CN111487535B - Parameter obtaining and switching method for liquid metal battery double-equivalent circuit model - Google Patents

Parameter obtaining and switching method for liquid metal battery double-equivalent circuit model Download PDF

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CN111487535B
CN111487535B CN202010359607.4A CN202010359607A CN111487535B CN 111487535 B CN111487535 B CN 111487535B CN 202010359607 A CN202010359607 A CN 202010359607A CN 111487535 B CN111487535 B CN 111487535B
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soc
equivalent circuit
battery
thevenin
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CN111487535A (en
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王康丽
林靖
蒋凯
李浩秒
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Huazhong University of Science and Technology
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/36—Arrangements for testing, measuring or monitoring the electrical condition of accumulators or electric batteries, e.g. capacity or state of charge [SoC]
    • G01R31/367—Software therefor, e.g. for battery testing using modelling or look-up tables
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/36—Arrangements for testing, measuring or monitoring the electrical condition of accumulators or electric batteries, e.g. capacity or state of charge [SoC]
    • G01R31/382—Arrangements for monitoring battery or accumulator variables, e.g. SoC
    • G01R31/3842—Arrangements for monitoring battery or accumulator variables, e.g. SoC combining voltage and current measurements
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/36—Arrangements for testing, measuring or monitoring the electrical condition of accumulators or electric batteries, e.g. capacity or state of charge [SoC]
    • G01R31/385—Arrangements for measuring battery or accumulator variables
    • G01R31/387—Determining ampere-hour charge capacity or SoC
    • G01R31/388—Determining ampere-hour charge capacity or SoC involving voltage measurements

Abstract

The invention discloses a parameter obtaining and switching method of a liquid metal battery double equivalent circuit model, belonging to the field of liquid metal battery application; the parameter acquisition method comprises the following steps: carrying out charge-discharge cycle test and constant current pulse test on the liquid metal battery; obtaining open-circuit voltage by using a standing method; calculating the direct current internal resistance of a Rint model in the charging and discharging directions according to the ohm law; calculating impedance parameters of the Thevenin model; and dividing the parameters of the double equivalent circuit model into four types according to the operating conditions. The problem that a single equivalent circuit model cannot comprehensively represent the multi-time scale dynamic characteristics of the liquid metal battery is solved; the switching method comprises the following steps: and (3) keeping or switching the parameters of the double equivalent circuit model according to the current input into the battery at the time t and the time (t-1), and further calculating the output voltage of the double equivalent circuit model at the time t. The parameter switching method can be used for rapidly switching to the corresponding model parameter library according to the working condition change of the battery, and is easy to implement in practical application.

Description

Parameter obtaining and switching method for liquid metal battery double-equivalent circuit model
Technical Field
The invention belongs to the technical field of liquid metal battery application, and particularly relates to a parameter obtaining and switching method of a liquid metal battery double equivalent circuit model.
Background
The liquid metal battery is a novel high-temperature battery, and has wide application prospect in the field of large-scale energy storage due to the advantages of low cost, large capacity, long service life and the like. The battery modeling is one of key technologies of battery application, and is the basis of work such as battery equalization and battery application system simulation. At present, the commonly used battery models are roughly divided into three types, namely a mathematical model, an electrochemical model and an equivalent circuit model, wherein the equivalent circuit model is widely applied due to the advantages of simple structure, easy acquisition of model parameters, high model precision and the like. According to different topological structures, equivalent circuit models are divided into different types such as a Rint model, a PNGV model and a Thevenin model.
The specification of the Chinese invention patent CN107248597A discloses a modeling method of a liquid metal battery, which constructs a second-order Thevenin equivalent circuit model and identifies the model parameters of the equivalent circuit through mixed pulse power performance test data. The invention fully considers the condition that the model parameters change violently under the high SoC and low SoC states, and the established equivalent circuit model can better reflect the short-time scale variable current charging and discharging characteristics of the liquid metal battery, but the model parameters are from the identification result of the mixed pulse power performance test data, so the long-time scale constant current charging and discharging characteristics of the liquid metal battery cannot be well reflected, and the precision in practical application is not ideal.
