Evaluation method for lfp battery pack soc calibration function based on hybrid working condition

Abstract

This invention relates to the field of new energy agricultural machinery battery management technology, specifically disclosing an evaluation method for the SOC calibration function of LFP battery packs based on hybrid operating conditions. This method includes a constant rate charge / discharge verification SOC calibration strategy and a road spectrum operating condition verification SOC calibration strategy. The constant rate charge / discharge verification SOC calibration strategy includes calibration verification under conditions of artificially high SOC at normal / low / high temperatures, calibration verification under conditions of artificially low SOC, and SOC miscalibration verification. The road spectrum operating condition verification SOC calibration strategy includes calibration verification under conditions of artificially high SOC during initial 10-30% SOC road spectrum discharge at normal / low / high temperatures, calibration verification under conditions of artificially low SOC during initial 70-90% SOC road spectrum charging, cyclic verification of whether SOC calibration is falsely triggered during initial 40-60% SOC road spectrum discharge, and cyclic verification of whether SOC calibration is falsely triggered during initial 50-70% SOC road spectrum charging. This invention is applicable to the quantitative evaluation and verification of the SOC calibration accuracy, operating condition adaptability, and robustness of LFP battery systems in hybrid tractors under heavy-load operating conditions.

Description

Technical Field

[0001] This invention relates to the field of new energy agricultural machinery battery management technology, specifically to an evaluation method for the SOC calibration function of an LFP battery pack based on hybrid operating conditions. Background Technology

[0002] Hybrid tractors are rapidly gaining popularity: Compared to traditional diesel tractors, hybrid models, with their advantages of 15%-30% fuel savings, high torque at low speeds, continuously variable transmission (CVT), and low emissions, have become the mainstream route for high-horsepower, high-end agricultural machinery. Leading domestic companies (such as YTO Group and Lovol) have already launched 200-400 horsepower hybrid tractor products in batches. LFP batteries are becoming the preferred energy storage solution: LFP batteries have low cost, long cycle life (5000+ cycles), good thermal stability, and high safety, perfectly meeting the needs of tractors for heavy-duty, continuous operation, and harsh environments (-35℃~60℃). Hybrid operating conditions are extremely complex: Tractor operating conditions (plowing, deep loosening, sowing, harvesting, and transport) involve drastic load fluctuations, large current surges, frequent start-stop cycles, and frequent energy recovery. The State of Charge (SOC) fluctuates frequently within a narrow range of 30%-80%, placing stringent requirements on the accuracy and stability of SOC estimation. At the same time, the evaluation and verification of corresponding SOC calibration strategies are also crucial.

[0003] LFP batteries possess advantages such as high safety, long cycle life, low cost, resistance to over-discharge, and adaptability to low-frequency, high-rate charging and discharging, making them highly suitable for the harsh operating environments of agricultural machinery. However, LFP batteries inherently exhibit an ultra-long voltage plateau characteristic of 30%~90% SOC, within which the open-circuit voltage of the battery shows almost no significant change, and the voltage gradient is extremely small. Traditional SOC calibration methods based on voltage correction are essentially ineffective, and SOC estimation relies heavily on ampere-hour integration calculations. Existing SOC calibration and evaluation technologies are mostly developed for hybrid and pure electric passenger vehicles, with mainstream technical solutions including ampere-hour integration methods, static OCV calibration methods, full charge or discharge calibration, extended Kalman filter (EKF) correction methods, and segmented threshold calibration methods. Among them, the ampere-hour integration method is simple to calculate and has strong real-time performance, but it has the defect of continuous error accumulation; static OCV calibration can only achieve accurate calibration under long-term static conditions and cannot be adapted to dynamic operation scenarios; full charge or discharge calibration is difficult to trigger under actual operation conditions; the filter correction algorithm observes the gain close to zero in the LFP voltage plateau region, the filter fails, and degenerates into pure ampere-hour integration and segmented threshold calibration method, but the SOC calibration accuracy, operating condition adaptability and reliability of this calibration strategy need to be rigorously evaluated and verified.

[0004] Existing industry evaluation systems are all designed based on passenger car road conditions, using smooth urban driving, constant speed cruising, and conventional start-stop conditions as test benchmarks, and only assessing single indicators such as static calibration accuracy and steady-state error. However, for the specific extreme conditions of hybrid tractors in agricultural fields, such as heavy loads, variable load operations, large temperature differences between day and night, long-term continuous hybrid coupling, frequent start-stop, intermittent energy recovery, and high-rate fluctuating charging and discharging, there is currently no dedicated SOC calibration function evaluation system, which cannot effectively verify the true performance of SOC calibration algorithms in agricultural machinery scenarios.

[0005] Currently, the industry's SOC calibration evaluation standards, test conditions, and evaluation indicators are all developed for passenger vehicles, resulting in a serious mismatch between scenarios: First, the test conditions are limited, only covering smooth driving steady-state conditions, and not encompassing core agricultural machinery operating conditions such as tractor heavy-load variable load, continuous hybrid coupling, field disturbance, and wide temperature range operation; second, the evaluation indicators are one-sided, only assessing static calibration accuracy and single-point error, without assessing core agricultural machinery indicators such as dynamic convergence, operating condition adaptability, anti-disturbance ability, long-term stability, and temperature range robustness; third, there is no quantitative scoring system, and the calibration standards of different manufacturers are not uniform, making it impossible to compare the advantages and disadvantages of different SOC calibration algorithms under agricultural machinery operating conditions, and making it impossible to accurately locate the calibration failure operating condition range and algorithm defects. Summary of the Invention

[0006] The purpose of this invention is to provide an evaluation method for the SOC calibration function of LFP battery packs based on hybrid operating conditions, which is particularly suitable for the quantitative evaluation and verification of the SOC calibration accuracy, operating condition adaptability and robustness of lithium iron phosphate (LFP) battery systems of hybrid tractors under heavy-load operation conditions in farmland.

