A method and system for balancing lithium iron phosphate batteries

Abstract

The technical solution adopted by the present invention is: a method and system for equalizing lithium iron phosphate batteries, including the following steps: obtaining a single cell whose voltage exceeds the equalization judgment voltage value at any time during the charging process of the battery system with the minimum trickle current value as a single cell to be equalized; obtaining the amount of electricity charged in each string of single cells to be equalized from the time when the voltage reaches the equalization judgment voltage value to the time when the battery system charging ends, and recording it as the corresponding charged amount of each string of single cells to be equalized; judging whether it is necessary to perform equalization on the single cell and the number of single cells to be equalized according to the available amount values ​​of all single cells in the lithium iron phosphate battery; selecting some or all of the single cells to be equalized to perform the equalization operation, and the required equalized capacity value is determined according to the difference in the charged amount of the single cells to be equalized. The present invention can accurately calculate the equalization amount and equalization time of the battery to be equalized.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy vehicles, and in particular relates to a method and system for balancing a lithium iron phosphate battery. Background Art

[0002] With the widespread application of electric vehicles, during long-term use, the consistency of batteries in the battery system will gradually deteriorate with the increase of usage time and mileage, thereby affecting the discharge capacity of the battery system and causing the battery capacity to not be effectively utilized. Batteries with large consistency differences in the battery system are passively balanced to narrow the difference in discharge capacity with other batteries, thereby improving the system consistency level and releasing more energy.

[0003] The existing mainstream battery balancing strategy mainly relies on judging the single cell voltage difference of the battery in the battery system, determining the battery that needs to be balanced, and starting the balancing until the battery voltage difference is less than the threshold for starting the balancing.

[0004] The existing balancing strategy is greatly affected by the voltage of the single cell. When the health of some strings of single cells decreases, the balancing cannot achieve the expected effect. The balancing strategy that relies on the voltage difference of the single cell to determine whether to enter or exit cannot accurately meet the actual balancing needs; at the same time, the amount of electricity that needs to be balanced for the single cell cannot be accurately estimated, which directly reduces the balancing effect. Summary of the invention

[0005] The purpose of the present invention is to solve the deficiencies of the above-mentioned background technology and provide a method and system for balancing lithium iron phosphate batteries to accurately calculate the balancing power and balancing time of the batteries to be balanced.

[0006] The technical solution adopted by the present invention is: a method for equalizing a lithium iron phosphate battery, comprising the following steps: obtaining a single cell whose voltage exceeds the equalization judgment voltage value at any time during the charging process of the battery system at the minimum trickle current value as a single cell to be equalized;

[0007] Obtain the amount of electricity charged into each string of cells to be balanced from the time when the voltage of the cells reaches the equalization determination voltage value to the time when the battery system charging ends, and record it as the corresponding amount of electricity charged into each string of cells to be balanced;

[0008] Determine whether it is necessary to perform balancing on the single cells and the number of single cells that need to be balanced according to the available value of all single cells in the lithium iron phosphate battery;

[0009] A part or all of the cells to be balanced are selected to perform a balancing operation, and the required balancing capacity value is determined according to the difference in the charged capacity of the cells to be balanced.

[0010] The above specific solution also includes the following steps: calculating the balancing time of each single cell according to the set balancing circuit current value, the number of single cells to be balanced and the corresponding required balancing capacity value.

[0011] In the above specific solution, after the single cell to be balanced and its corresponding capacity value required to be balanced are determined, they are first saved; and after it is determined that no fault that does not allow equalization is triggered, the equalization process of the single cell is executed.

[0012] In the above specific scheme, the process of obtaining the corresponding charged power of the single cells to be balanced includes: obtaining the moment when the voltage of each string of single cells to be balanced reaches the equalization judgment voltage value and the moment when the battery system charging is completed when the battery system is charged with the minimum trickle current value; calculating the time difference between the moment when the voltage of each string of single cells to be balanced reaches the equalization judgment voltage value and the moment when the battery system charging is completed, and integrating the calculated time difference and the minimum trickle current value to obtain the corresponding charged power of each string of single cells to be balanced.

