CN117148186A - Lithium battery safety early warning system and method based on lithium battery explosion-proof valve pressure monitoring - Google Patents

Abstract

The invention provides a lithium battery safety early warning system and a method based on lithium battery explosion-proof valve pressure monitoring, which belong to the field of lithium battery safety, wherein the system comprises: the system comprises a conditioning circuit, an analog-to-digital conversion module, a charge-discharge tester and an upper computer; the conditioning circuit is a Wheatstone bridge circuit; the opposite-side bridge arm is a metal strain gauge, the remaining two bridge arms are reference resistors with the same size, and the reference resistor is the resistance of the metal strain gauge when the metal strain gauge is not stressed at normal temperature; the metal strain gauge is attached to the surface of the explosion-proof valve of the lithium battery, and strain of the explosion-proof valve is detected; and analyzing the variation trend of the strain digital signal along with the SOC and the temperature of the lithium battery, comparing the theoretical curve between the strain of the metal strain gauge direction and the charge and discharge of the lithium battery, and judging whether the safety pre-warning of the lithium battery is carried out. The metal strain gauge is arranged outside the lithium battery, the internal characteristics of the lithium battery are not required to be changed, and a large amount of space is not required to be occupied.

Description

Lithium battery safety early warning system and method based on pressure monitoring of lithium battery explosion-proof valve

Technical field

The invention belongs to the field of lithium battery safety, and more specifically, relates to a lithium battery safety early warning system and method based on pressure monitoring of a lithium battery explosion-proof valve.

Background technique

The direct driving force for the rapid development of electrochemical energy storage is the widespread application of lithium-ion batteries. However, lithium-ion batteries have the risk of thermal runaway. Once thermal runaway occurs in a lithium-ion battery energy storage power station, it will lead to fire or explosion accidents, and the consequences will be disastrous. Therefore, early warning is the most important function in the safety protection system of energy storage power stations. In current energy storage engineering applications, early warning measures are not perfect. Although they are equipped with voltage and current power sensors that can estimate the safety status of a single module or battery cell, the accuracy is still low, so new parameter sensing is needed. It is very important to estimate the condition of lithium batteries.

In the existing energy storage engineering applications, early warning measures are not perfect, which is reflected in the lack of other physical quantities other than voltage, current, and temperature to characterize the battery status; when the voltage, current, and temperature sensors are interfered with or fail, there is a lack of other physical quantities. Physical quantity for battery warning. When a battery experiences thermal runaway, a series of side reactions will occur internally, some of which will generate gas. Before the battery explosion-proof valve is opened, these gases will accumulate inside the battery case, causing the battery to bulge and change the pressure inside the battery. Therefore, early warning can be provided by detecting the pressure inside the battery. The current more accurate method is to use embedded optical fibers to detect the internal pressure of the battery. This method is expensive and requires sensors to be embedded inside the battery. The process is very complex and will affect the internal characteristics of the battery; or it can detect battery expansion through customized external fixtures. This method is more expensive and requires a lot of space.

Contents of the invention

In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a lithium battery safety early warning system and method based on pressure monitoring of lithium battery explosion-proof valves, aiming to solve the high cost and need of the existing embedded optical fiber pressure detection method inside the battery. Embedding sensors inside the battery is a very complicated process and will affect the internal characteristics of the battery.

In order to achieve the above objectives, on the one hand, the present invention provides a lithium battery safety early warning system based on pressure monitoring of lithium battery explosion-proof valves, including: a conditioning circuit, an analog-to-digital conversion module, a charge and discharge tester and a host computer;

One end of the charge and discharge tester is connected to the lithium battery, and the other end is connected to the host computer; it is used to perform charge and discharge tests on the lithium battery, and at the same time transmits the charge and discharge status parameters to the host computer;

The conditioning circuit is a Wheatstone bridge circuit; the opposite bridge arm is a metal strain gauge, and the remaining two bridge arms are reference resistors of equal size. The size of the reference resistor is the resistance of the metal strain gauge when it is not stressed at room temperature; metal The strain gauge is attached to the surface of the lithium battery explosion-proof valve to detect the strain of the explosion-proof valve; the strain of the explosion-proof valve is equivalent to the strain mechanical signal of the metal strain gauge;