[0007] The technical solution adopted in this invention is: an evaluation method for the SOC calibration function of LFP battery pack based on hybrid operating conditions, including a constant rate charge and discharge verification SOC calibration strategy and a road spectrum operating condition verification SOC calibration strategy. The constant rate charge / discharge verification SOC calibration strategy includes: Calibration verification under conditions of artificially high SOC at room temperature / low temperature / high temperature; Calibration verification under falsely low SOC conditions at room temperature / low temperature / high temperature; Verification of miscalibration of SOC at room temperature / low temperature / high temperature; The road spectrum operating condition verification SOC calibration strategy includes: Calibration verification under simulated high SOC conditions in the circuit spectrum discharge at initial 10~30% SOC at room temperature / low temperature / high temperature; Calibration verification under simulated low SOC conditions in circuit spectrum charging at initial 70~90% SOC at room temperature / low temperature / high temperature; Verify whether SOC calibration is falsely triggered by discharge cycling at room temperature / low temperature / high temperature under initial SOC of 40~60%; Verify whether SOC calibration is falsely triggered by charging cycles at room temperature / low temperature / high temperature under initial 50~70% SOC.

[0008] As a further optimization of the evaluation method for the SOC calibration function of LFP battery pack based on hybrid operating conditions of the present invention, the constant rate charge and discharge verification SOC calibration strategy has a low temperature of -30℃ to 15℃ and a high temperature of 40℃ to 50℃; the road spectrum operating condition verification SOC calibration strategy has a low temperature of -10℃ to 15℃ and a high temperature of 40℃ to 50℃.

[0009] As a further optimization of the evaluation method for the SOC calibration function of LFP battery pack based on hybrid operating conditions of the present invention, the calibration verification under the condition of artificially high SOC at normal temperature / low temperature / high temperature is specifically as follows: Under normal temperature, low temperature or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 30% to 50%; the internal reference SOC of the BMS and the displayed SOC output by the BMS are manually modified so that the absolute SOC difference between the modified reference SOC and the displayed SOC and the initial system SOC of the battery pack is 20 to 50 percentage points; the battery pack is discharged using a constant current rate until the minimum voltage of the battery cell reaches the preset discharge cutoff voltage and then the discharge is stopped; wherein, the discharge current rate is selected as the current rate preset by the BMS that can trigger the SOC calibration logic; during the discharge process, it is verified whether the BMS can start the SOC calibration in time when the SOC calibration trigger condition is reached, and the calculation accuracy after SOC calibration is detected.

[0010] As a further optimization of the evaluation method for the SOC calibration function of LFP battery pack based on hybrid operating conditions of the present invention, the calibration verification under the falsely low SOC conditions at normal temperature / low temperature / high temperature is specifically as follows: Under normal temperature, low temperature, or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 70% to 90%; the internal reference SOC of the BMS and the displayed SOC output by the BMS are manually modified so that the absolute SOC difference between the modified reference SOC and the displayed SOC and the initial system SOC of the battery pack is 20 to 50 percentage points; the battery pack is charged using a constant current rate until the highest voltage of the individual battery cells reaches the preset charging cutoff voltage and then charging is stopped; wherein, the charging current rate is selected as the current rate preset by the BMS that can trigger the SOC calibration logic; during the charging process, it is verified whether the BMS can start the SOC calibration in time when the SOC calibration trigger condition is reached, and the calculation accuracy after SOC calibration is detected.

[0011] As a further optimization of the evaluation method for the SOC calibration function of LFP battery pack based on hybrid operating conditions of the present invention, the verification of SOC miscalibration at normal temperature / low temperature / high temperature includes that SOC miscalibration is not triggered during full charge and discharge within the calibration rate range and that SOC miscalibration is not triggered during full charge and discharge outside the calibration rate range.

[0012] As a further optimization of the evaluation method for the SOC calibration function of LFP battery pack based on hybrid operating conditions of the present invention, the specific steps for preventing SOC miscalibration during the full charge and discharge process within the calibration rate range are as follows: Under normal temperature, low temperature, or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 100%, and it is confirmed that both the internal reference SOC of the BMS and the displayed SOC output by the BMS are 100%; a constant rate discharge is performed at a current rate falling within the SOC calibration trigger rate range preset by the BMS, discharging until the lowest voltage of a single cell reaches the preset discharge cutoff voltage; then, it is left to stand for 10 minutes, and then charged at the same constant rate until the highest voltage of a single cell reaches the preset charging cutoff voltage; and then the charging is stopped; it is detected whether SOC miscalibration occurs in the BMS during the entire full charge and discharge process. The specific steps for preventing SOC miscalibration during full charge and discharge outside the calibration rate range are as follows: Under normal temperature, low temperature, or high temperature conditions, adjust the LFP battery pack to an initial system SOC of 100%, confirming that both the internal reference SOC of the BMS and the displayed SOC output by the BMS are 100%; select a current rate outside the SOC calibration trigger rate range preset by the BMS for constant rate discharge until the lowest voltage of a single cell reaches the preset discharge cutoff voltage; then let it stand for 10 minutes, and then charge it at the same constant rate until the highest voltage of a single cell reaches the preset charging cutoff voltage; and then stop charging. Detect whether the BMS miscalibrates SOC during the entire full charge and discharge process.

[0013] As a further optimization of the evaluation method for the SOC calibration function of LFP battery pack based on hybrid operating conditions of the present invention, the calibration verification under the simulated high SOC conditions of room temperature / low temperature / high temperature discharge spectrum at an initial SOC of 10-30% specifically involves: adjusting the LFP battery pack to an initial system SOC of 10%-30% under room temperature, low temperature, or high temperature conditions; manually modifying the internal reference SOC of the BMS and the displayed SOC output by the BMS, so that the absolute SOC difference between the modified internal reference SOC and displayed SOC of the BMS and the initial system SOC of the battery pack is 20-50 percentage points; discharging the battery pack using a hybrid discharge spectrum that matches the current test environment temperature; stopping the test when the minimum voltage of a single cell reaches the preset discharge cutoff voltage; detecting whether the BMS starts SOC calibration when the SOC calibration conditions are met during the discharge process, and detecting the calculation accuracy of the SOC after calibration.