Due to the defects and shortcomings, further improvement and improvement are urgently needed in the field, and a double equivalent circuit model is designed to improve the precision of the model aiming at the problem that a single equivalent circuit model cannot comprehensively represent the multi-time scale dynamic characteristics of the liquid metal battery.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a parameter obtaining and switching method of a liquid metal battery double equivalent circuit model, and aims to solve the technical problem that a single equivalent circuit model cannot comprehensively represent the multi-time scale dynamic characteristics of a liquid metal battery. According to the double equivalent circuit model provided by the invention, the Rint model and the Thevenin model are respectively utilized to represent the constant current charge-discharge characteristic and the variable current charge-discharge characteristic of the liquid metal battery, so that a more accurate model is provided for state estimation and balance control of a battery management system.
In order to achieve the above object, the present invention provides a method for obtaining parameters of a liquid metal battery double equivalent circuit model, comprising:
(1) respectively carrying out charge-discharge cycle test and constant current pulse test in the charging direction and the discharging direction of the liquid metal battery to obtain test data;
the test data comprises sampling time, battery terminal voltage and charging and discharging current;
preferably, the constant current pulse in the charging direction is tested as: performing constant-current charging on the liquid metal battery from a fully discharged state, standing at intervals of a fixed SoC once, and measuring the terminal voltage of the battery after standing as the open-circuit voltage in the corresponding SoC state (the inside of the battery after standing reaches thermodynamic equilibrium, and the terminal voltage of the battery is approximate to the open-circuit voltage of the battery at the moment) until the battery is fully charged;
preferably, the constant current pulse in the discharge direction is tested as: performing constant-current discharge on the liquid metal battery from a full charge state, standing at intervals of a fixed SoC once, and measuring the terminal voltage of the battery after standing as the open-circuit voltage in the corresponding SoC state (the inside of the battery after standing reaches thermodynamic equilibrium, and the terminal voltage of the battery is approximate to the open-circuit voltage of the battery at the moment) until the battery is discharged completely;
preferably, the open-circuit voltages in the charging direction and the discharging direction are respectively subjected to linear interpolation fitting about the SoC to obtain open-circuit voltage functions in the charging direction and the discharging direction;
(2) acquiring an open-circuit voltage according to the open-circuit voltage function of the charging direction and the discharging direction;
specifically, the open circuit voltage in the Rint model and the Thevenin model is the mean of the open circuit voltage function in the charging direction and the discharging direction; namely:
Figure BDA0002473047280000021
wherein, UOCV C(SoC) is an open circuit voltage function of the charging direction; u shapeOCV D(SoC) is an open circuit voltage function of the discharge direction; u shapeOCV(SoC) is open circuit voltage;
(3) calculating the direct current internal resistance of the Rint model in the charging direction and the direct current internal resistance of the Rint model in the discharging direction according to the ohm law based on the open-circuit voltage and the charging and discharging cycle test data;
discrete state space of step response and Thevenin model based on constant current pulse testFitting ohmic internal resistance R of Thevenin model by using linear least square method0And a polarization capacitor C1And a polarization resistance R1；
Specifically, the direct current internal resistance of the charging direction Rint model is:
Figure BDA0002473047280000031
the direct current internal resistance of the Rint model in the discharging direction is as follows:
Figure BDA0002473047280000032
wherein, Ut C(SoC) and Ut D(SoC) battery terminal voltages in a charging direction and a discharging direction, respectively; u shapeOCV(SoC) is open circuit voltage; i is constant current charge-discharge current, wherein I takes a positive sign in the charge process, and I takes a negative sign in the discharge process; battery terminal voltage U in charging directiont C(SoC), cell terminal voltage U in discharge directiont D(SoC) and constant current charging and discharging current I from the charging and discharging cycle test, open circuit voltage UOCV(SoC) from constant current pulse testing;
specifically, the discrete state space equation of Thevenin model is:
Figure BDA0002473047280000033
