Lithium iron phosphate battery SOC error correction method and error correction module

Abstract

The invention relates to a lithium iron phosphate battery SOC error correction method and an error correction module, and belongs to the technical field of battery management, and the method comprises the following steps: 1, calculating an SOC error in real time; step 2, judging whether an error of SOC calculation is greater than 5% of a standard value, if so, entering step 3, and otherwise, returning to step 1; step 3, entering calibration charging, judging whether small slope calibration is carried out on the SOC, if so, entering step 4, and if not, entering step 5; step 4, performing small slope calibration by SOC, and then entering step 7; 5, judging whether full charge calibration is carried out or not, if so, entering a step 6, and otherwise, returning to the step 3; 6, the SOC performs full charge calibration and then enters the step 7; and 7, exiting the calibration charging, resetting the accumulated SOC error, and returning to the step 1. The SOC error is calculated in real time, whether small slope correction or full charge correction is carried out or not is judged, and the accumulated error is reduced or eliminated to a certain degree.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery management, and in particular relates to a SOC error correction method for a lithium iron phosphate battery and a SOC error correction module for a lithium iron phosphate battery. Background Art

[0002] With the continuous and rapid development of electric vehicles, the requirements for low-voltage batteries of electric vehicles are getting higher and higher. Traditional lead-acid batteries can no longer meet the power supply needs of low-voltage devices of electric vehicles due to their low discharge power, slow charging speed, short life, serious false power and other problems. Therefore, low-voltage lithium batteries have become a key component of electric vehicles, and cooperate with electric vehicles to realize the key power supply during the low-voltage power supply period in the vehicle. Lithium iron phosphate batteries have become the main battery source for low-voltage lithium battery systems because of their advantages of high safety, long life and low cost.

[0003] SOC (State of charge) indicates the remaining power of a low-voltage lithium battery system. It is a key indicator for electric vehicles to manage the charge and discharge of low-voltage lithium battery systems. It is also a key indicator of low-voltage lithium battery systems and is generally expressed in percentage. At present, the general method for calculating SOC is based on ampere-hour integration, but ampere-hour integration has initial value errors and cumulative errors. The estimated error of SOC must be controlled within a certain range. Otherwise, the SOC error will be too large, causing disorder in the charge and discharge control of the low-voltage battery system, resulting in product failure and safety risks.

[0004] At present, there are mainly the following algorithms for SOC correction: one is to correct SOC under certain conditions based on the relationship between static voltage, OCV and SOC; the second is to calibrate the SOC value based on dynamic voltage according to certain characteristic points of the charging curve or discharge curve, and when the calibrated voltage, current and temperature conditions are met, the current estimated SOC is processed as a calibrated SOC value; the third is a filtering algorithm based on a first-order, second-order or even higher-order RC equivalent circuit model, such as EKF and UKF algorithms, to achieve rapid elimination of the initial value error of the estimated SOC and real-time update of the SOC. However, these correction algorithms have some non-negligible defects in the process of migrating to low-voltage lithium battery systems, especially lithium iron phosphate batteries, which makes it impossible to fundamentally solve the estimation error of the BMS SOC of low-voltage lithium battery systems. As mentioned above, the first SOC correction method, based on OCV SOC correction, requires that the voltage corresponding to the current SOC be in the non-platform area before it can be used. As a power source, the low-voltage lithium battery system will not allow the voltage to drop below the non-platform area (<30% SOC) during normal use to avoid the failure of power feeding to meet the power supply needs of the on-board low-voltage electrical appliances. In addition, this method requires the low-voltage lithium battery system to be stationary in the non-platform area for a long time, which does not match the current application conditions of the low-voltage lithium battery system (if the SOC is lower than 30%, the power replenishment will be started, and the system will not be allowed to stand at a low SOC). Therefore, this method is almost not triggered under normal use conditions on the low-voltage lithium battery system; the defect of the second method is that it requires special The test and calibration of the collection points has a large testing workload, and the calibration parameters will be distorted due to the differences between batteries. After the battery ages, the calibration value is not updated, the error of the SOC correction value will also become larger, and even the problem of miscorrection will occur; the third algorithm based on models and filtering, on the one hand, has a large amount of calculation, and has high requirements on the computing power and memory of the hardware MCU, which increases the cost of the product. In addition, due to the problem of a long platform area of ​​lithium iron phosphate and the characteristics of the low-voltage lithium battery system mainly working in the range above the platform area, this algorithm is not effective in improving the estimation accuracy of SOC. After the battery ages, the built-in battery model parameters cannot be updated, and the estimation error of SOC is difficult to control and cannot meet the needs. Summary of the invention

