A self-wake-up calibration method for SOC-OCV based on lithium iron phosphate batteries

Abstract

This invention discloses a SOC-OCV self-wake-up calibration method based on lithium iron phosphate (LFP) batteries. Based on offline conditions, the charging OCV curves and discharging OCV curves of the LFP battery at different temperatures are tested. It determines whether the LFP BMS state meets the prerequisites for SOC-OCV sleep-wake-up calibration. When the LFP BMS state meets the prerequisites, SOC-OCV self-wake-up calibration is performed. The Vt value of the LFP battery under sleep-wake-up calibration conditions is measured. Different SOC-OCV self-wake-up calibration strategies are adopted depending on whether the LFP battery is in a plateau region. This method solves the problem of difficult SOC calibration of LFP cells, improves the reliability of LFP battery SOC calibration, and increases the reliability and durability of LFP battery systems.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate battery technology, and in particular to a SOC-OCV self-wake-up calibration method based on lithium iron phosphate batteries. Background Technology

[0002] The SOC-OCV self-wake-up calibration method is a method for calculating battery capacity in a battery management system. It utilizes the relationship between the battery's open-circuit voltage (OCV) and known state of charge (SOC) to update the SOC value by monitoring voltage changes during idle states. This method requires periodic calibration to ensure accuracy and can be performed using an external reference battery or the charge / discharge process. As the closest prior art to this invention, patent number CN113933728A discloses a method for calibrating the static SOC of a lithium iron phosphate battery using the SOC-OCV curve. This method presents the SOC-OCV curves of at least two lithium iron phosphate batteries tested under different temperature conditions. Then, based on whether the difference between the target SOC value and the current SOC value is within 10%, it determines whether a voltage plateau region or a non-plateau region is needed, and decides whether calibration should be performed.

[0003] The above techniques do not consider the impact of hysteresis voltage on the OCV curve. In real-world calibration, the presence of hysteresis voltage can cause a deviation between the actual OCV curve and the average OCV curve, leading to miscalibration of SOC. Voltage measurement errors are also not considered. The SOC-OCV curves of the lithium iron phosphate batteries described above are measured at steady-state voltage. However, based on some lithium iron phosphate battery test data, a voltage measurement error of 3 mV can easily lead to a SOC deviation of >10% in the plateau region, resulting in miscalibration. Furthermore, the presence of a dormant BMS (Battery Management System) is not considered. If the BMS experiences current or polarization during calibration, the measured voltage will include polarization voltage, resulting in calibration errors. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to solve the problem of difficult SOC calibration of lithium iron phosphate cells, improve the reliability of SOC calibration of lithium iron phosphate batteries, and increase the reliability and durability of lithium iron phosphate battery systems.

[0005] Technical solution: To achieve the above-mentioned objectives, this invention provides a SOC-OCV self-wake-up calibration method based on lithium iron phosphate batteries.

[0006] Based on offline conditions, the charging OCV curves and discharging OCV curves of lithium iron phosphate batteries at different temperatures were tested.

[0007] Determine whether the status of the lithium iron phosphate BMS meets the prerequisites for SOC-OCV sleep self-wake calibration.

[0008] When the lithium iron phosphate BMS status meets the prerequisites for SOC-OCV sleep self-wake calibration, SOC-OCV self-wake calibration is performed.

[0009] Measure the Vt of a lithium iron phosphate battery under dormant self-wake calibration conditions;

[0010] Different SOC-OCV self-wake-up calibration strategies are adopted depending on whether the lithium iron phosphate battery is in the plateau region.

[0011] Furthermore, the prerequisites for the hibernation self-wake calibration include:

[0012] The lithium iron phosphate BMS is in sleep / wake-up mode;

[0013] The current flowing through the lithium iron phosphate BMS is less than I_min;

[0014] The sleep time of lithium iron phosphate BMS is longer than T_slp.

[0015] Furthermore, the sleep self-wake mode is a brief automatic wake-up state of the lithium iron phosphate BMS during sleep.

[0016] Furthermore, I_min is 10-50 times the noise level of the current sensor measurement. Furthermore, the measurement of Vt of the lithium iron phosphate battery under the sleep-wake calibration condition includes: the SOC-OCV sleep-wake calibration condition has three components, namely:

[0017] Condition 1: The current SOC is greater than the upper limit of the SOC calibration threshold, and the difference is greater than ΔSOC; at the same time, the difference between the current SOC obtained by looking up the OCV_max from the charging OCV curve and the actual measured voltage is greater than ΔVt.

