A lithium iron phosphate power battery system life estimation method and system and vehicle

Abstract

This invention belongs to the field of new energy vehicle battery management technology, specifically relating to a method, system, and vehicle for estimating the lifespan of a lithium iron phosphate power battery system. The method includes: determining the SOC1 corresponding to the inflection point of the lithium battery plateau period; and basing the SOC on the SOC at the start of charging at the highest voltage. 0max The relationship between the highest single-cell SOHmaxv and SOC1, and the highest single-cell capacity Q of the battery system. 3max With SOC 0max The relation, Q 3max SOHmaxv is derived from the relationship with SOHmaxv; based on the SOC at the start of charging at the lowest voltage. 0min The relationship between the lowest single-cell SOHminv and SOC1, and the lowest single-cell capacity Q of the battery system. 3min With SOC 0min The relation, Q 3min The relationship between SOHminv and SOHminv is used to derive SOHminv; thus, the system SOH is obtained. This invention improves the accuracy of SOH estimation.

Description

Technical Field

[0001] This invention belongs to the field of new energy vehicle battery management technology, specifically relating to a method, system, and vehicle for estimating the lifespan of a lithium iron phosphate power battery system. Background Technology

[0002] Pure electric vehicles need to acquire basic battery information in real time during operation, such as current, voltage, and temperature. Based on this information, the battery's State of Health (SOH) and other health data can be calculated. These data are used to determine the remaining driving range and maximum discharge power, ensuring safe driving. Therefore, the accuracy of the calculated battery SOH and health data is crucial. SOH (State of Health): The ratio of the battery's current actual capacity to its rated capacity.

[0003] The current main method for calculating the State of Charge (SOH) of lithium iron phosphate battery systems is to obtain the remaining system capacity based on the initial State of Charge (SOC), the final State of Charge (SOC), and charging capacity parameters during the charging process. The SOH is then calculated by comparing this remaining capacity with the battery system's rated capacity. SOC (State of Charge) is defined as the ratio of the battery's remaining capacity after a period of use or storage to its capacity when fully charged. This method is simple and universally applicable, but the accuracy of the calculation depends heavily on the accuracy of the initial SOC. For lithium iron phosphate batteries to accurately obtain the initial SOC before charging, they need to be stored for at least 2 hours before charging, during a non-plateau period (SOC < 35%). Currently, there is relatively little data on vehicles meeting these two conditions before charging, posing a significant challenge to timely and accurate calculation of the battery system's SOH.

[0004] Currently, some companies in the industry calculate battery SOH using the battery internal resistance method. This method requires not only accurate calculation of the battery internal resistance but also data on the relationship between internal resistance and remaining capacity. However, battery internal resistance is greatly affected by battery temperature, current, SOH, SOC, and battery connection status, making it difficult to accurately obtain the internal resistance. Furthermore, the relationship between battery internal resistance and capacity is generally based on laboratory measurements, but these laboratory-tested relationships do not accurately reflect real-world vehicle operating conditions, resulting in poor accuracy of the SOH calculated using this method. Summary of the Invention

[0005] The purpose of this invention is to provide a method, system, and vehicle for estimating the lifespan of a lithium iron phosphate power battery system, in order to solve the problem of poor accuracy in existing methods for estimating the state of harm (SOH) of lithium iron phosphate battery systems.

[0006] To solve the above-mentioned technical problems, the technical solution provided by this invention and the corresponding beneficial effects of the technical solution are as follows:

[0007] The present invention provides a method for estimating the lifespan of a lithium iron phosphate power battery system, comprising the following steps:

[0008] 1) During the charging process, identify the inflection point of the lithium iron phosphate battery plateau period and determine the battery state of charge corresponding to the voltage value at the inflection point, which is recorded as the battery state of charge SOC1 at the inflection point.