[0014] As a further optimization of the evaluation method for the SOC calibration function of LFP battery pack based on hybrid operating conditions of the present invention, the calibration verification under the simulated low SOC condition of the initial 70~90% SOC under normal temperature / low temperature / high temperature circuit spectrum charging is specifically as follows: Under normal temperature, low temperature or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 70%~90%; the internal reference SOC of the BMS and the displayed SOC output by the BMS are manually modified so that the absolute SOC difference between the modified internal reference SOC and the displayed SOC of the BMS and the initial system SOC of the battery pack is 20~50 percentage points; the battery pack is charged using a hybrid charging circuit spectrum that matches the current test environment temperature; the test is stopped when the highest voltage of a single cell reaches the preset charging cutoff voltage; when the SOC calibration conditions are reached during the charging process, it is detected whether the BMS starts SOC calibration, and the calculation accuracy after SOC calibration is detected.

[0015] As a further optimization of the evaluation method for the SOC calibration function of LFP battery pack based on hybrid operating conditions of the present invention, the specific steps of verifying whether the SOC calibration is falsely triggered by discharge cycle verification under the initial 40~60% SOC at room temperature / low temperature / high temperature are as follows: Under room temperature, low temperature or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 40%~60%; the battery pack is discharged using a hybrid discharge circuit spectrum that matches the current test environment temperature; the test is stopped when the minimum voltage of a single cell reaches the preset discharge cutoff voltage; and the BMS is checked for SOC miscalibration throughout the entire process from the initial system SOC of 40%~60% to the end of the discharge.

[0016] As a further optimization of the evaluation method for the SOC calibration function of LFP battery pack based on hybrid operating conditions of the present invention, the verification of whether the SOC calibration is falsely triggered by the charging cycle under the initial 50~70% SOC at room temperature / low temperature / high temperature is specifically as follows: under room temperature, low temperature or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 50%~70%; the battery pack is charged using a hybrid charging circuit that matches the current test environment temperature; the test is stopped when the highest voltage of a single cell reaches the preset charging cutoff voltage; and the BMS is checked for SOC miscalibration throughout the entire process from the initial system SOC of 50%~70% to the end of charging.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The evaluation method for SOC calibration function of LFP battery pack in hybrid operating conditions provided by this invention can verify and evaluate the timeliness, calibration rate, calibration accuracy and reliability of calibration, so that hybrid tractors can operate stably and reliably without frequent or erroneous calibration. It is especially suitable for the quantitative evaluation and verification of SOC calibration accuracy, operating condition adaptability and robustness of lithium iron phosphate (LFP) battery systems of hybrid tractors under heavy-load agricultural operating conditions. Attached Figure Description

[0018] Figure 1 This is a flowchart of the evaluation process based on the calibration function of this invention; Figure 2 This is the calibration verification curve for the present invention under the condition of artificially high SOC at room temperature; Figure 3 This is the calibration verification curve for the present invention under the condition of artificially low SOC at room temperature; Figure 4 This is the SOC miscalibration verification curve at room temperature (within the calibration range) of the present invention. Figure 5 This is the SOC miscalibration verification curve at room temperature (outside the calibration rate range) for the present invention. Figure 6 This is the calibration verification curve for the present invention under the condition of artificially high SOC at low temperature; Figure 7 This is the calibration verification curve under the condition of artificially low SOC in the present invention; Figure 8 This is the calibration verification curve for the present invention under the condition of artificially high SOC in the room temperature amplifier spectrum operating condition; Figure 9 This is the calibration verification curve for the present invention under the condition of low SOC during normal temperature charging circuit spectrum operation. Figure 10 This is to verify the SOC miscalibration under normal temperature discharge cycle conditions of the present invention (initial system SOC is about 40%). Figure 11 This is to verify the SOC miscalibration under normal temperature discharge cycle conditions of the present invention (initial system SOC is about 25%). Figure 12 This is to verify the SOC miscalibration under normal temperature discharge cycle conditions of the present invention (initial system SOC is about 15%). Figure 13 This invention is used to verify the SOC miscalibration under normal temperature circuit spectrum charging cycle conditions (initial system SOC is about 75%). Figure 14 This invention is used to verify the SOC miscalibration under normal temperature circuit spectrum charging cycle conditions (initial system SOC is about 90%). Figure 15 This is a calibration verification of the present invention under the condition of artificially high SOC in the low-temperature discharge circuit spectrum operating condition; Figure 16 This is the calibration and verification curve for the invention used in spring plowing operations with hybrid tractors. Detailed Implementation

[0019] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0020] The present invention provides several key evaluation indicators for the SOC calibration function of LFP battery packs suitable for hybrid tractors, as follows: 1) Calibration Timeliness. Assess whether calibration can be triggered promptly when calibration conditions are met. The characteristics of LFP batteries dictate that the calibration range at high SOC is 95%-100%, and at low SOC, it is 0%-25%. Therefore, calibration needs to be triggered promptly when calibration conditions are met so that the VCU can quickly adjust energy distribution after obtaining the battery's actual SOC, avoiding overcharging or over-discharging faults that could cause the entire vehicle to break down.

[0021] 2) Calibration rate. This refers to the rate at which the displayed SOC follows the actual SOC. Vehicle control typically uses the displayed SOC for regulation and instrument panel display. Once calibration is triggered, the displayed SOC immediately jumps to the target value. The displayed SOC needs to quickly catch up to the actual SOC, and the VCU (Vehicle Control Unit) adjusts and displays energy distribution based on the displayed SOC.

[0022] 3) Calibration accuracy. This refers to the deviation between the true SOC calculated by the BMS and the actual SOC when calibration is triggered.

[0023] 4) Reliability of calibration. Evaluate whether the SOC will be miscalibrated under operating conditions.

