CN114019387A

CN114019387A - Method for representing SOC of lithium ion battery by ultrasonic reflection coefficient

Info

Publication number: CN114019387A
Application number: CN202111315011.5A
Authority: CN
Inventors: 宋国荣; 乔新中; 吕炎; 张斌鹏; 何存富; 吴斌
Original assignee: Beijing University of Technology
Current assignee: Beijing University of Technology
Priority date: 2021-11-08
Filing date: 2021-11-08
Publication date: 2022-02-08

Abstract

The invention discloses a method for representing the SOC of a lithium ion battery by an ultrasonic reflection coefficient. The method comprises the steps of carrying out frequency spectrum analysis on received body wave signals, enabling the reflection coefficient of a lithium ion battery to change when the state of charge (SOC) of the lithium ion battery changes, obtaining the frequency spectrum of the lithium ion battery under different states of charge (SOC) through ultrasonic water immersion detection, establishing a mapping relation between the frequency spectrum and the state of charge of the lithium ion battery, and representing the state of charge (SOC) of the lithium ion battery by valley coordinates of the frequency spectrum and the horizontal distance of a y axis. The invention can realize the nondestructive representation of the state of charge (SOC) of the lithium ion battery; the local SOC measurement of the lithium ion battery can be realized.

Description

Method for representing SOC of lithium ion battery by ultrasonic reflection coefficient

Technical Field

The invention belongs to the field Of ultrasonic nondestructive testing, and particularly relates to a method for representing the State Of Charge (SOC) Of a lithium ion battery.

Background

Under the influence of energy crisis and environmental crisis, developing green new energy becomes one of the important technical fields determining the economic development of the world. The lithium ion battery has the advantages of large energy density, good cycle performance, low self-discharge rate and the like, is a secondary battery and a chemical energy storage power supply with great application prospects, and is widely applied to the fields of new energy automobiles, consumer electronics, aerospace and the like. However, the capacity of the lithium ion battery is attenuated in the charge and discharge cycle process of the lithium ion battery, and the safety performance detection of the lithium ion battery, particularly the state of charge (SOC) and the state of health (SOH), is always a hot point problem for the nondestructive detection of the lithium ion battery.

The lithium ion battery is formed by laminating different materials, ultrasonic signals are complex to propagate, the internal structure of the lithium ion battery is slightly changed in the circulating process, the acoustic impedance is also changed, and the reflection coefficients of ultrasonic waves are different due to the mismatch of the acoustic impedance. The method realizes the representation of the SOC of the lithium ion battery by using the ultrasonic reflection coefficient frequency spectrum based on the mapping relation between the ultrasonic reflection coefficient frequency spectrum and the SOC of the lithium ion battery under different SOCs.

Disclosure of Invention

The invention designs a detection method for ultrasonically measuring the state of charge (SOC) of a lithium ion battery aiming at the problems. The technical scheme of the invention is as follows:

a method for representing the SOC of a lithium ion battery by an ultrasonic reflection coefficient method comprises the following specific steps:

step one, performing constant current discharge on the soft package lithium ion battery with the laminated structure at room temperature by adopting a charge-discharge device until the discharge cut-off voltage is reached, standing for a certain time, and performing constant current constant voltage charge on the soft package lithium ion battery after standing until the charge cut-off voltage is reached;

and step two, discharging the set soft package lithium ion battery to a discharge cut-off voltage by adopting a constant current, and obtaining N lithium ion batteries in different charge states according to different discharge time. The total discharge time T is the time taken by the battery to discharge from the charge cut-off voltage to the discharge cut-off voltage, the charge state of the lithium ion battery is represented by the proportion of the discharge time T, and the charge state calculation formula is as follows:

state of charge (SOC) of the battery is (T-T)/T100% SOC;

step three, building a detection device: an ultrasonic water immersion detection platform is built, a broadband ultrasonic probe is adopted, and an included angle between the two ultrasonic probes is fixed by an angle clamp;

step four, adopting an ultrasonic water immersion detection method, placing the lithium ion battery in a certain charge state obtained in the step two in a water tank, and selecting a broadband ultrasonic probe with certain central frequency to obtain an acoustic reflection signal of the lithium ion battery at a certain angle in a first excitation and first receiving mode;

step five, performing data processing on the time domain reflection signals obtained in the step four, and obtaining the frequency spectrum of the sound reflection coefficient of the lithium ion battery at the incident angle through Fourier transform;

and step six, repeating the step four to the step five until the sound reflection coefficient detection of the lithium ion battery obtained in the step two under all the charge states (0-100% SOC) is completed.