[0018] Condition 2: The current SOC is less than the lower limit of the SOC calibration threshold, and the difference is greater than ΔSOC; at the same time, the difference between the current SOC obtained by looking up the OCV_min from the discharge OCV curve and the actual measured voltage is greater than ΔVt.

[0019] Condition 3: The difference between the current SOC and the non-platform calibration value of SOC is greater than ΔSOC; at the same time, the difference between the current SOC obtained by looking up the OCV_avg from the average OCV curve and the actual measured voltage is greater than ΔVt.

[0020] Preferably, the average OCV curve is obtained by averaging the charging OCV curve and the discharging OCV curve.

[0021] Furthermore, depending on whether the lithium iron phosphate battery is in the plateau region, different SOC-OCV self-wake-up calibration strategies are adopted, including:

[0022] If the slope of the SOC-OCV curve If the value is less than 15mv / %, the lithium iron phosphate battery is in the plateau region, and the SOC-OCV self-wake-up calibration strategy A is adopted.

[0023] If the slope of the SOC-OCV curve If the value is greater than 15mv / %, the lithium iron phosphate battery is in the non-plateau region, and the SOC-OCV self-wake-up calibration strategy B is adopted.

[0024] More specifically, the 15mv / % is determined based on the OCV test results.

[0025] More specifically, the self-wake-up calibration strategy A includes:

[0026] Based on the current cell temperature and Vt of the lithium iron phosphate battery, the lower bound of the SOC calibration threshold is obtained by looking up the table through the charging OCV curve; the upper bound of the SOC calibration threshold is obtained by looking up the table through the discharging OCV curve; it is then determined whether condition one is met. If condition one is met, the current SOC is calibrated to the upper bound of the SOC calibration threshold; if condition one is not met, it is then determined whether condition two is met. If condition two is met, the current SOC is calibrated to the lower bound of the SOC calibration threshold; if condition two is not met, the current SOC is not calibrated.

[0027] More specifically, the self-wake-up calibration strategy B includes:

[0028] Based on the current cell temperature and Vt of the lithium iron phosphate battery, if Vt < 3300mV, the discharge OCV curve is used to look up the table; if Vt > 3300mV, the charging OCV curve is used to look up the table. The SOC values ​​obtained from the table lookup are all non-platform calibration values. It is then determined whether condition three is met. If condition three is met, the current SOC is calibrated to the non-platform calibration value; if condition three is not met, the current SOC is not calibrated.

[0029] More specifically, SOC: State of Charge; OCV: Open Circuit Voltage; Vt: Measured voltage of a single lithium iron phosphate battery cell; OCV_max: Open circuit voltage obtained from the charging OCV curve at the current SOC and temperature; OCV_min: Open circuit voltage obtained from the discharging OCV curve at the current SOC and temperature; OCV_avg: Open circuit voltage obtained from the average OCV curve at the current SOC and temperature; BMS: Battery Management System; ΔSOC: SOC deviation threshold, depending on the specific project; ΔVt: Voltage deviation threshold, depending on the specific project; I_min: Low current threshold, depending on the specific project; T_slp: Dormant time threshold, depending on the specific project.

[0030] Beneficial effects: 1. This invention takes into account the influence of hysteresis voltage on the OCV curve and adds charging OCV curve and discharging OCV curve to determine the correction range of SOC, thus avoiding the influence of not considering hysteresis voltage on SOC calibration.

[0031] 2. This invention takes into account voltage measurement error. Calibration is only performed when the difference between the OCV value obtained from the table based on the current SOC and charge / discharge OCV curve and the actual measured voltage of the single lithium iron phosphate battery is greater than ΔVt, thus avoiding miscalibration caused by voltage measurement error.

[0032] 3. This invention takes into account the influence of polarization voltage and sets the SOC-OCV calibration during the self-wake-up period of the lithium iron phosphate BMS. The self-wake-up state ensures that there is no polarization voltage in the battery at this time, thereby avoiding calibration errors. Attached Figure Description

[0033] Figure 1 This is the overall flowchart of this embodiment;

[0034] Figure 2 These are the OCV charge-discharge curves and the average OCV curve tested in this embodiment;

[0035] Figure 3 This is the calibration curve of Example 1, which is in the plateau region and meets condition one;

[0036] Figure 4 This is the calibration curve diagram of Example 2, which is in the plateau region and meets condition two.