[0005] The purpose of the present invention is to provide a SOC error correction method for a lithium iron phosphate battery in view of the problems existing in the prior art, which can be applied to the SOC error correction of a lithium iron phosphate battery to meet the use requirements of a low-voltage lithium battery system.

[0006] The technical solution is as follows:

[0007] A method for correcting SOC error of a lithium iron phosphate battery, comprising the following steps:

[0008] Step 1: Calculate the SOC error in real time using the following formula based on the SOC initial error, current sampling gain error, and current sampling drift error of the lithium iron phosphate battery:

[0009] SoC error =SoC error_initial +I gain_error *Δt+I drift_error *Δt, where

[0010] SoC error_initial is the initial error of SOC, I gain_error is the current sampling gain error, I drift_error is the current sampling drift error, Δt is the operation and sleep time since the last SOC correction;

[0011] Step 2: Determine whether the error of SOC calculation is greater than 5% of the standard value, if yes, go to step 3, otherwise return to step 1;

[0012] Step 3: Enter calibration charging to determine whether to perform a small slope calibration on the SOC. If necessary, proceed to step 4, otherwise proceed to step 5;

[0013] Step 4: SOC performs small slope calibration and then proceeds to step 7;

[0014] Step 5: Determine whether to perform full charge calibration. If necessary, proceed to step 6, otherwise return to step 3;

[0015] Step 6: After the SOC is fully charged and calibrated, proceed to step 7;

[0016] Step 7: Exit calibration charging, reset the accumulated SOC error, and return to step 1.

[0017] Furthermore, the error setting value of the SOC calculation in step 2 is 5% different from the standard value.

[0018] Furthermore, the current of the calibration charge in step 3 is constant current charging in the range of 0.1C to 1C, where C is the current maximum capacity of the lithium iron phosphate battery.

[0019] Further, in step 3, whether to perform a small slope calibration on the SOC is determined by the following steps:

[0020] Step 31: Calculate the SOC change dSOC in real time based on the Ah integral method, and calculate the cell voltage change dV within a fixed SOC change range;

[0021] Step 32: Calculate the ratio dV / dSOC of the voltage change within the range of the fixed SOC change of the battery cell to the SOC change, and record the calculated value;

[0022] Step 33: based on the dV / dSOC values ​​calculated and recorded in real time, draw a dV / dSOC curve, and calculate the slope of the curve;

[0023] Step 34: When the slope of the curve increases first and then decreases, and the dV / dSOC value is within the set threshold, it is judged as the peak point of the dV / dSOC curve, and a small slope calibration is performed on the SOC based on the peak point; when the peak point is not obvious or there is no peak point, no small slope calibration is performed, and step 5 is entered.

[0024] Furthermore, in step 34, the threshold is set to 0.015-0.050, and the cell voltage value corresponding to the value is between 3.3V and 3.45V.