wherein, U1(k)、Ut(k) And I (k) the polarization voltage, terminal voltage and current flowing into the battery at time k, respectively; Δ T is the sampling period. Generally, the step response time length of the fitting is 30-300 s; sampling period delta T is 1 s-10 s; and respectively carrying out linear least square method estimation on the pulse rising edge and the pulse falling edge in the charging process and the discharging process, and carrying out linear interpolation fitting on the acquired impedance parameters on the SoC to obtain: ohmic internal resistance R of charging rising edge0 C_R(SoC), polarization capacitance C1 C_R(SoC) and polarization resistance R1 C_R(SoC); ohmic internal resistance R of charging falling edge0 C_F(SoC), polarization capacitance C1 C_F(SoC) and polarization resistance R1 C_F(SoC); ohmic internal resistance R of discharging rising edge0 D_R(SoC), polarization capacitance C1 D_R(SoC) and polarization resistance R1 D _R(SoC); ohmic resistance R of discharging falling edge0 D_F(SoC), polarization capacitance C1 D_F(SoC) and polarization resistance R1 D_F(SoC)；
(4) Dividing parameters of the double equivalent circuit model into four parameter libraries, namely a charging rising edge library, a charging falling edge library, a discharging rising edge library and a discharging falling edge library according to different operation conditions of the liquid metal battery;
wherein each parameter library comprises corresponding open-circuit voltage UOCVDirect current internal resistance R of Rint modeldOhmic internal resistance R of Thevenin model0Thevenin model polarization capacitance C1And polarization resistance R of Thevenin model1。
Specifically, four parameter libraries are used for describing the dynamic characteristics of the liquid metal battery under different operation conditions, wherein the CR library represents a charging rising edge library and comprises UOCV(SoC)、Rd C(SoC)、R0 C_R(SoC)、C1 C_R(SoC) and R1 C_R(SoC); CF bank stands for charge falling edge bank, including UOCV(SoC)、Rd C(SoC)、R0 C_F(SoC)、C1 C_F(SoC) and R1 C_F(SoC); DR bank represents a discharge rising edge bank including UOCV(SoC)、Rd D(SoC)、R0 D_R(SoC)、C1 D_R(SoC) and R1 D_R(SoC); DF library represents the bank of falling edges of discharge, including UOCV(SoC)、Ud D(SoC)、R0 D_F(SoC)、C1 D_F(SoC) and R1 D_F(SoC)。
Based on the parameter obtaining method of the liquid metal battery double equivalent circuit model, the invention provides a switching method of the liquid metal battery double equivalent circuit model, which comprises the following steps:
(1) updating the SoC at the time t by using an ampere-hour integration method according to the current I (t) input into the battery at the time t;
(2) judging whether the current working condition changes: judging whether the difference value between the current input into the battery at the time t and the current input into the battery at the time (t-1) is 0, if the difference value between the current input into the battery at the time t and the current input into the battery at the time (t-1) is 0, updating the parameters of the double equivalent circuit model according to the change of the SoC without switching a parameter library, and turning to the step (6); otherwise, the parameter library needs to be switched, and the working condition switching time t is updated0Turning to the step (3) when t-1 is satisfied;
(3) judging the sign of the difference value between the current input into the battery at the time t and the current input into the battery at the time (t-1): if the difference value between the current input into the battery at the time t and the current input into the battery at the time (t-1) is greater than 0, the pulse rising edge process is carried out at the moment, the step (4) is carried out, otherwise, the pulse falling edge process is carried out at the moment, and the step (5) is carried out;
(4) judging the sign of the current input into the battery at the time t: if the current of the input battery at the time t is larger than 0, switching the parameters of the double equivalent circuit model to a CR library, and turning to the step (6); otherwise, switching the parameters of the double equivalent circuit model to a DR library, and turning to the step (6);
(5) judging the sign of the current input into the battery at the time t: if the current input into the battery at the time t is less than 0, switching the parameters of the double equivalent circuit model to a DF library, and turning to the step (6); otherwise, switching the parameters of the double equivalent circuit model to the CF library, and turning to the step (6);
(6) calculating output voltages of a Rint model and a Thevenin model at the t moment according to the parameters of the double equivalent circuit model;
(7) and obtaining the output voltage of the double equivalent circuit model at the time t by using the attenuation factor based on the output voltages of the Rint model and the Thevenin model at the time t.
