CN107843846B - A kind of health state of lithium ion battery estimation method - Google Patents

Abstract

A method for estimating the state of health of a lithium-ion battery, the method is as follows: Obtain the discharge voltage, discharge current, and discharge time data of the lithium-ion battery before and after aging and charging and discharging with a certain working system; intercept the data obtained in the first step at the same load Discharge voltage, discharge current, and discharge time data in the battery state interval; define the expression of the health status indicator factor of the lithium-ion battery; obtain the V _{lower limit} value in the third step; obtain the HI value in the formula, and obtain the health status of the battery . The advantages of the present invention are: the present invention starts from the apparent data (discharge voltage, discharge current, discharge time) that can be measured in real time when the battery is working, and the obtained method for estimating the state of health of the lithium-ion battery has parameters that are easy to obtain, universal and applicable. The characteristics of real-time application, it is easy to embed into the battery management system to estimate the state of health of the battery, which greatly improves the shortcomings of the existing test system or test equipment, and is very practical.

Description

A method for estimating state of health of lithium ion battery

technical field

The invention relates to a method for estimating the state of health of a lithium ion battery.

Background technique

The state of health (State of Health, SOH) of a lithium-ion battery is often used to characterize the aging degree of the battery. At present, there are two main definition methods, one: the maximum available capacity of the battery after aging and the maximum available capacity of the battery when it is fully healthy Percentage; Two: The percentage of the impedance of the battery after aging and the impedance of the battery when it is fully healthy. Accurate and timely estimation of the state of health of the battery is not only of great significance for the safe and efficient operation of the battery, but also can provide data support for the prediction of the battery's remaining usable life.

In the existing methods for estimating the state of health of lithium-ion batteries, the acquisition of the capacity value needs to use the constant current and constant voltage method to charge the battery to a fully charged state under constant temperature conditions, and then use the constant current method to discharge the battery to the cut-off voltage, and Cycle charge and discharge at least 3 times, and take the average value as the maximum available capacity value of the current state of the battery. Not only does it take a long time, but in most cases the battery is in dynamic charge and discharge or shallow charge and discharge, so this method is difficult to achieve real-time online application. The acquisition of impedance value mainly includes pulse method and electrochemical impedance spectroscopy method, but these two methods have great disadvantages. On the one hand, it is necessary to ensure that the state of charge and temperature of the battery before and after aging are the same. On the other hand, the former requires the test equipment to have High enough measurement accuracy, while the latter requires specific test equipment. In addition, because the change of the impedance value of the battery before and after aging is small, the health state estimation method based on impedance is difficult to be practically applied.

Contents of the invention

The purpose of the present invention is to provide a method for estimating the state of health of a lithium-ion battery to solve the problem that the existing method for estimating the state of health of a lithium-ion battery requires specific test experiments or specific test equipment, which makes it difficult to obtain parameters and is not suitable for online real-time applications.

In order to achieve the above object, the technical scheme that the present invention takes is as follows:

A lithium ion battery state of health estimation method, the state estimation method may further comprise the steps:

Step 1: Obtain the discharge voltage, discharge current, and discharge time data of the lithium-ion battery before and after aging and charging and discharging with a certain working system; the working system includes charging and discharging mode, charging and discharging current and temperature;

Step 2: Intercept the discharge voltage, discharge current, and discharge time data of the data obtained in step 1 within the same SOC range;

Step 3: Propose the expression of the indicator factor of the state of health of the lithium-ion battery:

Wherein, the HI is the state of health value of the lithium-ion battery, _t01 is the starting moment of the battery corresponding to the same state of charge interval before aging, and _t0 is the starting moment of the battery corresponding to the same state of charge interval after aging, t _end1 is the termination time of the battery corresponding to the same state of charge range before aging, t _end is the termination time of the battery corresponding to the same state of charge range after aging, V _fresh (t) and V _aged (t) are respectively And the discharge voltage data in the same state of charge interval in step 2 after aging, V _{lower limit} is the discharge cut-off voltage value when the lithium-ion battery is fully discharged, and I(t) is the lithium ion battery in the same state of charge interval in step 2 battery discharge current;

Step 4: Set the V _{lower limit} value;

Step 5: Obtain the HI value according to the formula (3), so as to obtain the state of health of the battery.

The above-mentioned method for estimating the state of health of a lithium-ion battery is applicable to lithium-ion battery cells and battery packs of all material systems.

