CN114577847A - Lithium battery charging and discharging heat production measuring method based on isothermal differential calorimetry - Google Patents
Lithium battery charging and discharging heat production measuring method based on isothermal differential calorimetry Download PDFInfo
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Abstract
The invention discloses a lithium battery charging and discharging heat production measuring method based on isothermal differential calorimetry. The constant temperature boundary is provided by an external constant temperature oil bath, two consistent battery samples are symmetrically arranged in a constant temperature thermal cavity, one of the two consistent battery samples is used as an experimental sample for charging and discharging, and the other battery sample is used as a reference sample for not charging and discharging. And measuring a temperature difference signal between the experimental sample and the reference sample by using a temperature sensor, and combining an equivalent heat transfer model to obtain a heat flow signal generated in the charging and discharging process of the battery sample. The model parameters are obtained by the step response of heat generated by the battery surface heating sheet, the system time constant is obtained by the dynamic process, and the equivalent thermal resistance of the system is obtained by the integral result. Compared with a power compensation method, the isothermal differential calorimetry provided by the invention has the advantages that the baseline noise is reduced from 10mW to 0.2mW, the signal-to-noise ratio of heat production measurement of the battery is greatly improved, and the influence of external environment temperature fluctuation is reduced.
Description
Technical Field
The invention relates to the field of battery thermal characteristic detection, in particular to a lithium battery charging and discharging heat production measuring method based on isothermal differential calorimetry.
Background
With the wide application of lithium ion batteries in production and life and the continuous development of manufacturing technology of lithium ion batteries, a large variety of lithium ion batteries with excellent performance are developed, produced and used. The performance and safety problems of the lithium ion battery are closely related to the thermal characteristics of the lithium ion battery, and the research on the heat generation characteristics of the lithium ion battery has a guiding function on improving the performance and the use safety of the battery[1-4]。
At present, the experimental methods applied to the research of heat generation of the lithium ion battery mainly include adiabatic accelerated calorimetry and isothermal calorimetry[5-6]. The measurement principle of the adiabatic accelerated calorimetry is that the temperature of a calorimetric cavity and the temperature of a measured object are kept to be dynamically equal, so that the measured object is in an adiabatic state, when the measured object generates heat, the heat is only used for heating the measured object, so that adiabatic temperature rise is obtained, and finally the heat generation power of the measured object is obtained through calculation of the adiabatic temperature rise rate. According to the measurement principle of isothermal calorimetry, the temperature of a calorimetric cavity and a measured object is kept constant, so that the measured object is in an isothermal state, when the measured object generates heat, the heating power is adjusted in a real-time feedback mode, so that the temperature of the measured object is kept constant, the heat generation power of the measured object is obtained, and the heat absorption and release quantity of the measured object under the temperature condition is measured.
The adiabatic acceleration calorimeter based on the adiabatic acceleration calorimetry has rich functions, can perform thermal runaway test on the lithium ion battery, and is widely used in the field of safety test of the lithium ion battery[7-9]. However, there are some disadvantages of the adiabatic acceleration calorimeter in the measurement of heat generated during the charge and discharge of the battery. First, only the discharge power of the lithium ion battery can be measured, and the heat absorption power cannot be accurately measured. Second, the heat generating power of the battery at a certain fixed temperature point cannot be accurately obtained. Third, thermal hysteresis effects make it difficult for the instrument to truly achieve adiabatic conditions, thereby introducing measurement errors. In contrast, isothermal calorimetry has the following advantages in the measurement process of heat generation of the lithium ion battery during charging and discharging. Firstly, the heat absorption and release functions in the charging and discharging process of the battery can be realizedAnd (4) measuring the rate. Secondly, the charging and discharging heat generation power of the battery at a certain constant temperature can be accurately obtained[10]. And thirdly, the measurement accuracy of the heat generation power of the battery during charging and discharging is higher.