Specifically, the output voltage of the double equivalent circuit model is:
UOUT(t)＝(1-α)·URint(t)+α·UThevenin(t)
wherein, URint(t)、UThevenin(t) output voltages of the Rint model and the Thevenin model at the time t respectively;
the attenuation factor α is expressed as:
Figure BDA0002473047280000051
wherein, t0The working condition switching time is set; t is a short time scale dynamic response attenuation period, and generally, the value range of T is 2 min-5 min.
Generally, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects:
the parameter acquisition method of the liquid metal battery double equivalent circuit model provided by the invention is simple, does not relate to a complex algorithm, and can comprehensively reflect the dynamic characteristics of the liquid metal battery by considering the parameter change conditions of the liquid metal battery under different working conditions such as charging, discharging, pulse rising edge, pulse falling edge and the like.
The Rint model and the Thevenin model are respectively used for representing the constant-current charge-discharge characteristic and the variable-current charge-discharge characteristic of the liquid metal battery, the problem that a single equivalent circuit model cannot comprehensively represent the multi-time scale dynamic characteristic of the liquid metal battery is solved, and the method has high model precision.
The switching method of the liquid metal battery double equivalent circuit model provided by the invention is simpler, can be quickly switched to a corresponding model parameter library according to the working condition change of the battery, and is easier in practical application.
The Rint model and the Thevenin model related to the invention are both simple equivalent circuit models, and the calculated amount in practical application is small.
Drawings
FIG. 1 is a schematic diagram of a Rint equivalent circuit model provided by an embodiment of the present invention;
fig. 2 is a schematic diagram of Thevenin equivalent circuit model according to an embodiment of the present invention;
fig. 3 is a flow chart of parameter acquisition of a liquid metal battery dual equivalent circuit model according to an embodiment of the present invention;
FIG. 4 is a constant current pulse test of the charging direction provided by an embodiment of the present invention;
FIG. 5 is a constant current pulse test in the discharge direction provided by an embodiment of the present invention;
fig. 6 is a flow chart illustrating parameter switching of a dual equivalent circuit model of a liquid metal battery according to an embodiment of the present invention;
fig. 7 is a schematic diagram of a model effect of a liquid metal battery double-equivalent circuit model under a constant-current charging and discharging condition according to an embodiment of the present invention;
fig. 8 is a schematic diagram of a model effect of a liquid metal battery double-equivalent circuit model under a variable current charging and discharging condition according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The invention provides a parameter obtaining and switching method of a liquid metal battery double equivalent circuit model.
The charge-discharge cycle test is to perform charge-discharge cycle on the liquid metal battery under a constant current and voltage window; the constant current pulse test is alternately carried out by constant current charging (discharging) and standing; the Rint model is a constant voltage source UOCVAnd a DC internal resistance RdThe series structure is used for describing the constant current charging and discharging characteristics of the liquid metal battery; thevenin model is a constant voltage source UOCVAn ohmic internal resistance R0And a capacitor ringFor describing the variable current charging and discharging characteristics of the liquid metal battery, the capacitor ring is a capacitor C1And a resistor R1The parallel structure of (1) is used to describe the polarization process of the liquid metal battery.