[0009] 2) Determine the battery state of charge (SOC) corresponding to the start time of the highest voltage charging. 0max The relationship between the highest single-cell battery health state SOHmaxv and the battery state of charge SOC1 at the inflection point is used to determine the battery state of charge SOC corresponding to the start of charging at the lowest voltage. 0min The relationship between the lowest single-cell battery health state SOHminv and the battery charge state SOC1 at the inflection point;

[0010] 3) Based on the highest single-cell capacity Q of the battery system 3max State of charge (SOC) corresponding to the start of charging at the highest voltage 0max The relationship, and the highest single-cell capacity Q of the battery system. 3max The battery health state SOHmaxv of the highest-performing cell is calculated by combining the relationship with the relationship in step 2); based on the minimum cell capacity Q of the battery system... 3min State of Charge (SOC) corresponding to the start of charging at the lowest voltage 0min The relationship, and the minimum single-cell capacity Q of the battery system. 3min The battery health state SOHminv of the lowest single cell is calculated by combining the relationship between the relationship with the lowest single cell and the relationship in step 2).

[0011] 4) Determine the system's battery health state SOHsys based on the highest single-cell battery health state SOHmaxv and the lowest single-cell battery health state SOHminv.

[0012] The beneficial effects of the above technical solution are as follows:

[0013] 1) This invention fully utilizes the inflection point characteristics of lithium iron phosphate batteries during the plateau period. By identifying this inflection point, the battery's state of charge (SOC1) at the inflection point can be obtained; then, the highest and lowest single-cell SOCs before the start of charging can be accurately calculated. 0max SOC 0min This relaxes the requirement that the low-end SOC be left idle for a certain period of time before charging the actual vehicle, and then calculates the battery health status (SOHsys) of the system based on the formula.

[0014] 2) The present invention has a simple implementation method and simple parameter acquisition, which improves the calculation range of the actual vehicle life and obtains the actual vehicle battery health status (SOH) in advance, thereby ensuring safe driving of the car.

[0015] Furthermore, to improve accuracy, the battery state of charge (SOC) corresponding to the highest voltage charging start time in step 2) is... 0max The relationship between the highest single-cell battery health state SOHmaxv and the formula is:

[0016] SOC 0max =SOC1-Q 1max / (SOHmaxv*C0)

[0017] The state of charge (SOC) of the battery at the start of charging at the lowest voltage 0min The relationship between the lowest single cell's state of health (SOHminv) and the state of health (SOHminv) is as follows:

[0018] SOC 0min =SOC1-Q 1min / (SOHminv*C0)

[0019] The rated capacity of the battery system is C0.

[0020] Furthermore, to simplify the computational complexity, the battery health state SOHmaxv of the highest-performing cell and the battery health state SOHminv of the lowest-performing cell are calculated in step 3) using the following method:

[0021] According to Q 3max =Q2 / (100% - SOC) 0max ) and Q 3max =SOHmaxv*C0 to get Q2 / (100%-SOC1+Q) 1max / (SOHmaxv*C0))=SOHmaxv*C0, from which we can deduce SOHmaxv=((Q2 / C0)-(Q 1max / C0)) / (100%-SOC1);

[0022] According to Q 3min =Q2 / (X-SOC) 0min ) and Q 3min =SOHminv*C0 to get Q2 / (X-SOC1+Q) 1min / (SOHminv*C0))=SOHminv*C0, from which we can deduce SOHminv=((Q2 / C0)-(Q 1min / C0)) / (X-SOC1);

[0023] Where X is the SOC at the end of charging, Q2 is the total charging capacity of the battery cell, and Q1max Q represents the charging capacity before the highest single-cell inflection point of the battery. 1min The charging capacity before the lowest single-cell inflection point of the battery.

[0024] Furthermore, the charging capacity Q of the battery before the highest single-cell inflection point is obtained using the ampere-hour integration method. 1max The charging capacity Q of the battery before the lowest single-cell inflection point 1min 1. Total charging capacity of individual battery cells Q2.

