CN109188285B - A method for estimating internal and external thermal resistance of lithium batteries - Google Patents

Abstract

The invention discloses a method for estimating internal and external thermal resistances of a lithium battery, which comprises the steps of setting different environmental temperature gradients, and discharging the lithium battery at constant current with different discharge rates under each temperature gradient; respectively recording the discharge current, the internal temperature and the surface temperature of the battery when the lithium battery is in a thermal equilibrium state at each group of temperature; the accurate measurement of the internal and external thermal resistances is achieved by establishing a three-dimensional relation curve chart of the discharge current, the ambient temperature and the internal temperature, namely a thermal equilibrium surface, and combining the internal and external temperature difference and the surface and ambient temperature difference. The invention fully analyzes the main factors influencing the internal and external thermal resistance of the lithium battery and designs the experiment by a variable control method; by constructing the three-dimensional heat balance surface, the change conditions of the internal and external heat resistance along with various factors are easy to be determined.

Description

A method for estimating internal and external thermal resistance of lithium batteries

technical field

The invention relates to the technical field of power batteries, in particular to a method for estimating internal and external thermal resistance of a lithium battery.

Background technique

The power lithium battery lumped parameter thermal model is the main model used to describe the internal heat generation, heat conduction and heat dissipation process of the lithium battery. The internal thermal resistance in the thermal model parameters is used to represent the internal thermal conduction rate of the lithium battery, and the external thermal resistance represents the lithium battery Ambient heat exchange rate. The existing experimental methods for measuring the internal and external thermal resistance of lithium batteries are not comprehensive and representative, so they cannot accurately measure the internal and external thermal resistance of lithium batteries, and also directly affect the accuracy of real-time estimation of the internal temperature of the battery. Due to the special physical structure and chemical composition of lithium batteries, the actual working environment and working conditions of the battery have a great influence on the internal and external thermal resistance during the working process of the battery. Therefore, in order to improve the accuracy of the measurement of the internal and external thermal resistance of lithium batteries, it is extremely important to analyze the main factors that may affect the internal and external thermal resistance of lithium batteries, and to accurately estimate the internal and external thermal resistance of lithium batteries and the internal temperature of the battery. Battery performance, extending battery life and responding to the call for energy conservation and environmental protection are of great significance.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for estimating the internal and external thermal resistance of a lithium battery in order to remedy the defects of the prior art.

The present invention is achieved through the following technical solutions:

A method for estimating internal and external thermal resistance of a lithium battery, comprising the following steps:

(1) Set different ambient temperature gradients, and under each temperature gradient, the lithium battery uses different discharge rates for constant current discharge;

(2) Record the discharge current, the internal temperature of the battery and the surface temperature of the battery when the lithium battery is in thermal equilibrium at each temperature;

(3) Accurate determination of internal and external thermal resistance is achieved by establishing a three-dimensional relationship surface map of discharge current, ambient temperature and internal temperature, that is, thermal equilibrium surface, combined with internal and external temperature difference and surface and ambient temperature difference.

The specific method for establishing a three-dimensional relational surface graph in step (3) is as follows: a simplified thermoelectric parameter model of a power lithium battery, including an internal heat source Q, an internal heat capacity C _c of the battery, an external hot melt C _s , and an internal temperature of the battery T _c , The battery surface temperature T _s , the external ambient temperature Ta , the internal thermal resistance R _i represent the internal heat conduction rate of the lithium battery, the external thermal resistance _Ro represents the heat exchange rate between the battery surface and the environment, and _t represents the time constant; analogous to the first-order RC filter network, get formula (1);

Formula (1) is the functional relationship expression of internal and external thermal resistance, discharge rate and ambient temperature; combined with the internal resistance temperature model of lithium battery, and from formula (1), it is obtained through the calculation method of experimental data that the internal thermal resistance of lithium battery belongs to a fixed value , while the external thermal resistance has nothing to do with the discharge rate, and has a relationship with the ambient temperature, using a quadratic curve to fit R _o =pT _a ² +qT _a +h, where p=0.0002275, q=-0.08045, h=3.406.

The invention proposes a method of estimating internal and external thermal resistance through a thermal balance surface, establishes a lithium battery thermal balance surface through an experimental design method, and more comprehensively depicts the change of internal and external thermal resistance, so as to accurately measure the internal and external thermal resistance of a lithium battery. The reason for using this method is that, firstly, when the lithium battery is discharged with a certain discharge rate and constant current, there are thermal equilibrium and non-equilibrium states, and the difference between the two is obvious and cannot be ignored; secondly, other unknown factors are avoided by the method of controlling variables in the experimental design. effect, making the results more representative.