[0024] Explanation of terms in the instruction manual: Display SOC: The SOC output by the BMS after continuous processing of the actual SOC; Baseline SOC (True SOC): Battery pack SOC estimated internally by the BMS; Initial System SOC: refers to the actual SOC of the battery pack when the battery pack is charged using charging and discharging equipment.

[0025] Absolute SOC difference = Modified SOC - Initial system SOC.

[0026] The meaning of an absolute difference of 20 to 50 percentage points: Based on the initial SOC, directly add or subtract 20 to 50, for example, if the initial is 40%, it will be modified to 60% to 90%.

[0027] BMS Preset SOC Calibration Trigger Range: The current range that is preset by the BMS program and can trigger the SOC calibration logic.

[0028] Hybrid discharge circuit spectrum matched to the test environment temperature: the discharge simulation circuit spectrum of the hybrid vehicle under the corresponding temperature conditions.

[0029] Note: The discharge circuit spectrum is a simulated circuit spectrum under hybrid operating conditions. The low-temperature discharge circuit spectrum is used for low-temperature tests, and the high-temperature discharge circuit spectrum is used for high-temperature tests.

[0030] The preset discharge cutoff voltage is the specified discharge cutoff voltage for this LFP battery pack.

[0031] The battery pack is discharged using a constant current rate until the lowest voltage of a single battery cell reaches the preset discharge cutoff voltage, which is 2.5V.

[0032] The battery pack is charged using a constant current rate until the highest voltage of each battery cell reaches the preset charging cutoff voltage, which is 3.65V.

[0033] Miscalibration refers to the BMS performing a SOC calibration operation when the SOC calibration trigger conditions are not met.

[0034] The specific verification process is as follows: Figure 1 As shown.

[0035] The specific verification steps and methods are as follows: 1. Verification of SOC calibration strategy using constant rate charge / discharge 1.1 Verification of constant rate charge and discharge at room temperature 1.1.1 Calibration Verification under the Condition of Falsely High SOC at Room Temperature Under normal temperature conditions, adjust the LFP battery pack to an initial system SOC of 30%. Manually modify the actual SOC and displayed SOC of the BMS to 60%, i.e., increase it by 30 percentage points. Discharge at a constant rate until the minimum voltage of each cell reaches the cutoff condition (2.5V), stopping the discharge at a rate that meets the SOC calibration trigger. Verify whether the SOC can trigger calibration in time when the calibration conditions are met, and whether the calibration accuracy meets the requirements.

[0036] For example, a 10AH battery system is verified using a 0.3C discharge, and the test curve is as follows: Figure 2 As shown.

[0037] When the first calibration point specified in the calibration strategy at the discharge end—25% SOC—is reached, the BMS promptly triggers calibration. The actual SOC is calibrated to 25% SOC, at which point the theoretical SOC is 24.5%, with a calibration accuracy of 0.5%. The displayed SOC quickly follows the actual SOC. When the actual SOC drops from 25% to 23.7%, the displayed SOC catches up with the actual SOC, and the two are consistent.

[0038] Criteria for judging SOC calibration accuracy: | Actual SOC - Theoretical SOC| ≤ 8%; The criteria for judging the timeliness of calibration: from the start of calibration, within a 5% change in the actual SOC, the displayed SOC catches up with the actual SOC; Conclusion: The SOC calibration function at the discharge end at room temperature meets the requirements.

[0039] 1.1.2 Calibration Verification under Low SOC Conditions at Room Temperature Under normal temperature conditions, adjust the LFP battery pack to an initial system SOC of approximately 80%. Manually modify the actual SOC and displayed SOC in the BMS to 60%, a decrease of 20 percentage points. Charge at a constant rate until the highest voltage of each cell reaches the cutoff condition (3.65V), then stop. Select a charging rate that meets the SOC calibration trigger requirement. Verify whether the SOC triggers calibration promptly upon reaching the calibration conditions and whether the calibration accuracy meets the requirements.

[0040] For example, a 10AH battery system, using 0.3C charging verification, the test curve is as follows: Figure 3 As shown.

[0041] When the first calibration point specified in the calibration strategy for the end of charging—95% SOC—is reached, the BMS promptly triggers calibration. The actual SOC is calibrated to 95% SOC, at which point the theoretical SOC is 97.4%, with a calibration accuracy of 2.4%. The displayed SOC quickly catches up with the actual SOC. When the actual SOC rises from 95% to 95.2%, the displayed SOC catches up with the actual SOC, and the two are consistent.

[0042] Conclusion: The SOC calibration function at the charging end at room temperature meets the requirements.

[0043] 1.1.3 Verification of Room Temperature SOC Miscalibration 1) Under normal temperature conditions, adjust the LFP battery pack to an initial system SOC of 100%. Confirm that the actual SOC and displayed SOC of the BMS are also 100%. Discharge at a constant rate until the lowest voltage of a single cell reaches the cutoff condition (2.5V), then proceed to the next step. Let it rest for 10 minutes, then charge at a constant rate until the highest voltage of a single cell reaches the cutoff condition (3.65V), then stop. Select the charge and discharge rates that meet the SOC calibration trigger. Verify whether the SOC will trigger false calibration during the full charge and discharge process.

[0044] For example, a 10AH battery system is verified using a 0.5C charge-discharge cycle, and the test curve is as follows: Figure 4 As shown.

[0045] During the discharge process from 100% to 0%, the BMS did not trigger miscalibration, and the SOC accuracy was ≤5.8%.

[0046] Conclusion: Under normal temperature conditions, no miscalibration was triggered during full discharge within the calibration range, and the function was normal.

[0047] The analysis under 0-100% full-fill conditions is the same as that under full-discharge conditions.