[0025] Furthermore, X is obtained in the following way:

[0026] Obtain the charging voltage value at the end of charging, and then use the OCV-SOC table lookup method to obtain the SOC at the end of charging based on the charging voltage value at the end of charging.

[0027] Furthermore, to improve safety, step 4) uses the following formula to determine the system's battery health state (SOHsys):

[0028] SOHsys=min(SOHmaxv,SOHminv).

[0029] Furthermore, in order to accurately obtain the state of charge (SOC1) of the battery at the inflection point, step 1) uses the voltage change within unit capacity method to identify the inflection point of the lithium iron phosphate battery plateau period, obtains the voltage value at the inflection point, and determines the state of charge (SOC1) of the battery at the inflection point by looking up the OCV-SOC table.

[0030] To address the above problems, the present invention provides a lithium iron phosphate power battery system life estimation system, which includes a processor for executing computer instructions to implement a lithium iron phosphate power battery system life estimation method as described in the present invention.

[0031] To address the above problems, the present invention provides a vehicle comprising a vehicle body and a lithium iron phosphate power battery system, and further comprising a processor for executing computer instructions to implement a lithium iron phosphate power battery system life estimation method as described in the present invention. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the voltage change curve with SOC during the charging process of the lithium iron phosphate battery in the method embodiment of the present invention;

[0033] Figure 2 This is a schematic diagram showing the relationship between the extreme value dV / dQ ratio and the system SOC during the charging process of the lithium iron phosphate battery system in the method embodiment of the present invention.

[0034] Figure 3This is a flowchart of the system SOH calculation method during the charging process of a lithium iron phosphate battery system in an embodiment of the present invention. Detailed Implementation

[0035] This invention, based on the charging curve characteristics of lithium iron phosphate batteries, obtains the inflection point between two plateaus of the charging curve by measuring the rate of voltage change per unit capacity. It calculates the initial state of charge (SOC0) before charging by ampere-hour integration and then calculates the difference between the initial SOC0 and the final SOC. Finally, it calculates the usable capacity of the battery system using the ratio of the total capacity to the change in SOC during charging. The ratio of the usable capacity to the rated capacity of the battery yields the single-cell state of charge (SOH). This invention also simultaneously calculates the SOHmaxv and SOHminv corresponding to the highest and lowest voltages, respectively; the smaller of these two values ​​is the system SOH. This invention is applicable to pure electric vehicles, hybrid vehicles, and energy storage systems equipped with lithium iron phosphate battery systems.

[0036] Specifically, such as Figure 1 The figure shows the voltage variation curve of a lithium iron phosphate battery with SOC during charging. It can be seen that the voltage changes rapidly at the beginning and end of the charging process, followed by two plateau periods where the voltage remains relatively constant. Between these plateaus is a sloping inflection point. Since the SOC at this inflection point remains relatively constant throughout the battery's lifespan, the initial SOC0 can be calculated from this point. Figure 2 As shown, by processing the ratio of the extreme charging voltage to the charging capacity of the lithium iron phosphate battery system, the curves dVmax / dq and dVmin / dq are obtained. It can be seen that there is a rising edge when Vmax and Vmin enter the inflection point. This rising edge has a slope. By identifying that this slope drops to 0, the inflection points of the highest voltage and the lowest voltage are calculated respectively, and the starting SOC corresponding to the highest voltage and the lowest voltage is calculated.

[0037] The definitions of terms in this invention include the following:

[0038] Vmax: Maximum battery system voltage; Vmin: Minimum battery system voltage.

[0039] Km: Slope of voltage change per unit capacity during the highest voltage cell charging process;

[0040] Kn: Slope of voltage change per unit capacity during the charging process of the lowest voltage cell;

[0041] K0: The slope of the voltage change per unit capacity during the charging process when the extreme voltage enters the second plateau period from the inflection point;

[0042] C0: Rated capacity of the battery system;

[0043] Q1: Charging capacity of a single battery cell before the inflection point;

[0044] Q2: Total charging capacity of individual battery cells;

[0045] Q3: Actual capacity of a single battery cell;

[0046] Q0: Rated capacity of the battery system.

