CN108695570B - Self-healing-based lithium battery physical model construction method and monitoring system - Google Patents
Self-healing-based lithium battery physical model construction method and monitoring system Download PDFInfo
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- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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Abstract
The invention relates to a self-healing-based lithium battery physical model construction method and a self-healing-based lithium battery physical model monitoring systemocInternal resistance R of the batteryi，RiRespectively comprises discharge resistors RdisAnd a charging resistor RchParallel RC network and capacitor CsAnd a resistance Rs. Wherein the open circuit voltage UocDescribing the voltage characteristics of the battery, capacitance CsThe capacity characteristics of the battery are described. The charging and discharging monitoring system takes LabView2012 as a software development platform and comprises a voltage acquisition card, a power supply module, a current acquisition module and a relay module; meanwhile, the constant-current charging circuit also comprises a constant-current charging circuit, a constant-voltage charging circuit and a discharging circuit. According to the invention, the voltage description function after the battery is converted from the discharging state to the standing state is obtained, the model can accurately describe the voltage change condition of the battery in the standing state for different self-healing time, the error between the model analog value and the measured value is maintained in a stable range, and the model has higher accuracy.
Description
Technical Field
The invention relates to a self-healing-based lithium battery physical model construction method and a self-healing-based lithium battery physical model monitoring system, and belongs to the technical field of lithium battery modeling.
Background
In recent years, electric vehicles have attracted much attention and are widely popularized due to their characteristics of cleanliness, high efficiency, and the like. However, the power battery becomes an important influence factor for the performance and stability of the electric vehicle. The lithium ion battery has small volume, light weight and low self-discharge rate, and is one of the best choices of the power battery of the electric automobile. Therefore, research on safety and reliability of lithium ion batteries is a major concern. For example, the document "lithium ion battery online state of charge and state of health estimation model" proposes to estimate the state of charge and the state of health of a battery by using the variation of voltage per unit time in the battery discharge process, and the method realizes the online estimation of the state of charge and the state of health of the lithium battery and has better robustness; an estimation method combining the state of charge and the state of health of a lithium battery is provided in the document 'estimation of the combined state of charge and the state of health of a lithium ion battery of an electric vehicle', the state of charge and the real-time estimation are carried out by utilizing an extended Kalman algorithm, and the state of health is updated off line, and experiments prove that the method has better accuracy; however, the above articles indicate that the lithium battery works in an ideal state, and do not consider the discontinuous phenomenon of the lithium battery in the actual use process, that is, the capacity of the lithium battery rises when the lithium battery is in a static state, and the phenomenon is helpful for prolonging the service life of the lithium battery, and has important research significance.
Currently, the battery models in common use can be divided into two types: electrochemical models, and equivalent circuit models. The electrochemical model is based on electrochemical theory and adopts a mathematical method to describe the reaction process in the battery, but the method is complex in calculation and difficult to determine the parameter value. The equivalent circuit model describes the battery characteristics based on the external characteristics of the battery, and avoids the internal reaction of the battery and complex parameter calculation. Considering the accuracy and practicability of the model comprehensively, the equivalent circuit model is widely applied to online estimation of the battery management system. An online estimation method of a LiFePO4 battery based on a model is provided in the document 'online estimation of model parameters and charging state of the LiFePO4 battery by using novel open-circuit voltage under various environmental temperatures', the LiFePO4 battery is simulated by using a Thevenin equivalent model, and a corresponding mathematical expression equation is given; a new lithium battery equivalent model is proposed in the document 'on-line identification of lithium ion battery parameters and realization thereof in battery electric quantity state prediction based on an improved equivalent circuit model', noise is added into the equivalent model to simplify the calculation complexity of model parameters and have higher accuracy; a dynamic multi-parameter battery energy estimation method based on a model is provided in the document 'lithium ion battery peak power dynamic multi-parameter estimation method based on a model', so that ohmic internal resistance in a model worn in Vinan is improved, the accuracy of the model is improved, and the remaining available energy of the battery can be calculated more reliably. However, the model studied in the above-mentioned paper does not describe the state change of the battery in the self-healing phenomenon. Therefore, in order to better describe the state of the lithium battery in the self-healing process, a lithium battery physical equivalent model based on the self-healing characteristics is provided.
The lithium battery for the vehicle can be divided into three states of charging, discharging and standing. When the battery is charged, lithium ions are generated at the positive electrode of the battery, and simultaneously, the lithium ions pass through the electrolyte and reach the negative electrode of the battery to be combined with graphite (nano-scale carbon powder). In this process, the more lithium ions are combined with the graphite of the negative electrode, the greater the capacity of the battery. When the battery is discharged, lithium ions are separated from graphite at the negative electrode of the battery and return to the positive electrode of the battery, and the battery capacity is reduced.