The lithium battery isothermal calorimeter can be used as an instrument capable of accurately measuring the charging and discharging heat generation characteristics of the battery at different temperatures, and can provide accurate basic data for optimizing a lithium battery heat management system. The working principle of the isothermal calorimeter based on the power compensation method is as follows: the constant temperature boundary is maintained by the circulation of an external refrigerant, the temperature of the battery is controlled to be constant by the flexible heating sheet, and the electric power of the heating sheet is recorded in real time. When the heat generating power of the battery changes, the electric power of the heating plate is controlled to change so as to maintain constant heat dissipation power. By analyzing the output power change of the heating sheet and the data provided by the charging and discharging system in the process, parameters such as battery heat absorption and discharge power, total heat absorption and discharge amount and the like under different temperatures and use conditions can be obtained. It can be known from the working principle that the power of the heating plate is fed back and controlled in real time to control the temperature of the battery, so that the baseline power of the isothermal calorimeter is easily interfered by the fluctuation of the ambient temperature, and the baseline noise is too large. The base line noise of the common battery isothermal calorimeter based on the power compensation method is more than 10 mW. When the capacity of the battery is small or the charge-discharge multiplying power is low, the heat generation power of the battery is small, so that the measurement result error is large due to the interference of noise in the charge-discharge heat generation measurement of the battery by the isothermal calorimeter, and the reference value of experimental data is reduced.
Reference to the literature
[1] Duguang super, Zheng Li, Zhang Zhi super, research on thermal safety of Li-ion battery progresses [ J ]. science and technology of energy storage, 2019,8(03): 500-.
[2] The research dynamics of the hot model of the Lioubank Hui, Ganbaiyan and He pei Run lithium ion battery is J, Power Source declaration, 2019,17(01):95-103.
[3] Research on electrochemical-thermal characteristics of automotive high specific energy lithium ion batteries [ D ]. Jiangsu university, 2018.
[4] Wu Qing Yuan, Zhang Heng Yun, Lijun Wei, calibration calorimetry measures the specific heat capacity and heat generation rate of lithium battery [ J ]. automobile engineering, 2020,42(01):59-65.
[5] Wutangqin, Experimental study on heat generation and thermal induction runaway characteristics of lithium ion batteries [ D ]. university of China science and technology, 2018.
[6] The application of Wanghao, Li Jianjun, Wangli adiabatic accelerated calorimeter in the aspect of safety research of lithium ion battery [ J ]. new material industry, 2013(01):53-58.
[7]J.S.Gnanaraj,E.Zinigrad,L.Asraf,H.E.Gottlieb,M.Sprecher,D.Aurbach,M.Schmidt.The use of accelerating rate calorimetry(ARC)for the study of the thermal reactions of Li-ion battery electrolyte solutions[J].Journal of Power Source,318:200-219,2016.
[8]W.C.Chen,Y.W.Wang,C.M.Shu.Adiabatic calorimetry test of the reaction kinetics and self-heating model for 18650Li-ion cells in various states of charge[J].Journal of Power Source,318:200-209,2016.
[9]K.Murata,M.Ota.Proposal for evaluation method of bettery safety through thermal analysis[J].Journal of Energy Storage,20；576-580,2018.
[10] Design of an isothermal calorimetry experimental system for vehicle power lithium batteries [ J ] experimental technology and management, 37(3) 103 and 106 (2020).
Disclosure of Invention
The invention provides a lithium battery charging and discharging heat production measuring method based on isothermal differential calorimetry, aiming at the defects of the existing isothermal calorimetry in the lithium battery charging and discharging heat production test. The method reduces the influence of internal noise and external interference on the measurement of the heat generation of the battery, thereby improving the signal to noise ratio and providing a reliable data source for the establishment of a thermal model of the lithium ion battery and the development of a thermal management system of the battery.
In order to achieve the above purpose, the technical problem of the present invention can be achieved by the following technical solutions:
a lithium battery charging and discharging heat production measurement method based on isothermal differential calorimetry comprises the following steps:
step 1: and selecting a battery with the same model and size as a reference battery according to the model of the battery to be tested. And selecting a flexible heater, a uniform heating block and a heat-conducting silica gel pad with corresponding sizes according to the geometric size of the battery to be tested.
Step 2: and determining the isothermal calorimetric temperature and the charge-discharge parameters of the battery to be tested.