[0047] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0048] Method Implementation Examples:

[0049] An embodiment of the life estimation method for a lithium iron phosphate power battery system of the present invention, such as... Figure 3 As shown, the method includes the following steps:

[0050] Step 1: After the vehicle enters the charging process, calculate the voltage change per unit charging capacity of the highest and lowest single-cell voltages in the lithium iron phosphate battery system, i.e., dVmax / dQ and dVmin / dQ. Vmax: highest voltage of the battery system; Vmin: lowest voltage of the battery system.

[0051] Step 2: Calculate and record the change in dVmax / dQ / dVmin / dQ per unit time, i.e., the slope.

[0052] Step 3: If the value of dVmax / dQ changes from km to kx, record the time t at this moment. 1max If the slope of dVmin / dq changes from the slope of kn to kx, record the time at this point as t. 1min Where Km: the slope of voltage change per unit capacity during the charging process of the highest voltage cell; Kn: the slope of voltage change per unit capacity during the charging process of the lowest voltage cell. Then from t 1max The corresponding voltage value and t 1min The smaller voltage value is obtained from the corresponding voltage values, and the battery state of charge (SOC1) at the inflection point is obtained by looking up the OCV-SOC table.

[0053] Step 4: After charging begins, integrate the charging capacity, and take the integration time until the highest and lowest voltage inflection points t. 1max , t 1min Maximum voltage integral capacity Q 1max Also known as the battery's highest single-cell charging capacity before the inflection point, or the lowest voltage integral capacity Q. 1min Also known as the charging capacity before the lowest single-cell inflection point of the battery. The highest voltage integral capacity is... The lowest voltage integral capacity is

[0054] Step 5: After charging begins, integrate the charging capacity over time until charging ends (t2). The integrated capacity is... In this step, the integral capacity Q2 is also called the total charging capacity of the battery cell.

[0055] Step 6: SOC at the start of highest voltage charging 0max SOC 0max =SOC1-Q 1max / (SOHmaxv*C0), SOC at the start of minimum voltage charging. 0min SOC 0min =SOC1-Q 1min / (SOHminv*C0). Where SOC 0max Also known as the battery state of charge at the start of the highest voltage charging period, SOC. 0min Also known as the battery state of charge at the moment when charging begins at the lowest voltage.

[0056] Step 7: Maximum single-cell capacity Q of the battery system 3max =Q2 / (100% - SOC) 0max The minimum single-cell capacity of the battery system, Q 3min =Q2 / (X-SOC) 0min X: represents the SOC (State of Charge) at the end of charging.

[0057] Step 8: The highest usable capacity of a single battery cell in the battery system is Q. 3max Q 3max =SOHmaxv*C0; Minimum single-cell capacity Q of the battery system 3min Q 3min =SOHminv*C0

[0058] Step 9: Combine steps 6, 7, and 8 to obtain the highest monomer concentration: Q2 / (100%-SOC1+Q) 1max / (SOHmaxv*C0))=SOHmaxv*C0, minimum monomer: Q2 / (X-SOC1+Q 1min / (SOHminv*C0))=SOHminv*C0.

[0059] Step 10: Simplify the expression in step 9 to obtain:

[0060] For the highest voltage: Q2 / C0=SOHmaxv*(100%-SOC1)+Q 1max / C0, let Q2 / C0=C, Q1 / C0=A, 100%-SOC1=B, to obtain the highest monomer SOHmaxv=(CA) / B;

[0061] For the lowest voltage: Q2 / C0=SOHminv*(X-SOC1)+Q 1min / C0, let X-SOC1=B1, minimum single-cell SOHminv=(C-A1) / B1, X: is the SOC corresponding to the end of charging. Based on the charging voltage value at the end of charging, the SOC corresponding to the end of charging can be obtained by using the OCV-SOC table method.