The output end of the conditioning circuit is connected to the input end of the analog-to-digital conversion module, which is used to detect the strain of the lithium battery explosion-proof valve, read the strain mechanical signal transmitted by the metal strain gauge, and convert the strain mechanical signal into a strain electrical signal;

The analog-to-digital conversion module is used to convert strain electrical signals into strain digital signals;

The host computer is used to analyze the changing trend of the strain digital signal with SOC and lithium battery temperature based on the strain digital signal and the charge and discharge state parameters, compare the theoretical functional relationship between the strain in the direction of the metal strain gauge and the charge and discharge state parameters of the lithium battery, and judge Whether to carry out lithium battery safety warning;

Among them, the theoretical functional relationship indicates that the strain in the direction of the metal strain gauge is proportional to the SOC and lithium battery temperature changes.

Further preferably, the charging and discharging state parameters include: current and voltage during charging and discharging, lithium battery temperature and SOC, and charging and discharging time.

Further preferably, the theoretical functional relationship between the strain in the direction of the metal strain gauge and the charging and discharging state parameters of the lithium battery is:

Among them, ε ₁ is the strain in the direction of the metal strain gauge; ΔT is the temperature change of the lithium battery; SOC is the state of charge (SOC); h _pole is the initial thickness of the electrode; S _pole is the surface area after the electrode is expanded; ω _- is the volume fraction of the negative electrode material, ω ₊ is the volume fraction of the positive electrode material, α _- is the volume expansion coefficient of the negative electrode material, α ₊ is the volume expansion coefficient of the positive electrode material, V is the internal void of the original battery; μ is Poisson's ratio; n is the gas The amount of substance; R is the molar gas constant; P ₀ is the standard atmospheric pressure; T ₀ is the standard temperature, here it is 25°C.

Further preferably, the relationship between the strain of the lithium battery explosion-proof valve detected in the conditioning circuit and the reference resistance and the resistance change of the metal strain gauge is:

Among them, ΔR is the resistance change value caused by the force deformation of the metal strain gauge; R ₀ is the resistance value of the metal strain gauge when it is not stressed at room temperature; K ₀ is the strain sensitivity coefficient of the metal material, and ε ₁ is the mechanical resistance in the direction of the metal strain gauge. strain.

On the other hand, for the above-mentioned lithium battery safety early warning system based on lithium battery explosion-proof valve pressure monitoring, the present invention provides a corresponding lithium battery safety early warning method, which includes the following steps:

When the lithium battery is charging/discharging, the charging and discharging status parameters are transmitted to the host computer in real time;

Attach the metal strain gauge to the surface of the lithium battery explosion-proof valve to detect the strain of the lithium battery explosion-proof valve; where the strain of the explosion-proof valve is equivalent to the strain mechanical signal transmitted by the metal strain gauge;

Convert the strain mechanical signal transmitted by the metal strain gauge into a strain electrical signal, and then convert the strain electrical signal into a strain digital signal;

Based on the charging and discharging state parameters, combined with the strain digital signal, analyze the changing trend of the strain digital signal with SOC and lithium battery temperature, compare the theoretical functional relationship between the strain in the direction of the metal strain gauge and the lithium battery charging and discharging state parameters, and determine whether to proceed with lithium Battery safety warning;

Among them, the theoretical functional relationship indicates that the strain of the metal strain gauge is proportional to the SOC and lithium battery temperature changes.

Further preferably, the charging and discharging state parameters include: current and voltage during charging and discharging, lithium battery temperature and SOC, and charging and discharging time.