The invention provides a parameter acquisition method of a liquid metal battery double equivalent circuit model, which comprises the following steps:
(1) respectively carrying out charge-discharge cycle test and constant current pulse test in the charge direction and the discharge direction of the liquid metal battery to obtain charge-discharge cycle test data and constant current pulse test data;
the test data comprises sampling time, battery terminal voltage and charging and discharging current;
preferably, the constant current pulse in the charging direction is tested as: performing constant-current charging on the liquid metal battery from a fully discharged state, standing at intervals of a fixed SoC once, and measuring the terminal voltage of the battery after standing as the open-circuit voltage in the corresponding SoC state (the inside of the battery after standing reaches thermodynamic equilibrium, and the terminal voltage of the battery is approximate to the open-circuit voltage of the battery at the moment) until the battery is fully charged;
preferably, the constant current pulse in the discharge direction is tested as: performing constant-current discharge on the liquid metal battery from a full charge state, standing at intervals of a fixed SoC once, and measuring the terminal voltage of the battery after standing as the open-circuit voltage in the corresponding SoC state (the inside of the battery after standing reaches thermodynamic equilibrium, and the terminal voltage of the battery is approximate to the open-circuit voltage of the battery at the moment) until the battery is discharged completely;
preferably, the open-circuit voltages in the charging direction and the discharging direction are respectively subjected to linear interpolation fitting about the SoC to obtain open-circuit voltage functions in the charging direction and the discharging direction;
(2) acquiring an open-circuit voltage according to the open-circuit voltage function of the charging direction and the discharging direction;
specifically, the open circuit voltage in the Rint model and the Thevenin model is the mean of the open circuit voltage function in the charging direction and the discharging direction; namely:
Figure BDA0002473047280000081
wherein, UOCV C(SoC) is an open circuit voltage function of the charging direction; u shapeOCV D(SoC) is an open circuit voltage function of the discharge direction; u shapeOCV(SoC) is open circuit voltage;
(3) calculating the direct current internal resistance of the Rint model in the charging direction and the direct current internal resistance of the Rint model in the discharging direction according to the ohm law based on the open-circuit voltage and the charging and discharging cycle test data;
based on step response of constant current pulse test and discrete state space equation of Thevenin model, and linear least square method is utilized to fit ohmic internal resistance R of Thevenin model0And a polarization capacitor C1And a polarization resistance R1；
Specifically, the direct current internal resistance of the charging direction Rint model is:
Figure BDA0002473047280000082
the direct current internal resistance of the Rint model in the discharging direction is as follows:
Figure BDA0002473047280000083
wherein, Ut C(SoC) and Ut D(SoC) battery terminal voltages in a charging direction and a discharging direction, respectively; u shapeOCV(SoC) is open circuit voltage; i is constant current charge-discharge current, wherein I takes a positive sign in the charge process, and I takes a negative sign in the discharge process; battery terminal voltage U in charging directiont C(SoC), cell terminal voltage U in discharge directiont D(SoC) and constant current charging and discharging current I from the charging and discharging cycle test, open circuit voltage UOCV(SoC) from constant current pulse testing;
specifically, the discrete state space equation of Thevenin model is:
Figure BDA0002473047280000091
wherein, U1(k)、Ut(k) And I (k) the polarization voltage, terminal voltage and current flowing into the battery at time k, respectively; Δ T is the sampling period. Generally, the step response time length of the fitting is 30-300 s; sampling period delta T is 1 s-10 s; and respectively carrying out linear least square method estimation on the pulse rising edge and the pulse falling edge in the charging process and the discharging process, and carrying out linear interpolation fitting on the acquired impedance parameters on the SoC to obtain: ohmic internal resistance R of charging rising edge0 C_R(SoC), polarization capacitance C1 C_R(SoC) and polarization resistance R1 C_R(SoC); ohmic internal resistance R of charging falling edge0 C_F(SoC), polarization capacitance C1 C_F(SoC) and polarization resistance R1 C_F(SoC); ohmic internal resistance R of discharging rising edge0 D_R(SoC), polarization capacitance C1 D_R(SoC) and polarization resistance R1 D _R(SoC); ohmic resistance R of discharging falling edge0 D_F(SoC), polarization capacitance C1 D_F(SoC) and polarization resistance R1 D_F(SoC)；
(4) Dividing parameters of the double equivalent circuit model into four parameter libraries, namely a charging rising edge library, a charging falling edge library, a discharging rising edge library and a discharging falling edge library according to different operation conditions of the liquid metal battery;
wherein each parameter library comprises corresponding open-circuit voltage UOCVDirect current internal resistance R of Rint modeldOhmic internal resistance R of Thevenin model0Thevenin model polarization capacitance C1And polarization resistance R of Thevenin model1。
