Backup power supply health state estimation method based on direct current internal resistance

Abstract

The invention belongs to the technical field of fan variable pitch backup power supplies, and particularly discloses a backup power supply health state estimation method based on direct current internal resistance, which comprises the following steps: firstly, measuring the direct current internal resistance DCR, then obtaining a health state SOHR, and further processing and analyzing the health state SOHR. According to the method, the DCR of the lithium battery can be used as a direct physical quantity for representing life attenuation, and theoretically, the life of the lithium battery attenuates under any working condition, so that the internal resistance of the lithium battery is increased, and the effectiveness of an SOH estimation result is very high.

Description

Technical Field

[0001] The invention relates to the technical field of a wind turbine pitch-changing backup power supply, and in particular to a method for estimating the health state of a backup power supply based on direct current internal resistance. Background Art

[0002] The wind turbine pitch backup power supply contains an energy storage system, and its operating conditions are significantly different from common power energy storage and industrial and commercial energy storage. The backup power supply is only started and discharged under emergency conditions, and the number of energy storage charge and discharge cycles is very small. In addition to emergency power supply, the backup power supply usually does not supply the load and cannot discharge the stored energy. The ambient temperature range for storage and use of the backup power supply is wide. Affected by the temperature characteristics of the lithium battery, the state of charge (SOC) of the lithium battery after emergency retraction is different at different temperatures. The above situation poses a challenge to the development of the state of health (SOH) estimation method and end of life (EOL) judgment method for the backup power storage system. Therefore, it is necessary to estimate the health state SOH of the wind turbine pitch backup power supply product.

[0003] The state of health (SOH) is used to characterize the aging degree of lithium batteries in energy storage systems. It is one of the most important states for lithium battery energy storage systems used as backup power supplies. At present, the definition of SOH has not been completely unified, but the core idea is: compare the current measured value of a certain state parameter of the lithium battery to be tested with its initial value or rated value, and express it as a percentage as SOH. Among the various measurable state parameters, the maximum capacity available (MCA) of lithium batteries is a commonly used parameter for SOH estimation.

[0004] Define the state of health SOH from the perspective of maximum available capacity MCA, denoted as SOH C :

[0005]

[0006] Where C MCA Indicates the maximum available capacity, C nominal Indicates the initial or rated capacity. Ideally, the MCA of a lithium battery can be obtained through a capacity test. The test steps are as follows:

[0007] (1) The lithium battery is charged at a constant current with a rated current. When the rated charging cut-off voltage of the lithium battery is reached, the constant current charging is switched to constant voltage charging until the charging current drops to 0.05C;

[0008] (2) Leave to stand for 60 minutes;

[0009] (3) Discharge the lithium battery at a constant current with a rated current until the rated discharge cut-off voltage of the lithium battery is reached;

[0010] (4) Leave to stand for 60 minutes;

[0011] (5) The above test is repeated three times, and the average discharge capacity is taken as the current maximum available capacity of the lithium battery.

[0012] The capacity test is generally carried out at 25±2℃. The capacity test result or the capacity specified by the manufacturer after the lithium battery leaves the factory can be used as the C nominal ; The result of the capacity test of the lithium battery after running for a period of time is C MCA . In practical applications, lithium batteries are generally connected in series to form lithium battery clusters, and then the lithium battery clusters are connected in parallel to form a system. Therefore, it is difficult to accurately charge / discharge all lithium batteries to the same rated charge / discharge voltage. At the same time, the capacity test is time-consuming, and it is difficult for the lithium battery management system (BMS) to use it as a conventional means of estimating SOH. Instead, the capacity test is simplified as a supplementary means of regular correction of SOH. At present, the main way to estimate SOH in BMS is still to estimate the maximum available capacity MCA. There are two common methods: partial charge and discharge method and wear lookup table method.