The advantages of the present invention are: the present invention fully analyzes the main factors affecting the internal and external thermal resistance of lithium batteries, and designs experiments by controlling variables; by constructing a three-dimensional thermal balance surface, it is easy to clarify the changes of internal and external thermal resistance with various factors.

Description of drawings

Figure 1 is a simplified thermoelectric parameter model diagram of a lithium battery.

Figure 2 is a diagram of the thermal equilibrium constructed from the discharge current, ambient temperature and internal temperature.

Figure 3 is _a graph showing the relationship between the external thermal resistance _Ro and the ambient temperature Ta.

Figure 4 is a flow chart of the calculation.

Detailed ways

A method for estimating internal and external thermal resistance of a lithium battery, comprising the following steps:

(1) Set different ambient temperature gradients, and under each temperature gradient, the lithium battery uses different discharge rates for constant current discharge;

(2) Record the discharge current, the internal temperature of the battery and the surface temperature of the battery when the lithium battery is in thermal equilibrium at each temperature;

(3) Accurate determination of internal and external thermal resistance is achieved by establishing a three-dimensional relationship surface map of discharge current, ambient temperature and internal temperature, that is, thermal equilibrium surface, combined with internal and external temperature difference and surface and ambient temperature difference.

The specific method for establishing a three-dimensional relationship surface map in step (3) is as follows: as shown in Figure 1, the simplified thermoelectric parameter model of the power lithium battery includes the internal heat source Q, the internal heat capacity C _c of the battery, the external hot melt C _s , The battery surface temperature T _s , the battery internal temperature T _c , the external ambient temperature Ta , the internal thermal resistance R _i represents the internal thermal conduction rate of the lithium battery, the external thermal resistance _Ro represents the heat exchange rate between the battery surface and the environment, and _t represents the time constant; analogy The first-order RC filter network is obtained in formula (1);

Formula (1) is the functional relationship expression between internal and external thermal resistance, discharge rate and ambient temperature, as shown in Figure 2. Combined with the internal resistance temperature model of lithium battery, and from formula (1), it is obtained through experimental data verification method that lithium battery The internal thermal resistance is a fixed value, while the external thermal resistance has nothing to do with the discharge rate, and has a relationship with the ambient temperature, as shown in Figure 3, using a quadratic curve to fit R _o =pT _a ² +qT _a +h, where p = 0.0002275, q = -0.08045, h = 3.406. In FIG. 4 , T _cs and T _sa represent the temperature difference between the inside and the surface of the battery, and the temperature difference between the surface and the environment, respectively.

Claims (1)

1. A method for estimating internal and external thermal resistances of a lithium battery is characterized by comprising the following steps: the method comprises the following steps:

(1) setting different environmental temperature gradients, wherein the lithium battery adopts different discharge rates for constant current discharge under each temperature gradient;

(2) respectively recording the discharge current, the internal temperature and the surface temperature of the battery when the lithium battery is in a thermal equilibrium state at each group of temperature;

(3) measuring internal and external thermal resistances by establishing a three-dimensional relation curve chart of the discharge current, the ambient temperature and the internal temperature, namely a thermal balance plane, and combining the internal and external temperature difference and the surface and ambient temperature difference;

the method for measuring the internal and external thermal resistances in the step (3) comprises the following steps: simplified thermoelectric parametric model of power lithium battery including internal heat sourceQInternal heat capacity of batteryC _c External hot meltingC _s Internal temperature of batteryT _c Electricity, electricityTemperature of the surface of the poolT _s External ambient temperatureT _a Internal thermal resistanceR _iCharacterizing the internal heat conduction rate and the external thermal resistance of the lithium batteryR _o Characterize the rate of heat exchange between the cell surface and the environment,trepresents a time constant; analogy to first orderRCA filter network, obtaining a formula (1);

(1)

the formula (1) is a functional relation expression of internal and external thermal resistance, discharge multiplying power and environmental temperature; combining a lithium battery internal resistance temperature model, and obtaining the model by an experimental data checking method according to the formula (1), wherein the internal thermal resistance of the lithium battery belongs to a fixed value, the external thermal resistance is irrelevant to the discharge multiplying power and has a relation with the environment temperature, and fitting a secondary curve

Wherein, in the step (A),p=0.0002275,q=-0.08045,h=3.406， p、q、hrespectively represent coefficients of fitted quadratic curve terms.

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