Specifically, four parameter libraries are used for describing the dynamic characteristics of the liquid metal battery under different operation conditions, wherein the CR library represents a charging rising edge library and comprises UOCV(SoC)、Rd C(SoC)、R0 C_R(SoC)、C1 C_R(SoC) and R1 C_R(SoC); CF bank stands for charge falling edge bank, including UOCV(SoC)、Rd C(SoC)、R0 C_F(SoC)、C1 C_F(SoC) and R1 C_F(SoC); DR bank represents a discharge rising edge bank including UOCV(SoC)、Rd D(SoC)、R0 D_R(SoC)、C1 D_R(SoC) and R1 D_R(SoC); DF library represents the bank of falling edges of discharge, including UOCV(SoC)、Ud D(SoC)、R0 D_F(SoC)、C1 D_F(SoC) and R1 D_F(SoC)。
Based on the parameter obtaining method of the liquid metal battery double equivalent circuit model, the invention provides a switching method of the liquid metal battery double equivalent circuit model, which comprises the following steps:
(1) updating the SoC at the time t by using an ampere-hour integration method according to the current I (t) input into the battery at the time t;
(2) judging whether the current working condition changes: judging whether the difference value between the current input into the battery at the time t and the current input into the battery at the time (t-1) is 0, if the difference value between the current input into the battery at the time t and the current input into the battery at the time (t-1) is 0, updating the parameters of the double equivalent circuit model according to the change of the SoC without switching a model parameter library, and turning to the step (6); otherwise, the model parameter base needs to be switched, and the working condition switching time t is updated0Turning to the step (3) when t-1 is satisfied;
(3) judging the sign of the difference value between the current input into the battery at the time t and the current input into the battery at the time (t-1): if the difference value between the current input into the battery at the time t and the current input into the battery at the time (t-1) is greater than 0, the pulse rising edge process is carried out at the moment, the step (4) is carried out, otherwise, the pulse falling edge process is carried out at the moment, and the step (5) is carried out;
(4) judging the sign of the current input into the battery at the time t: if the current of the input battery at the time t is larger than 0, switching the parameters of the double equivalent circuit model to a CR library, and turning to the step (6); otherwise, switching the parameters of the double equivalent circuit model to a DR library, and turning to the step (6);
(5) judging the sign of the current input into the battery at the time t: if the current input into the battery at the time t is less than 0, switching the parameters of the double equivalent circuit model to a DF library, and turning to the step (6); otherwise, switching the parameters of the double equivalent circuit model to the CF library, and turning to the step (6);
(6) calculating output voltages of a Rint model and a Thevenin model at the t moment according to the parameters of the double equivalent circuit model;
(7) and obtaining the output voltage of the double equivalent circuit model at the time t by using the attenuation factor based on the output voltages of the Rint model and the Thevenin model at the time t.
Specifically, the output voltage of the double equivalent circuit model is:
UOUT(t)＝(1-α)·URint(t)+α·UThevenin(t)
wherein, URint(t)、UThevenin(t) output voltages of the Rint model and the Thevenin model at the time t respectively;
the attenuation factor α is expressed as:
Figure BDA0002473047280000101
wherein, t0The working condition switching time is set; t is a short time scale dynamic response attenuation period, and generally, the value range of T is 2 min-5 min.
Examples
FIG. 1 is a schematic diagram of an equivalent circuit model of Rint according to the present invention, which includes a constant voltage source UOCVAnd a DC internal resistance RdThe constant current charging and discharging characteristics of the liquid metal battery are described.
FIG. 2 is a schematic diagram of the Thevenin equivalent circuit model provided by the present invention, which includes a constant voltage source UOCVAn ohmic internal resistance R0A polarization capacitor C1And a polarization resistor R1The variable current charging and discharging characteristics of the liquid metal battery are described.
Fig. 3 is a flow chart for obtaining parameters of a liquid metal battery double equivalent circuit model provided by the present invention, and the main steps include:
(1) establishing a double equivalent circuit model;
(2) carrying out charge-discharge cycle test and constant current pulse test on the liquid metal battery;
(3) based on the constant current pulse test data, obtaining open circuit voltages in different SoC states by using a standing method;
(4) based on the charging and discharging cycle test data and the open-circuit voltage data, respectively calculating direct current internal resistances in different SoC states in the charging direction and the discharging direction by using the ohm's law;
(5) identifying impedance parameters of the Thevenin model by using a least square method, and respectively identifying the impedance parameters under four working conditions of a charging rising edge, a charging falling edge, a discharging rising edge and a discharging falling edge;
(6) obtaining fitting functions of different circuit parameters about the SoC by using a linear interpolation method;
(7) and establishing a double equivalent circuit model parameter library under different working conditions, wherein the double equivalent circuit model parameter library comprises a CR library, a CF library, a DR library and a DF library.