[0013] 1. Partial charge and discharge method

[0014] For lithium batteries with ternary lithium as the positive electrode material, in the SOC∈[0,100%] range, the terminal voltage (terminal voltage) or open circuit voltage (open circuit voltage, OCV) variation range is larger than that of lithium batteries with lithium iron phosphate as the positive electrode material, and there is no obvious voltage plateau period, such as Figure 1 Therefore, the maximum available capacity MCA can be estimated by the partial charge and discharge capacity between the two SOC points, and then the SOH can be calculated. This method is also called the bistable point method. The estimation process is as follows:

[0015] Utilize the maximum available capacity C at the current temperature MCA T Represents the SOC of point A and point B:

[0016]

[0017] Calculate C by partial charge and discharge capacity MCA_T :

[0018]

[0019] At different temperatures, the charging and discharging capabilities of lithium batteries are different, resulting in the maximum available capacity C at a certain temperature estimated by the partial charge and discharge method. MCA_T Different from other temperatures. In order to unify the estimation standard, when calculating SOH C It needs to be unified, and the conversion expression is shown in (4).

[0020]

[0021] Among them C nominal_T is the initial / nominal capacity at this temperature. The initial / nominal capacity table of a 134Ah ternary lithium battery at different temperatures is shown in Table 1.

[0022] Table 1 Initial / nominal capacity C at different temperatures nominal_T surface

[0023]

[0024] It can be seen that the SOH is estimated by the partial charge and discharge method C The results are greatly affected by temperature, and it is necessary to establish a comparison table of initial / nominal capacity at different temperatures in advance, as shown in Table 1. The accuracy of the standard values ​​in the table also has a significant impact on the estimation results.

[0025] 2. Wear table method

[0026] When using the wear lookup table method, a basic assumption must be followed: the current maximum available capacity MCA of the lithium battery decreases approximately linearly with the number of cycles, as shown in Figure 2.

[0027] Under this premise, it is approximately assumed that the total cumulative charging or discharging capacity of the lithium battery is approximately linearly related to the product of the nominal capacity and the nominal number of cycles, as shown in (5).

[0028]

[0029] In formula (5), C ACC Indicates the cumulative charge or discharge capacity, N nominal Indicates the rated number of cycles of a lithium battery. The wear table method is often used for lithium iron phosphate batteries, but due to the lack of effective measurement and accurate calculation, its accuracy is lower than the partial charge and discharge method.

[0030] Both the maximum available capacity MCA and the DC internal resistance DCR can be used as a measure of the health state SOH. The reason is that as the lithium battery is used and stored, its MCA continues to decay and DCR continues to increase. The two are basically monotonically changing throughout the life cycle of the lithium battery. Defining SOH from the perspective of MCA can dynamically reflect the performance of the lithium battery, and combined with the state of charge SOC estimation, the energy state (SOE) of the lithium battery can be quickly obtained. Defining SOH from the perspective of DCR can better reflect the safety of lithium batteries, because DCR is more closely related to lithium battery anomalies, failures and faults. At the same time, the linearity of the DCR change process is better than that of MCA, and the corresponding SOH estimation result has a smaller mutation. In addition, compared with alternating current impedance (ACI) and even electrochemical impedance spectroscopy (EIS), MCA and DCR can be obtained online in practical applications through relatively convenient methods.

[0031] The application object of the present invention is a wind turbine pitch backup power supply, in which the energy storage lithium battery has a long static time and a small number of startups (charge and discharge times). If the SOH estimation of the lithium battery is carried out based on the maximum available capacity MCA, due to the long-term static characteristics of the lithium battery system, the SOH calculation result is inaccurate, and cannot reflect the true state of the lithium battery, and cannot be used as a criterion for the end of life. In addition to the partial charge and discharge method and the wear lookup table method, if the SOH is obtained by time lookup based on the lithium battery calendar life decay table, since the temperature range of the lithium battery is very wide (-20 to 55°C), and the calendar decay rate at different temperatures is significantly different, especially the decay rate at high temperature is very fast, it is difficult to establish an accurate calendar decay curve, which will result in a large gap between the lookup result and the actual situation. If the MCA-based SOH estimation is combined with the calendar decay table lookup, a major problem is that the high-power discharge of the lithium battery after long-term static may cause a large capacity decay, and it is also difficult to obtain an accurate SOH value.