In actual use, after each charging, the state of the lithium battery is repeatedly switched between discharging and standing, as shown in fig. 1. When the battery is kept still, the self-healing phenomenon of the battery can occur, and due to the existence of the concentration difference of lithium ions between the anode and the cathode of the battery, the lithium ions of the cathode of the battery can reach the cathode of the battery under the driving of the concentration difference. This process is the reverse of battery discharge, and similar to battery charging, the state of charge of the battery is boosted. The self-healing phenomenon of the automobile lithium battery is common because the automobile is in a running and stagnation state for most of time. The self-healing strength is related to the time for which the battery is left standing and can change as the number of battery charges and discharges increases, and affects the remaining life of the battery. Therefore, a battery model that can describe the self-healing phenomenon of the lithium ion battery is very important.
Disclosure of Invention
The invention aims to provide a lithium battery physical model construction method and a monitoring system based on self-healing, wherein the model can better describe the state of a lithium battery in the self-healing process; the charge and discharge monitoring system can realize the programming control of the discharge process of the lithium battery, simulate the conversion from the discharge state to the standing state of the lithium battery in the actual use process, and monitor the self-healing phenomenon of the lithium battery.
The invention relates to a lithium battery physical model construction method based on self-healing, as shown in figure 2, the model comprises a battery open-circuit voltage UocAnd a batteryInternal resistance Ri，RiRespectively comprises discharge resistors RdisAnd a charging resistor RchParallel RC network and capacitor CsAnd a resistance Rs. Wherein the open circuit voltage UocDescribing the voltage characteristics of the battery, capacitance CsThe capacity characteristics of the battery are described. The model can be divided into two parts, wherein one part describes the voltage of the lithium battery in the discharging stage and the other part describes the self-healing phenomenon voltage of the lithium battery in the standing stage. The following is specifically introduced:
1.1. description of the model
When the battery reaches a stable state, the voltage of the battery does not change any more, and the terminal voltage is equal to the open-circuit voltage U of the batteryocIs also equal to the capacitance CsI.e.:
UL＝UOC＝UCs(1)
1.1.1. discharged state battery description
And describing the discharge state of the lithium battery physical model by using an HPPC test. When the battery passes the discharge pulse current, the voltage of the battery undergoes two changes, a rapid voltage change process and a slow voltage change process, and fig. 3 shows the change of the voltage of the battery when the battery passes the HPPC test.
In the stage of rapid voltage change, the sharp drop of the voltage is mainly caused by the internal resistance R of the batteryiInfluence of, internal resistance RiThe value of (c) is related to the variation value of the voltage, as shown in equation (2); when the battery is charged with ILDuring constant current discharge, the voltage changes slowly, in order to simplify the model, the capacitance of the battery can be considered to be in an open circuit state, the change value of the battery is represented by the parallel RC network part, and the voltage change value of the battery can be represented by formula (3):
load R connected across the batteryLIs connected with a resistorThe value can be found from the ratio of the terminal voltage and the current of the battery:
RL＝UL-F/IL(4)
1.1.2. static State Battery description
The values of all the parameters of the model cannot be obtained through the HPPC test alone, and in order to better describe the self-healing phenomenon of the battery, the self-healing test is necessary for the battery, and a schematic diagram of the self-healing test is shown in fig. 4. And carrying out a self-healing test once in a single charge-discharge cycle of the lithium battery. When the charging is finished, the battery is discharged at constant current, and t0At the moment when the battery voltage reaches UL-SRWhen the battery is in the static state, no current passes through the RC parallel network, so the model can be simplified into the form in fig. 2.