And 3, step 3: the battery to be tested is arranged in a heat sink of the isothermal heat cavity from top to bottom according to the structures of the heat-conducting silica gel pad, the heat-homogenizing block, the flexible heater, the battery to be tested, the flexible heater, the heat-homogenizing block and the heat-conducting silica gel pad; the reference battery is arranged in the heat sink of the isothermal calorimetry cavity from top to bottom according to the structures of the heat conduction silica gel pad, the heat equalizing block, the flexible heater, the reference battery, the flexible heater, the heat equalizing block and the heat conduction silica gel pad.
The temperature sensor is arranged in a groove of the upper and lower heat equalizing blocks close to the side of the battery, the upper and lower flexible heaters of the battery to be tested are connected in parallel and are connected with constant-current and constant-voltage source equipment outside the isothermal heat cavity, and the isothermal heat cavity is sealed after the temperature sensor is installed.
And 4, step 4: and starting the battery isothermal calorimeter, setting the oil bath temperature and setting the oil bath temperature as an external circulation temperature control mode, and controlling the heat sink temperature at a constant temperature.
And 5: after the temperature of the isothermal heat cavity is stable, the constant-current constant-voltage equipment outside the isothermal heat cavity outputs a step signal with constant power for a certain time to the flexible heater.
Step 6: after the step signal output of the constant power is finished, the flexible heater and the uniform heat block are operated for a period of time, and the temperature is stable. And recording the real-time temperature measured by the upper and lower side temperature sensors of the reference battery and the real-time temperature measured by the upper and lower side temperature sensors of the battery to be measured in the process, so as to obtain system parameters including a system equivalent heat capacity C and a system equivalent heat resistance R.
And 7: and after system parameters are obtained, connecting the battery to be tested with the isothermal heat cavity external charging and discharging equipment. After the installation is finished, the isothermal calorimetric cavity is sealed, the battery isothermal calorimeter is started, the oil bath temperature is set to be in an external circulation temperature control mode, and the heat sink temperature is controlled to be constant.
And 8: and after the temperature in the isothermal calorimetric cavity is stable, charging and discharging the battery to be tested according to a required mode. And after the charging and discharging are finished, waiting for the temperature of the reference battery side and the temperature of the battery side to be measured to be stable. And recording the real-time temperatures measured by the upper and lower side temperature sensors of the reference battery and the real-time temperatures measured by the upper and lower side temperature sensors of the battery to be measured in the process, and obtaining the real-time heat flow change of the battery to be measured easily.
And step 9: and integrating the real-time heat flow of the battery to be tested with time to obtain the heat production quantity of the battery to be tested in the charging and discharging processes.
To sum up, in order to improve the signal-to-noise ratio of the measurement data of the charge and discharge heat production of the isothermal calorimetry instrument of the battery, the invention provides the measurement method of the charge and discharge heat production of the lithium battery based on the isothermal calorimetry instrument, which makes up the defects of the existing isothermal calorimetry instrument, improves the sensitivity of the measurement of the thermal characteristics of the battery, and greatly enriches the application of the isothermal calorimetry technology in the research field of batteries.
Drawings
FIG. 1 is a schematic diagram of measurement of heat generation during charging and discharging of a lithium battery according to the present invention;
FIG. 2 is a schematic view of the system parameter measurement installation of the present invention;
FIG. 3 is an equivalent heat transfer model of the side of the battery to be tested;
FIG. 4 is a reference cell side equivalent heat transfer model;
FIG. 5 is system time constant acquisition;
FIG. 6 is a step response curve of the system of the present invention;
FIG. 7 is a graph of thermal current versus time for a 550mAh lithium battery of the present invention charged at 0.5C;
FIG. 8 is a graph of thermal current versus time for a 550mAh lithium battery of the present invention discharging at 0.5C;
FIG. 9 is a graph showing the baseline power noise of a 550mAh lithium battery at 0.5C charge and discharge measured by the method of the present invention;
illustration of the drawings: fig. 1, 1.1 battery under test; 1.2 reference cell; 1.3 a flexible heater; 1.4 temperature sensor; 1.5 heat homogenizing block; 1.6 heat-conducting silica gel pad; 1.7 heat sink; 1.8 charging and discharging wires; 1.9 oil bath pipeline; 1.10 calorimetric chambers; 1.11 high air tightness aviation plug; 1.12 an intake valve; 1.13 air outlet valve; 1.14 gas flow meter; 1.15 pressure relief valve.
