Lithium iron phosphate discharge end stage SOC accurate compensation method

Abstract

The invention discloses a lithium iron phosphate discharge end stage SOC accurate compensation method, which is used for compensating battery discharge end stage SOC, and comprises the following steps: based on SOC, discharge current, continuous discharge time, voltage deviation and load fluctuation state, determining whether the battery is in the discharge end stage; if so, acquiring the voltage of each monomer of the battery pack, and calculating the average voltage and the voltage deviation of the monomer voltages; acquiring current passing through the battery pack, calculating to obtain a current difference value so as to obtain a fluctuation mark, and updating the current value of the last discharge period; obtaining the average temperature of the battery, and obtaining the initial polarization voltage and the polarization voltage attenuation value through the current difference value. The method has the beneficial effects that the SOC estimation error at the final discharge stage is obviously reduced to be within + / -2% from + / -5% or above in the prior art, and the problem of sudden power failure in the vehicle running process caused by inaccurate SOC estimation is fundamentally avoided.

Description

Technical Field

[0001] This invention relates to the field of batteries, and more particularly to a method for accurate SOC compensation at the end of discharge of lithium iron phosphate batteries. Background Technology

[0002] Lithium iron phosphate batteries have become the mainstream power battery for electric two-wheelers and low-speed electric vehicles due to their significant advantages such as high safety, long cycle life, and controllable cost. However, at the end of the battery discharge period (usually corresponding to the SOC range of 0% to 20%), their inherent electrochemical characteristics pose a serious challenge to the real-time and accurate estimation of the state of charge (SOC), mainly in the following aspects:

[0003] During the final stage of discharge, the rate of change of battery OCV with SOC accelerates dramatically; typically, a 1% change in SOC corresponds to a voltage fluctuation of 8–11 mV. This high degree of nonlinearity significantly increases the error of traditional estimation methods based on a fixed voltage-SOC mapping relationship within this range. As SOC decreases, ohmic polarization, electrochemical polarization, and concentration polarization within the battery become increasingly pronounced. During the final stage of discharge, the proportion of total voltage loss caused by polarization voltage can rise to over 15%, and the establishment and relaxation processes of polarization are significantly affected by current and temperature, resulting in complex dynamic characteristics. The self-discharge rate of batteries in the low SOC range is approximately 30%–50% higher than that in the medium-to-high SOC range. Furthermore, the effect of temperature on self-discharge is more sensitive in this range, further increasing the uncertainty of SOC estimation. Electric two-wheelers and low-speed electric vehicles often experience frequent start-stop, acceleration, and hill climbing during actual operation, leading to drastic fluctuations in load current. Simultaneously, the consistency deviation between multiple cells within the battery pack is amplified during the final stage of discharge, directly affecting the accuracy of the overall SOC estimation.

[0004] Currently, commonly used SOC estimation methods in the industry (such as the ampere-hour integration method combined with open-circuit voltage correction method) have significant shortcomings in addressing the aforementioned characteristics at the end of discharge. Existing solutions mostly rely on fixed compensation coefficients, simplified equivalent circuit models, or coarse voltage-SOC mapping logic, failing to provide refined modeling and compensation for the rapid nonlinear changes in voltage at the end of discharge, the coupling effect of polarization and self-discharge, and complex dynamic operating conditions. This results in SOC estimation errors typically exceeding ±5% at the end of discharge.

[0005] Such errors cause a series of prominent problems at the application level: they can easily lead to the over-discharge protection function of the battery management system (BMS) being falsely triggered, causing the vehicle to suddenly lose power while driving, which seriously affects the user experience and driving safety; at the same time, the inability to accurately identify the true state of the battery may cause over-discharge of individual cells, thereby damaging battery health, shortening the overall cycle life of the battery pack, and increasing the user's subsequent replacement costs. Summary of the Invention

[0006] Purpose of the invention: The purpose of this invention is to solve the technical problems in the prior art and provide a method for accurate SOC compensation at the end of lithium iron phosphate discharge.

[0007] Technical solution: A method for accurate SOC compensation at the end of discharge of lithium iron phosphate batteries, used to compensate for the SOC at the end of battery discharge, including:

[0008] Step S1: Based on SOC, discharge current, continuous discharge time, voltage deviation and load fluctuation status, determine whether it is in the final stage of discharge;

[0009] Step S2: If it is in the final stage of discharge, obtain the voltage of each cell in the battery pack and calculate the average voltage and voltage deviation of the cell.