At t0At (-) time, battery is in discharge state, capacitor CsThe voltage across can be represented by equation (5); at t0At the moment (+) the battery is in a static state and the capacitor CsCan be expressed by equation (6). T is the voltage of the capacitor cannot be suddenly changed0Capacitance at (-) time CsVoltage value of and t0The voltage value at the time (+) is equal, i.e., as shown in equation (7):
UCS(t0-)＝UL-SR(t0-)+IL*RP＝UOC-SR-Ui(t0-) (5)
UCs(t0+)＝UL-SR(t0+)-URs(t0+) (6)
UCs(t0)＝UCs(t0-)＝UCs(t0+) (7)
from equations (5), (6) and (7), it can be derived that the voltage variation value when the battery is converted from the discharge state to the rest state:
△UL-SR＝URs(t0+)+IL*RP(8)
when the battery is in a stable state for a long enough time, the capacitance C is shown by the formula (1)sWill be equal to the open circuit voltage U of the batteryoc-SR. However, the capacitance CsAt t0The voltage value at the moment is less than Uoc-SRComparing equation (5) with equation (9) shows that the ideal voltage source U is at △ t when the battery is at restOCCharging the capacitor Cs and flowing through the capacitor CsHas a current of IsTime constant of τ, capacitance CsElectric quantity of Qcs：
UCs(△t→∞)＝UOC-SR(9)
τ＝CS*(Ri+RS) (11)
QCs＝UCs*CS(12)
When the lithium battery is converted from a discharge state to a standing state, the capacitor CsThe voltage across satisfies formula (11):
UCs＝UOC-SR-[UOC-SR-(UL-SR(t0-)+IL*RP)]*exp[-t/CS/(RS+Ri)](13)
terminal voltage U at the same timeL-SRAnd the capacitor voltage UcsSatisfies the relationship shown in formula (6), so that the terminal voltage U can be derived from formula (6), formula (8), formula (10) and formula (12)L-SRThe functional relation is satisfied:
in order to monitor the discharge state and the standing state of the physical model of the lithium battery, the invention designs a charge and discharge monitoring system, specifically a lithium battery charge and discharge monitoring system based on LabView is established, as shown in FIG. 5. The charging and discharging monitoring system takes LabView2012 as a software development platform and comprises a voltage acquisition card, a power supply module, a current acquisition module and a relay module; the charging circuit also comprises a constant current charging circuit, a constant voltage charging circuit and a discharging circuit, wherein I1 represents constant current charging current, I2 represents constant voltage charging current, and I3 represents discharging current. The three loops are controlled to be switched on or off through three relay switches of the relay module respectively. The current acquisition module converts a circuit current signal into a voltage signal, the output end of the current acquisition module is connected with the input end of a certain channel of the voltage acquisition card, and the voltage acquisition card is used for conditioning and performing analog-to-digital conversion on the signal and transmitting the signal to an upper computer. The voltage acquisition card transmits the voltage and current information of the lithium battery to the PC, and the upper computer programs and controls the state of the relay switch module, so that switching among different loops is realized. The system realizes the programming control of the discharging process of the lithium battery, simulates the conversion from the discharging state to the standing state of the lithium battery in the actual using process, and monitors the self-healing phenomenon of the lithium battery. The upper computer is responsible for switching of the circuit state, and then controls the conversion of the lithium battery among three states of charging, discharging and standing. Meanwhile, the upper computer can realize real-time monitoring of the circuit state of the lithium battery.
(1) The voltage acquisition module: PXIe-4300
The voltage acquisition module PXIe-4300 comprises 8 analog input channels, each channel is provided with an independent analog-to-digital converter, synchronous sampling can be carried out, and the high efficiency of data acquisition is ensured; the voltage input range of the voltage acquisition module is +/-300V, and six gears of 1V/2V/5V/10V/30V/300V can be selected respectively; the resolution of PXIe-4300 is 16 bits, and the data acquisition precision is high.
(2) The current acquisition module: siglent CP 401
The current acquisition module specifically adopts a non-contact current sensor Siglent CP 401 based on the Hall principle, which is produced by Dingyang science and technology Limited company (Siglent), and is used for monitoring the circuit current. The percentage error of the output signal of the Siglent CP 401 is 3% +/-5 mV, so that the accuracy is high, and the reliability of circuit current information is ensured; the current-voltage conversion ratio is 100mV/A, the sensitivity is high, and the linearity is better; the conversion rate can reach 0.3V/mu s, and the response time is faster. Siglent CP 401 can be used for measuring the current of the range of 50mA to 10A, and the system charge-discharge current is within the range of 2A, so that the requirements of experimental design are met. In the actual use process, the Siglent CP 401 converts the circuit current signal into a voltage signal, the output end of the Siglent CP 401 is connected with the input of a certain channel of the PXIe-4300, and the PXIe-4300 data acquisition card is used for signal conditioning and analog-to-digital conversion and transmitting the signal to the upper computer.
(3) A power supply module: NI PXI-4130
PXI-4130 is a programmable Source Measurement Unit (SMU) in which both channels can operate as either a constant voltage source or a constant current source. The constant voltage source mode and the constant current source mode both have settable clamp limit values, ensuring the stability of output voltage values and current values. The selection of the constant voltage source or constant current source may be controlled by programming. The charging process of the lithium battery can be divided into two steps of constant Current charging and constant voltage charging, wherein an Output Function (DC Current) is set to be Output by a Channel 0 of PXI-4130, and the Current value (Current Level) is 1A; the Channel 1 outputs a constant Voltage (Output Function: DC Voltage) with a Voltage Level of 4.2V.