[0025] Further, in step 6, when fully charged, the maximum voltage range of the single cell is 3.45V to 3.65V, and the total voltage range of the system is 3.45V*the number of lithium iron phosphate battery cells in series to 3.65V*the number of lithium iron phosphate battery cells in series. The specific steps of full charge calibration are as follows:

[0026] Step 61: Charge to full charge, determine the maximum voltage of the single cell and the total pressure of the system in real time in units of 1 second, if the maximum voltage of the single cell is greater than the single cell full charge threshold or the total pressure of the system is greater than the system full charge threshold, proceed to step 62, if the maximum voltage of the single cell is less than the single cell full charge threshold or the total pressure of the system is less than the system full charge threshold and the charging current drops to the set value or becomes a discharge current, proceed to step 63;

[0027] Step 62: Dynamically adjust the DC / DC output voltage to drop by 4 control levels each time the condition is determined to be met;

[0028] Step 63: Dynamically adjust the DC / DC output voltage to increase by one control level each time the condition is determined to be met;

[0029] Step 64: Full charge and consistency of cells in the system are judged in real time. After the pressure difference in the system is less than the set value and the cell voltage meets the cell full charge voltage, full charge calibration of SOC is performed.

[0030] Furthermore, the SOC error correction method for a lithium iron phosphate battery is characterized in that: in the step 61, the single cell full charge threshold is one of the following three situations at an average current of 0.5-1A: 3.6V for 10-15s, 3.55V for 30-60s, and 3.45V for 60-120s; the system full charge threshold is one of the following three situations at an average current of 0.5-1A: 3.6V*the number of lithium iron phosphate battery cells in series for 10-15s, 3.55V*the number of lithium iron phosphate battery cells in series for 30-60s, and 3.45V*the number of lithium iron phosphate battery cells in series for 60-120s, and the charging current drop constant is 3A.

[0031] Furthermore, in step 62 and step 63, each control level is 10mv to 20mv.

[0032] Furthermore, in step 64, the pressure difference in the system is set to 30-50 mV.

[0033] A lithium iron phosphate battery SOC error correction module is installed in a lithium iron phosphate battery control board, comprising a memory, a processor, and a computer program stored in the memory and capable of running on the processor. When the processor reads and executes the program, the lithium iron phosphate battery SOC error correction method as described above is implemented.

[0034] Beneficial effects:

[0035] 1) The present invention selects small slope correction or full charge correction based on the voltage error, current gain error, current zero drift error collected in real time and the SOC cumulative error in the real-time calculation process to reduce or eliminate the accumulated error to a certain extent.

[0036] 2) Based on the constant output current charging, in the small slope area of ​​the lithium iron phosphate battery, the charging dV / dSOC curve is calculated in real time, the peak point in the charging process is identified, and the peak corresponding SOC is calibrated.

[0037] 3) When calibrating the charging to the end, based on the maximum battery cell voltage value in the system, the DC / DC output voltage is adjusted in real time. On the one hand, the balancing speed is accelerated to quickly reduce the voltage difference between cells. On the other hand, the consistency between cells in the system is improved to ensure that the smallest cell in the system can be fully charged, avoiding the SOC error of the smallest cell in the system due to consistency problems, which affects the use of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a logic block diagram of the method of the present invention;

[0039] Figure 2 A logic block diagram for determining whether to perform a small slope calibration on the SOC;

[0040] Figure 3 The logic block diagram for full charge calibration;

[0041] Figure 4 This is a dV / dSOC-SOC relationship curve. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. The directions or positional relationships indicated by the terms "upper", "lower", "front", "back", "left", "right", "bottom", "inside", "outside", etc. are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the present invention:

[0043] like Figure 1 A method for correcting SOC error of a lithium iron phosphate battery is shown, and the following steps are adopted:

[0044] Step 1: Calculate the SOC error in real time using the following formula based on the SOC initial error, current sampling gain error, and current sampling drift error of the lithium iron phosphate battery:

[0045] SoC error =SoC error_initial +I gain_error *Δt+I drift_error *Δt, where

[0046] SoC error_initial is the initial error of SOC, I gain_error is the current sampling gain error, I drift_error is the current sampling drift error, Δt is the operation and sleep time since the last SOC correction;

[0047] Step 2: Determine whether the error of SOC calculation is greater than 5% of the standard value. If yes, enter calibration charging, otherwise return to step 1;

[0048] Step 3: Enter calibration charging to determine whether to perform a small slope calibration on the SOC. If necessary, enter step 4, otherwise enter step 5. The calibration charging current is a constant current charge in the range of 0.1C to 1C, where C is the current maximum capacity of the lithium iron phosphate battery;

[0049] Step 4: SOC performs small slope calibration and then proceeds to step 7;

[0050] Step 5: Determine whether to perform full charge calibration. If necessary, proceed to step 6, otherwise return to step 3;

[0051] Step 6: After the SOC is fully charged and calibrated, proceed to step 7;

[0052] Step 7: Exit calibration charging, reset the accumulated SOC error, and return to step 1.