The beneficial effect of the present invention relative to the prior art is: the present invention starts from the apparent data (discharge voltage, discharge current, discharge time) that can be measured in real time when the battery is working, and the obtained lithium-ion battery health state estimation method has parameters that are easy to obtain , Universal, and real-time application characteristics, easy to embed in the battery management system to estimate the health status of the battery, only need to use the discharge voltage, discharge current, and discharge time data in the same charge state interval when the battery is working to obtain the battery in real time The estimated value of the state of health overcomes the shortcomings of the existing state of health estimation methods based on capacity or impedance that require standard testing systems or testing equipment, and has strong practicability. And the influence of temperature on battery discharge performance can be further introduced into the lithium-ion battery state of health estimation method to obtain an improved lithium-ion battery state of health estimation method with a wider application range. In addition, this lithium-ion battery state-of-health estimation method can be used for rapid evaluation of the optimal operating temperature of the battery, rapid screening of battery consistency, and preliminary rapid evaluation of the performance of electrode materials.

Description of drawings

Figure 1 is a constant current discharge curve diagram of lithium-ion batteries in different health states;

The moment represented by baseline 1 indicates that the state of charge of the battery is 100%, the moment represented by baseline 2 represents the discharge cut-off voltage of the lithium-ion battery, the moment represented by baseline 3 indicates that the battery is discharged to the same state of charge, square marks, circle marks, upper regular triangle marks The curves marked with the regular triangle mark and the lower mark represent the constant current discharge curves when the battery health status is 100%, 91.7%, 85.3% and 80%, respectively. Here, for the convenience of comparison, the discharge current before and after battery aging is taken as a constant value.

Fig. 2 is the relationship diagram between the state of health and the number of cycles using the capacity value obtained by the standard performance test as the state of health factor;

Fig. 3 is a relationship diagram between the health status and the number of cycles obtained by the health status estimation method proposed by the present invention.

Detailed ways

The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and embodiments, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should cover In the protection scope of the present invention.

Embodiment 1: This embodiment describes a method for estimating the state of health of a lithium-ion battery, and the method for estimating the state includes the following steps:

Step 1: Obtain the discharge voltage, discharge current, and discharge time data of the lithium-ion battery before and after aging and charging and discharging with a certain working system; the working system includes charging and discharging mode, charging and discharging current and temperature;

Step 2: Intercept the discharge voltage, discharge current, and discharge time data of the data obtained in step 1 within the same SOC range;

Step 3: Propose the expression of the indicator factor of the state of health of the lithium-ion battery:

Wherein, the HI is the state of health value of the lithium-ion battery, _t01 is the starting moment of the battery corresponding to the same state of charge interval before aging, and _t0 is the starting moment of the battery corresponding to the same state of charge interval after aging, t _end1 is the termination time of the battery corresponding to the same state of charge range before aging, t _end is the termination time of the battery corresponding to the same state of charge range after aging, V _fresh (t) and V _aged (t) are respectively And the discharge voltage data in the same state of charge interval in step 2 after aging, V _{lower limit} is the discharge cut-off voltage value when the lithium-ion battery is fully discharged, and I(t) is the lithium ion battery in the same state of charge interval in step 2 battery discharge current;

Step 4: Set the V _{lower limit} value;

Step 5: Obtain the HI value according to the formula (3), so as to obtain the state of health of the battery.

Embodiment 2: In the method for estimating the state of health of a lithium-ion battery described in Embodiment 1, in step 1, the working system is that the temperature and current are the same or the changes are the same in the front and back cycles.

Specific embodiment three: a kind of method for estimating the state of health of a lithium-ion battery described in specific embodiment one, the specific process of step one is:

Use a certain working system to charge and discharge the lithium-ion battery before and after aging, and obtain the discharge voltage, discharge current, and discharge time data of the lithium-ion battery at different aging stages. As shown in Figure 1, it is an example diagram of charging a lithium-ion battery to 100% state of charge by constant current and constant voltage method, and then discharging to 70% state of charge by constant current method. The charging and discharging system adopted by the curve in the figure remains the same before and after the aging of the lithium-ion battery.

Embodiment 4: A method for estimating the state of health of a lithium-ion battery described in Embodiment 1, the specific process of step 2 is:

Set the start value and end value of the expected state of charge interval, and intercept the discharge voltage, discharge current, and discharge time data in this interval of the data obtained in step 1 as the input data of the state of health estimation method.