[0048] 2) Full discharge and full charge verification outside the calibration rate under normal temperature conditions. Adjust the LFP battery pack to an initial system SOC of 100%. Confirm that the actual SOC and displayed SOC of the BMS are also 100%. Discharge at a constant rate until the lowest voltage of a single cell reaches the cutoff condition, then proceed to the next step. Let it rest for 10 minutes, then charge at a constant rate until the highest voltage of a single cell reaches the cutoff condition. Select a current rate outside the SOC calibration trigger range for both charging and discharging. Verify whether the SOC will trigger incorrect calibration during the full discharge and full charge process.

[0049] For example, a 10AH battery system uses 2C charge / discharge verification, and the test curve is as follows: Figure 5 As shown.

[0050] During the charge / discharge process from 100% to 0% and back to 100%, the BMS did not trigger miscalibration, and the SOC accuracy was ≤2.8%.

[0051] Conclusion: Under normal temperature conditions, the SOC did not trigger miscalibration during the full charge and discharge process outside the calibration range, and the function was normal.

[0052] 1.2 Verification of constant rate charge and discharge at low temperature 1.2.1 Calibration Verification under Low-Temperature SOC (Falsely High) Conditions Under 0±2℃ conditions, adjust the LFP battery pack to an initial system SOC of approximately 30%. Manually modify the actual SOC and displayed SOC of the BMS to 60% SOC. Discharge at a constant rate until the minimum voltage of each cell reaches the cutoff condition, stopping when the discharge rate is selected to meet the current rate triggering SOC calibration. Verify whether SOC can trigger calibration in time when the calibration conditions are met, and whether the calibration accuracy meets the requirements.

[0053] For example, a 10AH battery system is verified using a 0.6C to 0.5C discharge cycle, and the test curve is as follows: Figure 6 As shown.

[0054] When the first calibration point specified in the calibration strategy at the discharge end—25% SOC—is reached, the BMS promptly triggers calibration. The actual SOC calibration is 24.7% SOC, while the theoretical SOC is 21.4%, resulting in a calibration accuracy of 3.3%. The displayed SOC quickly catches up with the actual SOC. When the actual SOC decreases from 24.7% to 23.3%, the displayed SOC catches up with the actual SOC, and the two are consistent.

[0055] Conclusion: The SOC calibration function at the discharge end at low temperature meets the requirements.

[0056] 1.2.2 Calibration Verification under Low-Temperature State of Charge (SOC) Conditions At 0±2℃, adjust the LFP battery pack to an initial system SOC of 70%. Manually adjust the actual SOC and displayed SOC of the BMS to approximately 40% SOC. Charge at a constant rate until the highest voltage of each cell reaches the cutoff condition, stopping the charging rate at a current rate that meets the SOC calibration trigger. Verify whether the SOC can trigger calibration in time when the calibration conditions are met, and whether the calibration accuracy meets the requirements.

[0057] For example, a 10AH battery system is verified using a 0.4C to 0.5C charging cycle, and the test curve is as follows: Figure 7 As shown.

[0058] When the BMS triggers 95% SOC calibration, the actual SOC calibration is 95% SOC, while the theoretical SOC is 69%, resulting in a calibration accuracy of 26%. The calibration accuracy does not meet the requirements.

[0059] Conclusion: The SOC calibration function at the charging end at low temperatures does not meet the requirements.

[0060] 1.2.3 Verification of Low-Temperature SOC Miscalibration 1) At 0±2℃, adjust the LFP battery pack to an initial system SOC of 100%. Confirm that the actual SOC and displayed SOC of the BMS are also 100%. Discharge at a constant rate until the lowest voltage of a single cell reaches the cutoff condition, then proceed to the next step. Let it stand for 10 minutes, then charge at a constant rate until the highest voltage of a single cell reaches the cutoff condition. Select the charge and discharge rates that meet the SOC calibration trigger. Verify whether the SOC will trigger incorrect calibration during the full charge and discharge process.

[0061] For example, a 10AH battery system is tested using a 0.5C charge-discharge cycle.

[0062] During the discharge process from 100% to 0%, the BMS did not trigger miscalibration, and the SOC accuracy was ≤3.2%; during the charging process from 0% to 100% SOC, the BMS did not trigger miscalibration, and the SOC accuracy was ≤4.3%.

[0063] Conclusion: Under low temperature conditions, the SOC did not trigger miscalibration during the full charge and discharge process within the calibration range, and the function was normal.

[0064] 2) Full charge / discharge verification outside the calibration rate at 0±2℃. Adjust the LFP battery pack to an initial system SOC of 100%. Confirm that the actual SOC and displayed SOC of the BMS are also 100%. Discharge at a constant rate until the lowest voltage of a single cell reaches the cutoff condition, then proceed to the next step. Let it stand for 10 minutes, then charge at a constant rate until the highest voltage of a single cell reaches the cutoff condition. Select a current rate outside the SOC calibration trigger range for both charge and discharge. Verify whether the SOC will trigger incorrect calibration during the full charge / discharge process.

[0065] During the discharge process from 100% to 0%, the BMS did not trigger miscalibration, and the SOC accuracy was ≤2.8%; during the charging process from 0% to 100% SOC, the BMS did not trigger miscalibration, and the SOC accuracy was ≤3.9%.

[0066] Conclusion: Under low temperature conditions, the SOC did not trigger miscalibration during full discharge and full charge outside the calibration range, and the function was normal.

[0067] 1.3 Verification of constant rate charge and discharge at high temperature Under conditions of 40~50℃, the constant rate charge and discharge verification process is the same as the low temperature verification process in 1.2.

[0068] 2. Road Spectrum Operating Condition Verification of SOC Calibration Strategy 2.1 Verification of SOC calibration strategy under ambient temperature road spectrum conditions 2.1.1 Selection of Charge / Discharge Circuit Spectrum When selecting ambient temperature, low temperature, and high temperature profiles, the following requirements should be met: 1) It can represent the typical operating conditions of the whole machine, including the maximum allowable feedback current and maximum discharge current under the maximum load at the target temperature; 2) The discharge spectrum generally shows a discharge trend, while the charging spectrum generally shows a charging trend; 3) The data interval of the road spectrum is ≤0.1s.