Further preferably, the theoretical functional relationship between the strain in the direction of the metal strain gauge and the charging and discharging state parameters of the lithium battery is:

Among them, ε ₁ is the strain in the direction of the metal strain gauge; ΔT is the temperature change of the lithium battery; SOC is the state of charge (SOC); h _pole is the initial thickness of the electrode; S _pole is the surface area after the electrode is expanded; ω _- is the volume fraction of the negative electrode material, ω ₊ is the volume fraction of the positive electrode material, α _- is the volume expansion coefficient of the negative electrode material, α ₊ is the volume expansion coefficient of the positive electrode material, V is the internal void of the original battery; μ is Poisson's ratio; n is the gas The amount of substance; R is the molar gas constant; P ₀ is the standard atmospheric pressure; T ₀ is the standard temperature, here it is 25°C.

Further preferably, the relationship between the strain of the lithium battery explosion-proof valve detected in the conditioning circuit and the reference resistance and the resistance change of the metal strain gauge is:

Among them, ΔR is the resistance change value caused by the force deformation of the metal strain gauge; R ₀ is the resistance value of the metal strain gauge when it is not stressed at room temperature; K ₀ is the strain sensitivity coefficient of the metal material, and ε ₁ is the mechanical resistance in the direction of the metal strain gauge. strain.

Generally speaking, compared with the prior art, the above technical solution conceived by the present invention has the following advantages:

Beneficial effects:

The invention provides a lithium battery safety early warning system and method based on pressure monitoring of a lithium battery explosion-proof valve. The conditioning circuit is a Wheatstone bridge circuit; the opposite bridge arm is a metal strain gauge, and the metal strain gauge is attached to the lithium battery explosion-proof valve. The surface of the valve is used to detect the strain of the explosion-proof valve; it is possible to install metal strain gauges outside the lithium battery without changing the internal characteristics of the lithium battery and without taking up a lot of space, which is of great application value.

The present invention provides a lithium battery safety early warning system and method based on lithium battery explosion-proof valve pressure monitoring, in which a theoretical functional relationship between the strain in the direction of the metal strain gauge and the charging and discharging state parameters of the lithium battery is constructed. A multi-physics model of lithium battery temperature and state of charge is established, and the multi-physics model can be applied to battery safety warning.

The invention provides a lithium battery safety early warning system and method based on pressure monitoring of a lithium battery explosion-proof valve. The explosion-proof valve strain signal can realize early warning of battery overcharging and thermal runaway, which can be about 10 minutes earlier than the explosion-proof valve opening and characteristic gas release. Minutes, battery safety warning is implemented.

Description of the drawings

Figure 1 is a schematic diagram of a lithium battery safety early warning method based on pressure monitoring of lithium battery explosion-proof valves provided by an embodiment of the present invention;

Figure 2 is a schematic diagram of a method for estimating battery safety status based on lithium battery explosion-proof valve pressure provided by an embodiment of the present invention;

Figure 3 is a measurement conditioning circuit diagram provided by an embodiment of the present invention;

Figure 4 is a curve of charging time and battery explosion-proof valve pressure under normal working conditions provided by the embodiment of the present invention;

Figure 5 is a curve of discharge time and battery explosion-proof valve pressure under normal working conditions provided by the embodiment of the present invention;

Figure 6 is a curve of charging time and battery explosion-proof valve pressure under overcharging out-of-control conditions provided by an embodiment of the present invention;

Figure 7 is a curve of explosion-proof valve pressure and characteristic gas detection under overcharge out-of-control conditions provided by the embodiment of the present invention.

Detailed ways

In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention and are not intended to limit the present invention.

Lithium-ion batteries are accompanied by temperature changes during use, causing thermal stress. The hard-shell lithium-ion battery has a layered wound structure. Due to the different thermal expansion coefficients between the winding layers, the thermal expansion of the materials in each layer will restrict each other and produce thermal stress; the thermal stress reflected on the battery core is the internal pressure of the battery. Changes: Compared with implanting a sensor inside the battery to detect the internal pressure of the battery, the invention proposes a method to estimate the battery safety state based on the pressure of the lithium battery explosion-proof valve, which can detect the internal characteristics of the battery by detecting the external surface of the battery without destroying the internal characteristics of the battery. The deformation caused by pressure reflects the pressure changes inside the battery.