(4) A relay module: NI PXI-2564
PXI-2564 is a generic type of switch module with independent Single Pole Single Throw (SPST) Form A relays. PXI-2564 has 16 channels, each relay is a non-latching relay, has extremely low on-resistance and low thermal offset. PXI-2564 is completely programmable by software. The control of the relay switches 1, 2 and 3 is realized through an upper computer LabVIEW programming in the lithium battery charging and discharging automatic monitoring system so as to realize the switching between the charging mode and the discharging mode.
The invention discloses a self-healing-based lithium battery physical model construction method and a self-healing-based lithium battery physical model monitoring system, which have the advantages and effects that: aiming at the self-healing phenomenon in the discontinuous discharging process of the lithium battery, a physical equivalent model of the lithium battery is provided, mathematical description is carried out on the model, and a voltage description function after the battery is converted from a discharging state to a standing state is obtained. A self-healing related experiment is designed by using a LabView-based lithium battery test platform, and parameters of the model are determined through experimental test data. The experimental result proves that the model can accurately describe the voltage change condition of the battery in the standing state for different self-healing time, the error between the model analog value and the measured value is maintained in a stable range, and the model has higher accuracy.
Drawings
Fig. 1 shows a lithium battery with continuous discharge compared to intermittent discharge.
Fig. 2 is a circuit diagram showing a physical equivalent model of a lithium battery.
Fig. 3 shows the voltage and current changes of the lithium battery cell in the HPPC test.
Fig. 4 is a schematic diagram of a self-healing test.
FIG. 5 is a block diagram of a LabView-based lithium battery monitoring system.
FIG. 6 is a flow chart of an embodiment of the present invention.
Fig. 7(a) to 7(c) are graphs comparing the model simulation value and the experimental measurement value when the self-healing time is 60min according to the embodiment of the present invention. Fig. 8 shows an error comparison between the model simulation value and the experimental measurement value when the self-healing time is 60min according to the embodiment of the invention.
Fig. 9(a) to 9(c) are graphs comparing the model simulation value and the experimental measurement value when the self-healing time is 15min according to the embodiment of the present invention. Fig. 10(a) to 10(c) are graphs comparing the model simulation value and the experimental measurement value when the self-healing time is 90min according to the embodiment of the present invention. FIG. 11 is a comparison of the error between the model simulation value and the experimental measurement value when the self-healing time is 15min according to the embodiment of the present invention.
FIG. 12 is a comparison of the error between the model simulation value and the experimental measurement value when the self-healing time is 90min according to the embodiment of the present invention.
Detailed Description
The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.
The invention relates to a lithium battery physical model construction method based on self-healing, as shown in figure 2, the model comprises a battery open-circuit voltage UocInternal resistance R of the batteryi，RiRespectively comprises discharge resistors RdisAnd a charging resistor RchParallel RC network and capacitor CsAnd a resistance Rs. Wherein the open circuit voltage UocDescribing the voltage characteristics of the battery, capacitance CsThe capacity characteristics of the battery are described. The model can be divided into two parts, wherein one part describes the voltage of the lithium battery in the discharging stage and the other part describes the self-healing phenomenon voltage of the lithium battery in the standing stage. The following is specifically introduced:
1.2. description of the model
When the battery reaches a stable state, the voltage of the battery does not change any more, and the terminal voltage is equal to the open-circuit voltage U of the batteryocIs also equal to the capacitance CsI.e.:
UL＝UOC＝UCs(1)
1.2.1. discharged state battery description
And describing the discharge state of the lithium battery physical model by using an HPPC test. When the battery passes the discharge pulse current, the voltage of the battery undergoes two changes, a rapid voltage change process and a slow voltage change process, and fig. 3 shows the change of the voltage of the battery when the battery passes the HPPC test.
In the stage of rapid voltage change, the sharp drop of the voltage is mainly caused by the internal resistance R of the batteryiInfluence of, internal resistance RiThe value of (c) is related to the variation value of the voltage, as shown in equation (2); when the battery is charged with ILDuring constant current discharge, the voltage changes slowly, in order to simplify the model, the capacitance of the battery can be considered to be in an open circuit state, the change value of the battery is represented by the parallel RC network part, and the voltage change value of the battery can be represented by formula (3):
load R connected across the batteryLCan be measured from the end of the batteryThe ratio of voltage to current yields:
RL＝UL-F/IL(4)
1.2.2. static State Battery description
The values of all the parameters of the model cannot be obtained through the HPPC test alone, and in order to better describe the self-healing phenomenon of the battery, the self-healing test is necessary for the battery, and a schematic diagram of the self-healing test is shown in fig. 4. And carrying out a self-healing test once in a single charge-discharge cycle of the lithium battery. When the charging is finished, the battery is discharged at constant current, and t0At the moment when the battery voltage reaches UL-SRWhen the battery is in the static state, no current passes through the RC parallel network, so the model can be simplified into the form in fig. 2.
