequency reference needs to be set to meet the acquisition requirements of the AC signal. To this end, the present embodiment sets an oscillation circuit to generate a square wave signal of a set frequency, which is transmitted to the AC reference unit on the one hand to generate an AC reference voltage signal with the same frequency as the square wave signal, and transmitted to the adjustable AC voltage source to adjust the frequency of the low-voltage AC power output by the adjustable AC voltage source; on the other hand, it is transmitted to the switch circuit to adjust the path switching frequency of the switch circuit.

[0098] The voltage measurement loop can be connected to both ends of the battery cell to collect the voltage at both ends of the battery cell and reduce it according to a certain ratio. In this embodiment, an operational amplifier can be set in the voltage measurement loop, such as an AD8510 chip with an 8MHz bandwidth, to accurately measure the rapid change of the voltage at both ends of the battery cell.

[0099] The current measurement circuit is used to detect the current flowing through the battery cell and amplify it according to a certain ratio. In some embodiments, a sampling resistor R1 can be connected in series in the power supply circuit of the battery cell, and the current measurement circuit can be connected to both ends of the sampling resistor R1. The current flowing through the battery cell is calculated by detecting the voltage difference across the sampling resistor R1 and the resistance value of the sampling resistor R1, that is, the voltage signal is used to reflect the current change.

[0100] An ADC chip and a third switch are provided in the control unit, and the voltage detection signal collected by the voltage measurement loop and the current detection signal collected by the current measurement loop can be directly transmitted to the ADC chip for analog-to-digital conversion to generate a digital quantity signal and send it to the control chip MCU. The third switch is used to perform gating control on the voltage detection signal output by the voltage measurement loop and the current detection signal output by the current measurement loop, so as to transmit the voltage detection signal and the current detection signal to the AC / DC separation circuit in a time-sharing manner.

[0101] The AC / DC separation circuit is used to perform AC / DC separation on the voltage detection signal output by the voltage measurement circuit and the current detection signal output by the current measurement circuit, so as to output an AC voltage component and an AC current component, which are transmitted to the switch circuit.

[0102] In this embodiment, the switch circuit is used to process the received AC voltage component or AC current component into two voltage signals or current signals with a phase difference of 90°, so as to calculate the AC impedance and equivalent capacitance of the battery cell.

[0103] In some embodiments, an inverter and a first analog switch and a second analog switch may be provided in the switch circuit. Figure 1 As shown. Among them, the inverter is used to invert the phase of the AC voltage component or the AC current component output by the AC / DC separation circuit to generate a voltage waveform with the same amplitude but opposite phase as the AC voltage component or the AC current component, hereinafter referred to as the inverted component. The inverter can be set as one or two paths, and the output inverted component is transmitted to one of the switch paths of the first analog switch and the second analog switch respectively. The other switch path of the first analog switch and the second analog switch is configured to receive the AC voltage component or the AC current component output by the AC / DC separation circuit respectively. The square wave signal output by the oscillation circuit is used to control the two switch paths of the second analog switch to be alternately turned on. For example, when the square wave signal is at a high level, the AC voltage component or the AC current component is output; when the square wave signal is at a low level, the inverted component is output. The phase shift circuit is configured to shift the phase of the square wave signal output by the oscillation circuit to generate a phase-shifted square wave with a phase difference of 90° from the square wave signal, and transmit it to the first analog switch to control the two switch paths of the first analog switch to be alternately turned on. For example, when the phase-shifted square wave is at a high level, an AC voltage component or an AC current component is output; when the phase-shifted square wave is at a low level, an anti-phase component is output. In this way, two voltage signals or current signals with a phase difference of 90° can be output through the first analog switch and the second analog switch.

[0104] Two voltage signals or current signals with a phase difference of 90° are transmitted to the first filter and the second filter respectively. After the high-frequency components are eliminated, they are sent to the ADC chip for analog-to-digital conversion to calculate the AC impedance and equivalent capacitance of the battery cell.