Embodiment 5: A method for estimating the state of health of a lithium-ion battery described in Embodiment 1, the specific basis for obtaining the expression of the indicator factor of the state of health of the lithium-ion battery in the step 3 is:

Lithium-ion batteries will age with cyclic charging and discharging or long-term standing. A solid electrolyte interface film is formed on the negative electrode of the battery and continues to thicken, resulting in an increase in the impedance of the battery and consuming electrolyte and available lithium ions at the same time. Therefore, when the battery is used When the same working system is charged and discharged, its polarization voltage gradually increases with aging, which is shown in the graph of the relationship between the discharge curve and the discharge time (as shown in Figure 1): at the same state of charge, the lithium-ion battery The discharge voltage curve of the battery gradually moves down in the direction of the vertical axis with the aging of the battery, that is to say, in the same charge state interval, the discharge curve of the battery is consistent with the discharge start axis (baseline 1), the end axis (baseline 3) and the battery The area of the graph enclosed by the discharge cut-off voltage axis (baseline 2) at full discharge gradually decreases as the battery ages, as shown in Figure 1

Wherein, the S _i is the area of the graph surrounded by the discharge curve and the baseline axis when the battery is discharged for the ith time, as shown in Figure 1, i is the number of cycles, _t0 and t _end are respectively corresponding to the baseline in Figure 1 1 and baseline 3, V _i (t) is the change data of the i-th discharge voltage V of the battery with time t, because it is generally recognized in the industry that when the discharge voltage of a lithium-ion battery drops to the discharge cut-off voltage, the battery cannot continue to Doing work, therefore, V _{lower limit} here takes the discharge cut-off voltage value when the lithium-ion battery is fully discharged;

In order to introduce the influence of the discharge current I(t) on the discharge curve of the lithium-ion battery, the formula (2) is obtained,

Wherein, the W _i is the working ability of the battery in the interval between t ₀ and t _end when the battery is discharged for the i time, it can be seen that this value also decreases with the aging of the battery, i is the number of cycles, t ₀ and t end t _end is the time represented by baseline 1 and baseline 3 in Figure 1, V _i (t) is the data of the i-th discharge voltage of the battery as a function of time, and V _{lower limit} is taken here when the lithium-ion battery is fully discharged discharge cut-off voltage value.

Based on the formula (2), the state of health estimation method of the lithium-ion battery is obtained, as shown in the formula (3)

Wherein, the HI is the state of health value of the lithium-ion battery, _t01 is the starting moment of the battery corresponding to the same state of charge interval before aging, and _t0 is the starting moment of the battery corresponding to the same state of charge interval after aging, t _end1 is the termination time of the battery corresponding to the same state of charge range before aging, t _end is the termination time of the battery corresponding to the same state of charge range after aging, V _fresh (t) and V _aged (t) are respectively And the discharge voltage data in the same state of charge interval in step 2 after aging, V _{lower limit} is the discharge cut-off voltage value when the lithium-ion battery is fully discharged, and I(t) is the lithium ion battery in the same state of charge interval in step 2 The discharge current of the battery.

Embodiment 6: In the method for estimating the state of health of a lithium-ion battery described in Embodiment 1, in step 4, the value requirement of the V _{lower limit} is:

The value of V _{lower limit} can be different according to different lithium-ion battery systems. The most convenient and general method that facilitates the comparison of HI changes in formula (3) is to take the discharge cut-off voltage when the lithium-ion battery is fully discharged as V The value of _{lower limit} , the discharge cut-off voltage of different lithium-ion battery systems when the battery is fully discharged has different values, but the discharge cut-off voltage of lithium-ion battery systems with specific electrode materials is fixed, such as lithium cobalt oxide/graphite batteries, The discharge cut-off voltage of its complete discharge is 3V. In addition, the value of this parameter is not limited to the discharge cut-off voltage when the battery is fully discharged, it can take any value in theory, but for the sake of comparison, the principle of V _{lower limit} is that the selected value of V _{lower limit} needs to make the formula The HI value in (3) should only change significantly when the battery ages.

Embodiment 7: A method for estimating the state of health of a lithium-ion battery described in any of Embodiments 1 to 6 is applicable to all material systems (such as lithium manganate system, lithium nickelate system, ternary system, cobalt Lithium-ion battery, single cell and battery pack.

Example 1:

A method for estimating the state of health of a lithium-ion battery, the specific steps are as follows:

Step 1: Obtain the discharge voltage, discharge current, and discharge time data of the lithium-ion battery before and after aging and charging and discharging with the same working system;

A lithium cobalt oxide/graphite lithium-ion battery with a rated capacity of 1.2Ah was used as the experimental object. Before the experiment, the standard performance test (Reference Performance Test, RPT) was used to calibrate the capacity value of the battery. The specific method was: using the constant current and constant voltage method (0.6C constant current rate charge to 4.2V, maintain constant voltage 4.2V charge until the current drops to 0.05C) Charge it to a fully charged state, let it stand for 2min, then discharge it to 3V at a 0.6C constant current rate, cycle 3 times , taking the average value as the capacity value of the battery. Then, carry out cycle aging test on the battery, the specific method is: use constant current constant voltage method (1.5C constant current rate charge to 4.2V, keep constant voltage 4.2V charge until the current drops to 0.05C) to charge it to full charge state, let it stand for 2 minutes, and then discharge it at a constant current rate of 1.5C for 12 minutes, conduct a standard performance test on the battery every 100 cycles, and obtain the capacity value of the battery as a reference for battery aging, as shown in Figure 2.