At t0At (-) time, battery is in discharge state, capacitor CsThe voltage across can be represented by equation (5); at t0At the moment (+) the battery is in a static state and the capacitor CsCan be expressed by equation (6). T is the voltage of the capacitor cannot be suddenly changed0Capacitance at (-) time CsVoltage value of and t0The voltage value at the time (+) is equal, i.e., as shown in equation (7):
UCS(t0-)＝UL-SR(t0-)+IL*RP＝UOC-SR-Ui(t0-) (5)
UCs(t0+)＝UL-SR(t0+)-URs(t0+) (6)
UCs(t0)＝UCs(t0-)＝UCs(t0+) (7)
from equations (5), (6) and (7), it can be derived that the voltage variation value when the battery is converted from the discharge state to the rest state:
△UL-SR＝URs(t0+)+IL*RP(8)
when the batteryWhen the battery is left standing for a sufficiently long time, the battery is in a stable state, and the capacitance C is known from the formula (1)sWill be equal to the open circuit voltage U of the batteryoc-SR. However, the capacitance CsAt t0The voltage value at the moment is less than Uoc-SRComparing equation (5) with equation (9) shows that the ideal voltage source U is at △ t when the battery is at restOCCharging the capacitor Cs and flowing through the capacitor CsHas a current of IsTime constant of τ, capacitance CsElectric quantity of Qcs：
UCs(△t→∞)＝UOC-SR(9)
τ＝CS*(Ri+RS) (11)
QCs＝UCs*CS(12)
When the lithium battery is converted from a discharge state to a standing state, the capacitor CsThe voltage across satisfies formula (11):
UCs＝UOC-SR-[UOC-SR-(UL-SR(t0-)+IL*RP)]*exp[-t/CS/(RS+Ri)](13)
terminal voltage U at the same timeL-SRAnd the capacitor voltage UcsSatisfies the relationship shown in formula (6), so that the terminal voltage U can be derived from formula (6), formula (8), formula (10) and formula (12)L-SRThe functional relation is satisfied:
in order to monitor the discharge state and the standing state of the physical model of the lithium battery, the invention designs a charge and discharge monitoring system, specifically a lithium battery charge and discharge monitoring system based on LabView is established, as shown in FIG. 5. The charging and discharging monitoring system takes LabView2012 as a software development platform and comprises a voltage acquisition card, a power supply module, a current acquisition module and a relay module; the charging circuit also comprises a constant current charging circuit, a constant voltage charging circuit and a discharging circuit, wherein I1 represents constant current charging current, I2 represents constant voltage charging current, and I3 represents discharging current. The three loops are controlled to be switched on or off through three relay switches of the relay module respectively. The current acquisition module converts a circuit current signal into a voltage signal, the output end of the current acquisition module is connected with the input end of a certain channel of the voltage acquisition card, and the voltage acquisition card is used for conditioning and performing analog-to-digital conversion on the signal and transmitting the signal to an upper computer. The voltage acquisition card transmits the voltage and current information of the lithium battery to the PC, and the upper computer programs and controls the state of the relay switch module, so that switching among different loops is realized. The system realizes the programming control of the discharging process of the lithium battery, simulates the conversion from the discharging state to the standing state of the lithium battery in the actual using process, and monitors the self-healing phenomenon of the lithium battery. The upper computer is responsible for switching of the circuit state, and then controls the conversion of the lithium battery among three states of charging, discharging and standing. Meanwhile, the upper computer can realize real-time monitoring of the circuit state of the lithium battery.
(1) The voltage acquisition module: PXIe-4300
The voltage acquisition module PXIe-4300 comprises 8 analog input channels, each channel is provided with an independent analog-to-digital converter, synchronous sampling can be carried out, and the high efficiency of data acquisition is ensured; the voltage input range of the voltage acquisition module is +/-300V, and six gears of 1V/2V/5V/10V/30V/300V can be selected respectively; the resolution of PXIe-4300 is 16 bits, and the data acquisition precision is high.