[0013] In the above specific scheme, the process of determining whether it is necessary to perform balancing on the single cells includes: recording the available power values ​​of all single cells in the lithium iron phosphate battery at the end of charging, and calculating their standard deviation; if the standard deviation is less than the first threshold, there is no need to start the balancing process; if the standard deviation is greater than the second threshold, manual intervention is required; if the standard deviation is greater than or equal to the first threshold and less than or equal to the second threshold, it is determined that it is necessary to perform balancing on the single cells.

[0014] In the above technical solution, the process of determining the number of single cells that need to be balanced includes: determining the basic number of balanced single cells according to the number of battery strings in the battery system according to a set ratio, denoted as n; counting the number of single cells whose voltage exceeds the equalization judgment voltage value at any time during the charging process of the battery system with the minimum trickle current value, denoted as m; if m is greater than n, selecting n single cells for balancing; if m is less than or equal to n, selecting m single cells for balancing.

[0015] In the above technical solution, for all single cells whose voltage exceeds the equalization judgment voltage value at any time during the charging process of the battery system with the minimum trickle current value, they are sorted and numbered from roughly small to large according to their corresponding charged capacities: the numbers are recorded as SN1, SN2...SNm, and the corresponding charged capacities are recorded as Qc1, Qc2...Qcm.

[0016] In the above technical solution, if m is greater than n, n single cells numbered SN1, SN2, ..., SNn are selected for balancing; based on the charged power Qc(n+1) of the single cell numbered SN(n+1), the capacity values ​​Qb1, Qb2, ..., Qbn required to be balanced for the n single cells to be balanced are calculated:

[0017] Qb1=Qc1–Qc(n+1), Qb2=Qc2–Qc(n+1)…Qbn=Qcn–Qc(n+1).

[0018] In the above technical solution, if m≤n, the capacity values ​​Qb1, Qb2, ..., Qbm required to be balanced for the m single cells to be balanced are calculated based on the charging capacity Qcm of the single cell numbered SNm:

[0019] Qb1=Qc1-Qcm, Qb2=Qc2-Qcm,...Qbm=0.

[0020] The present invention also provides a lithium iron phosphate battery equalization system, which is used in the lithium iron phosphate battery equalization method described in the above technical solution.

[0021] The beneficial effects of the present invention are as follows: the present invention adopts a completely different single cell balancing strategy from the prior art: setting a balancing judgment voltage value (calibratable value) of a single cell, at the end of charging, when the charging current of the battery system drops to the minimum trickle value, recording the time when the single cell voltage is greater than the balancing judgment voltage value and the time when the battery system charging ends, and calculating the amount of electricity charged into each string of batteries exceeding the balancing judgment voltage value during this period according to the integral of the charging current and time. According to the number of batteries that need to be balanced in the entire system, select some or all of the batteries with higher charged electricity from the batteries exceeding the balancing judgment voltage value for balancing, reduce the voltage difference with other batteries at the end of charging, and improve the consistency of the battery at the high end of electricity. The selection principle of balancing batteries and the balancing electricity calculation method adopted by the present invention can accurately calculate the amount of electricity that needs to be balanced for each string of batteries, and the balancing time can be accurately quantified, completing the balancing function while reducing energy loss. The present invention does not make rigid requirements on hardware conditions, and can be brought into the method proposed by the present invention according to specific application scenarios and adapted by itself. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the method flow of the present invention;

[0023] Figure 2 The figure is a schematic diagram of a method flow of a specific embodiment. DETAILED DESCRIPTION

[0024] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments to facilitate a clear understanding of the present invention, but they do not constitute a limitation on the present invention.

[0025] like Figure 1 As shown, a method for equalizing a lithium iron phosphate battery of the present invention comprises the following steps: obtaining a single cell whose voltage exceeds an equalization determination voltage value at any time during the charging process of the battery system at a minimum trickle current value as a single cell to be equalized;

[0026] Obtain the amount of electricity charged into each string of cells to be balanced from the time when the voltage of the cells reaches the equalization determination voltage value to the time when the battery system charging ends, and record it as the corresponding amount of electricity charged into each string of cells to be balanced;

[0027] According to the available value of all the single cells in the lithium iron phosphate battery, it is determined whether the single cells need to be balanced and the number of single cells that need to be balanced;

[0028] Some or all of the cells to be balanced are selected for balancing, and the required balancing capacity value is determined according to the difference in the charged capacity of the cells to be balanced.

[0029] The specific selection principle of the single battery to be balanced and the calculation of the balanced power refer to the detailed description below. The power required to be balanced for each battery string and the balancing time can be accurately quantified, completing the balancing function while reducing energy loss.