Step 2: Intercept the discharge voltage, discharge current, and discharge time data of the data obtained in step 1 in the same charge state range:

In order to illustrate the feasibility of a method for estimating the state of health of a lithium-ion battery proposed in the present invention, the cycle aging test in step 1 is carried out between 100% state of charge and 70% state of charge, so intercept step 1 The discharge voltage, discharge current (1.5C), and discharge time (12min, or 720s) data between 100% state of charge (baseline 1) and 70% state of charge (baseline 3) are shown in Figure 1.

Step 3: Obtain the expression of the health status indicator factor of the lithium-ion battery;

Wherein, the HI is the state of health value of the lithium-ion battery, _t01 is the starting moment of the battery corresponding to the same state of charge interval before aging, and _t0 is the starting moment of the battery corresponding to the same state of charge interval after aging, t _end1 is the termination time of the battery corresponding to the same state of charge range before aging, t _end is the termination time of the battery corresponding to the same state of charge range after aging, V _fresh (t) and V _aged (t) are respectively And the discharge voltage data in the same state of charge interval in step 2 after aging, V _{lower limit} is the discharge cut-off voltage value when the lithium-ion battery is fully discharged, and I(t) is the lithium ion battery in the same state of charge interval in step 2 battery discharge current;

Step 4: Take the value of the V _{lower limit} in Step 3;

For the lithium cobalt oxide/graphite lithium ion battery system, V _{lower limit} =3V, which is the discharge cut-off voltage value when the battery is fully discharged.

Step 5: Obtain the HI value in formula (3) according to step 2, step 3 and step 4, and obtain the state of health of the battery;

As shown in Figure 3, it is the relationship between the state of health of the lithium ion battery obtained according to a method for estimating the state of health of the lithium ion battery proposed by the present invention along with the number of cycles. From the figure, it can be seen that this method can be used for the estimation of the state of health of the lithium ion battery , and the required input data (discharge voltage, discharge current, discharge time) can be measured in real time, so it can be used for real-time online estimation of the state of health of lithium-ion batteries, which greatly improves the existing methods for estimating the state of health of lithium-ion batteries that require specific calibration tests , or the disadvantage of requiring specific test equipment.

Claims (4)

1. A lithium-ion battery state of health estimation method, characterized in that: the state estimation method may further comprise the steps: Step 1: Obtain the discharge voltage, discharge current, and discharge time data of the lithium-ion battery before and after aging and charging and discharging with a certain working system; the working system includes charging and discharging mode, charging and discharging current and temperature; Step 2: Intercept the discharge voltage, discharge current, and discharge time data of the data obtained in step 1 within the same SOC range; Step 3: Propose the expression of the indicator factor of the state of health of the lithium-ion battery: Wherein, the HI is the state of health value of the lithium-ion battery, _t01 is the starting moment of the battery corresponding to the same state of charge interval before aging, and _t0 is the starting moment of the battery corresponding to the same state of charge interval after aging, t _end1 is the end time of the battery corresponding to the same state of charge interval before aging, t _end is the end time of the same state of charge interval after battery aging, V _fresh (t) and V _aged (t) are respectively And the discharge voltage data in the same state of charge interval in step 2 before aging, V _{lower limit} is the discharge cut-off voltage value when the lithium-ion battery is fully discharged, and I(t) is the lithium ion battery in the same state of charge interval in step 2 battery discharge current; Step 4: Set the V _{lower limit} value; Step 5: Obtain the HI value according to the formula (3), so as to obtain the state of health of the battery. 2. A method for estimating the state of health of a lithium-ion battery according to claim 1, wherein in step 1, the working system is that the temperature and current are the same or the changes are the same during the front and back cycles. 3. a kind of lithium-ion battery state of health estimation method according to claim 1, is characterized in that: the concrete process of step 1 is: Use a certain working system to charge and discharge the lithium-ion battery before and after aging, and obtain the discharge voltage, discharge current, and discharge time data of the lithium-ion battery at different aging stages. 4. a kind of lithium-ion battery state of health estimation method according to claim 1 is characterized in that: the concrete process of step 2 is: Set the start value and end value of the expected state of charge interval, and intercept the discharge voltage, discharge current, and discharge time data in this interval of the data obtained in step 1 as the input data of the state of health estimation method.