Lithium ion battery entropy thermal coefficient calculation method based on heat generation rate model

Abstract

The invention discloses a lithium ion battery entropy thermal coefficient calculation method based on a heat generation rate model, and relates to the technical field of batteries. The method comprises the following steps: measuring the internal resistance of the battery at different temperatures through experiments, and calculating to obtain the internal resistance heat generation rate at the 1C discharge rate; respectively measuring the temperature rise of the battery under 1C discharge rate under adiabatic and room temperature conditions, and calculating to obtain the total heat generation rate; and indirectly calculating to obtain the entropy thermal coefficient of the battery based on a Bernardi heat generation rate model formula by combining the internal resistance heat generation rate under the 1C discharge rate. The entropy thermal coefficient calculation method provided by the invention has the advantages of short experimental period, high accuracy, capability of comprehensively reflecting the thermal characteristics of the battery and the like, and is of great significance for evaluating the thermal safety of the lithium ion battery and optimizing a battery management system.

Description

Technical Field

[0001] The present invention relates to the field of battery technology, and in particular to a method for calculating the entropy thermal coefficient of a lithium-ion battery based on a heat generation rate model. Background Art

[0002] As the preferred power source for electric vehicles, lithium-ion batteries have the advantages of high specific energy, long cycle life, low self-discharge rate, and environmental protection. However, lithium-ion batteries generate a lot of heat during the charging and discharging process. If the heat is not dissipated in time, it will seriously affect the working performance and life of the battery, and may even cause thermal runaway, fire or explosion. Therefore, it is of great significance to accurately estimate the heat generated by lithium-ion batteries during the charging and discharging process. In the calculation of reversible heat, the entropy thermal coefficient (dE / dT) of the battery is one of the key parameters.

[0003] The open circuit voltage method is currently the main method for measuring the entropy thermal coefficient of batteries. The principle of the open circuit voltage method is to fix the state of charge (SOC) of the battery, change the ambient temperature, and measure the open circuit voltage of the battery at different temperatures. By fitting the voltage-temperature curve, the entropy thermal coefficient of the battery at this SOC is obtained. This method is easy to measure the entropy thermal coefficient, but there are still some shortcomings: ① For the measurement of open circuit voltage, it is necessary to achieve electrochemical equilibrium at different ambient temperatures, and the larger the battery volume and capacity, the longer it takes; ② Affected by the self-discharge effect of the battery itself, when the measurement time is long, it will affect the accuracy of the open circuit voltage measurement.

[0004] Therefore, a simple, accurate and applicable method for calculating entropy change of various batteries is needed. Summary of the invention

[0005] In order to overcome the problems in the prior art of directly measuring the entropy thermal coefficient, such as the long experimental period and the influence of slight self-discharge on the accuracy of the test results, a method for indirectly measuring the entropy thermal coefficient is proposed.

[0006] This method measures the total internal resistance of the battery at different charge states and temperatures, and uses a fitting function to calculate the entropy change coefficient of the battery, thereby obtaining the entropy change of the battery. This method has the advantages of short experimental cycle and high accuracy. The specific steps are as follows:

[0007] Step 1: The heat generation rate model formula adopts the heat generation rate model formula established by Bernardi, that is, Q=I(E-E0)+IT(dE / dT), where Q is the total heat generation rate, I is the charge and discharge current of the single cell, E is the terminal voltage, E0 is the open circuit voltage, T is the battery surface temperature, and dE / dT is the entropy thermal coefficient.

[0008] Step 2: The total internal resistance R of discharge under different ambient temperatures and different SOCs is measured through experiments, and the internal resistance heat generation rate is calculated using the formula E-E0=IR.

[0009] Step 3: Test the temperature rise of the battery during discharge under adiabatic conditions and room temperature respectively, and use thermodynamic theory to calculate the total heat generation rate Q1 under adiabatic conditions, and use the equivalent heat transfer coefficient h comb Calculate the total heat generation rate Q2 at room temperature.