[0105] In this embodiment, a divider may also be provided in the battery cell measuring device, such as Figure 1 As shown, it is used to receive the voltage detection signal output by the voltage measurement circuit and the current detection signal output by the current measurement circuit in the high-voltage holding stage. After division, a voltage signal reflecting the size of the resistance can be obtained, which is transmitted to the ADC chip for analog-to-digital conversion and then sent to the control chip MCU to reduce the data processing workload of the control chip MCU.

[0106] Combine the following Figure 1 The battery cell measuring device shown in the figure illustrates the battery cell measuring method, such as Figure 2 As shown, it is divided into four testing stages.

[0107] (1) Voltage rise stage

[0108] That is, a gradually increasing DC voltage is applied to the two ends of the battery cell under test, and the current flowing through the battery cell and the voltage at both ends of the cell are collected at the same time. The change in current and voltage is used to determine whether there is a micro-short circuit problem caused by metal impurities inside the battery cell.

[0109] Specifically, the unfilled battery cell is connected to the measuring device, the control chip MCU outputs the high voltage setting value, and the adjustable DC high voltage source is controlled to gradually increase its output DC voltage to the high voltage setting value within the set time, such as Figure 3 Section a in .

[0110] During the voltage rising process, the voltage across the battery cell is collected in real time through the voltage measurement circuit, and the current flowing through the battery cell is collected in real time through the current measurement circuit. The control chip MCU notifies the ADC chip to receive the voltage detection signal and current detection signal output by the voltage measurement circuit and the current measurement circuit, and after analog-to-digital conversion, generates voltage data and current data and sends them to the control chip MCU.

[0111] The control chip MCU calculates the current change and voltage change according to the received voltage data and current data. If the current change or voltage change exceeds the set threshold, it means that there is a micro short circuit problem inside the battery cell, and the micro short circuit problem is caused by metal impurities introduced into the battery cell. Otherwise, continue to the next test stage.

[0112] In this embodiment, the control chip MCU can be configured to directly generate the test results and output them through a display device for easy viewing by operators.

[0113] (2) High-voltage maintenance stage

[0114] That is, a DC high voltage is continuously applied to both ends of the battery cell under test, and the current flowing through the battery cell and the voltage at both ends of the battery cell are collected at the same time. The resistance change of the battery cell is calculated, and it is determined based on the resistance change whether there is a micro-short circuit problem inside the battery cell or whether the diaphragm inside the battery cell has a low insulation resistance problem due to being thin or punctured by insulating material.

[0115] Specifically, the control chip MCU controls the adjustable DC high voltage source to maintain the DC voltage outputted by the adjustable DC source when the DC voltage reaches the high voltage setting value, such as Figure 3 Section b in the figure is used to continuously apply DC high voltage to both ends of the battery cell.

[0116] During the high-voltage holding stage, the voltage across the battery cell is collected in real time through the voltage measurement circuit, and the current flowing through the battery cell is collected in real time through the current measurement circuit. The divider is started to divide the received voltage detection signal and current detection signal to generate a voltage signal reflecting the cell resistance and send it to the ADC chip.

[0117] The control chip MCU notifies the ADC chip to receive the voltage signal output by the divider, convert it into a digital signal (resistance value) and send it to the control chip MCU.

[0118] The control chip MCU detects the change in the resistance value of the battery cell. If the resistance change exceeds the set normal threshold, it means that the battery cell either has a micro short circuit problem or the thickness of the diaphragm inside the battery cell does not meet the requirements, such as the diaphragm thickness is too thin or the diaphragm has been punctured by insulating materials. A thin diaphragm thickness or puncture by insulating materials will cause the insulation resistance value of the battery cell to be low.

[0119] Some battery cells have relatively thin internal diaphragms, but they do not cause micro short circuits inside the cells. Therefore, such defects cannot be detected by only testing the voltage changes at both ends of the cells and the current changes flowing through the cells. In other words, such defects cannot be detected during the voltage rise phase. For this reason, this embodiment adopts a method of calculating the insulation resistance value of the cell during the high voltage holding phase to facilitate the discovery of cells with such defects, thereby achieving the purpose of improving the accuracy of the test results.

[0120] If the resistance change calculated during the high voltage holding stage is normal, the subsequent testing process can be continued to further screen for battery defects.