Detailed Description
In order to make the steps, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be described more clearly, in detail and completely in the following with reference to the accompanying drawings in the embodiments of the present invention.
As shown in fig. 1, the main technical idea of the present invention is as follows:
according to the geometric dimensions of the selected to-be-tested battery 1.1 and the reference battery 1.2, the flexible heater 1.3, the uniform heating block 1.5 and the heat-conducting silica gel pad 1.6 with corresponding dimensions are selected. According to the model of the battery to be tested and the charging and discharging parameters, a charging and discharging lead 1.8 with a proper wire diameter is selected and is connected to the outside through a high-air-tightness aviation plug 1.11. When the charging and discharging heat production of the battery is measured, the side of the battery to be measured is sequentially arranged between an upper heat sink 1.7 and a lower heat sink 1.7 in a calorimetric cavity 1.10 from top to bottom according to the modes of a heat conducting silica gel pad, a heat equalizing block, a flexible heater, the battery to be measured, the flexible heater, the heat equalizing block and the heat conducting silica gel pad. The reference battery side is installed from top to bottom according to the structures of the heat-conducting silica gel pad, the uniform heating block, the flexible heater, the battery to be tested, the flexible heater, the uniform heating block and the heat-conducting silica gel pad. In order to reduce the measurement error, the side mounting structure of the battery to be measured should be symmetrical to the side mounting structure of the reference battery. The positive and negative electrodes of the battery to be tested are connected with external equipment through the wires and the electrical connectors on the calorimetric cavity. The temperature sensor 1.4 is mounted in a groove on the battery side of the upper and lower thermoblocks, and is well fitted in the groove. And closing the calorimetric cavity after the battery is installed.
In order to maintain the isothermal state, the heat sink with the refrigerant is connected with an external oil bath through an oil bath pipeline 1.9. In the experimental process, the oil bath starts external circulation to circularly pump the silicone oil with a certain temperature into the heat sink, so that the heat sink is controlled at a fixed temperature point. And if the isothermal calorimetric temperature required by the battery to be tested is lower than the room temperature, dry gas replacement needs to be carried out on the calorimetric cavity. During replacement, the heat metering cavity is closed, external dry gas is connected to the air inlet valve 1.12 on the heat metering cavity through a pipeline, and then the air inlet valve and the air outlet valve 1.13 on the wall of the heat metering cavity are opened in sequence. The dry gas replacement time can be adjusted according to the gas flow rate indicated by the gas flow meter 1.14 on the wall of the calorimeter chamber. And closing the inlet and outlet air valves after replacement is finished. In the gas replacement process, when the air pressure in the cavity is overlarge, the pressure release valve 1.15 arranged on the calorimetric cavity is opened.
And starting the isothermal calorimeter after checking that the circuit is connected without errors, setting the temperature of the oil bath, setting the oil bath to be in an external circulation temperature control mode, and controlling the temperature of the heat sink to be at a constant temperature point required by the experiment. And (3) acquiring and recording the real-time temperatures of the upper surface and the lower surface of the battery to be measured and the reference battery in the charging and discharging processes by using upper computer software, thereby calculating the real-time heat flow and the heat generation quantity of the battery to be measured in the charging and discharging processes.
The real-time heat flow generated by the battery to be tested in the charging and discharging process is calculated according to the formula (1), and the heat generation quantity of the battery to be tested is obtained according to the formula (2).
Phi is real-time heat flow generated by the battery to be tested in the charging and discharging processes; c and R are equivalent heat capacity and equivalent thermal resistance of the measurement system, and the equivalent heat capacity C and the equivalent thermal resistance R of the system are obtained through experimental calculation; the delta T is the sum of the temperature difference between the upper surface and the lower surface of the reference battery and the battery to be measured; q is t3To t4Generating heat of the battery to be detected within a time period; t is t3The charge and discharge starting time point of the battery to be tested is set; t is t4And the time point of system recovery and stabilization after the charge and discharge of the battery to be tested are the.