The battery explosion-proof valve serves as a release channel for the internal pressure of the battery during the battery thermal runaway process, and its sensitivity to the internal pressure should be much greater than other parts of the battery surface. Therefore, the present invention reflects the status of the battery by detecting the deformation of the battery explosion-proof valve due to internal pressure changes during battery charging and discharging (hereinafter collectively referred to as explosion-proof valve pressure), and obtains a result by comparing the battery explosion-proof valve pressure under different charging and discharging conditions. A method for estimating battery safety status based on lithium battery explosion-proof valve pressure. Figure 1 is a flow chart of a battery safety early warning method based on lithium battery explosion-proof valve pressure monitoring provided by the present invention. The method specifically includes:

Step S1: Build a lithium battery safety warning system based on metal strain gauges and conditioning circuits to monitor the pressure of the explosion-proof valve of hard-shell lithium batteries;

Figure 2 is a lithium battery safety early warning system for monitoring the pressure of explosion-proof valve of hard-shell lithium batteries provided by an embodiment of the present invention, including: a conditioning circuit, a high-precision ADC, a charge and discharge tester and a host computer; Figure 3 is an embodiment of the present invention The measurement and conditioning circuit diagram used uses the conditioning circuit in Figure 3 to process the sensor signal to improve sensitivity and anti-interference ability; among them, metal strain gauges are placed on the No. 1 bridge arm and No. 3 bridge arm; the No. 2 bridge arm and The corresponding reference resistor is placed on the No. 4 bridge arm; the metal strain gauge used is a metal strain gauge with a resistance value of R ₀ when it is not stressed at room temperature, and the reference resistor is selected as the corresponding R ₀ ;

The input differential mode voltage of the amplifier circuit is calculated according to the formula:

Among them, ΔU is the differential mode voltage of the input amplifier circuit, VCC is the power supply voltage of the bridge circuit, R ₀ is the resistance value of the metal strain gauge when it is not stressed at room temperature, and is also the reference resistance value, ΔR is the force of the metal strain gauge The resistance change value caused by deformation; at the same time, the stress on the metal strain gauge has the following relationship with the resistance change value:

Among them, K ₀ is the strain sensitivity coefficient of the metal material, and ε ₁ is the strain in the direction of the metal strain gauge;

Taking the derivative of formula (1) with respect to ΔR, we get:

The obtained (ΔU)′ is a parameter that reflects the rate of stress change inside the battery; then integrating formulas (2) and (3), we get:

Step S2: Obtain the composite parameter reflecting the internal temperature and SOC of the battery based on the explosion-proof valve pressure, and construct a theoretical curve between the strain in the direction of the metal strain gauge and the charge and discharge of the lithium battery;

The battery explosion-proof valve pressure is a symptom of changes in the internal stress of the battery, and the internal stress of the battery changes due to the deintercalation and heat generation of electrode materials. Therefore, through experiments under different working conditions, the relationship between the explosion-proof valve pressure and the battery internal temperature and SOC change trend can be obtained. relationship, so the battery explosion-proof valve pressure can be used as a composite parameter that reflects the battery internal temperature and SOC based on the explosion-proof valve pressure;

The expansion and contraction of the internal core of the battery caused by internal stress changes the volume of the internal pores of the battery, which is recorded as ΔV. Assume that the change in the thickness of the battery core caused by charging and discharging is Δh _pole . Assuming that the internal temperature of the battery remains unchanged, the lithium-ion battery expands. Without constraints, the separator and current collector are not compressed and deformed, then the electrode thickness change Δh _pole is linearly related to the change in SOC:

Among them, h _pole is the initial thickness of the electrode, ω _- is the volume fraction of the negative electrode material, ω ₊ is the volume fraction of the positive electrode material, α _- is the volume expansion coefficient of the negative electrode material, α ₊ is the volume expansion coefficient of the positive electrode material, β is the negative electrode excess coefficient, both is a constant, assuming the initial SOC is 0, then ΔSOC = SOC;

In the same way, assuming that the SOC remains unchanged, there is:

Among them, Δh _temp is the change in thickness during the resting stage, ΔT is the temperature change, α is the thermal expansion coefficient, and α is approximately considered to be a constant; combining formulas (5) and (6) can be obtained:

Assuming that the volume change of the internal void of the battery is ΔV, then:

ΔV＝Δh _pole S _pole (8)

V′=V-ΔV (9)

Among them, V′ is the changed internal void in the battery, V is the internal void in the original battery, and S _pole is the surface area after the electrode is expanded, which remains unchanged when the internal stress changes;

The pressure of the explosion-proof valve is generated by the internal stress acting on the explosion-proof valve through the internal gas of the battery. According to the ideal gas equation of state:

P _in V＝nRT (10)

as well as

F _in ＝P _in S _valve (11)

and generalized Hooke's law

Among them, P _in is the internal pressure of the battery; n is the amount of gas substance; R is the molar gas constant. When the battery is in a safe state, it is approximately considered that no obvious gas is generated, so the above three values are all constants; σ _valve is the battery The direction of the stress on the explosion-proof valve is perpendicular to the surface of the explosion-proof valve; F _in is the pressure inside the battery acting on the explosion-proof valve; F _out is the pressure outside the battery acting on the explosion-proof valve, usually F _out =P ₀ S _valve , P ₀ is the standard atmospheric pressure; S _valve is the area of the explosion-proof valve; E is the elastic modulus; μ is Poisson's ratio; ε ₁ is the strain in the same direction as the metal strain gauge, σ ₁ is the stress in the direction of the metal strain gauge; σ ₂ is The stress perpendicular to the surface direction of the explosion-proof valve can be considered as σ ₂ =σ _valve ; σ ₃ is the stress in the direction perpendicular to both σ ₁ and σ ₂ ; in the safe state of the battery, it is approximately considered that the explosion-proof valve has only one direction. Principal strain, that is, ε ₂ =ε ₃ =0;

Integrating all the above formulas, the theoretical functional relationship between the strain in the direction of the metal strain gauge and the characteristics and state parameters of the lithium battery is obtained:

In the safe state of the battery, it is approximately believed that the explosion-proof valve has only one main strain in one direction, that is, ε ₂ =ε ₃ =0, and σ ₂ =σ _valve . Simplifying the above formula and specifying the strain direction, we get:

in,

Among them, K ₀ , K ₁ , K _T1 , K _V , K _SOC , and K _T2 are all approximately constant coefficients, which can be determined through experimental data;

Select the lithium iron phosphate square hard-shell lithium battery as the test object. As shown in Figure 1, first charge the battery with constant current and then constant voltage, charge until it is fully charged, let it stand for 1 hour, then discharge it with constant current to 0% SOC, and let it stand for 2 hours. Then build the sensor circuit according to step S1, and paste the two metal strain gauges on the explosion-proof valve; place the battery and test circuit together in a dry and sealed 25°C constant temperature environment, and transmit the ADC signal to the host computer through the data line. Then let it sit for 2 hours;

Record the explosion-proof valve pressure in the initial state, connect the charge-discharge tester and battery, charge-discharge tester and host computer, then turn on the charge-discharge tester, and then charge with constant current for 2 hours. At the same time, record the explosion-proof valve pressure changes and battery charging curve, and draw the battery. The curve of charging time and battery core explosion-proof valve pressure is shown in Figure 4. In Figure 4, the explosion-proof valve pressure curve first decreases, and then shows a slow upward trend; from (Among them, K _T1 , K _SOC , and K _T2 are all less than zero) It can be seen that the explosion-proof valve pressure increases monotonically with respect to SOC and ΔT; during the charging process, the explosion-proof valve pressure tends to increase due to the increase in SOC, and at the same time There is also an increasing trend with the increase of ΔT; in the t ₀ ~ t ₁ stage, the battery temperature change and the SOC change work together to increase the battery explosion-proof valve pressure. At this time, the temperature dominates the battery explosion-proof valve pressure change, and the explosion-proof valve pressure Showing a rapid increasing trend; from t ₁ to t ₂ , the heat generated inside the battery gradually tends to dissipate, and the temperature increases slowly. At this time, the explosion-proof valve pressure is affected by the joint action of SOC and temperature, showing a slowly increasing trend; after t ₂ , the temperature no longer Increase, the battery's heat production and heat dissipation reach a balance. At this time, the battery changes under the action of SOC. Since the SOC is larger at this time, a small amount of gas production begins during the battery charging process. Under the joint action of SOC and chemical reaction gas production, The pressure of the battery explosion-proof valve still increases rapidly, which is an irreversible process. However, during the battery charging and discharging process, this process does not dominate; however, as the number of battery cycles increases, the impact of this irreversible process cannot be ignored.