[0121] Similarly, the control chip MCU can be configured to directly generate test results and output them through a display device for easy viewing by operators.

[0122] (3) AC / DC superposition stage

[0123] That is, a low-voltage AC power source is superimposed on the DC high voltage applied to the two ends of the battery cell to control the battery cell to charge and discharge repeatedly. At the same time, the current flowing through the battery cell and the voltage at both ends of the cell are collected, and the presence of metal impurities inside the cell is determined based on the voltage change; the AC impedance and equivalent capacitance of the cell are calculated using the collected voltage and current, and the thickness of the diaphragm inside the cell is determined based on the equivalent capacitance value to determine whether it meets the requirements; based on the AC impedance, it is determined whether there is agglomeration between the positive and negative plates of the cell due to uneven dispersion of the slurry.

[0124] Specifically, the control chip MCU controls the adjustable DC high-voltage source to continuously output DC high voltage and output the DC bus voltage value, and controls the adjustable DC high-voltage source to generate the DC bus voltage required by the adjustable AC voltage source. During this period, the control chip MCU starts the oscillation circuit to output a square wave signal, and provides a reference frequency to the adjustable AC voltage source through the AC reference unit. At the same time, the control chip MCU controls the first switching switch and the second switching switch to be turned on, and transmits the DC bus voltage output by the adjustable DC high-voltage source to the adjustable AC voltage source through the first switching switch, controls the adjustable AC voltage source to invert and output a low-voltage AC power source, and superimposes it to both ends of the battery cell through the second switching switch. That is, an alternating high voltage with a voltage that is always positive is applied to both ends of the battery cell, such as Figure 3 Section c in .

[0125] In the AC / DC superposition stage, this embodiment uses an AC voltage with a high-voltage bias to power the battery cells, so that the battery cells can be controlled to operate in a repeated charge and discharge state, so that defective batteries can be detected, whose data in the charging stage are the same as those of normal batteries, but whose discharge speed suddenly increases during the discharge stage.

[0126] The main reason why this type of defective battery cell exhibits this abnormal phenomenon is the presence of metal impurities inside the battery cell. These metal impurities have not pierced the diaphragm during the voltage rise stage and the high-voltage maintenance stage, causing the battery cell to have a micro-short circuit problem. Therefore, the various data in the charging stage are the same as those of the normal battery cell, which leads to the inability to detect such defects during the voltage rise stage and the high-voltage maintenance stage. After entering the AC / DC superposition stage, because the unfilled battery cell is repeatedly charged and discharged, the metal impurities inside it will pierce the diaphragm under the action of the repeatedly changing electric field force, which will cause a micro-short circuit inside the battery cell, causing the voltage at both ends of the battery cell to drop rapidly during the discharge stage, even below the set lower limit. Therefore, in the AC / DC superposition stage, it is possible to judge whether there are metal impurities inside the battery cell based on the voltage changes at both ends of the battery cell. Specifically, if the voltage at both ends of the battery cell drops rapidly to the lower limit, it can be considered that there is a micro-short circuit problem caused by metal impurities inside the battery cell.

[0127] For unliquid-filled battery cells, the existing insulation test method and pulse test method cannot accurately detect problem cells with a small amount of metal impurities and unclear short circuits. However, by using the AC / DC superposition test method of this embodiment, even if the metal impurities introduced into the battery cell are small and few, the hidden micro-short circuit problem of the battery cell will be made obvious during the repeated charge and discharge process of the battery cell, thereby facilitating the discovery and identification of the problem.

[0128] Therefore, in terms of detecting and identifying the micro short circuit defects in battery cells caused by the introduction of metal impurities inside the battery cells, the AC / DC superposition test method of this embodiment is obviously more accurate and comprehensive than the traditional battery cell measurement method.

[0129] According to the test results, if the battery cell does not have a micro-short circuit problem, further testing and identification will be carried out to determine whether the battery cell has a diaphragm thickness that does not meet the requirements or whether the slurry between the positive and negative electrodes has agglomeration due to uneven dispersion.