The steps of this example are as follows:
step 1: and selecting a battery with the same model and size as a reference battery according to the model of the battery to be tested. And selecting a flexible heater, a uniform heating block and a heat conduction silica gel pad with corresponding sizes according to the geometric size of the battery to be tested.
Step 2: and determining the isothermal calorimetric temperature and the charge and discharge parameters of the battery to be tested, and selecting a charge and discharge conductor with a proper wire diameter according to the charge and discharge parameters.
And step 3: and installing a system parameter measuring structure. The side of the battery to be tested is arranged in the isothermal heat cavity heat sink from top to bottom according to the structures of the heat-conducting silica gel pad, the heat-homogenizing block, the flexible heater, the battery to be tested, the flexible heater, the heat-homogenizing block and the heat-conducting silica gel pad; the reference battery is arranged in the heat sink of the isothermal calorimetry cavity from top to bottom according to the structures of the heat conduction silica gel pad, the heat equalizing block, the flexible heater, the reference battery, the flexible heater, the heat equalizing block and the heat conduction silica gel pad. The temperature sensor is installed in the groove of the upper and lower uniform heating blocks close to the battery side and well embedded with the groove. The reference battery side and the battery side to be measured are installed in the same symmetrical structure, so that the measurement error is reduced. The upper and lower flexible heaters of the battery to be tested are connected in parallel and are connected with constant-current and constant-voltage source equipment outside the isothermal thermal cavity through a lead. The mounting structure is shown in fig. 2, and the calorimetric cavity is sealed after the mounting is finished.
And 4, step 4: if the isothermal calorimetric temperature required by the battery to be tested is lower than the room temperature, dry gas replacement needs to be carried out on the calorimetric cavity, and the short circuit of the anode and the cathode of the battery caused by cold water generated in the calorimetric cavity is prevented. And after the calorimetric cavity is sealed, connecting an external dry gas source to the gas inlet valve, sequentially opening the gas inlet valve and the gas outlet valve on the calorimetric cavity, and adjusting the gas replacement time according to the gas flow rate displayed by the gas flow meter on the wall of the calorimetric cavity. In the gas replacement process, if the air pressure in the cavity is too high, the pressure release valve is opened. And after the replacement is finished, the inlet and outlet air valves and the pressure release valve are closed.
And 5: and starting the battery isothermal calorimeter, setting the oil bath temperature and setting the oil bath temperature as an external circulation temperature control mode, and controlling the heat sink temperature at a constant temperature.
Step 6: after the temperature of the calorimetric cavity is stabilized, the constant-current constant-voltage equipment outside the calorimetric cavity outputs a step signal with constant power for a certain time to the flexible heater through the lead in the calorimetric cavity.
And 7: after the step signal output of the constant power is finished, the flexible heater and the uniform heat block are operated for a period of time, and the temperature is stable. And the upper computer records the real-time temperature measured by the upper and lower side temperature sensors of the reference battery and the real-time temperature measured by the upper and lower side temperature sensors of the battery to be measured in the process. And substituting the data into a formula to obtain system parameters, namely the equivalent heat capacity C of the system and the equivalent heat resistance R of the system.
And 8: and after system parameters are obtained, the battery to be tested is connected with the charging and discharging equipment outside the isothermal heat cavity through the charging and discharging conducting wires. After the installation is finished, the calorimetric cavity is sealed, the battery isothermal calorimeter is started, the oil bath temperature is set to be in an external circulation temperature control mode, and the heat sink temperature is controlled to be constant.
And step 9: and after the temperature in the calorimetric cavity is stable, charging and discharging the battery to be tested according to the mode required by the experiment. And after the charging and discharging are finished, waiting for the temperature of the reference battery side and the temperature of the battery side to be measured to be stable. And the upper computer records the real-time temperature measured by the upper and lower side temperature sensors of the reference battery and the real-time temperature measured by the upper and lower side temperature sensors of the battery to be measured in the process, and substitutes the real-time temperature into a formula to obtain the real-time heat flow change of the battery to be measured.
Step 10: and integrating the real-time heat flow of the battery to be tested with time to obtain the heat production quantity of the battery to be tested in the charging and discharging processes.
The structures of the side of the battery to be tested and the side of the reference battery are analyzed, and a mathematical model of the side of the battery to be tested and the side of the reference battery is deduced in detail through an equivalent circuit diagram.