[0010] Step S3: Obtain the current passing through the battery pack, calculate the current difference to obtain the fluctuation indicator, and update the current value of the previous discharge cycle.

[0011] Step S4: Obtain the battery average temperature and obtain the initial polarization voltage and polarization voltage decay value through the current difference;

[0012] Step S5: Calculate the fluctuation loss and the total damage at the end of the discharge period using the average voltage and voltage deviation of the individual cell voltage.

[0013] Step S6: Obtain the true voltage of the battery pack by polarization voltage decay and average voltage of individual cells, and obtain the compensated SOC by using the preset battery pack true voltage-SOC comparison table. Repeat steps S1-S6.

[0014] Preferably, the criteria for determining the end of the discharge phase include:

[0015] When all the conditions are met—SOC≤20%, discharge current≥0.1C, continuous discharge time≥5 seconds, individual cell voltage deviation does not exceed the set threshold, and load fluctuation is marked as true—it is determined to be the end of the discharge period.

[0016] Preferably, the voltage of each cell in the battery pack is obtained, and the average voltage and voltage deviation of the cell voltage are calculated, including:

[0017] The formula for calculating the sum of the voltages of all individual cells is as follows:

[0018] ;

[0019] The average voltage of a single cell is calculated by summing the voltages of all individual cells, as shown in the following formula:

[0020] ;

[0021] The formulas for calculating the highest and lowest voltage values ​​among all individual cells are as follows:

[0022] ;

[0023] ;

[0024] The maximum voltage deviation is calculated using the individual cell voltages at the maximum and minimum voltages, as shown in the following formula:

[0025] ;

[0026] in, Let n be the voltage of the k-th individual unit in the string, and n be the number of individual units.

[0027] Preferably, the process involves acquiring the current flowing through the battery pack, calculating the current difference to obtain a fluctuation indicator, and updating the current value from the previous discharge cycle, including:

[0028] The current difference between the current discharge cycle and the previous discharge cycle is calculated using the following formula:

[0029] ;

[0030] The fluctuation indicator is determined based on the current difference, using the following formula:

[0031] ;

[0032] Update the previous cycle current with the current cycle current.

[0033] Preferably, obtaining the battery average temperature to obtain the initial polarization voltage and polarization voltage decay value through the current difference includes:

[0034] The average battery temperature is obtained by using the current difference to derive the initial polarization voltage and the polarization voltage decay value, including:

[0035] The initial revised formula for calculating polarization resistance is as follows:

[0036]

[0037] The formula for calculating the temperature-corrected polarization time constant is:

[0038] ;

[0039] If the fluctuation is indicated as 1

[0040] The formula for calculating the polarization resistance corrected for load fluctuation is:

[0041] ;

[0042] The formula for calculating the load fluctuation correction polarization time constant is:

[0043] ;

[0044] The initial polarization voltage is obtained based on the temperature-corrected polarization resistance and the temperature-corrected polarization time constant, as shown in the following formula:

[0045] ;

[0046] The polarization voltage decay is obtained based on the initial polarization voltage, using the following formula:

[0047] ;

[0048] in, Polarization voltage reference value, This is the reference value for the polarization time constant.

[0049] Preferably, the fluctuation loss and total damage at the end of the discharge period are calculated using the average voltage and voltage deviation of the individual cell voltage, including:

[0050] The self-discharge SOC loss is calculated using the following formula:

[0051] ;

[0052] The polarization-self-discharge coupling loss is calculated using the following formula:

[0053] ;

[0054] The consistency loss correction is calculated using the following formula:

[0055] ;

[0056] The formula for calculating fluctuation loss is as follows:

[0057] ;

[0058] The total losses calculated only at the end of the discharge period are as follows:

[0059] ;

[0060] in, t is the cumulative discharge time. This represents the final self-discharge K value. γ represents the voltage change corresponding to 1% SOC at the end of the period, and γ is the polarization-self-discharge coupling coefficient.

[0061] Preferably, the true voltage of the battery pack is obtained through polarization voltage decay and average cell voltage, and a new compensated SOC is obtained through a preset battery pack true voltage-SOC lookup table, including:

[0062] The actual voltage of the battery pack is calculated using the following formula:

[0063] ;

[0064] The reference SOC value was obtained by referring to the battery pack's actual voltage-SOC reference table with a 1% SOC accuracy.