[0130] Specifically, the control chip MCU can be configured to adjust the conduction branch of the third switching switch first, and the voltage detection signal collected by the voltage measurement circuit can be sent to the AC / DC separation circuit. The AC / DC separation circuit is started to perform AC / DC separation on the voltage detection signal to extract the AC voltage component and transmit it to the first analog switch, the second analog switch and the inverter respectively. The first analog switch and the second analog switch are controlled by the oscillation circuit and the phase shift circuit to alternately conduct their two switch paths to output two voltage signals with a phase difference of 90°. After the high-frequency components are filtered out by the first filter and the second filter respectively, they are transmitted to the ADC chip for analog-to-digital conversion, and then the voltage data is generated and sent to the control chip MCU.

[0131] After collecting voltage data for a sufficient period of time, the control chip MCU is configured to switch the conduction branch of the third switch, and the current detection signal collected by the current measurement circuit is sent to the AC / DC separation circuit for AC / DC separation. After that, two current signals with a phase difference of 90° are transmitted to the ADC chip through the inverter, analog switch, and filter. After the ADC chip performs analog-to-digital conversion, the current data is generated and sent to the control chip MCU.

[0132] The control chip MCU generates two voltage components U1 and U2 with a phase difference of 90° and two current components I1 and I2 with a phase difference of 90° according to the received voltage signal and current signal, and calculates the AC impedance and equivalent capacitance value of the battery cell according to the voltage components U1 and U2 and the current components I1 and I2. The specific process is as follows:

[0133] Use the following formula to calculate the maximum value of the cell voltage U z And the phase angle β:

[0134] U1=U z sin(ωt+β)

[0135] U2=U z sin(ωt+β+90°);

[0136] Where ω is the angular frequency, which is related to the frequency of the low-voltage AC power supply; t is the sampling time.

[0137] Use the following formula to calculate the maximum current I flowing through the battery cell: z And the phase angle γ:

[0138] I1=I z sin(ωt+γ)

[0139] I2=I z sin(ωt+γ+90°);

[0140] The equivalent impedance Z and the corresponding phase angle θ of the battery cell are calculated using the following formula:

[0141] Z=U z / I z

[0142] θ = β - γ;

[0143] The AC impedance R and equivalent capacitance C of the battery cell are calculated using the following formula:

[0144] R=Zcosθ

[0145] C=Zsinθ.

[0146] The equivalent model of an unfilled battery cell is a capacitor, to which a high voltage can be applied at both ends; the positive and negative electrodes of the unfilled battery cell are equivalent to the positive and negative electrodes of the capacitor; the diaphragm between the positive and negative electrodes is equivalent to the insulating medium between the positive and negative electrodes of the capacitor, and the thickness of the diaphragm reflects the distance between the positive and negative electrodes of the capacitor.

[0147] According to the formula of factors affecting the size of capacitance:

[0148] C' = εs / (4πkd);

[0149] Among them, ε is the dielectric constant of the dielectric, which indicates the conductivity of the material; s is the area of ​​the two parallel metal plates; k is the electrostatic force constant; and d is the distance between the two parallel metal plates. Since the greater the distance between the positive and negative electrodes of the capacitor, the smaller the capacitance value, the greater the thickness of the diaphragm between the positive and negative electrodes of the equivalent unfilled battery cell, the smaller the equivalent capacitance value of the battery cell. Therefore, by detecting whether the equivalent capacitance value of the unfilled battery cell meets the threshold, it is possible to determine whether the diaphragm thickness meets the requirements. Battery cells whose diaphragm thickness does not meet the requirements are defective products with quality problems.

[0150] Based on this, this embodiment determines the set capacitance value according to the equivalent capacitance value corresponding to the unfilled battery cell whose diaphragm thickness meets the requirements, and writes it into the control chip MCU. After calculating the equivalent capacitance value C of the battery cell, the control chip MCU can directly compare it with the set capacitance value. If the equivalent capacitance value C is greater than the set capacitance value, it can be considered that the thickness of the diaphragm inside the battery cell does not meet the requirements, that is, the thickness of the diaphragm is too thin, which can easily cause the problem of low insulation resistance of the battery cell and can be regarded as a defective product.