[0037] Figure 5 This is the calibration curve of Example 3, which is in the non-platform region and meets condition three. Detailed Implementation

[0038] like Figure 1As shown, a SOC-OCV self-wake-up calibration method based on lithium iron phosphate batteries is presented.

[0039] S1: Based on offline conditions, test the charging OCV curve and discharging OCV curve of lithium iron phosphate battery at different temperatures;

[0040] S2: Determine whether the lithium iron phosphate BMS status meets the prerequisites for SOC-OCV sleep self-wake calibration.

[0041] The prerequisites for sleep-wake calibration are: the lithium iron phosphate BMS is in sleep-wake mode, the current flowing through the lithium iron phosphate BMS is less than I_min, and the sleep time of the lithium iron phosphate BMS is greater than T_slp. Sleep-wake mode refers to the brief automatic wake-up state of the lithium iron phosphate BMS during sleep. I_min is 10-50 times the noise of the current sensor measurement.

[0042] S3: When the lithium iron phosphate BMS meets the prerequisites for SOC-OCV sleep self-wake calibration, perform SOC-OCV self-wake calibration;

[0043] S4: Measure Vt of lithium iron phosphate battery under dormant self-wake calibration conditions;

[0044] There are three SOC-OCV sleep / wake-up calibration conditions:

[0045] Condition 1: The current SOC is greater than the upper limit of the SOC calibration threshold, and the difference is greater than ΔSOC; at the same time, the difference between the current SOC obtained by looking up the OCV_max from the charging OCV curve and the actual measured voltage is greater than ΔVt.

[0046] Condition 2: The current SOC is less than the lower limit of the SOC calibration threshold, and the difference is greater than ΔSOC; at the same time, the difference between the current SOC obtained by looking up the OCV_min from the discharge OCV curve and the actual measured voltage is greater than ΔVt.

[0047] Condition 3: The difference between the current SOC and the non-platform calibration value of SOC is greater than ΔSOC; simultaneously, the difference between the OCV_avg obtained from the average OCV curve and the actual measured voltage is greater than ΔVt. The average OCV curve is obtained by averaging the charging OCV curve and the discharging OCV curve.

[0048] S5: Different SOC-OCV self-wake-up calibration strategies are adopted depending on whether the lithium iron phosphate battery is in the plateau region.

[0049] If the slope of the SOC-OCV curve If the value is less than 15mv / %, the lithium iron phosphate battery is in the plateau region, and the SOC-OCV self-wake-up calibration strategy A is adopted.

[0050] Calibration Strategy A: Based on the current cell temperature and Vt of the lithium iron phosphate battery, the lower bound of the SOC calibration threshold is obtained by looking up the table through the charging OCV curve; the upper bound of the SOC calibration threshold is obtained by looking up the table through the discharging OCV curve; it is then determined whether condition one is met. If condition one is met, the current SOC is calibrated to the upper bound of the SOC calibration threshold; if condition one is not met, it is then determined whether condition two is met. If condition two is met, the current SOC is calibrated to the lower bound of the SOC calibration threshold; if condition two is not met, the current SOC is not calibrated.

[0051] If the slope of the SOC-OCV curve If the value is greater than 15mv / %, the lithium iron phosphate battery is in the non-plateau region, and the SOC-OCV self-wake-up calibration strategy B is adopted.

[0052] Calibration Strategy B: Based on the current cell temperature and Vt of the lithium iron phosphate battery, if Vt < 3300mV, use the discharge OCV curve to look up the table; if Vt > 3300mV, use the charging OCV curve to look up the table. The SOC values ​​obtained from the table lookup are all non-platform calibration values. Determine whether condition three is met. If condition three is met, then calibrate the current SOC to the non-platform calibration value. If condition three is not met, then do not calibrate the current SOC.

[0053] 15mV / % is determined based on OCV test results; SOC: State of Charge; OCV: Open Circuit Voltage; Vt: Measured voltage of a single lithium iron phosphate battery cell; OCV_max: Open circuit voltage obtained from the charging OCV curve at the current SOC and temperature; OCV_min: Open circuit voltage obtained from the discharging OCV curve at the current SOC and temperature; OCV_avg: Open circuit voltage obtained from the average OCV curve at the current SOC and temperature; BMS: Battery Management System; ΔSOC: SOC deviation threshold, depending on the specific project; ΔVt: Voltage deviation threshold, depending on the specific project; I_min: Low current threshold, depending on the specific project; T_slp: Dormant time threshold, depending on the specific project.