Similarly, let the lithium battery stand for 2 hours, connect the charge and discharge tester to the lithium battery, the charge and discharge tester and the host computer, then turn on the charge and discharge tester, and then discharge at a constant current of 0.3C for 2 hours, while recording the pressure changes of the explosion-proof valve and Battery discharge curve, the curve of battery discharge time and battery core explosion-proof valve pressure is drawn as shown in Figure 5. In Figure 5, the pressure curve of the explosion-proof valve first decreases, and then shows a slow upward trend. Depend on (Among them, K _T1 , K _SOC , and K _T2 are all less than zero) It can be seen that the explosion-proof valve pressure increases monotonically with respect to SOC and ΔT. During the discharge process, the explosion-proof valve pressure has a decreasing trend due to the decrease of SOC. At the same time, There is an increasing trend with the increase of ΔT; from t ₀ to t ₁ , the battery temperature changes more rapidly. At this time, temperature dominates the change of battery explosion-proof valve pressure, and the explosion-proof valve pressure shows an increasing trend; from t ₁ to t ₂ , The internal heat generation of the battery is equal to the heat dissipation, and the temperature no longer increases. At this time, the explosion-proof valve pressure is dominated by SOC, showing a decreasing trend; the temperature is slowly increasing again from t ₂ to t ₃ , and at this time the battery changes under the dual effects of SOC and temperature. , tends to be gentle; the battery ends charging at time t ₃ , when the SOC no longer changes, the battery explosion-proof valve pressure decreases monotonically with temperature, and the rate of change is faster than when SOC and temperature interact together, indicating that during the discharge process, SOC and Temperature will have an impact on the pressure of the battery explosion-proof valve. The decrease in SOC will reduce the pressure of the explosion-proof valve, while the increase in temperature will increase the pressure of the explosion-proof valve. Since the SOC does not change during the standing process, it is only affected by temperature;

S3: Battery safety warning based on the theoretical curve equation between the strain in the direction of the metal strain gauge and the charge and discharge of the lithium battery (lithium battery explosion-proof valve pressure curve);

The explosion-proof valve pressure of the lithium battery in the safe state is as explained in S2. During the thermal runaway process of the battery, it is accompanied by a sharp increase in heat production. When the negative electrode reacts with the electrolyte to form a film, a large amount of gas is produced, which will affect the battery explosion-proof valve. There is a significant trend on the pressure curve, which can provide safety warning before the battery thermal runaway and provide safety alarm when the battery thermal runaway;

The hard-shell lithium iron phosphate battery is overcharged at a constant current (0.4C) until the explosion-proof valve opens, and the explosion-proof valve pressure curve during the overcharge stage is shown in Figure 6; before overcharging, the explosion-proof valve pressure shows a slow change with temperature and SOC. The upward trend, with small-scale fluctuations as the temperature fluctuates, is consistent with the qualitative analysis results in S2; when t = t0, the change rate of the explosion-proof valve pressure increases significantly, indicating that gas production begins to occur inside the battery, resulting in the explosion-proof valve There is an inflection point when the pressure suddenly changes. At this time, the explosion-proof valve has not yet opened; when t = t1, due to the large amount of gas production, the battery side wall bulges, and the explosion-proof valve pressure shows a downward trend due to the increase in the internal volume of the battery core, and begins to deviate from S2. Qualitative analysis results during the normal charging and discharging process of the battery show a second sudden turning point, at which time it is believed that the battery thermal runaway is about to occur; at t = t2, the pressure of the explosion-proof valve suddenly increases, the explosion-proof valve opens, and the gas detector begins to detect Hydrogen gas is generated and an alarm is issued. The gas detection results and explosion-proof valve pressure are shown in Figure 7;