[0053] like Figure 2 As shown, in step 3, whether to perform a small slope calibration on the SOC is determined by the following steps:

[0054] Step 31: Calculate the SOC change dSOC in real time based on the Ah integral method, and calculate the cell voltage change dV within a fixed SOC change range;

[0055] Step 32: Calculate the ratio dV / dSOC of the voltage change within the range of the fixed SOC change of the battery cell to the SOC change, and record the calculated value;

[0056] Step 33: based on the dV / dSOC values ​​calculated and recorded in real time, draw a dV / dSOC curve, and calculate the slope of the curve;

[0057] Step 34: When the slope of the curve increases first and then decreases, and the dV / dSOC value is between 0.015-0.050 and the cell voltage value corresponding to the value is between 3.3V and 3.45V, it is judged as the peak point of the dV / dSOC curve, and a small slope calibration is performed on the SOC based on the peak point; when the peak point is not obvious or there is no peak point, no small slope calibration is performed, and step 5 is entered.

[0058] like Figure 3 As shown, in step 6, when fully charged, the maximum voltage range of the single cell is 3.45V~3.65V, and the total voltage range of the system is 3.45V*the number of lithium iron phosphate battery cells in series~3.65V*the number of lithium iron phosphate battery cells in series. The specific steps of full charge calibration are as follows:

[0059] Step 61: Charge to full charge, determine the maximum voltage of the single cell and the total pressure of the system in real time in units of 1 second, if the maximum voltage of the single cell is greater than the single cell full charge threshold or the total pressure of the system is greater than the system full charge threshold, proceed to step 62, if the maximum voltage of the single cell is less than the single cell full charge threshold or the total pressure of the system is less than the system full charge threshold and the charging current drops to 3A or becomes a discharge current, proceed to step 63, wherein the single cell full charge threshold is one of the following three conditions under the average current of 0.5-1A: 3.6V for 10-15s, 3.55V for 30-60s, and 3.45V for 60-120s; the system full charge threshold is one of the following three conditions under the average current of 0.5-1A: 3.6V*the number of lithium iron phosphate battery cells in series for 10-15s, 3.55V*the number of lithium iron phosphate battery cells in series for 30-60s, and 3.45V*the number of lithium iron phosphate battery cells in series for 60-120s;

[0060] Step 62: Each time after determining that the condition is met, dynamically adjust the DC / DC output voltage to drop by 4 control levels, each control level being 10mv to 20mv;

[0061] Step 63: Each time after it is determined that the condition is met, the voltage of the DC / DC output is dynamically adjusted to increase by one control level, and each control level is 10mv to 20mv;

[0062] Step 64: Perform real-time judgment on full charge and consistency of cells in the system. After the pressure difference in the system is less than 30-50mV and the cell voltage meets the cell full charge voltage, perform full charge calibration of SOC.

[0063] A lithium iron phosphate battery SOC error correction module is installed in a lithium iron phosphate battery control board, comprising a memory, a processor, and a computer program stored in the memory and capable of running on the processor. When the processor reads and executes the program, the lithium iron phosphate battery SOC error correction method as described above is implemented.