[0069] 2.1.2 Room temperature discharge at 10% SOC, simulating calibration verification under artificially high SOC conditions. Under normal temperature conditions, adjust the LFP battery pack to an initial system SOC of 10%, and manually adjust the BMS actual SOC and displayed SOC to 60%. Discharge the battery pack using a room temperature discharge circuit spectrum. Stop the test when the minimum voltage of a single cell reaches the cutoff condition. Verify whether the SOC can trigger calibration in time when the calibration conditions are met, and whether the calibration accuracy meets the requirements.

[0070] For example, the discharge end spectrum test curve of a 10AH battery system at room temperature is as follows: Figure 8 As shown.

[0071] The BMS underwent two calibrations. During the first calibration, the actual SOC was 14.4%, and the theoretical SOC was 9.23%, with a deviation of 5.17%. During the second calibration, the actual SOC was 5.5%, and the theoretical SOC was 4.18%, with a deviation of 1.32%. At the end of the road spectrum test, the deviation between the actual and theoretical SOC was 0.7%, and the SOC calibration accuracy met the requirements.

[0072] During the first calibration trigger, the theoretical SOC dropped from 9.23% to 8.96%, indicating that the SOC caught up with the actual SOC. During the second calibration trigger, the theoretical SOC dropped from 4.18% to 4.06%, again indicating that the SOC caught up with the actual SOC. The SOC calibration rate meets the requirements. Conclusion: Under normal temperature discharge circuit spectrum conditions, the SOC calibration function meets the requirements.

[0073] 2.1.3 Room temperature circuit spectrum charging at 90% SOC, simulating calibration verification under artificially low SOC conditions. Under normal temperature conditions, adjust the LFP battery pack to an initial system SOC of approximately 90%. Manually adjust the BMS actual SOC and displayed SOC to 60%. Charge the battery pack using a normal temperature charging circuit. Stop the test when the highest voltage of a single cell reaches the cutoff condition. Verify whether the SOC can trigger calibration in time when the calibration conditions are met, and whether the calibration accuracy meets the requirements.

[0074] For example, the power spectrum test curve of a 10AH battery system at the end of normal temperature charging is as follows: Figure 9 As shown.

[0075] When BMS triggers calibration, the actual SOC of BMS is 95%, and the theoretical SOC is 93.38%, with a deviation of 1.62%. At the end of the road spectrum test, the deviation between the actual SOC of BMS and the theoretical SOC is 2.2%, and the SOC calibration accuracy meets the requirements.

[0076] When calibration was triggered, the theoretical SOC increased from 93.38% to 93.62%, indicating that the SOC caught up with the actual SOC. The SOC calibration rate met the requirements. Conclusion: Under normal temperature charging circuitry conditions, the SOC calibration function meets the requirements.

[0077] 2.1.4 Perform room temperature discharge cycles at 40% SOC to verify whether SOC calibration is falsely triggered. Under normal temperature conditions, the LFP battery pack was adjusted to an initial system SOC of approximately 40%. The pack was then discharged using a room-temperature discharge spectrum. The test was stopped when the minimum voltage of each individual cell reached the cutoff condition. This verifies whether the SOC triggers miscalibration during the discharge process from 40% to 0% SOC.

[0078] For example, the discharge spectrum cycle test curve of a 10AH battery system at room temperature is as follows: Figure 10 , Figure 11 , Figure 12 As shown.

[0079] During the discharge process from 40% to 0% of the road spectrum, the BMS did not trigger miscalibration, and the SOC accuracy was ≤3.5%. Conclusion: During the room temperature discharge cycle, the SOC did not trigger miscalibration, and the function was normal.

[0080] 2.1.5 Perform a room temperature circuit spectrum charging cycle at 70% SOC to verify whether SOC calibration is falsely triggered. Under normal temperature conditions, the LFP battery pack is adjusted to an initial system SOC of approximately 70%. The battery pack is then charged using a normal temperature charging circuit. The test is stopped when the highest voltage of a single cell reaches the cutoff condition. This verifies whether the SOC triggers miscalibration during the discharge process within the 70%-100% SOC circuit. For example, for a 10AH battery system, the normal temperature charging circuit cyclic test curve is as follows: Figure 13 , Figure 14 As shown.

[0081] During the charging process from 70% to 100% of the road spectrum, the BMS did not trigger miscalibration, and the SOC accuracy was ≤1.54%. Conclusion: During the room temperature circuit spectrum charging cycle, the SOC did not trigger a miscalibration, and the function was normal.

[0082] 2.2 Low-Temperature Road Spectrum Condition Verification of SOC Calibration Strategy 2.2.1 Low-temperature path spectrum discharge at 10% SOC, simulating calibration verification under artificially high SOC conditions. At 5±2℃, the LFP battery pack was adjusted to an initial system SOC of 10%. The actual SOC and displayed SOC of the BMS were manually adjusted to 60%. The battery pack was discharged using a low-temperature discharge spectrum. The test was stopped when the minimum voltage of a single cell reached the cutoff condition. This verified whether the SOC could trigger calibration in time when the calibration conditions were met, and whether the calibration accuracy met the requirements.

[0083] For example, the low-temperature discharge end-of-life spectrum test curve of a 10AH battery system is as follows: Figure 15 As shown.

[0084] The BMS underwent four calibrations. During the first calibration trigger, the actual SOC was 24.1%, and the theoretical SOC was 10%, with a deviation of 14.1%. During the second calibration trigger, the actual SOC was 19%, and the theoretical SOC was 11.7%, with a deviation of 7.3%. During the third calibration trigger, the actual SOC was 15.7%, and the theoretical SOC was 11.6%, with a deviation of 4.1%. During the fourth calibration trigger, the actual SOC was 8.4%, and the theoretical SOC was 12.7%, with a deviation of 4.3%. At the end of the road spectrum test, the deviation between the actual and theoretical SOC was 3.8%, and the SOC calibration accuracy met the requirements.