According to the structure of the isothermal differential calorimetry, an analysis object is divided into four parts, namely a heat sink, a heat conduction silica gel pad, a heat homogenizing block and a battery. Fig. 3 and 4 show equivalent circuit diagrams of isothermal calorimetry structures. T is0The boundary temperature provided for the heat sink is in units of K; the equivalent thermal resistance of the heat-conducting silica gel pad and the heat-homogenizing block between the temperature measuring point and the heat sink is R in the unit of K/W, the equivalent thermal resistance is C in the unit of J/K; t is a unit of1And T2The temperature measured by the temperature sensors on the upper surface and the lower surface of the battery to be measured is K; t'1And T'2The temperature measured by the temperature sensors on the upper and lower surfaces of the reference cell is given in K. Φ is the heat flow generated during charging and discharging of the battery, and is expressed in units of W.
The battery to be tested can generate heat flow in the charging and discharging processes, the heat flow on the upper side of the battery to be tested is equal to the sum of the heat flow flowing through the heat capacity on the upper side and the heat flow of the heat resistance on the upper side, and the formula (1) shows that:
because the upper part and the lower part are of symmetrical structures, the heat capacity of the upper side and the lower side is considered to be equal to the heat resistance, and the heat flow formula of the lower side of the battery to be tested can be written as follows:
because the reference battery is not connected with an external battery charge and discharge tester, no internal heat source exists, and the heat flow generated by the battery is 0. The structure of the reference battery side is the same as that of the battery to be tested, and the heat capacities and the heat resistances of the reference battery side and the battery to be tested are considered to be equal, so that the heat flow formula of the reference battery side is as shown in formula (3):
the heat flow equation at the underside of the reference cell can be written as:
equation (1) is subtracted from equation (3) to obtain the upper heat flux difference:
equation (2) minus equation (4) yields the lower heat flow difference:
the heat flow generated by the battery to be tested in the charging and discharging process is the sum of the heat flows of the upper part and the lower part of the battery, and the total real-time heat flow generated by the battery in the charging and discharging process can be obtained by adding the formula (5) and the formula (6):
ΔT＝ΔT1+ΔT2 (8)
therefore, when the battery to be tested is connected with the charging and discharging equipment for charging and discharging, the real-time heat flow generated by the battery to be tested is as shown in the formula (9):
integrating the real-time heat flow of the battery to be tested with time to obtain the heat production quantity Q of the battery in the charging and discharging process, wherein the formula (10) is as follows:
q is t3To t4Generating heat of the battery to be detected within a time period; t is t3The charge and discharge starting time point of the battery to be tested is set; t is t4And the time point of system recovery and stabilization after the charge and discharge of the battery to be tested are the.
In order to calculate the real-time heat flow and heat production amount measurement generated in the battery charging and discharging process, the equivalent heat capacity C and the equivalent heat resistance R of the system need to be obtained. In the process, an upper flexible heater and a lower flexible heater on the side of the battery to be tested are connected in parallel and are connected with external constant voltage and constant current equipment through leads. The constant-current constant-voltage equipment outside the calorimetric cavity starts to output a step signal with certain time and power. The real-time heat flow at this time is:
wherein phirThe heat flow is provided after the constant current and constant voltage equipment is connected with the flexible heater; delta T0Is the sum of the temperature difference between the upper side and the lower side of the reference battery side and the battery side to be measured. Integrating the two sides of the formula (11), and when the equivalent thermal cavity temperature is stable, the temperature difference between the upper part and the lower part of the side of the battery to be measured and the side of the reference batteryThe equivalent thermal resistance R of the system is as follows:
wherein t is1To t2In order to stabilize the temperature of the calorimetric cavity, the temperature difference between the upper part and the lower part of the side of the battery to be tested and the side of the reference battery is constant for a period of time.
From the step response curve, the time constant t of the system can be found, as shown in fig. 5. The product of the equivalent heat capacity and the equivalent thermal resistance is the instrument time constant, and the system equivalent heat capacity can be obtained by the formula (13) after the instrument time constant and the system equivalent thermal resistance are known.