[0064] Example 1: 4 lithium iron phosphate batteries are connected in series, with a single cell voltage of 3.45V to 3.65V and a total voltage of 13.8 to 14.6V. The battery control board is equipped with an SOC error correction module. The error correction module calculates the SOC error in real time according to the SOC initial error, current sampling gain error and current sampling drift error of the lithium iron phosphate battery using the following formula:

[0065] SoC error =SoC error_initial +I gain_error *Δt+I drift_error *Δt, if the error is greater than 5% of the standard value, then enter calibration charging, calculate the SOC change dSOC in real time based on the Ah integration method, calculate the change dV of the cell voltage within the fixed SOC change range, calculate the ratio of the voltage change to the SOC change dV / dSOC within the fixed SOC change range of the cell, and record the calculated value, draw the dV / dSOC curve based on the real-time calculated and recorded dV / dSOC value, and calculate the slope of the curve, such as Figure 4 As shown, when the slope of the curve increases first and then decreases, and the dV / dSOC value is 0.025 and the corresponding cell voltage value is 3.4V, it is judged as the peak point of the dV / dSOC curve, and a small slope calibration is performed on the SOC based on the peak point, the calibration charge is exited, and the accumulated SOC error is reset at the same time. After the reset is completed, the SOC error is recalculated in real time based on the initial value error and the accumulated error.

[0066] Example 2: 4 lithium iron phosphate batteries are connected in series, with a single cell voltage of 3.45V to 3.65V and a total voltage of 13.8 to 14.6V. The battery control board is equipped with an SOC error correction module. The error correction module calculates the SOC error in real time according to the SOC initial error, current sampling gain error and current sampling drift error of the lithium iron phosphate battery using the following formula:

[0067] SoC error =SoC error_initial +I gain_error *Δt+I drift_error *Δt, if the error is greater than 5% of the standard value, then enter the calibration charge, calculate the SOC change dSOC in real time based on the Ah integral method, calculate the change dV of the cell voltage within the fixed SOC change range, calculate the ratio of the voltage change to the SOC change dV / dSOC based on the fixed SOC change range of the cell, and record the calculated value, draw the dV / dSOC curve based on the real-time calculated and recorded dV / dSOC value, and calculate the slope of the curve. If there is no peak point, it is determined to perform full charge calibration and charge to full charge. When fully charged, the maximum voltage of the cell is 3.65V, and the total system voltage is 14.6V. The maximum voltage of the cell and the system in the system are determined in real time in units of 1 second. Total pressure, if the maximum voltage of the single cell is greater than 3.65V or the total voltage of the system is greater than 14.42V, the DC / DC output voltage is dynamically adjusted to decrease by 60mv each time the conditions are met; if the maximum voltage of the single cell is less than the full charge threshold and the total voltage of the system is less than 3.6V and the charging current drops to 3A, the DC / DC output voltage is dynamically adjusted to increase by 20mv each time the conditions are met; the full charge and consistency of the single cells in the system are judged in real time. After the pressure difference in the system is less than the set value and the single cell voltage meets the full charge voltage, the SOC full charge calibration is performed and then the calibration charge is exited. At the same time, the accumulated SOC error is reset. After the reset is completed, the SOC error is recalculated in real time based on the initial value error and the accumulated error.

[0068] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the principles and spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for correcting SOC error of a lithium iron phosphate battery, characterized in that: Step 1: Calculate the SOC error in real time using the following formula based on the SOC initial error, current sampling gain error, and current sampling drift error of the lithium iron phosphate battery: SoC error =SoC error_initial +I gain_error *Δt+I drift_error *Δt, where SoC error_initial is the initial error of SOC, I gain_error is the current sampling gain error, I drift_error is the current sampling drift error, Δt is the operation and sleep time since the last SOC correction; Step 2: Determine whether the error of SOC calculation is greater than 5% of the standard value, if yes, go to step 3, otherwise return to step 1; Step 3: Enter calibration charging to determine whether to perform a small slope calibration on the SOC. If necessary, proceed to step 4, otherwise proceed to step 5; Step 4: SOC performs small slope calibration and then proceeds to step 7; Step 5: Determine whether to perform full charge calibration. If necessary, proceed to step 6, otherwise return to step 3; Step 6: After the SOC is fully charged and calibrated, proceed to step 7; Step 7: Exit calibration charging, reset the accumulated SOC error, and return to step 1.