[0032] Considering that the MCA parameter is needed in the estimation of SOC and SOE, the SOH estimation method based on the maximum available capacity MCA is the choice of mainstream BMS at this stage. However, for backup power supply scenarios with high safety requirements and low usage, the SOH estimation method based on DC internal resistance DCR is more valuable.

[0033] The following is an analysis and comparison of the traditional MCA-based SOH estimation method and the DCR-based SOH estimation method proposed in the present invention.

[0034] 1. Sources of Error

[0035] Traditional SOH estimation methods based on the maximum available capacity MCA, such as the partial charge and discharge method, first need to obtain the SOC value, so the main error comes from the SOC. There are two methods for calculating SOC in mainstream BMS - the ampere-hour integration method and the open circuit voltage method. Take the ampere-hour integration method as an example, as shown in (6).

[0036]

[0037] Among them SOH Ah It indicates the SOC value estimated by the ampere-hour integration method, and SOC0 indicates the initial value when calculating the ampere-hour integration. It can be seen that the SOC value is also obtained by estimation. Therefore, when estimating SOH, the error first comes from the selection of the initial SOC value, that is, SOC0. In addition, in the ampere-hour integration method, it is necessary to know the current C value of the lithium battery in advance. MCA , and according to formula (2)-(4), the partial charge and discharge method is to use C MCA To estimate SOH C , which is equivalent to the unknown number C MCA If the SOH estimated by BMS last time is used to reversely calculate C MCA , first substituting it into SOC and then calculating the SOH value this time will introduce a large error.

[0038] If the open circuit voltage method is used to estimate SOC, it is difficult to obtain OCV in practical applications, so the terminal voltage is usually used to approximate OCV, and the relaxation phenomenon of lithium battery voltage will introduce a large error. Even if the one-to-one correspondence between OCV and SOC is considered, and it is assumed that there is no error in the acquisition of OCV, the OCV-SOC correspondence is not a simple linear mapping, but is calculated through C MCA The nonlinear mapping of calibration cannot avoid errors even if the SOC is estimated by the open circuit voltage method first and the SOH is estimated by the partial charge and discharge method in an ideal situation. Therefore, the ampere-hour integration method and the open circuit voltage method are essentially unable to decouple from the current maximum available capacity MCA, resulting in the inevitability of errors in the partial charge and discharge method.

[0039] 2. Fluctuating results

[0040] According to the constant power and constant current cycle aging experiments of lithium batteries and their fitting curves, there are two phenomena worth noting about SOH estimation. First, the SOH of lithium batteries C There are fluctuations within a certain range, and there are cases where the MCA of the later cycle is greater than the MCA of the previous cycle. Second, as the number of cycles increases, the rate of SOH decay changes. According to the test of lithium iron phosphate batteries, the experimental results show that the SOH decay rate changes, such as Figure 3 shown.

[0041] The first phenomenon shows that the MCA of lithium batteries decreases dynamically over a long time scale, but in adjacent cycles, due to the dynamic changes in the electrochemical characteristics of lithium batteries, MCA may recur. The second phenomenon shows that in long-term cycle tests, the steady state of the lithium battery itself will change. Although the test conditions are the same, the capacity decay rates in different stages are different. These phenomena show that the errors of traditional MCA-based SOH estimation methods, especially the wear lookup table method, cannot be avoided. More importantly, if the cumulative charge or discharge capacity is used for calculation, due to the obvious difference between the actual charge and discharge depth and conditions and experimental measurements, such an approximate method has a large error. Therefore, the accuracy of the SOH estimation results based on MCA is limited, so it is difficult to use it to judge the end of life conditions.

[0042] 3. Capacity Diving

[0043] For lithium batteries using ternary lithium as the positive electrode material, when the number of cycles of charge and discharge reaches a certain number, there is a "capacity diving" phenomenon in which the capacity rapidly decays, such as Figure 4 If the SOH estimation method based on the maximum available capacity MCA is used for such lithium batteries, it may also bring safety risks to the energy storage system.