[0062] Step 11: System SOHsys = min(SOHmaxv, SOHminv).

[0063] The present invention has the following beneficial effects:

[0064] 1) Fully utilize the inflection point characteristics of lithium iron phosphate batteries during the plateau period and identify this inflection point by using the voltage change per unit capacity during charging. It can accurately calculate the highest and lowest single-cell SOC before charging begins, relaxing the requirement that vehicles need to be left at a low SOC for a certain period of time before charging.

[0065] 2) Based on the SOC0 corresponding to the inflection point, the SOC (also known as Q in this formula) of the charge from the start time to before the inflection point is obtained by the ampere-hour integration method. 1max The method of this invention calculates the SOC at the start of charging by using the value obtained from (SOH*C0) and then working backwards to calculate the accurate SOC at the start of charging. This method accurately calculates the SOC corresponding to the highest and lowest charging start times for each individual cell, and then calculates the corresponding SOH for that cell.

[0066] 3) The method of the present invention is simple to implement and easy to obtain parameters, which improves the calculation range of the actual vehicle life and allows the actual vehicle battery health status (SOH) to be obtained in advance.

[0067] System Implementation Example:

[0068] An embodiment of a lithium iron phosphate power battery system life estimation system according to the present invention includes a memory, a processor, and an internal bus. The processor and the memory communicate and interact with each other via the internal bus. The memory includes at least one software function module stored in the memory. The processor executes various functional applications and data processing by running the software programs and modules stored in the memory, thereby implementing the lithium iron phosphate power battery system life estimation method described in the method embodiment of the present invention.

[0069] The processor can be a microprocessor (MCU), a programmable logic device (FPGA), or other processing devices. The memory can be any type of memory that uses electrical energy to store information, such as RAM or ROM.

[0070] Vehicle Example:

[0071] One embodiment of the present invention includes a vehicle body and a lithium iron phosphate (LFP) battery system, and further includes a memory, a processor, and an internal bus. The processor and the memory communicate and interact with each other via the internal bus. The memory includes at least one software function module stored in the memory. The processor executes various functional applications and data processing by running the software programs and modules stored in the memory, thereby implementing the LFP battery system life estimation method described in the method embodiment of the present invention. The processor can be a microprocessor (MCU), a programmable logic device (FPGA), or other processing device. The memory can be any type of memory that stores information using electrical energy, such as RAM or ROM. The vehicle is a pure electric vehicle or a hybrid vehicle equipped with a LFP battery system.

Claims

1. A method for estimating the lifespan of a lithium iron phosphate power battery system, characterized in that: Includes the following steps: 1) During the charging process, identify the inflection point of the lithium iron phosphate battery plateau period and determine the battery state of charge corresponding to the voltage value at the inflection point, which is recorded as the battery state of charge SOC1 at the inflection point. 2) Determine the battery state of charge (SOC) corresponding to the start time of the highest voltage charging. 0max The relationship between the highest single-cell battery health state SOHmaxv and the battery state of charge SOC1 at the inflection point is used to determine the battery state of charge SOC corresponding to the start of charging at the lowest voltage. 0min The relationship between the lowest single-cell battery health state SOHminv and the battery charge state SOC1 at the inflection point; 3) Based on the highest single-cell capacity Q of the battery system 3max State of charge (SOC) corresponding to the start of charging at the highest voltage 0max The relationship, and the highest single-cell capacity Q of the battery system. 3max The battery health state SOHmaxv of the highest-performing cell is calculated by combining the relationship with the relationship in step 2); based on the minimum cell capacity Q of the battery system... 3min State of Charge (SOC) corresponding to the start of charging at the lowest voltage 0min The relationship, and the minimum single-cell capacity Q of the battery system. 3min The battery health state SOHminv of the lowest single cell is calculated by combining the relationship between the relationship with the lowest single cell and the relationship in step 2). 4) Determine the system's battery health state SOHsys based on the highest single-cell battery health state SOHmaxv and the lowest single-cell battery health state SOHminv.