[0151] The equivalent model of the battery cell before liquid injection and the battery cell after liquid injection is very different. The equivalent capacitance of the battery cell after liquid injection is very small and difficult to detect, and high voltage cannot be applied to both ends of the battery cell after liquid injection. If high voltage is applied, the battery cell will be broken down and cause spontaneous combustion or explosion. Therefore, only a low-voltage AC power supply can be applied to both ends of the battery cell after liquid injection, and the quality of the battery cell can be judged by measuring the AC resistance of the battery cell. The equivalent model of the battery cell before liquid injection is a capacitor and can withstand high voltage. Therefore, the above-mentioned AC power supply with high-voltage bias can be used to measure the equivalent capacitance value of the battery cell to determine whether the diaphragm between the positive and negative plates of the battery cell meets the thickness requirements. The detection and identification of whether the thickness of the diaphragm meets the requirements of this battery defect is a function that the existing battery cell measurement method does not have.

[0152] In addition, during the AC / DC superposition test stage, it is also possible to detect and identify whether the battery cell has uneven slurry dispersion problems.

[0153] Specifically, if the slurry between the positive and negative electrodes of the battery cell is unevenly dispersed, serious agglomeration will occur, affecting the electrochemical performance of the battery. When the same voltage is applied to both ends of an unfilled battery cell, the AC current inside the unfilled battery cell with uniform slurry dispersion is larger than that of the unfilled battery cell with uneven slurry dispersion. In other words, the equivalent resistance of the battery cell with uneven slurry dispersion will be larger. Therefore, the AC impedance of the unfilled battery cell can be detected to determine whether the battery cell has agglomeration caused by uneven slurry dispersion.

[0154] Based on this, this embodiment first performs an AC / DC superposition test on the non-liquid-injected battery cell with uniform slurry dispersion between the pole pieces, determines a reasonable set resistance value, and writes it into the control chip MCU. In the actual test stage, after the configuration control chip MCU calculates the AC impedance R of the tested battery cell, it can be directly compared with the set resistance value. If the AC impedance R is greater than the set resistance value, it can be considered that there is agglomeration between the positive and negative pole pieces of the battery cell due to uneven slurry dispersion, and the battery cell is an unqualified battery cell. Otherwise, it can be considered that the slurry between the pole pieces of the tested battery cell is uniformly dispersed and there is no agglomeration phenomenon.

[0155] The existing methods for measuring the AC internal resistance of battery cells all measure the AC internal resistance of the battery cells after liquid injection. According to the size of the AC internal resistance of the battery cells after liquid injection, it can only be judged whether the battery cells after liquid injection are good or bad, and it cannot be tested whether it is caused by the uneven dispersion of the slurry between the pole pieces, because the electrolyte inside the battery cell will cause the internal resistance of the battery cell to be too small, affecting the accuracy of the measurement of the internal resistance of the battery cell. In addition, high-voltage power cannot be applied to both ends of the battery cell after liquid injection, otherwise the battery cell will be burned, which also makes it difficult to measure the internal resistance of the liquid-injected battery cell. In this embodiment, the above-mentioned AC power supply with high-voltage bias is applied to both ends of the non-liquid-injected battery cell, and then the AC component is separated for the calculation of the internal resistance of the battery cell. In this way, the influence of the electrolyte on the measurement of the internal resistance of the battery cell can be eliminated, and then the problem of large AC resistance caused by uneven dispersion of the slurry between the pole pieces can be accurately detected.

[0156] Using AC and DC superposition power to test unfilled cells can more accurately test the factors that cause cell quality problems, which is conducive to the improvement of production technology, improves the accuracy of cell measurement results before filling, and can effectively prevent the waste of electrolyte caused by problematic batteries after filling. Improving the accuracy of cell measurement before filling can reduce the probability of defective products entering the market and reduce the occurrence of battery spontaneous combustion accidents.

[0157] Similarly, the control chip MCU can be configured to directly generate test results and output them through a display device for easy viewing by operators.

[0158] (4) Self-discharge stage

[0159] That is, stop applying voltage to the battery cell under test, so that the cell enters the self-discharge process, and by detecting the voltage drop at both ends of the cell, it is determined whether there is a micro-short circuit problem or low insulation resistance problem inside the cell.