[0065] The new SOC value, obtained by comparing the SOC value with the total change in SOC, includes the following formula:

[0066] ;

[0067] The formula for the total change in SOC is as follows:

[0068] ;

[0069] For base voltage compensation, The change is expressed in coulombs.

[0070] Preferably, the boundary handling method when obtaining the reference SOC value from the battery pack's actual voltage according to the battery pack's actual voltage-SOC lookup table is as follows:

[0071] ;

[0072] in, This refers to the voltage value corresponding to a SOC of 0 in the reference table. This refers to the voltage value corresponding to SOC of 20 in the reference table.

[0073] Preferably, the interpolation calculation method between corresponding adjacent 1% SOC values ​​when obtaining the reference SOC value from the battery pack's actual voltage according to the battery pack's actual voltage-SOC comparison table is as follows:

[0074] ;

[0075] Where s is the current SOC, and s+1 indicates the next SOC in the lookup table. This is the current open-circuit voltage of the battery pack. The table shows the open-circuit voltage corresponding to SOC as s. In the reference table, SOC is the open-circuit voltage corresponding to s+1.

[0076] Preferably, the new SOC after compensation also includes accumulated discharge time storage for SOC compensation in the next process.

[0077] Beneficial effects:

[0078] By employing a high-precision OCV-SOC mapping table with 1% SOC intervals, the defects caused by the strong nonlinearity of the voltage at the end of the discharge can be effectively overcome. This invention significantly reduces the SOC estimation error at the end of the discharge from more than ±5% in the prior art to within ±2%, fundamentally avoiding the problem of sudden power outage during vehicle operation caused by inaccurate SOC estimation.

[0079] This method is specifically optimized for the actual operating characteristics of electric two-wheelers and low-speed electric vehicles. Its dynamic correction mechanism can adapt well to complex operating conditions such as frequent start-stop, drastic load fluctuations, and wide temperature range (-20℃ to 60℃). Even under extreme conditions such as low temperature or high rate (e.g., 2C ramping), the SOC estimation error can still be controlled within ±3.5%, ensuring the reliability of estimation in various real-world scenarios.

[0080] Accurate SOC estimation is fundamental to battery over-discharge protection. This invention, by precisely reflecting the true state of the battery pack, significantly reduces the false trigger rate of over-discharge protection from the industry-common approximately 3% to below 0.5%, substantially improving driving safety and user experience. Simultaneously, by preventing over-discharge damage to individual cells, it helps extend the overall cycle life of the battery pack by at least 10%, reducing long-term operating costs for users.

[0081] This technical solution is an optimized and upgraded version of the conventional SOC estimation framework, with clear logic and controllable computational load. No additional hardware sensors are required, and the algorithm can be directly ported to mainstream low-cost embedded platforms such as STM32F1 / F4 / L4. Single-run time is short (≤1ms), fully meeting the stringent requirements of automotive BMS for real-time performance and cost control, significantly shortening the R&D and deployment cycle.

[0082] This invention fully considers the integration with existing battery management systems (BMS), retaining conventional parameter configurations and basic mapping logic. Therefore, this solution can seamlessly integrate with mainstream electric two-wheelers and low-speed electric vehicle BMS systems on the market without complex hardware modifications, providing OEMs and battery pack manufacturers with a low-cost, high-efficiency technology upgrade path. Attached Figure Description

[0083] Figure 1 A schematic diagram of the method flow is provided for this invention.

[0084] Figure 2 A circuit structure diagram is provided for this invention.

[0085] Figure 3A schematic diagram of the polarization voltage decay curve is provided for this invention. Detailed Implementation

[0086] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0087] Example 1

[0088] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art. The terms "comprising" and similar expressions used herein mean that the element or object preceding the word covers the element or object listed following the word and its equivalents, but do not exclude other elements or objects.

[0089] In response to the problems existing in the current technology, such as Figures 1-3 As shown, a precise SOC compensation method for lithium iron phosphate batteries at the end of discharge is proposed to compensate for the SOC at the end of battery discharge, including:

[0090] Step S1: Based on SOC, discharge current, continuous discharge time, voltage deviation and load fluctuation status, determine whether it is in the final stage of discharge;

[0091] In some specific embodiments, the criteria for determining the end of the discharge phase include:

[0092] When all the conditions are met—SOC≤20%, discharge current≥0.1C, continuous discharge time≥5 seconds, individual cell voltage deviation does not exceed the set threshold, and load fluctuation is marked as true—it is determined to be the end of the discharge period.