2. The lifespan estimation method for lithium iron phosphate power battery systems according to claim 1, characterized in that: The battery state of charge (SOC) at the start of the highest voltage charging in step 2) is... 0max The relationship between the highest single-cell battery health state SOHmaxv and the formula is: SOCIETY 0max =SOC1-Q 1max / ( SOHmaxv *C0) The state of charge (SOC) of the battery at the start of charging at the lowest voltage 0min The relationship between the lowest single cell's state of health (SOHminv) and the state of health (SOHminv) is as follows: SOCIETY 0min =SOC1-Q 1min / ( SOHminv *C0) Among them, the charging capacity Q before the highest single-cell inflection point of the battery 1max The charging capacity Q before the lowest single-cell inflection point of the battery 1min The rated capacity of the battery system is C0.

3. The lifespan estimation method for lithium iron phosphate power battery systems according to claim 2, characterized in that: In step 3), the battery health state of the highest-performing cell (SOHmaxv) and the battery health state of the lowest-performing cell (SOHminv) are calculated using the following method: According to Q 3max = Q2 / (100%-SOC 0max ) and Q 3max = SOHmaxv *C0 to get Q2 / (100%- SOC1+Q) 1max / (SOHmaxv *C0))= SOHmaxv *C0, therefore SOHmaxv=(( Q2 / C0)-( Q 1max / C0)) / ( 100%-SOC1); According to Q 3min = Q2 / (X -SOC) 0min ) and Q 3min = SOHminv *C0 gives Q2 / (X- SOC1+Q) 1min / ( SOHminv*C0))= SOHminv *C0, therefore SOHminv=(( Q2 / C0)- (Q 1min / C0)) / (X- SOC1); Where X is the SOC at the end of charging, Q2 is the total charging capacity of the battery cell, and Q 1max Q represents the charging capacity before the highest single-cell inflection point of the battery. 1min The charging capacity before the lowest single-cell inflection point of the battery.

4. The lifespan estimation method for lithium iron phosphate power battery systems according to claim 3, characterized in that: The charging capacity Q of the battery before the highest single-cell inflection point is obtained using the ampere-hour integration method. 1max The charging capacity Q of the battery before the lowest single-cell inflection point 1min 1. Total charging capacity of individual battery cells Q2.

5. The lifespan estimation method for lithium iron phosphate power battery systems according to claim 3, characterized in that: X is obtained in the following way: Obtain the charging voltage value at the end of charging, and then use the OCV-SOC table lookup method to obtain the SOC at the end of charging based on the charging voltage value at the end of charging.

6. The lifespan estimation method for lithium iron phosphate power battery systems according to claim 1, characterized in that: In step 4), the following formula is used to determine the system's battery health status (SOHsys): SOHsys=min(SOHmaxv,SOHminv).

7. The method for estimating the lifespan of a lithium iron phosphate power battery system according to any one of claims 1 to 6, characterized in that: In step 1), the voltage change within a unit capacity method is used to identify the inflection point of the lithium iron phosphate battery plateau period, and the voltage value at the inflection point is obtained. The battery state of charge SOC1 at the inflection point is determined by looking up the OCV-SOC table.

8. A life estimation system for lithium iron phosphate power battery systems, characterized in that: The system includes a processor for executing computer instructions to implement the life estimation method for lithium iron phosphate power battery systems as described in any one of claims 1 to 7.

9. A vehicle, comprising a vehicle body and a lithium iron phosphate power battery system, characterized in that: It also includes a processor for executing computer instructions to implement the lithium iron phosphate power battery system life estimation method as described in any one of claims 1 to 7.

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