[0160] Specifically, after the AC / DC superposition test phase is completed, the configuration control chip MCU controls the adjustable DC high voltage source to stop output and enter the battery self-discharge phase. Since the battery cell under test has been fully charged, the voltage at both ends will slowly drop after the power supply is disconnected. Figure 3 The d-segment waveform in .

[0161] In the self-discharge stage, the control chip MCU can be configured to notify the ADC chip to receive the voltage detection signal output by the voltage measurement circuit, perform analog-to-digital conversion, and then send it to the control chip MCU.

[0162] The control chip MCU detects the voltage change at both ends of the battery cell based on the received voltage data. If it is detected that the voltage drop at both ends of the battery cell exceeds the set threshold within the set time, that is, the self-discharge speed is too fast, it can be considered that there is a micro-short circuit problem caused by metal impurities inside the battery cell, or the battery cell self-discharges too fast because the thickness of the internal diaphragm is too thin or the diaphragm has been punctured by insulating materials, resulting in low insulation resistance.

[0163] The specific type of defect can be determined by combining the test results of the above four stages. For example, in the AC / DC superposition stage, it can be tested whether there are metal impurities inside the battery cell, whether the thickness of the diaphragm meets the requirements, and whether the slurry between the pole pieces is evenly dispersed; if these defects do not exist, that is, the battery cell performs normally in the AC / DC superposition stage, then when entering the self-discharge stage, if it is detected that the battery cell self-discharges too fast, it can be considered that there is insulating material inside the battery cell, thereby realizing the battery cell defect type identification function.

[0164] Similarly, the control chip MCU can be configured to directly generate test results and output them through a display device for easy viewing by operators.

[0165] This embodiment can accurately detect problematic cells and identify the defect types of problematic cells by detecting the voltage and current changes of unfilled battery cells in the boost stage, the internal resistance changes in the voltage holding stage, the AC impedance and equivalent capacitance changes in the AC / DC superposition stage, and the voltage changes in the self-discharge stage. The defect types include the introduction of metal impurities and insulating impurities into the cell, the thickness of the diaphragm inside the cell not meeting the requirements, and uneven dispersion of the slurry between the positive and negative electrodes.

[0166] Of course, the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the above embodiments, it is still possible for a person skilled in the art to modify the technical solutions described in the above embodiments, or to replace some of the technical features therein by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions claimed to be protected by the present invention.

Claims

1. A battery cell measurement method, applied to a battery cell without liquid injection, characterized in that: include: Applying a DC high voltage across the unfilled battery cell; A low-voltage AC power supply is superimposed on the DC high voltage to control the battery cells to charge and discharge repeatedly, entering the AC / DC superposition test phase: Detecting the voltage across the battery cell and the current flowing through the battery cell; When the voltage at both ends of the battery cell drops to the lower limit, it is determined that there is a micro short circuit problem caused by metal impurities inside the battery cell; Calculate the AC impedance and equivalent capacitance of the battery cell; When the equivalent capacitance value is greater than the set capacitance value, it is determined that the thickness of the diaphragm inside the battery cell is too thin; When the AC impedance is greater than the set resistance, it is determined that there is agglomeration between the positive and negative electrodes of the battery cell due to uneven dispersion of the slurry.

2. The battery cell measurement method according to claim 1, characterized in that: The calculation process of the AC impedance and equivalent capacitance value of the battery cell includes: The voltage at both ends of the battery cell is collected and separated into AC and DC, and the AC voltage component U1 is extracted, and another AC voltage component U2 is formed which is 90° out of phase with U1; The current flowing through the battery cell is collected and separated into AC and DC, the AC current component I1 is extracted, and another AC current component I2 is formed which is 90° out of phase with I1; Use the following formula to calculate the maximum cell voltage U z And the phase angle β: U1=U z sin(ωt+β) U2=U z sin(ωt+β+90°); Where, ω is the angular frequency and t is the sampling time; Use the following formula to calculate the maximum current I flowing through the battery cell: z And the phase angle γ: I1=I z sin(ωt+γ) I2=I z sin(ωt+γ+90°); Calculate the equivalent impedance Z and corresponding phase angle θ of the battery cell: Z=U z / I z θ = β - γ; Calculate the AC impedance R and equivalent capacitance C of the battery cell: R=Zcosθ C=Zsinθ.