Step seven, drawing a lithium ion battery reflection coefficient frequency spectrum under 0% SOC-100% SOC according to all the reflection coefficients obtained in the step six;

and step eight, analyzing the change of the valley position of the reflection coefficient frequency spectrum, and acquiring the mapping relation between the reflection coefficient frequency spectrum and the state of charge of the lithium ion battery so as to represent the state of charge (SOC) of the lithium ion battery.

The invention has the following advantages:

1) the method realizes the detection of the state of charge (SOC) of the lithium ion battery by using a set of water immersion ultrasonic equipment, does not need to damage the battery structure in the detection process, realizes the nondestructive representation of the SOC of the lithium ion battery, and has obvious advantages in the aspects of implementation convenience, detection cost and detection safety.

2) According to the method, the change of the body wave frequency domain spectrum is utilized to represent the state of charge (SOC) of the lithium ion battery, external parameters such as current and voltage in the charging and discharging process of the battery do not need to be recorded, and the SOC of the battery can be accurately evaluated only by simply processing the received body wave signals; the invention provides a new idea for solving the technical problem aiming at the real-time, quick and accurate detection of the state of charge (SOC) of the soft package lithium ion battery.

Drawings

FIG. 1: 11 layers of structural models of the lithium ion battery;

FIG. 2: experimental device connection diagram;

FIG. 3: an angle clamp schematic;

FIG. 4: reflecting coefficient frequency spectrum three-dimensional graphs under different charge states;

FIG. 5: reflection coefficient frequency spectrum at a certain SOC:

Detailed Description

The working principle of the invention is as follows:

by utilizing ultrasonic water immersion detection, when the angle of an ultrasonic water immersion probe is fixed, the invention obtains the frequency domain spectrums of the reflection coefficients of the lithium ion batteries under different charge States (SOC) by detecting the acoustic reflection coefficients of the soft package lithium ion batteries under different charge states, and establishes the mapping relation between the reflection coefficients of the lithium ion batteries and the charge states so as to represent the charge states of the batteries.

The present invention will be described in further detail with reference to specific examples below:

example 1:

the method comprises the following steps:

taking a 0.7mm lithium ion soft package battery (laminated structure), and charging and discharging the lithium ion battery with the laminated structure by adopting ultrasonic charging and discharging equipment, wherein the lithium ion battery is discharged to a cut-off voltage of 3V, stands for 2min and is charged to a charging cut-off voltage of 4.2V.

Step two, the total time T for discharging the lithium ion battery in a full state to 3V at a rate of 1C is 50min, and the total time T is represented by a formula:

the State Of Charge (SOC) Of the battery is (T-T)/T100% SOC, wherein T is the discharge time, and T ranges from 0 to 50 min;

taking the time interval as 5min, and taking the discharge time t as 0min, 5min, 10min, 15min, 20min, 25min, 30min, 35min, 40min, 45min and 50min respectively; the states of charge of the lithium ion battery are respectively 100% SOC, 90% SOC, 80% SOC, 70% SOC, 60% SOC, 50% SOC, 40% SOC, 30% SOC, 20% SOC, 10% SOC and 0% SOC;

step four, the design of experimental device, for the measurement of ultrasonic reflection coefficient frequency spectrum under a certain contained angle, set up one set of measurement system that carries out variable angle ultrasonic reflection coefficient, this measurement system includes: the device comprises two ultrasonic water immersion probes (1, 2), an embedded controller (3), a digital oscilloscope (4), a battery detection system (5), a middle position machine (6), a water tank (7), an angle clamp (8), a lithium ion single cell test piece (9) and an upper computer (10). The coupling is shown in fig. 2. Two tabs of a lithium ion battery to be detected are connected into a battery detection system (5), the two tabs are subjected to waterproof treatment and immersed into a water tank filled with water together with an ultrasonic probe, the two ultrasonic probes (1 and 2) are fixed by an angle clamp (8), ultrasonic signals are excited and received at a certain angle, reflected signals are measured, an embedded controller (3) is connected with a transmitting ultrasonic probe (1) to send out the ultrasonic signals, and a digital oscilloscope (4) is connected with a receiving ultrasonic probe (2) and is connected with the embedded controller (3) to perform signal processing and acquisition. The battery detection system (5) monitors and controls the charge state of the lithium ion battery to be detected in real time, can change the SOC of the lithium ion battery, and connects the upper computer with the battery detection system through the intermediate computer (6) to perform signal transmission.

And step five, the angle clamp can realize the excitation and the receiving of ultrasonic waves of the ultrasonic transducer at different angles, and the angle can be adjusted and fixed. As shown in fig. 5, the angle variation ranges from (0-60 °).