[0054] Example 1

[0055] like Figure 2As shown, firstly, based on the offline state, the charging OCV curve and discharging OCV curve of the lithium iron phosphate battery cell are obtained in advance through OCV testing. After obtaining the charging OCV curve and discharging OCV curve, it is determined whether the lithium iron phosphate battery meets the calibration prerequisites. If the lithium iron phosphate BMS meets the SOC-OCV sleep self-wake calibration prerequisites, SOC-OCV self-wake calibration is performed; the current temperature is 25℃, the SOC of the lithium iron phosphate battery cell is 57%, and Vt is approximately 3280mV. Figure 2 The OCV test results shown are based on the slope of the SOC-OCV curve. If the value is less than 15mv / %, the lithium iron phosphate battery is considered to be in a plateau phase, therefore calibration strategy A is initiated. Figure 3 As shown, at 25℃, the lower and upper bounds of the SOC calibration threshold obtained based on the charging OCV curve and the discharging OCV curve are approximately 25% and 36%, respectively. Since the current SOC is 57%, it is greater than the upper bound of the SOC calibration threshold, and the difference is greater than ΔSOC. In this project, ΔSOC is taken as 3%. Therefore, the voltage difference comparison should be based on the charging OCV curve. The OCV_max obtained by looking up the table through the charging OCV curve is 3310mV, which is greater than ΔVt compared with the current Vt. In this project, ΔVt is taken as 5mV, thus satisfying condition one. At this time, the lithium iron phosphate battery BMS calibrates the current SOC (57%) to the upper bound of the SOC calibration threshold (36%).

[0056] Example 2

[0057] like Figure 2 As shown, firstly, based on the offline state, the charging OCV curve and discharging OCV curve of the lithium iron phosphate battery cell are obtained in advance through OCV testing. After obtaining the charging OCV curve and discharging OCV curve, it is determined whether the lithium iron phosphate battery has entered the calibration prerequisite. If the lithium iron phosphate BMS meets the SOC-OCV sleep self-wake calibration prerequisite, SOC-OCV self-wake calibration is performed; the current temperature is 25℃, the SOC of the lithium iron phosphate battery cell is 14%, and Vt is approximately 3280mV. Figure 2 The OCV test results shown are based on the slope of the SOC-OCV curve. If the value is less than 15mv / %, the lithium iron phosphate battery is considered to be in a plateau phase, therefore calibration strategy A is initiated. Figure 4As shown, at 25℃, the lower and upper bounds of the SOC calibration threshold obtained based on the charging OCV curve and the discharging OCV curve are approximately 25% and 36%, respectively. Since the current SOC is 14%, it is less than the lower bound of the SOC calibration threshold, and the difference is greater than ΔSOC. In this project, ΔSOC is taken as 3%. Therefore, the voltage difference comparison should be based on the discharging OCV curve. The OCV_min obtained by looking up the table through the charging OCV curve is 3205mV, which is greater than the difference between it and the current Vt. In this project, ΔVt is taken as 5mV, thus satisfying condition two. At this time, the lithium iron phosphate battery BMS calibrates the current SOC (14%) to the lower bound of the SOC calibration threshold (25%).

[0058] Example 3

[0059] like Figure 2 As shown, firstly, based on the offline state, the charging OCV curve and discharging OCV curve of the lithium iron phosphate battery cell are obtained in advance through OCV testing. After obtaining the charging OCV curve and discharging OCV curve, it is determined whether the lithium iron phosphate battery has entered the calibration prerequisite. If the lithium iron phosphate BMS meets the SOC-OCV sleep self-wake calibration prerequisite, SOC-OCV self-wake calibration is performed; the current temperature is 25℃, the SOC of the lithium iron phosphate battery cell is 20%, and Vt is approximately 3150mV. Figure 2 The OCV test results shown are based on the slope of the SOC-OCV curve. A value greater than 15mV / %, indicating the lithium iron phosphate battery is in a non-plateau phase, therefore calibration strategy B is initiated. Since Vt < 3300mV at this point, if... Figure 5 As shown, at 25℃, the SOC non-platform calibration value obtained based on the discharge OCV curve is 8%. Since the current SOC is 14%, the difference between it and the SOC non-platform calibration value is greater than ΔSOC. In this project, ΔSOC is taken as 3%. The OCV_avg obtained by looking up the average OCV curve is 3250mV, which is greater than the difference between it and the current Vt. In this project, ΔVt is taken as 5mV. Therefore, condition three is satisfied. At this time, the lithium iron phosphate battery BMS calibrates the current SOC (20%) to the SOC non-platform calibration value (8%).