(2) The current acquisition module: siglent CP 401
The current acquisition module specifically adopts a non-contact current sensor Siglent CP 401 based on the Hall principle, which is produced by Dingyang science and technology Limited company (Siglent), and is used for monitoring the circuit current. The percentage error of the output signal of the Siglent CP 401 is 3% +/-5 mV, so that the accuracy is high, and the reliability of circuit current information is ensured; the current-voltage conversion ratio is 100mV/A, the sensitivity is high, and the linearity is better; the conversion rate can reach 0.3V/mu s, and the response time is faster. Siglent CP 401 can be used for measuring the current of the range of 50mA to 10A, and the system charge-discharge current is within the range of 2A, so that the requirements of experimental design are met. In the actual use process, the Siglent CP 401 converts the circuit current signal into a voltage signal, the output end of the Siglent CP 401 is connected with the input of a certain channel of the PXIe-4300, and the PXIe-4300 data acquisition card is used for signal conditioning and analog-to-digital conversion and transmitting the signal to the upper computer.
(3) A power supply module: NI PXI-4130
PXI-4130 is a programmable Source Measurement Unit (SMU) in which both channels can operate as either a constant voltage source or a constant current source. The constant voltage source mode and the constant current source mode both have settable clamp limit values, ensuring the stability of output voltage values and current values. The selection of the constant voltage source or constant current source may be controlled by programming. The charging process of the lithium battery can be divided into two steps of constant Current charging and constant voltage charging, wherein an Output Function (DC Current) is set to be Output by a Channel 0 of PXI-4130, and the Current value (Current Level) is 1A; the Channel 1 outputs a constant Voltage (Output Function: DC Voltage) with a Voltage Level of 4.2V.
(4) A relay module: NI PXI-2564
PXI-2564 is a generic type of switch module with independent Single Pole Single Throw (SPST) Form A relays. PXI-2564 has 16 channels, each relay is a non-latching relay, has extremely low on-resistance and low thermal offset. PXI-2564 is completely programmable by software. The control of the relay switches 1, 2 and 3 is realized through an upper computer LabVIEW programming in the lithium battery charging and discharging automatic monitoring system so as to realize the switching between the charging mode and the discharging mode.
Example (b):
experiment one experiment was conducted on a NCR18650B lithium cell.
The test protocol is shown in figure 6. In order to obtain the initial capacity of the lithium battery, a SCT test (static capacity test) was performed on the lithium battery, and the SCT test was repeated 3 times; and then, carrying out a discharge state on the battery, carrying out an HPPC (hybrid pulse power performance test) test, and recording test data of the battery. The key of the experimental scheme is to carry out self-healing characteristic test on the lithium battery. And when the voltage of the lithium battery is firstly reduced to U, the working state of the battery is changed, and the discharging state is converted into the standing state. The value of the conversion voltage is related to the SOC and can influence the self-healing strength of the battery in a standing state, the value of the conversion voltage is selected to be 3V in an experiment, namely when the voltage of the battery is firstly reduced to 3V, the battery enters the standing state to perform self-healing test, and the value of the SOC can be obtained by a data recorder of an experiment system through a coulomb counting method.
The test is carried out on the self-healing time (namely the time in a standing state) of 15min,60min and 90min respectively, and each self-healing time test is repeated for 3 times.
(II) analysis of results
There is a relationship between the self-healing characteristics and the self-healing time, so it is very important to select the proper self-healing time. In order to observe obvious self-healing characteristics, first test data with the self-healing time of 60min are selected, the model is subjected to parameter identification, and the simulation value and the measured value of the battery model in the standing stage are compared. Meanwhile, the model and the parameters obtained by the experiment are applied to the same self-healing time and different experiments and multiple groups of experiments with different self-healing times, and the result is analyzed.
1. Model parameter identification
The unknown parameters of the battery model are identified by using the test data, which is the basis of result analysis and discussion. Matlab provides a number of methods to achieve a fit to a function, such as Cftool. The Cftool has a visual interactive interface and provides a plurality of fitting functions, so the Cftool is selected as a fitting tool by the method. The parameter identification results are shown in table 1 below:
TABLE 1
2. Battery model evaluation
After model parameters are obtained from the first experimental test data with the self-healing time of 60min, the simulation values of the models are compared with the actual measurement values, and fig. 7(a) to 7(c) represent the comparison results of experiment 1, experiment 2 and experiment 3 when the simulation values of the models and the self-healing time of 60 min. After the lithium battery is converted from the discharging state to the standing state, the model can better describe the state of the lithium battery at the voltage value rising stage. And when the standing time is long enough, the model analog value is still close to the terminal voltage of the lithium battery. The experiment with the standing time of 60min is repeated, the test data is compared with the simulation value of the model, the model can still simulate the data of the experiment of the same lithium battery at different times within the same self-healing time, and the results are shown in fig. 7(b) and 7 (c).