[0093] The purpose of this step is to accurately determine whether the battery has entered the final stage of discharge. The final stage of discharge (SOC 0%~20%) exhibits significant nonlinear characteristics and polarization effects, therefore, accurately determining whether the battery has entered the final stage of discharge is crucial for subsequent SOC estimation. By using parameters such as SOC, discharge current, continuous discharge time, voltage deviation, and load fluctuation, the system can confirm whether the battery's operating state meets the conditions for entering the final stage of discharge.

[0094] Step S2: If it is in the final stage of discharge, obtain the voltage of each cell in the battery pack and calculate the average voltage and voltage deviation of the cell.

[0095] In some specific embodiments, the voltage of each cell in the battery pack is obtained, and the average voltage and voltage deviation of the cell voltage are calculated, including:

[0096] The formula for calculating the sum of the voltages of all individual cells is as follows:

[0097] This step sums the voltages of all individual cells to obtain the total voltage information of the battery pack, providing a basis for subsequent calculations of average voltage and voltage deviation.

[0098] The average voltage of a single cell is calculated by summing the voltages of all individual cells, as shown in the following formula:

[0099] The average value of the individual cell voltage is used to represent the overall voltage level of the battery pack, helping to determine the health status of the battery pack and providing a reference for subsequent voltage compensation.

[0100] The formulas for calculating the highest and lowest voltage values ​​among all individual cells are as follows:

[0101] ;

[0102] ;

[0103] By identifying the maximum and minimum individual cell voltages, we can gain further insight into the voltage consistency of the battery pack. Significant differences in individual cell voltages within the pack may indicate inconsistencies or damage.

[0104] The maximum voltage deviation is calculated using the individual cell voltages at the maximum and minimum voltages, as shown in the following formula:

[0105] The maximum voltage difference is calculated, reflecting the voltage consistency between individual cells within the battery pack. A large voltage difference may indicate significant differences among some individual cells within the battery pack, which could affect the accuracy of SOC estimation, especially during SOC compensation at the end of discharge.

[0106] in, Let n be the voltage of the k-th individual unit in the string, and n be the number of individual units.

[0107] Step S3: Obtain the current passing through the battery pack, calculate the current difference to obtain the fluctuation indicator, and update the current value of the previous discharge cycle.

[0108] In some specific embodiments, the current flowing through the battery pack is acquired, the current difference is calculated to obtain a fluctuation indicator, and the current value of the previous discharge cycle is updated, including:

[0109] The current difference between the current discharge cycle and the previous discharge cycle is calculated using the following formula:

[0110] This step calculates the difference between the current in the current cycle and the current in the previous cycle. This difference reflects the degree of load fluctuation during battery discharge. The larger the current difference, the more significant the load fluctuation.

[0111] The fluctuation indicator is determined based on the current difference, using the following formula:

[0112] ;

[0113] Update the previous cycle current with the current cycle current.

[0114] The degree of load fluctuation is determined by the current difference. If the current change exceeds a set threshold, the system is considered to be experiencing significant load fluctuation. This is crucial for subsequent battery management systems (BMS), especially during battery SOC estimation and voltage correction, where the impact of load fluctuations needs to be considered.

[0115] Step S4: Obtain the battery average temperature and obtain the initial polarization voltage and polarization voltage decay value through the current difference;

[0116] In some specific embodiments, obtaining the initial polarization voltage and polarization voltage decay value from the battery average temperature through the current difference includes: obtaining the initial polarization voltage and polarization voltage decay value from the battery average temperature through the current difference, including:

[0117] The initial revised formula for calculating polarization resistance is as follows:

[0118] ;

[0119] When the temperature is below 25℃, the resistance to ion migration increases, and the polarization resistance also increases accordingly. This formula takes into account the effect of temperature change on polarization resistance.

[0120] The formula for calculating the temperature-corrected polarization time constant is:

[0121] ;

[0122] At low temperatures, the polarization decay process slows down, and the polarization time constant increases. This formula simulates the effect of temperature on the battery polarization process by adjusting the time constant.