2. The SOC error correction method and detection method of a lithium iron phosphate battery according to claim 1, characterized in that: The error setting value of the SOC calculation in step 2 is 5% different from the standard value.

3. The SOC error correction method for a lithium iron phosphate battery according to claim 1, characterized in that: The current for calibrating the charge in step 3 is constant current charging in the range of 0.1C to 1C, where C is the current maximum capacity of the lithium iron phosphate battery.

4. The SOC error correction method for a lithium iron phosphate battery according to claim 1, characterized in that: In step 3, whether to perform a small slope calibration on the SOC is determined by the following steps: Step 31: Calculate the SOC change dSOC in real time based on the Ah integral method, and calculate the cell voltage change dV within a fixed SOC change range; Step 32: Calculate the ratio dV / dSOC of the voltage change within the range of the fixed SOC change of the battery cell to the SOC change, and record the calculated value; Step 33: based on the dV / dSOC values ​​calculated and recorded in real time, draw a dV / dSOC curve, and calculate the slope of the curve; Step 34: When the slope of the curve increases first and then decreases, and the dV / dSOC value is within the set threshold, it is judged as the peak point of the dV / dSOC curve, and a small slope calibration is performed on the SOC based on the peak point; when the peak point is not obvious or there is no peak point, no small slope calibration is performed, and step 5 is entered.

5. The SOC error correction method for a lithium iron phosphate battery according to claim 4, characterized in that: In step 34, the threshold is set to 0.015-0.050, and the cell voltage value corresponding to the threshold is between 3.3V and 3.45V.

6. The SOC error correction method for a lithium iron phosphate battery according to claim 1, characterized in that: In step 6, when fully charged, the maximum voltage range of the single cell is 3.45V to 3.65V, and the total voltage range of the system is 3.45V*the number of lithium iron phosphate battery cells in series to 3.65V*the number of lithium iron phosphate battery cells in series. The specific steps of full charge calibration are as follows: Step 61: Charge to full charge, determine the maximum voltage of the single cell and the total pressure of the system in real time in units of 1 second, if the maximum voltage of the single cell is greater than the single cell full charge threshold or the total pressure of the system is greater than the system full charge threshold, proceed to step 62, if the maximum voltage of the single cell is less than the single cell full charge threshold or the total pressure of the system is less than the system full charge threshold and the charging current drops to the set value or becomes a discharge current, proceed to step 63; Step 62: Dynamically adjust the DC / DC output voltage to drop by 4 control levels each time the condition is determined to be met; Step 63: Dynamically adjust the DC / DC output voltage to increase by one control level each time the condition is determined to be met; Step 64: Full charge and consistency of cells in the system are judged in real time. After the pressure difference in the system is less than the set value and the cell voltage meets the cell full charge voltage, full charge calibration of SOC is performed.

7. The SOC error correction method for a lithium iron phosphate battery according to claim 6, characterized in that: In the step 61, the single cell full charge threshold is one of the following three conditions at an average current of 0.5-1A: 3.6V for 10-15s, 3.55V for 30-60s, and 3.45V for 60-120s; the system full charge threshold is one of the following three conditions at an average current of 0.5-1A: 3.6V*number of lithium iron phosphate battery cells in series for 10-15s, 3.55V*number of lithium iron phosphate battery cells in series for 30-60s, and 3.45V*number of lithium iron phosphate battery cells in series for 60-120s, and the charging current drop constant is 3A.

8. The SOC error correction method for a lithium iron phosphate battery according to claim 6, characterized in that: In step 62 and step 63, each control level is 10mv to 20mv.

9. The SOC error correction method for a lithium iron phosphate battery according to claim 6, characterized in that: In step 64, the pressure difference in the system is set to 30-50 mV.

10. An error correction module for lithium iron phosphate battery SOC, installed in a lithium iron phosphate battery control board, characterized in that: The invention comprises a memory, a processor and a computer program stored in the memory and capable of running on the processor, wherein when the processor reads and executes the program, the SOC error correction method for a lithium iron phosphate battery as described in any one of claims 1 to 9 is implemented.

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