[0010] Step 4: Combining the experimentally measured internal resistance heat generation rate and the calculated total heat generation rate, the entropy thermal coefficient of the battery is indirectly calculated according to the Bernardi heat generation rate model formula.

[0011] Beneficial Effects

[0012] The present invention provides a method for calculating the entropy thermal coefficient of a lithium-ion battery based on a heat generation rate model. Compared with the prior art, the beneficial effect of the present invention is that an indirect calculation method is adopted to avoid the need for long-term testing of open circuit voltage at different ambient temperatures and SOCs. By comprehensively considering the discharge temperature rise of the battery under adiabatic and room temperature conditions, as well as the influence of the heat generation rate of internal resistance, the entropy thermal coefficient of the battery can be calculated more accurately. The method of the present invention can reflect the transient changes in the entropy thermal coefficient of the battery at different discharge depths, providing more accurate data support for the thermal management of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a flow chart of the entropy heat coefficient calculation method;

[0014] Figure 2 It is the fitting curve of the average temperature of the battery surface changing with time when the lithium-ion battery is discharged at 1C rate at room temperature and then cooled to ambient temperature;

[0015] Figure 3 The variation of discharge temperature rise with depth of discharge (DOD) of lithium-ion batteries under adiabatic and room temperature conditions;

[0016] Figure 4 This is the curve of the entropy thermal coefficient of lithium-ion batteries changing with DOD under adiabatic and room temperature conditions. DETAILED DESCRIPTION

[0017] 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.

[0018] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0019] The specific embodiments of the present invention are described in further detail below:

[0020] This embodiment uses a ternary soft-pack lithium-ion power battery with a rated capacity of 36Ah as the research object. Figure 1 As shown, a method for calculating the entropy thermal coefficient of lithium-ion batteries based on the heat generation rate model is established, as follows:

[0021] 1. To calculate the total heat generation rate of lithium-ion batteries, it is required to test the temperature rise during the discharge process. The test of the discharge temperature rise is carried out under adiabatic conditions and room temperature conditions respectively.

[0022] Step 1, the total heat generation rate formula during battery discharge under adiabatic conditions is as follows:

[0023] Q1=C p mΔT (1)

[0024] In formula (1), Q1 represents the total heat generation rate of the battery under adiabatic conditions; m represents the mass of the single battery, C p The specific heat capacity of a single cell is 1.2 J·g, which is obtained by ARC's constant power heating test method. -1 ℃ -1 .

[0025] Step 2: Under room temperature conditions, considering the convection heat transfer and electromagnetic wave radiation heat transfer between the battery surface and the surrounding environment, the convection heat transfer and radiation heat transfer are converted into a reduced heat transfer coefficient h comb Simplified, the expression is as shown in formula (2):

[0026] Q c +Q r =h comb A(TT a ) (2)

[0027] Where: Q c and Q r denote the convective heat transfer and the radiative heat transfer, respectively, h comb It represents the reduced heat transfer coefficient considering convection and radiation, Ta Indicates the ambient temperature of the battery. The specific heat capacity of the battery does not change much within a certain range and can be treated as a constant. The following formula can be used to calculate h when the battery is at rest after discharge. comb :

[0028] C p mΔT+h comb A(TT a )=0 (3)

[0029] Based on the above formula, the reduced heat transfer coefficient h comb , the total heat generation rate during battery discharge at room temperature can be calculated as follows:

[0030] Q2=C p mΔT+h comb A(TT a ) (4)

[0031] In formula (4): Q2 represents the total heat generation rate of the battery at room temperature.

[0032] 2. When the lithium-ion battery is discharged at 1C rate at room temperature and cooled to ambient temperature, the data curve of the average temperature of the battery surface is obtained. The curve is exponentially fitted and the fitting curve equation is: θ = 23.50678 + 27.68657 × exp (-2.76667 × 10 －4 t) where: θ represents temperature, °C; t represents time, s. Combining (3), we have:

[0033]

[0034] Get the reduced heat transfer coefficient h comb 5.498W·m －2 ·K －1 , which is used to further calculate the total heat generation rate Q2 of the battery at room temperature.