[0085] During the first calibration trigger, the theoretical SOC rose from 10% to 11.65%, indicating that the SOC caught up with the actual SOC, and the SOC calibration rate met the requirements. During the second calibration trigger, the theoretical SOC rose from 11.65% to 11.66%, again indicating that the SOC caught up with the actual SOC, and the SOC calibration rate met the requirements. During the third calibration trigger, the theoretical SOC fell from 11.6% to 11.56%, indicating that the SOC caught up with the actual SOC, and the SOC calibration rate met the requirements. During the fourth calibration trigger, the theoretical SOC fell from 12.69% to 12.43%, again indicating that the SOC caught up with the actual SOC, and the SOC calibration rate met the requirements. Conclusion: Under low-temperature discharge circuit spectrum conditions, the SOC calibration function meets the requirements.

[0086] 2.2.2 Low-temperature path spectrum charging at 90% SOC: Calibration verification under simulated low SOC conditions At 5±2℃, the LFP battery pack was adjusted to an initial system SOC of 90%. The actual SOC and displayed SOC of the BMS were manually adjusted to 60%. The battery pack was charged using a low-temperature charging path spectrum. The test was stopped when the highest voltage of a single cell reached the cutoff condition. This verified whether the SOC could trigger calibration in time when the calibration conditions were met, and whether the calibration accuracy met the requirements.

[0087] The analysis process is described in section 2.1.3.

[0088] 2.2.3 Low-temperature discharge cycling at 40% SOC to verify whether SOC calibration is falsely triggered. At 5±2℃, the LFP battery pack was adjusted to an initial system SOC of approximately 40%. The battery pack was then discharged using a low-temperature discharge spectrum. The test was stopped when the minimum voltage of each individual cell reached the cutoff condition. This verifies whether the SOC triggers miscalibration during the discharge process within the 40%-0% SOC spectrum.

[0089] The analysis process is described in section 2.1.4.

[0090] 2.2.4 Low-temperature path spectrum charging cycle at 70% SOC to verify whether SOC calibration is falsely triggered. At 5±2℃, the LFP battery pack was adjusted to an initial system SOC of approximately 70%. The battery pack was then charged using a low-temperature charging circuit. The test was stopped when the highest voltage of a single cell reached the cutoff condition. This verifies whether SOC triggers miscalibration during discharge in a 70%-100% SOC circuit.

[0091] The analysis process is described in section 2.1.5.

[0092] 2.3 Verification of SOC Calibration Strategy under High-Temperature Road Spectrum Conditions Under conditions of 40~50℃, the verification process for high-temperature road spectrum conditions is the same as that under low-temperature conditions, see 2.2.

[0093] If all of the above requirements are met, the SOC calibration function of the BMS is considered to meet the requirements.

[0094] The cumulative error of the ampere-hour integral was verified using road spectrum operating conditions to calculate the theoretical calibration frequency. The test data are shown in Table 1.

[0095] The test results show that after 14 hours of road spectrum testing, the BMS SOC was 1.84% higher than the theoretical value. During spring and autumn planting seasons, assuming 14 hours of operation per day, the cumulative SOC error after two weeks of continuous operation would be approximately 1.84% * 14 = 25.8%. The vehicle's SOC control range is between 50-60%. Theoretically, after two weeks of continuous operation, the BMS SOC would be 25.8% higher than the theoretical value. The actual SOC was 50% - 25.8% = 24.2%, triggering a calibration during the SOC calibration period.

[0096] Table 1. Accumulated error of ampere-hour integral under road spectrum conditions

[0097] The SOC calibration strategy, validated by this evaluation method, was applied to hybrid tractors, resulting in stable and reliable overall performance during spring plowing, with no frequent or erroneous calibrations. In practice, discharge-end calibration was triggered approximately once every two weeks, consistent with theoretical predictions. No overcharge or over-discharge faults caused by incorrect or missed SOC calibrations occurred during the spring plowing period.

[0098] A 260-horsepower hybrid tractor equipped with battery pack number 711F6601032 accumulated 304.1 hours of work time during spring plowing operations in Wuwei, Gansu Province in April 2026, averaging 10.49 hours per day, with a maximum single-day work time of 23.5 hours. SOC calibration was triggered once on April 15th, and the SOC remained relatively stable between 50-60% for the rest of the time. After calibration, the SOC quickly returned to the convergence range, and the individual cell voltage was controlled between 3.15-3.48V, without triggering overcharge or over-discharge fault alarms. The tractor demonstrated stable and reliable performance during spring plowing operations, as verified by the following curve: Figure 16 As shown.