[0030] The present invention performs a balancing process based on the calculated amount of electricity and balancing time required to be balanced for each battery string. The method for performing the balancing process adopts the prior art, that is, a set constant current is used to discharge the single battery to be balanced within the balancing time.

[0031] Specifically, the following steps are also included: calculating the balancing time of each single cell according to the set balancing circuit current value, the number of single cells to be balanced and the corresponding required balancing capacity value.

[0032] Specifically, after determining the single cells that need to be balanced and their corresponding capacity values ​​that need to be balanced, they are first saved; and after determining that no fault that does not allow equalization is triggered, the equalization process of the single cells is executed.

[0033] Specifically, the process of obtaining the corresponding charged power of each string of cells to be balanced includes: obtaining the moment when the voltage of each string of cells to be balanced reaches the equalization judgment voltage value and the moment when the battery system ends charging when the battery system is charged with the minimum trickle current value; calculating the time difference between the moment when the voltage of each string of cells to be balanced reaches the equalization judgment voltage value and the moment when the battery system ends charging, and performing integral calculation using the calculated time difference and the minimum trickle current value to obtain the corresponding charged power of each string of cells to be balanced.

[0034] Specifically, the process of determining whether it is necessary to perform balancing on the single cells includes: recording the available power values ​​of all single cells in the acid-iron lithium battery at the end of charging, and calculating the standard deviation; if the standard deviation is less than a first threshold, there is no need to start the balancing process; if the standard deviation is greater than a second threshold, manual intervention is required; if the standard deviation is greater than or equal to the first threshold and less than or equal to the second threshold, it is determined that it is necessary to perform balancing on the single cells.

[0035] Specifically, the process of determining the number of single cells that need to be balanced includes: determining the basic number of balanced single cells according to the number of battery strings in the battery system according to a set ratio, recorded as n; counting the number of single cells whose voltage exceeds the equalization judgment voltage value at any time during the charging process of the battery system with the minimum trickle current value, recorded as m; if m is greater than n, selecting n single cells for balancing; if m is less than or equal to n, selecting m single cells for balancing.

[0036] Specifically, for all single cells whose voltage exceeds the equalization judgment voltage value at any time during the charging process of the battery system at the minimum trickle current value, they are sorted and numbered from roughly small to large according to their corresponding charged capacities: the numbers are recorded as SN1, SN2...SNm, and the corresponding charged capacities are recorded as Qc1, Qc2...Qcm.

[0037] Specifically, if m is greater than n, select n single cells numbered SN1, SN2, ..., SNn for balancing; based on the charged capacity Qc(n+1) of the single cell numbered SN(n+1), calculate the capacity values ​​Qb1, Qb2, ..., Qbn required to be balanced for the n single cells to be balanced:

[0038] Qb1=Qc1–Qc(n+1), Qb2=Qc2–Qc(n+1)…Qbn=Qcn–Qc(n+1).

[0039] Specifically, if m≤n, taking the charging capacity Qcm of the single cell numbered SNm as a reference, calculate the capacity values ​​Qb1, Qb2, ..., Qbm required to be balanced for the m single cells to be balanced:

[0040] Qb1=Qc1-Qcm, Qb2=Qc2-Qcm,...Qbm=0.

[0041] The principle of the present invention is further explained below with reference to specific embodiments.

[0042] First, set the equalization judgment voltage value of the single cell: the power of the lithium iron phosphate battery has a voltage plateau period. In the platform area, the characteristic relationship between the available power of the battery and the single cell voltage is not obvious, while the voltage change amplitude at the end of charge and discharge is large, which can more accurately obtain the relationship between the battery power and the single cell voltage. Combined with the actual charging and discharging conditions (the full charge frequency is generally higher than the battery system discharge to below 20% SOC), select the high SOC interval for battery consistency evaluation and equalization power calculation, and align the upper end voltage of the single cell; combined with the battery system charging strategy, and considering that the battery needs to be depolarized before reaching the equalization judgment voltage value, set 3.55V as the equalization judgment voltage value (calibrable).

[0043] The charging strategy adopted by the battery system of this specific embodiment is: in order to take into account the charged amount and charging time, the current charging strategy will enter a small current trickle process after charging with a large current to a certain single cell voltage value (which is also the balance judgment voltage value). The trickle current changes to the minimum current value and then maintains it until the full charging condition is met and charging stops.