Fig. 8 shows the error characteristics between the model simulation values and the experimental measurement values at a self-healing time of 60 min. When the battery starts to be converted from a discharge state to a static state, the voltage of the battery changes sharply, and the model has a large error in the short period. However, after the voltage of the battery changes for a short time, the model can better describe the voltage state of the lithium battery, and as the voltage change trend of the battery approaches to be flat, the model error also slowly decreases and approaches to 0.
Meanwhile, in order to prove that the model can describe the states of the lithium batteries with different self-healing times, experiments with the self-healing times of 15min and 90min are designed, parameters obtained from experimental test data with the self-healing time of 60min are applied to the experimental data with the self-healing times of 15min and 90min, and comparison results of the model and the experimental data are shown in fig. 9(a) (b) (c) and fig. 10(a) (b) (c). For experimental data with self-healing time of 15min and 90min, the model can still better describe the change of the battery voltage.
Fig. 11 and 12 are error characteristic curves between the model simulation values and the experimental measurement values for the self-healing time of 15min and 90min, respectively. Similar to the error curve when the self-healing time is 60min, a large error exists between the model and the experimental data in the state transition stage. But the error between the model and the test data slowly decreases over time and the variation in error tends to level off.
3. Battery model accuracy analysis
The accuracy of the battery model will be analyzed by the maximum error (maximum value of voltage error), the mean of the errors (mean of voltage errors), RMSE (root mean square error of voltage values). The experimental test data with the self-healing time of 15min,60min and 90min are analyzed, and the results are shown in table 2:
TABLE 2
In 3 tests with the self-healing time of 60min, the maximum errors are all less than 70mV, the maximum error rates are not more than 2%, and the root mean square errors are all less than 80 mV; when the error analysis is carried out on experimental test data with the self-healing time of 15min and 90min by using a model, the maximum error does not exceed 70mV, the maximum error rates of a third group of experiments except the experimental test with the self-healing time of 90min are slightly larger than 2%, the maximum error rates of other tests are smaller than 2%, and the root mean square error of 6 times of experiments with the self-healing time of 2 groups is smaller than 15 mV. Therefore, the model has higher accuracy and efficiency.
Claims (2)
1. A lithium battery physical model construction method based on self-healing is characterized in that: the model includes a battery open circuit voltage UocInternal resistance R of the batteryi，RiRespectively comprises discharge resistors RdisAnd a charging resistor RchParallel RC network and capacitor CsAnd a resistance Rs(ii) a Wherein the open circuit voltage UocDescribing the voltage characteristics of the battery, capacitance CsCharacterizing the capacity of the battery; the model can be divided into two parts, and one part describes lithium cell discharge phase voltage and another part describes lithium cell self-healing phenomenon voltage of stage of stewing, specifically as follows:
1.1. description of the model
When the battery reaches a stable stateIn this state, the battery voltage no longer changes, and the terminal voltage is equal to the open-circuit voltage U of the batteryocIs also equal to the capacitance CsI.e.:
UL＝UOC＝UCs(1)
1.1.1. discharged state battery description
Describing the discharge state of the lithium battery physical model by utilizing an HPPC test; when the battery passes through the discharge pulse current, the voltage of the battery can go through two change processes, namely a voltage rapid change process and a voltage slow change process;
in the stage of rapid voltage change, the sharp drop of the voltage is mainly caused by the internal resistance R of the batteryiInfluence of, internal resistance RiThe value of (c) is related to the variation value of the voltage, as shown in equation (2); when the battery is charged with ILDuring constant current discharge, the voltage changes slowly, in order to simplify the model, the capacitance of the battery can be considered to be in an open circuit state, the change value of the battery is represented by the parallel RC network part, and the voltage change value of the battery can be represented by formula (3):
load R connected across the batteryLThe resistance value of (a) can be obtained from the ratio of the terminal voltage and the current of the battery:
RL＝UL-F/IL(4)
1.1.2. static State Battery description
The values of all the parameters of the model can not be obtained through HPPC test alone, and the self-healing test needs to be carried out on the battery: performing a self-healing test within a single charge-discharge period of the lithium battery; when the charging is finished, the battery is discharged at constant current, and t0At the moment when the battery voltage reaches UL-SRWhen the battery is in the standing state, the battery is switched to the standing state and is subjected to a self-healing process with the time interval of △ t, and when the battery is in the standing state, the battery is subjected to RC combinationNo current passes through the network, so the model can be simplified;