Fig. 4 is a constant current pulse test in a charging direction provided in the example, and fig. 5 is a constant current pulse test in a discharging direction provided in the example. Acquiring open-circuit voltages of the liquid metal battery in different SoC states by using a standing process of constant current pulse testing; and identifying impedance parameters of the Thevenin model by using the step response of the constant current pulse test.
In the embodiment, the charge and discharge multiplying power of the battery cycle test and the constant current pulse test is 0.1C, the voltage window is 0.65V-1.10V, the SoC interval of the constant current pulse test is 5% SoC, and the standing time is 30 min.
In this embodiment, the step response time length of the least squares fitting is 60s, and the sampling period is 1 s.
Fig. 6 is a flow chart of parameter switching of a liquid metal battery dual equivalent circuit model according to an embodiment, which includes the following specific steps:
(1) updating the SoC at the time t according to the current I (t) input into the battery at the time t;
(2) judging whether the working condition of the battery changes or not by comparing the I (t) with the I (t-1), and directly calculating the output voltage of the battery if the working condition does not change; if the working condition changes, the working condition switching time t is updated0And carrying out step (3);
(3) and judging the operation condition of the battery through the symbols of I (t) -I (t-1) and I (t), switching the parameters of the double equivalent circuit model to a parameter library corresponding to the operation condition, and calculating the output voltage.
In this embodiment, the short time scale dynamic response decay period T is taken to be 3 min;
fig. 7 is a schematic diagram of a model effect of a liquid metal battery double-equivalent circuit model under a constant-current charging and discharging condition. The operation condition of the liquid metal battery is that 6 times of charge-discharge cycles are carried out under the charge-discharge rate of 0.1C, the error between the simulation voltage of the double equivalent circuit model provided by the embodiment and the actually measured voltage of the battery is within +/-0.006V, and the dynamic characteristic of the liquid metal battery in the constant current charge-discharge condition can be better reflected.
FIG. 8 is a schematic diagram of a model effect of a liquid metal battery double-equivalent circuit model under a variable current charging and discharging condition. The operation working conditions of the liquid metal battery comprise charging and discharging processes with different multiplying powers and a standing process under different SoC states. The error between the simulated voltage of the double equivalent circuit model and the actual measurement voltage of the battery is within-0.01V-0.02V, and the dynamic characteristic of the liquid metal battery in the variable current charging and discharging working condition can be well reflected.
The present invention is not limited to the above-described embodiments. Compared with the prior art, the invention has the following advantages:
the parameter acquisition method of the liquid metal battery double equivalent circuit model provided by the invention is simple, does not relate to a complex algorithm, and can comprehensively reflect the dynamic characteristics of the liquid metal battery by considering the parameter change conditions of the liquid metal battery under different working conditions such as charging, discharging, pulse rising edge, pulse falling edge and the like.
The Rint model and the Thevenin model are respectively used for representing the constant-current charge-discharge characteristic and the variable-current charge-discharge characteristic of the liquid metal battery, the problem that a single equivalent circuit model cannot comprehensively represent the multi-time scale dynamic characteristic of the liquid metal battery is solved, and the method has high model precision.
The switching method of the liquid metal battery double equivalent circuit model provided by the invention is simpler, can be quickly switched to a corresponding model parameter library according to the working condition change of the battery, and is easier in practical application.