Using the above method, a lithium ion battery charge and discharge experiment was performed with experimental parameters as shown in table 1.
TABLE 1 lithium cell Charge and discharge Experimental parameters
FIG. 6 is a graph of step response of the system of the present invention, in which the step signal is a curve (i), the temperature difference is a curve (ii), the heat sink temperature is 20 deg.C, the output power of the constant current and constant voltage device is 0.097W, and the time is a step signal of 3600S. When the temperature of the calorimetric cavity is stable, the difference between the upper and lower side temperatures of the reference battery and the upper and lower side temperatures of the battery to be measured is 0.562 ℃, and the equivalent thermal resistance R of the system obtained by the formula (12) is 5.794K/W. From the system step response curve, the system time constant t is 183S. The equivalent heat capacity C of the system is calculated according to equation (13) to be 31.5844J/K. And (3) carrying the system parameters into a formula (9) for calculating the real-time heat flow in the battery charging and discharging process. Fig. 7 is a graph of thermal current versus time during charging of a 550mAh lithium battery at 0.5C, and fig. 8 is a graph of thermal current versus time during discharging of a 550mAh lithium battery at 0.5C. Table 2 shows the heat generation measured using isothermal calorimetry based on the charging and discharging of lithium batteries.
TABLE 2 Heat production quantity of lithium battery during charging and discharging
FIG. 9 shows the baseline power noise of a 550mAh lithium battery at 0.5C charge/discharge, measured by the method of the present invention, where the baseline noise is 0.2 mW. The experimental result shows that compared with a power compensation method, the isothermal differential calorimetry provided by the invention has the advantages that the base line noise is reduced from 10mW to 0.2mW, and the signal-to-noise ratio of the heat production measurement of the battery is greatly improved.
Compared with the existing lithium battery charging and discharging calorimetric method, the method has the following advantages: the invention is based on the isothermal principle, and can measure and determine the charge-discharge thermal characteristics of the battery at the temperature. The invention integrates the differential calorimetric principle, reduces the influence of the noise interference of the isothermal calorimeter on the measurement result and improves the signal-to-noise ratio.
In conclusion, the method for measuring the charge and discharge heat production of the lithium battery based on the isothermal calorimetry provided by the invention makes up the defects of the existing isothermal calorimetry, improves the signal-to-noise ratio of the charge and discharge heat production data measured by the isothermal calorimetry, improves the sensitivity of the thermal characteristic measurement of the battery, and greatly enriches the application of the isothermal calorimetry technology in the field of battery research.
Claims (6)
1. A lithium battery charging and discharging heat production measuring method based on isothermal differential calorimetry is characterized by comprising the following steps:
step 1: selecting a battery with the same type and size as a reference battery according to the type of the battery to be tested; selecting a flexible heater, a uniform heating block and a heat-conducting silica gel pad with corresponding sizes according to the geometric size of the battery to be tested;
step 2: determining the isothermal calorimetric temperature and the charge-discharge parameters of the battery to be tested;
and step 3: the battery to be tested is arranged in a heat sink of the isothermal heat cavity from top to bottom according to the structures of the heat-conducting silica gel pad, the heat-homogenizing block, the flexible heater, the battery to be tested, the flexible heater, the heat-homogenizing block and the heat-conducting silica gel pad; the reference battery is arranged in the isothermal heat cavity heat sink from top to bottom according to the structures of the heat-conducting silica gel pad, the heat-homogenizing block, the flexible heater, the reference battery, the flexible heater, the heat-homogenizing block and the heat-conducting silica gel pad;
installing a temperature sensor in a groove of which the upper and lower uniform heating blocks are close to the side of the battery, connecting an upper flexible heater and a lower flexible heater of the battery to be tested in parallel and connecting the two flexible heaters with constant-current and constant-voltage source equipment outside the isothermal thermal cavity, and sealing the isothermal thermal cavity after the installation is finished;
and 4, step 4: starting the battery isothermal calorimeter, setting the oil bath temperature and setting an external circulation temperature control mode, and controlling the heat sink temperature at a constant temperature;
and 5: after the temperature of the isothermal heat cavity is stable, a constant-current constant-voltage device outside the isothermal heat cavity starts to output a step signal with constant power for a certain time to the flexible heater;