[0123] If the fluctuation is indicated as 1

[0124] The formula for calculating the polarization resistance corrected for load fluctuation is:

[0125] ;

[0126] The formula for calculating the load fluctuation correction polarization time constant is:

[0127] ;

[0128] Load fluctuations (such as frequent start-stop cycles and rapid current switching) can cause changes in polarization resistance and time constant. By correcting these parameters, the polarization characteristics of the battery under load fluctuations can be reflected more accurately.

[0129] The initial polarization voltage is obtained based on the temperature-corrected polarization resistance and the temperature-corrected polarization time constant, as shown in the following formula:

[0130] ;

[0131] The polarization voltage decay is obtained based on the initial polarization voltage, using the following formula:

[0132] ;

[0133] in, Polarization voltage reference value, This is the baseline value for the polarization time constant. The polarization voltage decays over time. This formula describes the decay process of the polarization voltage over time using an exponential decay model, taking into account load fluctuations and temperature corrections. Figure 3 T represents the average battery temperature.

[0134] Step S5: Calculate the fluctuation loss and the total damage at the end of the discharge period using the average voltage and voltage deviation of the individual cell voltage.

[0135] In some specific embodiments, fluctuation losses and total damage at the end of discharge are calculated using the average voltage and voltage deviation of the individual cell voltage, including:

[0136] The self-discharge SOC loss is calculated using the following formula:

[0137] The formula calculates the State of Charge (SOC) loss caused by battery self-discharge. Batteries undergo self-discharge at the end of their discharge cycle, resulting in a loss of charge. This loss is related to time and changes in battery voltage. This formula allows for the quantitative calculation of the SOC loss caused by self-discharge.

[0138] The polarization-self-discharge coupling loss is calculated using the following formula:

[0139] The coupling effect between polarization voltage and self-discharge accelerates battery charge loss. This formula is used to calculate the state-of-the-art (SOC) loss caused by this coupling effect. A higher polarization voltage exacerbates self-discharge, resulting in greater charge loss.

[0140] The consistency loss correction is calculated using the following formula:

[0141] ;

[0142] The greater the voltage difference between individual cells within a battery pack, the greater the error in SOC estimation. This formula quantifies the impact of voltage inconsistency within the battery pack on SOC estimation by calculating consistency loss.

[0143] The formula for calculating fluctuation loss is as follows:

[0144] ;

[0145] This formula calculates the SOC loss caused by load fluctuations. If the current difference (load fluctuation) is large, the load fluctuation will cause additional SOC loss. Fluctuation loss is proportional to the discharge time, and significant load fluctuations will increase SOC loss.

[0146] The total losses calculated only at the end of the discharge period are as follows:

[0147] ;

[0148] in, The discharge time t (in seconds) is converted to hours. This is to ensure that subsequent calculations use hours as the time unit, especially when calculating time-based losses (such as self-discharge, polarization decay, etc.), where hours more accurately reflect the actual performance of the battery. t represents the cumulative discharge time. This represents the final self-discharge K value. γ represents the voltage change corresponding to 1% SOC at the end of the period, and γ is the polarization-self-discharge coupling coefficient.

[0149] This formula combines various loss sources (fluctuation loss, consistency loss, polarization-self-discharge coupling loss, and self-discharge loss) to calculate the total damage at the end of battery discharge. This is the main source of error in SOC estimation and a significant factor affecting battery performance and range.

[0150] Step S6: Obtain the true voltage of the battery pack by polarization voltage decay and average voltage of individual cells, and obtain the compensated SOC by using a preset battery pack true voltage-SOC comparison table. Repeat steps S1-S6 to continuously compensate the SOC as the power is consumed.

[0151] In some specific embodiments, the true voltage of the battery pack is obtained through polarization voltage decay and average cell voltage, and a new compensated SOC is obtained through a preset battery pack true voltage-SOC lookup table, including:

[0152] The actual voltage of the battery pack is calculated using the following formula:

[0153] By correcting the average voltage, polarization voltage, and base voltage of individual cells, the true open-circuit voltage of the battery pack is obtained. This voltage value can more accurately reflect the actual state of the battery pack, providing a basis for subsequent SOC compensation.

[0154] The reference SOC value was obtained by referring to the battery pack's actual voltage-SOC reference table with a 1% SOC accuracy.