The Rint model and the Thevenin model related to the invention are both simple equivalent circuit models, and the calculated amount in practical application is small.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (5)

1. A parameter obtaining method for a liquid metal battery double equivalent circuit model is characterized by comprising the following steps:
(1) performing a charge-discharge cycle test and a constant current pulse test in the charging direction and the discharging direction of the liquid metal battery respectively at fixed SoC intervals, and obtaining open-circuit voltages in corresponding SoC states under the constant current pulse test;
(2) based on the open-circuit voltage in each corresponding SoC state, obtaining open-circuit voltage functions in the charging direction and the discharging direction by using a standing method and interpolation fitting, and obtaining the open-circuit voltage;
(3) calculating the direct current internal resistance of the Rint model in the charging direction and the direct current internal resistance of the Rint model in the discharging direction according to the ohm law based on the open-circuit voltage and the charging and discharging cycle test data;
fitting ohmic internal resistance, polarization capacitance and polarization resistance of the Thevenin model by using a linear least square method based on step response of constant current pulse test and a discrete state space equation of the Thevenin model;
(4) dividing parameters of the double equivalent circuit model into four parameter libraries, namely a charging rising edge library, a charging falling edge library, a discharging rising edge library and a discharging falling edge library according to different operation conditions of the liquid metal battery;
wherein, each parameter base comprises open circuit voltage, direct current internal resistance of a Rint model, ohmic internal resistance of a Thevenin model, polarization capacitance of the Thevenin model and polarization resistance of the Thevenin model, and output voltage U of the double equivalent circuit model at time tOUT(t) is:
UOUT(t)＝(1-α)·URint(t)+α·UThevenin(t)
Figure FDA0003001044190000011
wherein, URint(t)、UThevenin(t) output voltages of the Rint model and the Thevenin model at t time, respectively, where α is an attenuation factor, and t is0And T is a short time scale dynamic response attenuation period at the working condition switching moment.
2. The parameter obtaining method according to claim 1, wherein the open circuit voltage function in the charging direction and the discharging direction is obtained by using a stationary method and interpolation fitting in step (2), and the obtained open circuit voltage is:
Figure FDA0003001044190000021
wherein, UOCV C(SoC) is an open circuit voltage function of the charging direction; u shapeOCV D(SoC) is an open circuit voltage function of the discharge direction; u shapeOCVAnd (SoC) obtaining the open-circuit voltage by using a standing method and interpolation fitting to obtain an open-circuit voltage function in the charging direction and the discharging direction.
3. The parameter obtaining method according to claim 2, wherein the direct current internal resistance of the charging direction Rint model is:
Figure FDA0003001044190000022
the direct current internal resistance of the discharge direction Rint model is as follows:
Figure FDA0003001044190000023
wherein, Ut C(SoC) and Ut D(SoC) battery terminal voltages in a charging direction and a discharging direction, respectively; i is constant current charge-discharge current, and the charge direction is the positive direction.
4. The parameter acquisition method according to claim 2 or 3, wherein the discrete state space equation of the Thevenin model is:
Figure FDA0003001044190000024
wherein, U1(k)、Ut(k) And I (k) the polarization voltage, terminal voltage and current flowing into the battery at time k, respectively; Δ T is the sampling period; r0Ohmic internal resistance of Thevenin model; c1Polarization capacitance for Thevenin model; r1Polarization resistance for Thevenin model; u shapeOCV(k) Is UOCV(SoC) value at time k; u shape1(k-1) is the polarization voltage of the battery at the time k-1; i (k-1) is the current flowing into the battery at time k-1.
5. A handover method based on the parameter acquisition method of claim 1, characterized by comprising the steps of:
(1) updating the SoC at the time t by using an ampere-hour integration method according to the current I (t) input into the battery at the time t;
(2) if I (t) -I (t-1) ═ 0, updating the parameters of the double equivalent circuit model according to the change of the SoC, and turning to the step (4), otherwise, updating the working condition switching time t0Turning to the step (3) when t-1 is satisfied;
(3) if I (t) -I (t-1) >0 and I (t) >0, switching the parameters of the double equivalent circuit model to the charge rising edge library; if I (t) -I (t-1) >0 and I (t) is less than or equal to 0, switching the parameters of the double equivalent circuit model to the discharge rising edge library; if I (t) -I (t-1) <0 and I (t) <0, switching the parameters of the dual equivalent circuit model to the discharging falling edge library; if I (t) -I (t-1) <0 and I (t) ≧ 0, switching the double equivalent circuit model parameters to the charge falling edge library;
(4) calculating output voltages of a Rint model and a Thevenin model at the t moment according to the parameters of the double equivalent circuit model;
(5) obtaining the output voltage U of the double equivalent circuit model at the t moment by using the attenuation factor alpha based on the output voltages of the Rint model and the Thevenin model at the t momentOUT(t)：
UOUT(t)＝(1-α)·URint(t)+α·UThevenin(t)
Figure FDA0003001044190000031
Wherein, URint(t)、UThevenin(t) output voltages of the Rint model and the Thevenin model at time t, t0And T is a short time scale dynamic response attenuation period at the working condition switching moment.
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