step 6: after the step signal with constant power is output, the flexible heater and the uniform heat block are operated for a period of time, and the temperature of the flexible heater and the uniform heat block is stable; recording real-time temperatures measured by the upper and lower side temperature sensors of the reference battery and real-time temperatures measured by the upper and lower side temperature sensors of the battery to be measured in the process, thereby obtaining system parameters including a system equivalent heat capacity C and a system equivalent heat resistance R;
and 7: after system parameters are obtained, connecting a battery to be tested with charging and discharging equipment outside the isothermal calorimetry cavity; after the installation is finished, the isothermal heat cavity is sealed, the battery isothermal heat meter is started, the oil bath temperature is set to be in an external circulation temperature control mode, and the heat sink temperature is controlled to be constant;
and 8: after the temperature in the isothermal calorimetric cavity is stable, charging and discharging the battery to be tested according to a required mode; after charging and discharging are finished, waiting for the temperature of the reference battery side and the temperature of the battery side to be measured to be stable; recording real-time temperatures measured by the upper and lower side temperature sensors of the reference battery and real-time temperatures measured by the upper and lower side temperature sensors of the battery to be measured in the process, so as to obtain real-time heat flow change of the battery to be measured;
and step 9: and integrating the real-time heat flow of the battery to be tested with time to obtain the heat production quantity of the battery to be tested in the charging and discharging processes.
2. The method for measuring the heat generation of the lithium battery based on the isothermal differential calorimetry as claimed in claim 1, wherein: and the reference battery side and the battery side to be tested are symmetrically arranged.
3. The method for measuring the charge and discharge heat generation of the lithium battery based on the isothermal differential calorimetry as claimed in claim 1, wherein: if the isothermal calorimetric temperature required by the battery to be tested is lower than the room temperature, dry gas replacement is carried out on the isothermal calorimetric cavity, and the short circuit of the anode and the cathode of the battery caused by cold water generated in the isothermal calorimetric cavity is prevented; after the isothermal calorimetric cavity is sealed, connecting an external drying gas source to the gas inlet valve, sequentially opening the gas inlet valve and the gas outlet valve on the calorimetric cavity, and adjusting gas replacement time according to the gas flow rate displayed by the gas flowmeter on the isothermal calorimetric cavity wall; in the gas replacement process, if the air pressure in the cavity is overlarge, the pressure release valve is opened; and after the replacement is finished, the inlet and outlet air valves and the pressure release valve are closed.
4. The method for measuring the charge and discharge heat generation of the lithium battery based on the isothermal differential calorimetry as claimed in claim 1, wherein: the system equivalent thermal resistance R in the step 6 is calculated as follows:
wherein R is the system equivalent thermal resistance; delta T0The sum of the temperature difference of the upper side and the lower side of the reference side battery and the battery to be tested is obtained; phirThe heat flow is provided after the constant current and constant voltage equipment is connected with the flexible heater; t is t1To t2The temperature of the isothermal calorimetric cavity is stable, and the temperature difference between the upper part and the lower part of the side of the battery to be tested and the side of the reference battery is constant for a period of time;
the equivalent heat capacity C of the system is calculated as follows:
and t is a system time constant.
5. The method for measuring the heat generation of the lithium battery based on the isothermal differential calorimetry as claimed in claim 1, wherein: the real-time heat flow of the battery to be tested in the step 8 is calculated as follows:
phi is real-time heat flow generated by the battery to be tested in the charging and discharging processes; c is equivalent heat capacity; r is equivalent thermal resistance; and delta T is the sum of the temperature difference between the upper side and the lower side of the reference side battery and the battery to be tested.
6. The method for measuring the charge and discharge heat generation of a lithium battery based on isothermal differential calorimetry as claimed in claim 1, wherein the method comprises: in step 9, the heat generation amount of the battery to be tested in the charging and discharging process is calculated as follows:
wherein Q is t3To t4Generating heat of the battery to be detected within a time period; t is t3The charge and discharge starting time point of the battery to be tested is set; t is t4And the time point of recovering the stability of the battery to be tested after the charging and discharging are finished is the time point of recovering the stability of the battery to be tested.
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