[0044] From the perspective of the positive electrode, the reason for the capacity dive is that the positive electrode particles are broken. After the particles are broken, the metal elements dissolve and react with the electrolyte, which causes the rapid consumption of the electrolyte. When the electrolyte is exhausted, the capacity dive phenomenon begins to become significant. From the perspective of the negative electrode, as the number of lithium battery cycles increases, the negative electrode surface induces lithium precipitation side reactions, resulting in a sharp increase in the loss of active lithium ions, forming a passivation film, and causing the impedance of the solid electrolyte interface (SEI) film to surge, thus entering a stage of rapid capacity decay. In order to avoid the rapid decline in lithium battery availability and safety hazards caused by the capacity dive phenomenon, the MCA-based SOH estimation method, especially the wear lookup table method, is not suitable for ternary lithium batteries.

[0045] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of the present invention, and therefore may include information that does not constitute the prior art known to ordinary technicians in the field. Summary of the invention

[0046] The purpose of the present invention is to solve the technical problems existing in the background technology. To this end, a method for estimating the health status of a backup power supply based on DC internal resistance is provided.

[0047] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0048] The backup power supply health status estimation method based on DC internal resistance includes:

[0049] First, measure the DC internal resistance DCR:

[0050] After the lithium battery is charged at a constant current of 1C to the cut-off voltage, it is switched to constant voltage charging until the charging current reaches 0.05C;

[0051] Set aside for 30 minutes;

[0052] Discharge at a constant current of 1 / 3C for 90 minutes and adjust the SOC to a fixed value, where SOC represents the state of charge of the lithium battery;

[0053] Set aside for 2 hours;

[0054] Record the voltage U1 at the end of the shelf period, discharge with a constant current of 1C for 10s, and record the voltage U2 at the end of the discharge period. The DC internal resistance DCR when the SOC is the corresponding fixed value is:

[0055]

[0056] Where, I is the step current;

[0057] Then, based on the DC internal resistance DCR, the health state SOH is obtained R The most basic form is:

[0058]

[0059] Where R EOL , R present and R nominal They represent the end-of-life, current and initial DC internal resistance DCR respectively.

[0060] The following is a technical solution further defined by the present invention. The DC internal resistance DCR is related to SOC, OCV, MCA and temperature. The health state SOH R The general expression is:

[0061] SOH R =f(SOC,MCA,T)or SOH R =f(OCV,MCA,T)

[0062] Among them, SOC represents the state of charge of the lithium battery, OCV represents the open circuit voltage, and MCA represents the maximum available capacity of the lithium battery.

[0063] The following is a technical solution further defined by the present invention, taking room temperature 25±2℃ as the standard, health status SOH R It is expressed as:

[0064]

[0065]

[0066] Where R EOL (SOC), R present (SOC) and R nominal (SOC) represents the three DCR values ​​at a fixed SOC, R EOL (OCV), R present (OCV) and R nominal (OCV) represents the three DCR values ​​at a fixed OCV.

[0067] Compared with the prior art, the present invention has the following technical effects:

[0068] The SOH estimation method based on the DC internal resistance DCR proposed in the present invention has the advantage that the DCR of a lithium battery can be used as a direct physical quantity to characterize the life decay. Theoretically, no matter under what operating conditions the life of a lithium battery decays, it will lead to an increase in the internal resistance of the lithium battery. Therefore, the validity of the SOH estimation result is very high. Qualitatively speaking at a microscopic scale, the life decay of lithium batteries is attributed to the loss of lithium inventory (LLI) and the loss of active material (LAM). The loss of these substances often means the thickening of the SEI film, lithium precipitation at the negative electrode, the destruction of the electrode structure, etc. On the one hand, it means a reduction in the charge-carrying carrier - active lithium, and on the other hand, it means a decrease in the particle diffusion rate, which can be manifested as an increase in resistance on a macroscopic scale. Therefore, through the SOH R Estimation can reduce the negative impact of the special operating conditions of the variable pitch backup power supply on the SOH estimation, obtain more accurate estimation results, and more effectively reflect the safety status of the energy storage lithium battery.