[0155] The battery pack actual voltage-SOC comparison table can be set as follows:

[0156] ={2.50,2.58,2.65,2.71,2.78,2.83,2.88,2.92,2.95,2.98,3.01,3.04,3.06,3.08,3.10,3.11,3.12,3.13,3.14,3.15,3.16};

[0157] Specifically, 0% SOC corresponds to 2.50V, 20% SOC represents 3.16V, and so on.

[0158] The new SOC value, obtained by comparing the SOC value with the total change in SOC, includes the following formula:

[0159] ;

[0160] The formula for the total change in SOC is as follows:

[0161] ;

[0162] For base voltage compensation, The change is expressed in coulombs.

[0163] In some specific embodiments, the boundary handling method when obtaining the reference SOC value from the battery pack's actual voltage according to the battery pack's actual voltage-SOC lookup table is as follows:

[0164] ;

[0165] in, This refers to the voltage value corresponding to a SOC of 0 in the reference table. This refers to the voltage value corresponding to a SOC of 20 in the reference table. The compensated SOC value is calculated by combining the reference SOC value with the SOC change. The max and min functions are used to ensure the SOC value is between 0% and 100%, avoiding values ​​outside the reasonable range.

[0166] In some specific embodiments, the interpolation calculation method between corresponding adjacent 1% SOC values ​​when obtaining the reference SOC value from the battery pack's actual voltage according to the battery pack's actual voltage-SOC lookup table is as follows:

[0167] ;

[0168] Where s is the current SOC, and s+1 indicates the next SOC in the lookup table. This is the current open-circuit voltage of the battery pack. The table shows the open-circuit voltage corresponding to SOC as s. In the reference table, SOC is the open-circuit voltage corresponding to s+1.

[0169] The current SOC value is calculated using linear interpolation. This method is suitable when the actual battery pack voltage falls between two known SOC values. Interpolation improves the accuracy of SOC estimation, ensuring a more detailed SOC estimate.

[0170] Assuming the actual voltage U of the current battery pack OCV,t The value is 2.52V, and the corresponding SOC value can be estimated using interpolation. In the comparison table, 2.852V falls between 2.50V and 2.58V, corresponding to SOC values ​​of 16% and 17%, respectively.

[0171] U OCV,es =2.83 (corresponding to SOC 16%), U OCV,es+1 =2.88 (corresponding to SOC 17%), the SOC value can be obtained by interpolation calculation.

[0172] In some specific embodiments, the compensated new SOC also includes the accumulated discharge time t stored for SOC compensation in the next process.

[0173] The above description is merely a specific implementation of the embodiments of the present invention, but the protection scope of the embodiments of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed in the embodiments of the present invention should be covered within the protection scope of the embodiments of the present invention. Therefore, the protection scope of the embodiments of the present invention should be determined by the protection scope of the claims.

Claims

1. A method for accurate SOC compensation at the end of discharge of lithium iron phosphate batteries, used to compensate for the SOC at the end of battery discharge, characterized in that, include: Step S1: Based on SOC, discharge current, continuous discharge time, voltage deviation and load fluctuation status, determine whether it is in the final stage of discharge; Step S2: If it is in the final stage of discharge, obtain the voltage of each cell in the battery pack and calculate the average voltage and voltage deviation of the cell. Step S3: Obtain the current passing through the battery pack, calculate the current difference to obtain the fluctuation indicator, and update the current value of the previous discharge cycle. Step S4: Obtain the battery average temperature and obtain the initial polarization voltage and polarization voltage decay value through the current difference; Step S5: Calculate the fluctuation loss and the total damage at the end of the discharge period using the average voltage and voltage deviation of the individual cell voltage. Step S6: Obtain the true voltage of the battery pack by polarization voltage decay and average voltage of individual cells, and obtain the compensated SOC by using the preset battery pack true voltage-SOC comparison table. Repeat steps S1-S6.

2. The method for accurate SOC compensation at the end of discharge of lithium iron phosphate according to claim 1, characterized in that, The criteria for determining the end of the discharge phase include: When all the conditions are met—SOC≤20%, discharge current≥0.1C, continuous discharge time≥5 seconds, individual cell voltage deviation does not exceed the set threshold, and load fluctuation is marked as true—it is determined to be the end of the discharge period.