3. The battery cell measurement method according to claim 1 or 2, characterized in that: The process of applying a DC high voltage to both ends of the battery cell that is not filled with liquid includes: Voltage rising stage: applying a DC voltage to both ends of the battery cell, and controlling the DC voltage to gradually rise to a high voltage setting value within a set time; detecting the current flowing through the cell and the voltage at both ends of the cell during the voltage rising process. If the current change or the voltage change exceeds the set threshold, it is considered that there is a micro short circuit problem caused by metal impurities inside the cell; High-voltage holding stage: the DC voltage is maintained at the high-voltage setting value, the current flowing through the battery cell and the voltage across the battery cell are collected in real time, and the resistance change of the battery cell is calculated. When the resistance change exceeds the normal threshold, it is determined that the battery cell has a micro-short circuit problem or the diaphragm inside the battery cell is thin or punctured by insulating material, causing a low insulation resistance problem.

4. The battery cell measurement method according to claim 1 or 2, characterized in that: After the AC / DC superposition test phase is completed, the self-discharge phase begins, including: Stop applying voltage to the battery cells to allow the cells to self-discharge; Detect the voltage at both ends of the battery cell. If the voltage drops by more than the set threshold within the set time, it is considered that there is a micro short circuit problem caused by metal impurities inside the battery cell or the diaphragm inside the battery cell is too thin or punctured by insulating materials, causing low insulation resistance.

5. A battery cell measuring device for quality inspection of unfilled battery cells, characterized in that: include: An adjustable DC high voltage source, which is used to apply a DC high voltage across the two ends of the unfilled battery cell; An adjustable AC voltage source, which is used to superimpose a low-voltage AC power source on the DC high voltage output by the adjustable DC high voltage source, so as to control the battery cell to charge and discharge repeatedly and enter the AC / DC superposition test phase; A voltage measurement circuit, which is used to detect the voltage across the battery cell; A current measurement circuit for detecting the current flowing through the battery cell; an AC / DC separation circuit, which is used to perform AC / DC separation on the voltage detection signal output by the voltage measurement circuit or the current detection signal output by the current measurement circuit, and output an AC voltage component or an AC current component; An oscillator circuit, which is used to generate a square wave signal of a set frequency, and use the square wave signal to adjust the frequency of the low-voltage AC power output by the adjustable AC voltage source; A phase shift circuit receives the square wave signal output by the oscillation circuit and generates a phase-shifted square wave having a phase difference of 90° with the square wave signal; A switch circuit receives the AC voltage component or the AC current component and outputs two voltage signals or current signals with a phase difference of 90° according to the square wave signal and the phase-shifted square wave; A control unit receives the voltage detection signal output by the voltage measurement circuit during the AC / DC superposition test phase, and generates a test result indicating that a micro short circuit problem exists inside the battery cell due to metal impurities when the voltage detection signal drops to a lower limit value; the control unit receives the voltage signal and current signal output by the switch circuit, calculates the AC impedance and equivalent capacitance of the battery cell; generates a test result indicating that the thickness of the diaphragm inside the battery cell is too thin when the equivalent capacitance value is greater than a set capacitance value; and generates a test result indicating that agglomeration exists between the positive and negative electrodes of the battery cell due to uneven dispersion of the slurry when the AC impedance is greater than a set resistance value.

6. The battery cell measuring device according to claim 5, characterized in that: The control unit generates two voltage components U1 and U2 with a phase difference of 90° and two current components I1 and I2 with a phase difference of 90° according to the voltage signal and current signal output by the switch circuit; then, the maximum cell voltage U is calculated using the following formula: z And the phase angle β: U1=U z sin(ωt+β) U2=U z sin(ωt+β+90°); Where, ω is the angular frequency and t is the sampling time; Use the following formula to calculate the maximum current I flowing through the battery cell: z And the phase angle γ: I1=I z sin(ωt+γ) I2=I z sin(ωt+γ+90°); Calculate the equivalent impedance Z and corresponding phase angle θ of the battery cell: Z=U z / I z θ = β - γ; Calculate the AC impedance R and equivalent capacitance C of the battery cell: R=Zcosθ C=Zsinθ.