[0060] The above embodiments Figure 2 In this context, OCVmean is the average OCV curve, OCVcharge is the charging OCV curve, and OCVdischarge is the discharging OCV curve.

[0061] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A SOC-OCV self-wake-up calibration method based on lithium iron phosphate batteries, characterized in that, Based on offline conditions, the charging OCV curves and discharging OCV curves of lithium iron phosphate batteries at different temperatures were tested. Determine whether the status of the lithium iron phosphate BMS meets the prerequisites for SOC-OCV sleep self-wake calibration. When the lithium iron phosphate BMS status meets the prerequisites for SOC-OCV sleep self-wake calibration, SOC-OCV self-wake calibration is performed. Measure the Vt of a lithium iron phosphate battery under dormant self-wake calibration conditions; Different SOC-OCV self-wake-up calibration strategies are adopted depending on whether the lithium iron phosphate battery is in the plateau region. Hibernation self-wake calibration conditions include: Condition 1: The current SOC is greater than the upper limit of the SOC calibration threshold, and the difference is greater than ΔSOC; at the same time, the difference between the current SOC obtained by looking up the OCV_max from the charging OCV curve and the actual measured voltage is greater than ΔVt. Condition 2: The current SOC is less than the lower limit of the SOC calibration threshold, and the difference is greater than ΔSOC; at the same time, the difference between the current SOC obtained by looking up the OCV_min from the discharge OCV curve and the actual measured voltage is greater than ΔVt. Condition 3: The difference between the current SOC and the non-platform calibration value of SOC is greater than ΔSOC; at the same time, the difference between the current SOC obtained by looking up the OCV_avg from the average OCV curve and the actual measured voltage is greater than ΔVt. SOC-OCV self-wake-up calibration strategies include: If the slope of the SOC-OCV curve is less than 15mv / %, the lithium iron phosphate battery is in the plateau region, and the SOC-OCV self-wake-up calibration strategy A is adopted. If the slope of the SOC-OCV curve is greater than 15mv / %, the lithium iron phosphate battery is in the non-plateau region, and the SOC-OCV self-wake-up calibration strategy B is adopted. The self-wake-up calibration strategy A includes: Based on the current cell temperature and Vt of the lithium iron phosphate battery, the lower bound of the SOC calibration threshold is obtained by looking up the table through the charging OCV curve; the upper bound of the SOC calibration threshold is obtained by looking up the table through the discharging OCV curve; it is then determined whether condition one is met. If condition one is met, the current SOC is calibrated to the upper bound of the SOC calibration threshold; if condition one is not met, it is then determined whether condition two is met. If condition two is met, the current SOC is calibrated to the lower bound of the SOC calibration threshold; if condition two is not met, the current SOC is not calibrated. The self-wake-up calibration strategy B includes: Based on the current cell temperature and Vt of the lithium iron phosphate battery, if Vt < 3300mV, the discharge OCV curve is used to look up the table; if Vt > 3300mV, the charging OCV curve is used to look up the table. The SOC values ​​obtained from the table lookup are all non-platform calibration values. It is then determined whether condition three is met. If condition three is met, the current SOC is calibrated to the non-platform calibration value; if condition three is not met, the current SOC is not calibrated.

2. The SOC-OCV self-wake-up calibration method based on lithium iron phosphate battery according to claim 1, characterized in that, The prerequisites for the hibernation self-wake calibration include: The lithium iron phosphate BMS is in sleep / wake-up mode; The current flowing through the lithium iron phosphate BMS is less than I_min; The sleep time of lithium iron phosphate BMS is longer than T_slp.

3. The SOC-OCV self-wake-up calibration method based on lithium iron phosphate battery according to claim 2, characterized in that, The sleep self-wake mode is a brief automatic wake-up state of the lithium iron phosphate BMS during sleep.

4. The SOC-OCV self-wake-up calibration method based on lithium iron phosphate battery according to claim 2, characterized in that, The I_min is 10 to 50 times the noise of the current sensor measurement.

Images