at t0At (-) time, battery is in discharge state, capacitor CsThe voltage across can be represented by equation (5); at t0At the moment (+) the battery is in a static state and the capacitor CsCan be expressed by equation (6); t is the voltage of the capacitor cannot be suddenly changed0Capacitance at (-) time CsVoltage value of and t0The voltage value at the time (+) is equal, i.e., as shown in equation (7):
UCS(t0-)＝UL-SR(t0-)+IL*RP＝UOC-SR-Ui(t0-) (5)
UCs(t0+)＝UL-SR(t0+)-URs(t0+) (6)
UCs(t0)＝UCs(t0-)＝UCs(t0+) (7)
from equations (5), (6) and (7), it can be derived that the voltage variation value when the battery is converted from the discharge state to the rest state:
△UL-SR＝URs(t0+)+IL*RP(8)
when the battery is in a stable state for a long enough time, the capacitance C is shown by the formula (1)sWill be equal to the open circuit voltage U of the batteryoc-SR(ii) a However, the capacitance CsAt t0The voltage value at the moment is less than Uoc-SRComparing equation (5) with equation (9) shows that the ideal voltage source U is at △ t when the battery is at restOCCharging the capacitor Cs and flowing through the capacitor CsHas a current of IsTime constant of τ, capacitance CsElectric quantity of Qcs：
UCs(△t→∞)＝UOC-SR(9)
τ＝CS*(Ri+RS) (11)
QCs＝UCs*CS(12)
When the lithium battery is converted from a discharge state to a standing state, the capacitor CsThe voltage across satisfies formula (13):
UCs＝UOC-SR-[UOC-SR-(UL-SR(t0-)+IL*RP)]*exp[-t/CS/(RS+Ri)](13)
terminal voltage U at the same timeL-SRAnd the capacitor voltage UcsSatisfies the relationship shown in formula (6), so that the terminal voltage U can be derived from formula (6), formula (8), formula (10) and formula (12)L-SRThe functional relation is satisfied:
2. a monitoring system using the self-healing-based lithium battery physical model construction method according to claim 1, wherein the monitoring system comprises: the charging and discharging monitoring system for the lithium battery based on LabView takes LabView2012 as a software development platform and comprises a voltage acquisition card, a power supply module, a current acquisition module and a relay module; the three loops are controlled to be switched on or switched off through three relay switches of the relay module respectively; the current acquisition module converts a circuit current signal into a voltage signal, the output end of the current acquisition module is connected with the input of a certain channel of the voltage acquisition card, and the voltage acquisition card is used for conditioning and performing analog-to-digital conversion on the signal and transmitting the signal to an upper computer; the voltage acquisition card transmits the voltage and current information of the lithium battery to the PC, and the upper computer programs and controls the state of the relay switch module, so that switching among different loops is realized; the system realizes the control of the discharging process of the lithium battery, simulates the conversion from the discharging state to the standing state of the lithium battery in the actual using process, and monitors the self-healing phenomenon of the lithium battery; the upper computer is responsible for switching the circuit state, and then controls the conversion of the lithium battery among three states of charging, discharging and standing; meanwhile, the upper computer can realize real-time monitoring of the circuit state of the lithium battery; the method comprises the following specific steps:
(1) the voltage acquisition module: PXIe-4300
The voltage acquisition module PXIe-4300 comprises 8 analog input channels, each channel is provided with an independent analog-to-digital converter, and synchronous sampling can be carried out; the voltage input range of the voltage acquisition module is +/-300V, and six gears of 1V/2V/5V/10V/30V/300V can be selected respectively; the resolution ratio of PXIe-4300 is 16 bits, and the data acquisition precision is high;
(2) the current acquisition module:
the current acquisition module specifically adopts a non-contact current sensor based on the Hall principle and is used for monitoring the circuit current; the percentage error of the output signal is 3% +/-5 mV; the current-voltage conversion ratio is 100 mV/A; the slew rate was 0.3V/. mu.s; the current sensor converts a circuit current signal into a voltage signal, the output end of the current sensor is connected with the input of a certain channel of the voltage acquisition module, the data acquisition module is used for carrying out signal conditioning and analog-to-digital conversion and transmitting the signal to an upper computer;
(3) a power supply module:
the power supply module is specifically a programmable source measurement unit, and both channels can be used as constant voltage sources or constant current sources to operate; the constant voltage source mode and the constant current source mode both have settable clamp limit values; the selection of the constant voltage source or the constant current source can be controlled by programming; the lithium battery charging process can be divided into two steps of constant current charging and constant voltage charging, wherein a channel 0 of a voltage acquisition module is set to output a constant current, and the current value is 1A; 1 channel outputs constant voltage, and the voltage value is 4.2V;
(4) a relay module:
the relay module is a universal switch module and is provided with an independent single-pole single-throw Form A relay; the relay module is provided with 16 channels, and each relay is a non-latching relay; the control of a relay switch is realized through an upper computer in the automatic monitoring system for the charging and discharging of the lithium battery so as to realize the switching between a charging mode and a discharging mode.
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