7. The battery cell measuring device according to claim 5, characterized in that: The switch circuit comprises: an inverter, which receives the AC voltage component or the AC current component output by the AC / DC separation circuit and inverts the phase of the AC voltage component or the AC current component; A first analog switch, comprising two switch paths, one of which receives the AC voltage component or the AC current component output by the AC / DC separation circuit, and the other receives the inverted component output by the inverter; the first analog switch controls the two switch paths to be alternately connected according to the phase-shifted square wave output by the phase-shifting circuit; The second analog switch includes two switch paths, one of which receives the AC voltage component or the AC current component output by the AC / DC separation circuit, and the other receives the inverted component output by the inverter; the second analog switch controls the two switch paths to be alternately connected according to the square wave signal output by the oscillation circuit; and outputs two voltage signals or current signals with a phase difference of 90° through the first analog switch and the second analog switch.

8. The battery cell measuring device according to claim 7, characterized in that: Also includes: a first filter, which receives the voltage signal or current signal output by the first analog switch, filters out the high-frequency component therein, and then sends it to the control unit; The second filter receives the voltage signal or current signal output by the second analog switch, filters out the high-frequency component therein, and then sends it to the control unit.

9. The battery cell measuring device according to any one of claims 5 to 8, characterized in that: The control unit comprises: A control chip, which outputs a high voltage setting value to the adjustable DC high voltage source, and controls the adjustable DC high voltage source to gradually increase its output DC voltage to the high voltage setting value and maintain it within a set time; a first switching switch connected between the adjustable DC high voltage source and the adjustable AC voltage source, wherein the control chip controls the first switching switch to be turned on during the AC / DC superposition test phase, and controls the adjustable DC high voltage source to transmit a DC bus voltage to the adjustable AC voltage source; A second switching switch receives a control signal output by the control chip during an AC / DC superposition test phase to superimpose the low-voltage AC power output by the adjustable AC voltage source onto the DC high voltage output by the adjustable DC high voltage source; a third switching switch, which receives the voltage detection signal and the current detection signal output by the voltage measurement circuit and the current measurement circuit respectively, and under the control of the control chip, selects one of the detection signals to be transmitted to the AC / DC separation circuit; An ADC chip communicates with the control chip, receives different analog signals in different test phases, converts them into digital signals, and sends them to the control chip; wherein, When the DC voltage output by the adjustable DC high-voltage source is in the rising stage and the high-voltage holding stage, the control chip controls the ADC chip to perform analog-to-digital conversion on the voltage detection signal and the current detection signal output by the voltage measurement circuit and the current measurement circuit; in the voltage rising stage, the control chip calculates the current change and the voltage change according to the received voltage data and current data, and when the current change or the voltage change exceeds the set threshold, generates a test result indicating that there is a micro-short circuit problem caused by metal impurities inside the battery cell; in the high-voltage holding stage, the control chip calculates the resistance change of the battery cell according to the received voltage data and current data, and when the resistance change exceeds the normal threshold, generates a test result indicating that the battery cell is micro-short circuited or the diaphragm inside the battery cell is thin or punctured by insulating material, causing low insulation resistance; During the AC / DC superposition test phase, the control chip controls the ADC chip to perform analog-to-digital conversion on the voltage signal and the current signal output by the switch circuit, and identifies the battery defect type according to the received voltage data and current data.

10. The battery cell measuring device according to claim 9, characterized in that: After the AC / DC superposition test phase is completed, the control chip controls the adjustable DC high voltage source to stop running, so that the battery cell self-discharges and enters the self-discharge phase; During the self-discharge stage, the control chip controls the ADC chip to perform analog-to-digital conversion on the voltage detection signal output by the voltage measurement circuit. When the decrease amplitude of the received voltage data exceeds a set threshold within a set time, a test result is generated indicating that there is a micro-short circuit problem inside the battery cell due to metal impurities or that the insulation resistance of the diaphragm inside the battery cell is low due to thin thickness or puncture by insulating material.

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