3. The method for accurate SOC compensation at the end of discharge of lithium iron phosphate according to claim 1, characterized in that, The voltage of each individual cell in the battery pack is obtained, and the average voltage and voltage deviation of each cell are calculated, including: The formula for calculating the sum of the voltages of all individual cells is as follows: ； The average voltage of a single cell is calculated by summing the voltages of all individual cells, as shown in the following formula: ； The formulas for calculating the highest and lowest voltage values ​​among all individual cells are as follows: ； ； The maximum voltage deviation is calculated using the individual cell voltages at the maximum and minimum voltages, as shown in the following formula: ； in, Let n be the voltage of the k-th individual unit in the string, and n be the number of individual units.

4. The method for accurate SOC compensation at the end of discharge of lithium iron phosphate according to claim 3, characterized in that, The current flowing through the battery pack is acquired, the current difference is calculated to obtain a fluctuation indicator, and the current value of the previous discharge cycle is updated, including: The current difference between the current discharge cycle and the previous discharge cycle is calculated using the following formula: ； The fluctuation indicator is determined based on the current difference, using the following formula: ； Update the previous cycle current with the current cycle current. This is the load fluctuation threshold.

5. The method for accurate SOC compensation at the end of discharge of lithium iron phosphate according to claim 4, characterized in that, The average battery temperature is obtained by using the current difference to derive the initial polarization voltage and the polarization voltage decay value, including: The initial revised formula for calculating polarization resistance is as follows: The formula for calculating the temperature-corrected polarization time constant is: ； If the fluctuation is indicated as 1 The formula for calculating the polarization resistance corrected for load fluctuation is: ； The formula for calculating the load fluctuation correction polarization time constant is: ； The initial polarization voltage is obtained based on the temperature-corrected polarization resistance and the temperature-corrected polarization time constant, as shown in the following formula: ； The polarization voltage decay is obtained based on the initial polarization voltage, using the following formula: ； in, Polarization voltage reference value, Here, T is the baseline value for the polarization time constant, T is the average battery temperature, and t is the cumulative discharge time.

6. The method for accurate SOC compensation at the end of discharge of lithium iron phosphate according to claim 5, characterized in that, Fluctuation losses and total damage at the end of discharge are calculated using the average voltage and voltage deviation of the individual cell voltage, including: The self-discharge SOC loss is calculated using the following formula: ； The polarization-self-discharge coupling loss is calculated using the following formula: ； The consistency loss correction is calculated using the following formula: ； The formula for calculating fluctuation loss is as follows: ； The total losses calculated only at the end of the discharge period are as follows: ； in, t is the cumulative discharge time. This represents the final self-discharge K value. γ represents the voltage change corresponding to 1% SOC at the end of the period, and γ is the polarization-self-discharge coupling coefficient.

7. The method for accurate SOC compensation at the end of discharge of lithium iron phosphate according to claim 6, characterized in that, The true voltage of the battery pack is obtained by polarization voltage decay and average cell voltage, and the compensated new SOC is obtained by using a preset battery pack true voltage-SOC lookup table, including: The actual voltage of the battery pack is calculated using the following formula: ； The reference SOC value was obtained by referring to the battery pack's actual voltage-SOC reference table with a 1% SOC accuracy. The new SOC value, obtained by comparing the SOC value with the total change in SOC, includes the following formula: ； The formula for the total change in SOC is as follows: ； For base voltage compensation, The change is expressed in coulombs.

8. The method for accurate SOC compensation at the end of discharge of lithium iron phosphate according to claim 7, characterized in that, The boundary handling method when obtaining the reference SOC value from the battery pack's actual voltage using the battery pack's actual voltage-SOC comparison table is as follows: ； in, This refers to the voltage value corresponding to a SOC of 0 in the reference table. This refers to the voltage value corresponding to SOC of 20 in the reference table.

9. A method for precise SOC compensation at the end of discharge of lithium iron phosphate batteries according to claim 7, characterized in that, The interpolation method between corresponding adjacent 1% SOC values ​​when obtaining the reference SOC value from the battery pack's actual voltage using the battery pack's actual voltage-SOC comparison table is as follows: ； Where s is the current SOC, and s+1 indicates the next SOC in the lookup table. This is the current open-circuit voltage of the battery pack. The table shows the open-circuit voltage corresponding to SOC as s. In the reference table, SOC is the open-circuit voltage corresponding to s+1.

10. A method for precise SOC compensation at the end of discharge of lithium iron phosphate batteries according to claim 1, characterized in that, The new SOC after compensation also includes accumulated discharge time storage for SOC compensation in the next process.

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