[0099] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. An evaluation method for the SOC calibration function of an LFP battery pack based on hybrid operating conditions, characterized in that: This includes a constant rate charge / discharge verification SOC calibration strategy and a road spectrum operating condition verification SOC calibration strategy; The constant rate charge / discharge verification SOC calibration strategy includes: Calibration verification under the condition of artificially high SOC at room temperature / low temperature / high temperature. Artificially high SOC means that the internal reference SOC of BMS and the displayed SOC output by BMS are manually modified so that the modified reference SOC and displayed SOC are higher than the initial system SOC. Calibration verification under falsely low SOC conditions at room temperature / low temperature / high temperature. Falsely low conditions refer to manually modifying the internal reference SOC of the BMS and the displayed SOC output by the BMS so that the modified reference SOC and displayed SOC are lower than the initial system SOC. Verification of SOC miscalibration at room temperature / low temperature / high temperature; including SOC not triggering miscalibration during full charge and discharge within the calibration range and SOC not triggering miscalibration during full charge and discharge outside the calibration range; The road spectrum operating condition verification SOC calibration strategy includes: Calibration verification under simulated high SOC conditions in the circuit spectrum discharge at initial 10~30% SOC at room temperature / low temperature / high temperature; Specifically, under normal, low, or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 10%–30%; the internal reference SOC and the displayed SOC output by the BMS are manually modified so that the absolute SOC difference between the modified internal reference SOC and the displayed SOC and the initial system SOC of the battery pack is 20–50 percentage points; the battery pack is discharged using a hybrid discharge circuit spectrum that matches the current test environment temperature; the test is stopped when the minimum voltage of a single cell reaches the preset discharge cutoff voltage; when the SOC calibration conditions are met during the discharge process, it is checked whether the BMS initiates SOC calibration and the accuracy of the calculated SOC after calibration is checked. Calibration verification under simulated low SOC conditions using a road spectrum charging method at initial 70-90% SOC at room temperature, low temperature, and high temperature. Specifically, under normal, low, or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 70%–90%; the internal reference SOC and the displayed SOC output by the BMS are manually modified so that the absolute SOC difference between the modified internal reference SOC and the displayed SOC and the initial system SOC of the battery pack is 20–50 percentage points; the battery pack is charged using a hybrid charging circuit that matches the current test environment temperature; the test is stopped when the highest voltage of a single cell reaches the preset charging cutoff voltage; when the SOC calibration conditions are met during charging, it is checked whether the BMS initiates SOC calibration and the accuracy of the calculated SOC after calibration is checked. Verify whether SOC calibration is falsely triggered by discharge cycling at room temperature / low temperature / high temperature under initial SOC of 40~60%; Specifically, under normal temperature, low temperature, or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 40%–60%; the battery pack is discharged using a hybrid discharge circuit spectrum that matches the current test environment temperature; the test is stopped when the lowest voltage of a single cell reaches the preset discharge cutoff voltage; and the BMS is checked for SOC miscalibration throughout the entire process from the initial system SOC of 40%–60% to the end of the discharge. Verify whether SOC calibration is falsely triggered by charging cycles at room temperature / low temperature / high temperature under initial 50~70% SOC. Specifically, under normal temperature, low temperature, or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 50%–70%; the battery pack is charged using a hybrid charging path that matches the current test environment temperature; the test is stopped when the highest voltage of a single cell reaches the preset charging cutoff voltage; and the BMS is checked for SOC miscalibration throughout the entire process from the initial 50%–70% SOC to the end of charging.

2. The evaluation method for the SOC calibration function of an LFP battery pack based on hybrid operating conditions as described in claim 1, characterized in that, The constant rate charge / discharge verification SOC calibration strategy specifies a low temperature of -30℃ to 15℃ and a high temperature of 40℃ to 50℃; the road spectrum operating condition verification SOC calibration strategy specifies a low temperature of -10℃ to 15℃ and a high temperature of 40℃ to 50℃.

3. The evaluation method for the SOC calibration function of an LFP battery pack based on hybrid operating conditions as described in claim 1, characterized in that, The calibration verification under the conditions of artificially high SOC at normal / low / high temperatures is specifically as follows: Under normal, low, or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 30%–50%; the internal reference SOC of the BMS and the displayed SOC output by the BMS are manually modified so that the absolute SOC difference between the modified reference SOC and the displayed SOC and the initial system SOC is 20–50 percentage points; the battery pack is discharged using a constant current rate until the minimum voltage of the individual battery cells reaches the preset discharge cutoff voltage and then the discharge is stopped; wherein, the discharge current rate is selected as the current rate preset by the BMS that can trigger the SOC calibration logic; during the discharge process, it is verified whether the BMS can start the SOC calibration in time when the SOC calibration trigger condition is reached, and the calculation accuracy after SOC calibration is detected.

4. The evaluation method for the SOC calibration function of an LFP battery pack based on hybrid operating conditions as described in claim 1, characterized in that, The calibration verification under the falsely low SOC conditions at normal / low / high temperatures is specifically as follows: Under normal, low, or high temperature conditions, the LFP battery pack is adjusted to an initial system SOC of 70%–90%; the internal reference SOC of the BMS and the displayed SOC output by the BMS are manually modified so that the absolute SOC difference between the modified reference SOC and the displayed SOC and the initial system SOC of the battery pack is 20–50 percentage points; the battery pack is charged using a constant current rate until the highest voltage of the individual battery cells reaches the preset charging cutoff voltage, at which point charging is stopped; the charging current rate is selected as the current rate preset by the BMS that can trigger the SOC calibration logic; during the charging process, it is verified whether the BMS can promptly initiate SOC calibration when the SOC calibration trigger condition is met, and the calculation accuracy after SOC calibration is detected.

5. The evaluation method for the SOC calibration function of an LFP battery pack based on hybrid operating conditions as described in claim 1, characterized in that, The specific steps for ensuring that the SOC does not trigger miscalibration during the full charge and discharge process within the calibration rate range are as follows: Under normal temperature, low temperature, or high temperature conditions, adjust the LFP battery pack to an initial system SOC of 100%, and confirm that both the internal reference SOC of the BMS and the displayed SOC output by the BMS are 100%. Select a current rate that falls within the SOC calibration trigger rate range preset by the BMS for constant rate discharge, and discharge until the lowest voltage of the single cell reaches the preset discharge cutoff voltage. After that, let it stand for 10 minutes, then charge it at the same constant rate until the highest voltage of the single cell reaches the preset charging cutoff voltage and then stop. Detect whether the BMS experiences SOC miscalibration during the full charge and discharge process; The specific details of not triggering miscalibration of SOC during full charge and discharge outside the calibration rate range are as follows: Under normal temperature, low temperature or high temperature conditions, adjust the LFP battery pack to the initial system SOC of 100%, and confirm that the internal reference SOC of BMS and the displayed SOC output by BMS are both 100%; Select a current rate that falls outside the SOC calibration trigger rate range preset by the BMS for constant rate discharge, and discharge until the lowest voltage of the single cell reaches the preset discharge cutoff voltage. After that, let it stand for 10 minutes, then charge it at the same constant rate until the highest voltage of the single cell reaches the preset charging cutoff voltage and then stop. Detect whether the BMS experiences SOC miscalibration during the entire full charge and discharge process.

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