As can be seen from Figure 6, the first inflection point of the explosion-proof valve pressure (the sign that the battery core starts to produce gas) occurs about 12 minutes earlier than the explosion-proof valve opens; the second inflection point of the explosion-proof valve pressure (the sign that the side wall of the battery core begins to bulge and deform) appears. The time is about 10 minutes earlier than the explosion-proof valve opening, so an advance warning of at least 10 minutes can be achieved through the two characteristic inflection points of the explosion-proof valve pressure.

To sum up, compared with the existing ones, the present invention has the following advantages:

The invention provides a lithium battery safety early warning system and method based on pressure monitoring of a lithium battery explosion-proof valve. The conditioning circuit is a Wheatstone bridge circuit; the opposite bridge arm is a metal strain gauge, and the metal strain gauge is attached to the lithium battery explosion-proof valve. The surface of the valve is used to detect the strain of the explosion-proof valve; it is possible to install metal strain gauges outside the lithium battery without changing the internal characteristics of the lithium battery and without taking up a lot of space, which is of great application value.

The invention provides a lithium battery safety early warning system and method based on lithium battery explosion-proof valve pressure monitoring, in which a theoretical curve between the strain in the direction of the metal strain gauge and the charge and discharge of the lithium battery is constructed. A multi-physics model of lithium battery temperature and state of charge is established, and the multi-physics model can be applied to battery safety warning.

The invention provides a lithium battery safety early warning system and method based on pressure monitoring of a lithium battery explosion-proof valve. The explosion-proof valve strain signal can realize early warning of battery overcharging and thermal runaway, which can be about 10 minutes earlier than the explosion-proof valve opening and characteristic gas release. Minutes, battery safety warning is implemented.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements, etc., made within the spirit and principles of the present invention, All should be included in the protection scope of the present invention.

Claims (8)

1. Lithium cell safety precaution system based on explosion-proof valve pressure monitoring of lithium cell, its characterized in that includes: the system comprises a conditioning circuit, an analog-to-digital conversion module, a charge-discharge tester and an upper computer;

one end of the charge-discharge tester is connected with the lithium battery, and the other end of the charge-discharge tester is connected with the upper computer and is used for carrying out charge-discharge test on the lithium battery and transmitting charge-discharge state parameters to the upper computer;

the conditioning circuit is a Wheatstone bridge circuit; the opposite-side bridge arm is a metal strain gauge, the remaining two bridge arms are reference resistors with the same size, and the reference resistor is the resistance of the metal strain gauge when the metal strain gauge is not stressed at normal temperature; the metal strain gauge is attached to the surface of the explosion-proof valve of the lithium battery and used for detecting the strain of the explosion-proof valve; strain mechanical signals of the metal strain gauge are equivalent to strain of the explosion-proof valve;

the output end of the conditioning circuit is connected with the input end of the analog-to-digital conversion module and is used for detecting the strain of the lithium battery explosion-proof valve, reading the strain mechanical signal transmitted by the metal strain gauge and converting the strain mechanical signal into a strain electrical signal;

the analog-to-digital conversion module is used for converting the strain electric signal into a strain digital signal;

the upper computer is used for analyzing the change trend of the strain digital signal along with the SOC and the lithium battery temperature based on the strain digital signal and the charge-discharge state parameter, comparing the theoretical function relation between the strain of the metal strain gauge direction and the lithium battery charge-discharge state parameter, and judging whether to perform lithium battery safety pre-warning;

wherein, the theoretical functional relation characterizes that the strain of the metal strain gauge direction is in direct proportion to the SOC and the temperature change of the lithium battery.

2. The lithium battery safety precaution system of claim 1, wherein the charge-discharge state parameters comprise: current and voltage at charge and discharge, lithium battery temperature and SOC, and charge and discharge time.