[0069] The present invention is further described below in conjunction with the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0071] Figure 1 It is a schematic diagram of the partial charge and discharge method;

[0072] FIG2 is a schematic diagram of a SOH versus cycle number curve of a lithium iron phosphate battery, wherein FIG2(a) is a schematic diagram of a capacity retention rate curve during a 0.5P constant power cycle at 25°C, and FIG2(b) is a schematic diagram of a capacity retention rate curve during a 1C constant current cycle at 25°C;

[0073] Figure 3 It is a schematic diagram of the lithium battery capacity retention rate test curve;

[0074] Figure 4 This is a schematic diagram of the "capacity diving" curve of a ternary lithium battery;

[0075] Figure 5 is a schematic diagram of the Thevenin equivalent circuit of the present invention;

[0076] Figure 6 It is a schematic diagram of the process of testing DCR by step current in the present invention;

[0077] FIG. 7 is a schematic diagram of the broken line of the DCR value of the lithium battery of the present invention at different SOCs, wherein FIG. 7( a) is a schematic diagram of the broken line of the DCR value of the lithium iron phosphate battery at different SOCs, and FIG. 7( b) is a schematic diagram of the broken line of the DCR value of the ternary lithium battery at different SOCs;

[0078] Figure 8 : is a schematic diagram of the DCR value of the lithium iron phosphate battery of the present invention at different rates;

[0079] Fig. 9 It is a comparative diagram of various health status estimation indicators of lithium batteries. DETAILED DESCRIPTION

[0080] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed below.

[0081] 1.DCR test method

[0082] The SOH estimation method based on the DC internal resistance DCR proposed in the present invention first requires the measurement of the internal resistance. The standard method for measuring the DC internal resistance DCR is:

[0083] (1) After the lithium battery is charged at a constant current of 1C to the cut-off voltage, it is switched to constant voltage charging until the charging current reaches 0.05C;

[0084] (2) Leave for 30 minutes;

[0085] (3) Then discharge at a constant current of 1 / 3C for 90 min (adjust SOC to 50%);

[0086] (4) Leave for 2 hours;

[0087] (5) Record the voltage U1 at the end of the shelf period, discharge with a constant current of 1C for 10s, and record the voltage U2 at the end of the discharge period. The DCR when the SOC is 50% is shown in formula (7).

[0088]

[0089] Further analysis based on the Thevenin equivalent circuit model shows that the Thevenin circuit is Figure 5 shown.

[0090] Thevenin equivalent circuit consists of a series resistor and an RC network. The series resistor R0 is called the ohmic internal resistance, and the resistor R in the RC network p becomes the polarization internal resistance, capacitance C p It is called polarization capacitance. In steady state, the DCR of lithium battery is equal to R0+R p .

[0091] In practical applications, the ohmic internal resistance R0 of a small-capacity lithium battery is generally measured by applying a step current and measuring the change in the lithium battery voltage to calculate. When the capacity of the lithium battery is large, a larger current is required to obtain a significant voltage change. However, due to the limitation of the contactor switching time in the equipment, it is difficult to generate a large current in a very short time (0.1s). At the same time, measurement within this time is also prone to cause large errors. Therefore, the object of the DC internal resistance test is determined to be R0+R p . Due to the polarization capacitance C p , it takes some time for the circuit to reach a steady state after power is turned on. In the equivalent circuit, the time constant τ of the lithium battery is about hundreds of milliseconds. After 3 to 5 times the time constant τ, the circuit can be considered to reach a steady state. On this basis, a certain redundancy is added, and the duration of the step current is taken as 10s.

[0092] like Figure 6 As shown in the figure, the DCR test process is described in detail. Discharge starts at time t0, and the terminal voltage drops by ΔU due to the influence of the ohmic internal resistance R0 of the lithium battery. However, the voltage drop on the ohmic internal resistance lasts for a short time, and then due to the polarization phenomenon, the lithium battery voltage begins to slowly drop, corresponding to the period t0 to t1 in the figure. At this stage, the lithium battery voltage drop includes the voltage drop of the ohmic resistance R0 and the voltage drop of the polarization resistance R p If the voltage at the starting point of the step current is recorded as U0, the DCR can be calculated by the voltage difference between U1 and U2.