[0035] 3. Based on the total heat generation rate under adiabatic and room temperature conditions obtained above, combined with the internal resistance heat generation rate obtained by experimental measurement of internal resistance, the entropy thermal coefficient dE / dT under the two conditions can be calculated respectively according to the heat generation rate model formula. Figure 4 The curve of battery dE / dT changing with DOD shows that the changing trend of dE / dT with DOD calculated under the two conditions is basically the same. It can be seen that this method can be used to calculate the entropy thermal coefficient of the battery.

[0036] The present invention proposes a method for calculating the entropy thermal coefficient of lithium-ion batteries based on a heat generation rate model. By accurately simulating the heat generation rate of lithium-ion batteries during operation and combining the principles of thermodynamics, the entropy thermal coefficient of lithium-ion batteries can be accurately calculated. This method not only avoids the complexity and limitations of traditional direct measurement methods, but also improves calculation efficiency and accuracy, providing strong support for thermal management, performance optimization and safety assessment of lithium-ion batteries. Experimental verification shows that the calculation results of this method are highly consistent with the measured data, with high reliability and practicality, and is expected to be widely used in the research and development and production of lithium-ion batteries.

[0037] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for calculating the entropy thermal coefficient of a lithium-ion battery based on a heat generation rate model, characterized in that: The specific steps include: Step 1: According to the heat generation principle of lithium-ion batteries, based on the Bernardi heat generation rate model formula: In the formula, Q represents the total heat generation rate, I represents the charge and discharge current of the single battery, E represents the terminal voltage, E0 represents the open circuit voltage, T represents the surface temperature of the battery, and dE0 represents the entropy thermal coefficient; Step 2: Calculate the internal resistance heat generation rate. Internal resistance heat generation refers to the irreversible heat generated by the existence of internal resistance when current flows through the internal conductor of the battery. Therefore, the heat generation rate formula can be expressed by the product of internal resistance and current intensity: In the formula, Q R represents the internal resistance heat generation rate, R represents the total internal resistance of discharge; Step 3: Calculate the total heat generation rate Q1 of the lithium battery under adiabatic conditions and the total heat generation rate Q2 under room temperature conditions through a lithium battery temperature rise test.

2. The method for calculating the entropy thermal coefficient of a lithium-ion battery based on a heat generation rate model according to claim 1, characterized in that: The heat generation rate model formula described in step 1 is based on battery temperature rise test data.

3. The method for calculating the entropy thermal coefficient of a lithium-ion battery based on a heat generation rate model according to claim 1, characterized in that: The temperature rise test in step 3 requires the temperature rise of the lithium battery during its discharge process, and the test of its discharge temperature rise is carried out under adiabatic conditions and room temperature conditions respectively.

4. The method for calculating the entropy thermal coefficient of a lithium-ion battery based on a heat generation rate model according to claim 1, characterized in that: The total heat generation rate calculation formula during battery discharge under the adiabatic condition described in step 3 is as follows: Q1=C p mΔT Where Q1 represents the total heat generation rate of the battery under adiabatic conditions, m represents the mass of the single battery, C p It represents the specific heat capacity of a single cell, and ΔT represents the change in battery surface temperature.

5. The method for calculating the entropy thermal coefficient of a lithium-ion battery based on a heat generation rate model according to claim 1, characterized in that: The total heat generation rate calculation formula during battery discharge under room temperature conditions described in step 3 takes into account the convection heat transfer between the battery surface and the surrounding environment and the electromagnetic wave radiation heat transfer, and converts the convection heat transfer and radiation heat transfer into a reduced heat transfer coefficient h comb To simplify, the total heat generation rate during battery discharge at room temperature is calculated as follows: Q2=C p mΔT+h comb A(T-T a ) Where Q2 represents the total heat generation rate of the battery at room temperature, h comb represents the reduced heat transfer coefficient considering convection and radiation, A represents the battery surface area, T a Indicates the ambient temperature of the battery.

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