[0044] The specific implementation process of this specific embodiment is as follows: Figure 2 As shown:

[0045] The equalization judgment voltage value is set to 3.55V, and the battery pack is charged according to the set charging strategy until charging is completed.

[0046] At the end of charging, after the charging current drops to the minimum current value, record the single battery in each string whose voltage reaches the equalization judgment voltage value (3.55V, calibrable) during the process, and the system time when it reaches the equalization judgment voltage value: t1, t2, t3...tm, and the battery system charging end time t0. Among them, t1, t2, t3...tm are arranged in order from the first to the last.

[0047] According to the time difference between the time values ​​t1, t2, t3 ... tm and t0, and the real-time current value Ic, the charged power Qc1, Qc2 ... Qcm of each battery string reaching the equalization judgment voltage value in this time period is calculated by integration. The calculation formula is as follows:

[0048]

[0049] They are sorted from large to small according to their corresponding charged capacities, and corresponding serial numbers are generated: SN1, SN2...SNm.

[0050] At the end of charging, record the available power value Qnow of all cells in the lithium iron phosphate battery, and calculate the standard deviation σ of Qnow at this time:

[0051] If the standard deviation σ is low (σ<0.1Ah can be calibrated), it is determined that the battery system consistency is good and there is no need to start balancing;

[0052] If the standard deviation σ is too high (σ>4.8Ah can be calibrated), it is determined that the battery system consistency is poor and exceeds the range of passive battery balancing, requiring manual intervention and alarm prompts;

[0053] When the standard deviation σ is within a certain range (0.1≤σ≤4.8Ah), the balancing circuit is started to discharge the single cells that meet the balancing conditions in a balanced manner.

[0054] After determining that some single cells need to be discharged, first select the cells to be balanced and calculate the corresponding balanced power:

[0055] According to the number of battery strings in the entire battery system, it is expected that 2% (the converted number is n, which can be calibrated) of all the single cells in the lithium iron phosphate battery will be selected for balancing; at the end of charging, the number of batteries that exceed the balancing judgment voltage value during charging with a small current Ic at the charging end is counted as m.

[0056] (1) If m>n, select n cells with serial numbers SN1, SN2, ... SNn for balancing. Based on the capacity Qc(n+1) charged into the SN(n+1) battery, calculate the capacity value that needs to be balanced for these n cells:

[0057] Qb1=Qc1–Qc(n+1); Qb2=Qc2–Qc(n+1);……;

[0058] Qbn=Qcn−Qc(n+1);

[0059] (2) If m≤n, select all cells that exceed the equalization judgment voltage value, that is, m cells with serial numbers SN1…SNm for equalization. Based on the capacity Qcm charged into the SNm battery, calculate the capacity value that needs to be equalized for these m cells:

[0060] Qb1=Qc1-Qcm, Qb2=Qc2-Qcm,...Qbm=0.

[0061] After determining the number and serial number of the cells to be balanced, calculate the balancing time required for the single cells to be balanced:

[0062] After screening, the number and power of batteries to be balanced are obtained as Qb1, Qb2, ...; according to the set current value Ib of the balancing circuit, the balancing time Tbn is calculated:

[0063] Tb1=Qb1 / Ib, Tb2=Qb2 / Ib...Tbn=Qbn / Ib.

[0064] After charging is completed, the single cells that need to be balanced, the power level and the balancing time are saved before power is turned off. After determining that no fault that does not allow balancing is triggered, the discharge balancing process is performed to complete the balancing of the static battery.

[0065] Under the condition that no fault is triggered, the equalization process in the prior art can be performed in the discharge and high current charging stages. The equalization process is no longer performed when the current starts to decrease at the end of charging; after the equalized battery and the corresponding equalization power and time are determined after charging, the equalization process is performed again according to actual needs.

[0066] This specific embodiment is based on the characteristics of the battery system using 230Ah lithium iron phosphate batteries, sets the equalization judgment voltage value to 3.55V, the upper and lower limits of the standard deviation judgment value to 0.1-4.8Ah, the number of battery strings in the battery system is 192, and 2% of the batteries are selected for balancing, and the value of n is 4. The current load of the balancing circuit is 100mA, and the balancing time is obtained by the balancing power of each single battery being balanced / 100mA.