[0093] 2.SOH R Estimation method

[0094] Based on the definition of DCR based on DC internal resistance, SOH can be defined R as follows

[0095]

[0096] Formula (8) defines SOH R The most basic form, where R EOL , R present and R nominal They represent the end-of-life, current, and initial DCR, respectively. More precisely, DCR is a physical quantity related to SOC or OCV, MCA, and temperature, etc., so SOH R It can be written as the following general expression

[0097] SOH R =f(SOC, MCA, T) or SOH R =f(OCV, MCA, T) (9)

[0098] Similar to SOH C The method used in the estimation is to take the room temperature of 25±2℃ as the standard and SOH R More precisely,

[0099]

[0100]

[0101] Where R EOL (SOC), R present (SOC) and R nominal (SOC) represents the three DCR values ​​at a fixed SOC, R EOL (OCV), R present (OCV) and R nominal (OCV) represents the three DCR values ​​at a fixed OCV.

[0102] 3.SOH R attention

[0103] In the interval of SOC∈[0,100%], the value of DCR is not fixed, as shown in Figure 7, so the SOH is estimated based on DCR. R It needs to be done at a fixed SOC or OCV. The value of DCR is also related to the rate at which the lithium battery is charged and discharged, such as Figure 8 shown.

[0104] For conventional energy storage systems, it is difficult to perform DCR tests at a fixed SOC. At the same time, due to the coupling relationship between SOC and MCA, the DCR results obtained also have certain errors. It is almost impossible to perform DCR tests at a fixed OCV, because it requires the energy storage system to be able to frequently stabilize at the OCV and also requires the energy storage system to have the conditions for conducting DCR tests.

[0105] For the wind turbine pitch backup power supply, it may be idle for a long time with a cycle of one year. Due to the self-discharge of the energy storage lithium battery, when the self-discharge reaches a certain level, in order to ensure that the energy stored in the backup power supply meets the requirements of two emergency pitch retractions, it is necessary to recharge. This working condition fully meets the SOH R The need for estimation, its operation logic is as follows:

[0106] ① Detect the OCV of the lithium battery pack. If the OCV reaches the charging threshold, start charging;

[0107] ②End charging according to the BMS settings.

[0108] At this time, the judgment condition of ① satisfies SOH R To meet the estimation requirements, we only need to add a DCR test step between ① and ②. Since the self-discharge of lithium batteries is cyclical, SOH R The estimation can also be performed periodically without interfering with the original logic of the BMS and the entire wind turbine pitch backup power system.

[0109] SOH R With SOH C Comparison:

[0110] Estimating SOH R There are two prerequisites. First, it is necessary to test at a fixed SOC or OCV to obtain DCR, which is difficult in practice. Otherwise, it is necessary to conduct DCR at a large number of SOC values ​​or OCV values ​​in advance to establish a table and determine R by looking up the table. EOL and R nominal To get SOH R value; second is SOH R It has nothing to do with the maximum available capacity MCA. Under the existing embedded SOX estimation system, SOC, SOE and current MCA are strongly coupled. If SOH estimation is based on DCR, it is necessary to modify the SOC and SOE estimation methods synchronously to ensure the overall status monitoring accuracy of the BMS.

[0111] SOH C With SOH R Comparing the two, the advantages and disadvantages are as follows:

[0112] Schedule 1SOHR With SOH C Comparative advantages and disadvantages

[0113]

[0114]

[0115] SOH R Mapping with MCA:

[0116] If the current MCA of lithium batteries can be obtained through capacity testing, it should be the preferred choice for the management of the entire SOX module and BMS. However, there are four main difficulties in carrying out fixed capacity testing in actual applications:

[0117] (1) From the perspective of lithium battery characteristics: After lithium batteries are grouped into a system, due to the "barrel effect" caused by inconsistency, it is impossible to test the capacity of all lithium batteries, that is, fully charge and discharge. Instead, the lithium batteries with weaker performance are fully charged first during charging and discharged first during discharge. For safety reasons, the BMS will stop the system's charging and discharging when the lithium batteries with weaker performance are fully / empty.