3. The lithium battery safety precaution system of claim 1, wherein the theoretical functional relationship between the strain of the metal strain gauge direction and the lithium battery charge-discharge state parameter is:

wherein ε ₁ Strain in the direction of the metal strain gauge; delta T is the temperature change of the lithium battery; SOC is state of charge; h is a _pole An initial thickness of the electrode; s is S _pole Surface area after electrode deployment; omega _- Is the volume fraction of the anode material omega ₊ Alpha is the volume fraction of the positive electrode material _- Is the volume expansion coefficient of the anode material, alpha ₊ The positive electrode material volume expansion coefficient is V, which is the internal gap of the original battery; μ is poisson's ratio; n is the amount of gaseous species; r is molar gas constant; p (P) ₀ Is the standard atmospheric pressure; t (T) ₀ Is the standard temperature.

4. The lithium battery safety precaution system according to claim 1 or 2, wherein the relation between the strain of the detected lithium battery explosion-proof valve in the conditioning circuit and the resistance change of the reference resistance and the metal strain gauge is:

wherein DeltaR is a resistance change value generated by the forced deformation of the metal strain gauge; r is R ₀ The resistance value of the metal strain gauge is the resistance value of the metal strain gauge when the metal strain gauge is not stressed at normal temperature; k (K) ₀ Is the strain sensitivity coefficient epsilon of the metal material ₁ Is the mechanical strain in the direction of the metal strain gauge.

5. The lithium battery safety precaution method based on the lithium battery safety precaution system of claim 1, characterized by comprising the following steps:

when the lithium battery performs a charging/discharging process, transmitting a charging/discharging state parameter to the upper computer in real time;

attaching a metal strain gauge to the surface of the lithium battery explosion-proof valve, and detecting the strain of the lithium battery explosion-proof valve; the strain of the explosion-proof valve is equal to a strain mechanical signal transmitted by the metal strain gauge;

converting the strain mechanical signal transmitted by the metal strain gauge into a strain electrical signal, and converting the strain electrical signal into a strain digital signal;

based on the charge and discharge state parameters, the change trend of the strain digital signals along with the SOC and the lithium battery temperature is analyzed by combining the strain digital signals, and the theoretical functional relation between the strain of the metal strain gauge direction and the lithium battery charge and discharge state parameters is compared to judge whether the lithium battery safety precaution is carried out;

wherein, the theoretical functional relation characterizes that the strain of the metal strain gauge is in direct proportion to the SOC and the temperature change of the lithium battery.

6. The method of claim 5, wherein the charge-discharge state parameters include: current and voltage at charge and discharge, lithium battery temperature and SOC, and charge and discharge time.

7. The method of claim 5 or 6, wherein the functional relationship between the strain in the direction of the metal strain gauge and the charge-discharge state parameter of the lithium battery is:

wherein ε ₁ Strain in the direction of the metal strain gauge; delta T is the temperature change of the lithium battery; SOC is state of charge; h is a _pole An initial thickness of the electrode; s is S _pole Surface area after electrode deployment; omega _- Is the volume fraction of the anode material omega ₊ Alpha is the volume fraction of the positive electrode material _- Is the volume expansion coefficient of the anode material, alpha ₊ The positive electrode material volume expansion coefficient is V, which is the internal gap of the original battery; μ is poisson's ratio; n is the amount of gaseous species; r is molar gas constant; p (P) ₀ Is the standard atmospheric pressure; t (T) ₀ Is standard toTemperature.

8. The method for early warning safety of a lithium battery according to claim 7, wherein the relation between the strain of the explosion-proof valve of the lithium battery detected in the conditioning circuit and the reference resistance and the resistance variation of the metal strain gauge is as follows:

wherein DeltaR is a resistance change value generated by the forced deformation of the metal strain gauge; r is R ₀ The resistance value of the metal strain gauge is the resistance value of the metal strain gauge when the metal strain gauge is not stressed at normal temperature; k (K) ₀ Is the strain sensitivity coefficient epsilon of the metal material ₁ Is the mechanical strain in the direction of the metal strain gauge.