And step six, selecting two ultrasonic water immersion probes with the center frequency of 1MHz, fixing the angle of 19 degrees by an angle clamp, sequentially selecting the lithium ion battery in a certain charge state in the step two, obtaining a time domain reflection signal, and obtaining a reflection coefficient frequency spectrum curve under a certain SOC through Fourier transform. And repeating the steps until obtaining the frequency spectrum curve of the reflection coefficient of the lithium ion battery under 11 charge states in the step three.

And step seven, drawing reflection coefficient frequency spectrums under different SOC, and fitting 11 reflection coefficient frequency curve graphs obtained under different charge states to form a three-dimensional graph, as shown in FIG. 4. From the obtained three-dimensional graph of the reflection coefficient frequency spectrum, the curve equation obtained by tracking the coordinates of the valley point is as follows:

F(x)＝-17.31*x²+3359 x +2.072e + 06; x is the frequency

As the state of charge of the lithium ion single cell increases, the three-dimensional frequency spectrum valley of the reflection coefficient shifts to a high frequency. And establishing a mapping relation between the ultrasonic reflection coefficient frequency spectrum and the charge state of the lithium ion battery so as to represent the SOC of the lithium ion battery.

And step eight, representing the state of charge (SOC) of an unknown lithium ion battery, wherein the change of the state of charge of the lithium ion battery is represented by the change of the horizontal distance between the valley value curve of the frequency spectrum obtained in the step seven and the y axis, so that the mapping relation between the SOC of the lithium ion battery and the frequency spectrum of the reflection coefficient is established, and the SOC of the lithium ion battery is represented. And (3) charging the same type of lithium ion battery by using charging and discharging equipment, and obtaining a reflection coefficient frequency spectrum curve (shown in figure 5) of the lithium ion battery in the charge state from the sixth step to the seventh step. And obtaining a frequency spectrum change curve of the reflection coefficient of the lithium ion battery under the SOC, calculating the horizontal distance between the valley value and the y axis, and comparing the horizontal distance between the valley value line of the reflection coefficient frequency spectrum and the y axis in the step seven to determine the charge state of the lithium ion battery to be measured. The comparison shows that the state of charge is 53.5% SOC.

Claims

1. The method for representing the SOC of the lithium ion battery by the ultrasonic reflection coefficient is characterized by comprising the following steps: the method comprises the following specific steps:

step two, discharging the set soft package lithium ion battery to a discharge cut-off voltage by adopting a constant current, and obtaining N lithium ion batteries in different charge states according to different discharge time; the total discharge time T is the time taken by the battery to discharge from the charge cut-off voltage to the discharge cut-off voltage, the charge state of the lithium ion battery is represented by the proportion of the discharge time T, and the charge state calculation formula is as follows:

battery state of charge SOC ═ T/T × 100% SOC;

step six, repeating the step four to the step five until the sound reflection coefficient detection of the lithium ion battery obtained in the step two under all the charge states is completed;

and step eight, analyzing the change of the valley position of the reflection coefficient frequency spectrum, and acquiring the mapping relation between the reflection coefficient frequency spectrum and the charge state of the lithium ion battery so as to represent the charge state SOC of the lithium ion battery.

2. The method of claim 1 for ultrasonic reflectance characterization of lithium ion battery SOC, wherein: the detection device for realizing the detection method comprises water immersion probes (1, 2), an embedded controller (3), a digital oscilloscope (4), a battery detection system (5), a middle position machine (6), a water tank (7), an angle clamp (8), a lithium ion battery test piece (9) and an upper computer (10); the upper computer (10) is connected with the middle position machine (6) and the embedded control machine (3), the embedded control machine (3) is connected with the water immersion probe (1) and the digital oscilloscope (4), the middle position machine (6) is connected with the battery detection system (5), the battery detection system (5) is connected with the lithium ion battery test piece (9), the digital oscilloscope (4) is connected with the water immersion probe (2), and the angle clamp (8) fixes the water immersion probes (1 and 2).

3. The method of claim 1 for ultrasonic reflectance characterization of lithium ion battery SOC, wherein: a group of water immersion probes is characterized in that: the center frequency of the water immersion probes is 1MHz, and the two water immersion probes are arranged on one side of the lithium ion battery test piece at a certain angle.

4. The method of claim 1 for ultrasonic reflectance characterization of lithium ion battery SOC, wherein: the characterization method is characterized in that: and processing the received reflection time domain signals to establish a three-dimensional reflection coefficient frequency spectrum, and representing the charge state of the soft package lithium ion battery by the position relation of a valley value in the frequency spectrum.