[0067] For a multi-branch battery system with a large capacity, each branch acts as an independent system, and the calculations related to balancing within the system are completely consistent with the above strategy.

[0068] The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field.

Claims

1. A method for equalizing a lithium iron phosphate battery, characterized in that: The following steps are involved: Acquire a single cell whose voltage exceeds the equalization determination voltage value at any time during the process of charging the battery system at the minimum trickle current value as a single cell to be equalized; Obtain the amount of electricity charged into each string of cells to be balanced from the time when the voltage of the cells reaches the equalization determination voltage value to the time when the battery system charging ends, and record it as the corresponding amount of electricity charged into each string of cells to be balanced; Determine whether it is necessary to perform balancing on the single cells and the number of single cells that need to be balanced according to the available value of all single cells in the lithium iron phosphate battery; Select some or all of the cells to be balanced to perform a balancing operation, and the required balancing capacity value is determined according to the difference in the charged capacity of the cells to be balanced; The process of determining the number of single cells that need to be balanced includes: determining the basic number of balanced single cells according to the number of battery strings in the battery system according to a set ratio, which is recorded as n; counting the number of single cells whose voltage exceeds the equalization judgment voltage value at any time during the battery system is charged at the minimum trickle current value, which is recorded as m; if m is greater than n, then selecting n single cells for balancing; if m is less than or equal to n, then selecting m single cells for balancing; For all the single cells whose voltage exceeds the equalization judgment voltage value at any time during the charging process of the battery system at the minimum trickle current value, they are sorted and numbered from small to large according to their corresponding charged capacities: the numbers are recorded as SN1, SN2...SNm, and the corresponding charged capacities are recorded as Qc1, Qc2...Qcm; If m is greater than n, select n cells numbered SN1, SN2, ..., SNn for balancing; based on the charged capacity Qc(n+1) of the cell numbered SN(n+1), calculate the capacity values ​​Qb1, Qb2, ..., Qbn required for balancing the n cells to be balanced: Qb1=Qc1–Qc(n+1), Qb2=Qc2–Qc(n+1)…Qbn=Qcn–Qc(n+1).

2. The equalization method for a lithium iron phosphate battery according to claim 1, characterized in that: The following steps are also included: The balancing time of each single cell is calculated according to the set balancing circuit current value, the number of single cells to be balanced and the corresponding required balancing capacity value.

3. The equalization method of a lithium iron phosphate battery according to claim 1, characterized in that: After determining the single cell to be balanced and its corresponding capacity value required for equalization, it is first saved; and after determining that no fault that does not allow equalization is triggered, the equalization process of the single cell is executed.

4. The equalization method for a lithium iron phosphate battery according to claim 1, characterized in that: The process of obtaining the corresponding charged power of the single cells to be balanced includes: obtaining the time when the voltage of each string of single cells to be balanced reaches the equalization judgment voltage value and the time when the battery system charging is completed when the battery system is charged with the minimum trickle current value; calculating the time difference between the time when the voltage of each string of single cells to be balanced reaches the equalization judgment voltage value and the time when the battery system charging is completed, and integrating the calculated time difference and the minimum trickle current value to obtain the corresponding charged power of each string of single cells to be balanced.

5. The equalization method for a lithium iron phosphate battery according to claim 1, characterized in that: The process of determining whether it is necessary to perform balancing on the single cells includes: recording the available power values ​​of all single cells in the lithium iron phosphate battery at the end of charging, and calculating their standard deviation; if the standard deviation is less than a first threshold, there is no need to start the balancing process; if the standard deviation is greater than a second threshold, manual intervention is required; if the standard deviation is greater than or equal to the first threshold and less than or equal to the second threshold, it is determined that it is necessary to perform balancing on the single cells.

6. The equalization method for a lithium iron phosphate battery according to claim 1, characterized in that: If m≤n, take the charged capacity Qcm of the single cell numbered SNm as the benchmark, calculate the capacity values ​​Qb1, Qb2, ..., Qbm required to be balanced for the m single cells to be balanced: Qb1=Qc1-Qcm, Qb2=Qc2-Qcm,...Qbm=0.

7. A lithium iron phosphate battery equalization system, characterized in that: The system is used to implement the equalization method for the lithium iron phosphate battery described in any one of claims 1 to 6.

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