[0118] (2) Discharge load perspective: The discharge operation of the capacity test is not easy to implement. In practice, discharge requires a load. Even in industrial and commercial energy storage, when the grid is used as the load, the requirements of the grid must be met.

[0119] (3) From the perspective of maintenance time: For conventional lithium battery energy storage, capacity testing takes a long time. Even if only one capacity test is performed with full charge and discharge, a total of more than 3 hours is required at a 1C rate plus a 60-minute rest period in between. At a 0.5C rate, it takes more than 5 hours, and the time cost of maintenance is high.

[0120] (4) From the perspective of maintenance personnel: For lithium battery energy storage, full charging and discharging operations have certain risks. This is also the reason why lithium battery manufacturers recommend using lithium batteries in the range of 5% to 95% SOC. According to actual engineering experience, the SOC displayed by the BMS is not the actual SOC of the lithium battery. Therefore, full charging and discharging still need to be performed by professional technicians, resulting in high maintenance labor costs.

[0121] In addition to the traditional MCA-based method and the DCR-based method of the present invention, SOH can also be represented by a variety of indicators, such as Fig. 9 The figure contains the SOH based on MCA. C (Curve ①), SOH based on ohmic internal resistance Ω (Curve ②), SOH based on self-discharge resistance RSD (Curve ③), SOH based on the average weighted average of the above three indicators AW(Curve ④) and SOH based on the dynamic weighting of the above three indicators DW (curve ⑤), where the ohmic internal resistance is generally the internal resistance that is not connected in parallel with the capacitor in the lithium battery equivalent circuit model (ECM). The results show that with the goal of adapting applications, the representation and estimation of SOH can be flexible, but it is necessary to combine experiments to associate single or multiple different indicators to reflect the changing trend of SOH in different scenarios.

[0122] The above is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Any technician familiar with the art can make many possible changes and modifications to the technical solution of the present invention by using the above disclosed methods and technical contents without departing from the scope of the technical solution of the present invention, or modify it into an equivalent embodiment of equivalent changes. Therefore, all equivalent changes made according to the shape, structure and principle of the present invention without departing from the content of the technical solution of the present invention should be included in the protection scope of the present invention.

Claims

1. A backup power supply health status estimation method based on DC internal resistance, characterized in that: include: First, measure the DC internal resistance DCR: After the lithium battery is charged at a constant current of 1C to the cut-off voltage, it is switched to constant voltage charging until the charging current reaches 0.05C; Set aside for 30 minutes; Discharge at a constant current of 1 / 3C for 90 minutes and adjust the SOC to a fixed value, where SOC represents the state of charge of the lithium battery; Set aside for 2 hours; Record the voltage U1 at the end of the shelf period, discharge with a constant current of 1C for 10s, and record the voltage U2 at the end of the discharge period. The DC internal resistance DCR when the SOC is the corresponding fixed value is: Where, I is the step current; Then, based on the DC internal resistance DCR, the health state SOH is obtained R The most basic form is: Where R EOL , R present and R nominal They represent the end-of-life, current and initial DC internal resistance DCR respectively.

2. The method for estimating the health status of a backup power supply based on DC internal resistance according to claim 1, characterized in that: The DC internal resistance DCR is related to SOC, OCV, MCA and temperature, so the health state SOH R The general expression is: SOH R =f(SOC,MCA,T)or SOH R =f(OCV,MCA,T) Among them, SOC represents the state of charge of the lithium battery, OCV represents the open circuit voltage, and MCA represents the maximum available capacity of the lithium battery.

3. The method for estimating the health status of a backup power supply based on DC internal resistance according to claim 2, characterized in that: Taking room temperature 25±2℃ as standard, health status SOH R It is expressed as: Where R EOL (SOC), R present (SOC) and R nominal (SOC) represents the three DCR values ​​at a fixed SOC, R EOL (OCV), R present (OCV) and R nominal (OCV) represents the three DCR values ​​at a fixed OCV.

Images