CN107839500B

CN107839500B - Lithium battery pack balance control method and system for dynamically correcting SOC

Info

Publication number: CN107839500B
Application number: CN201710562904.7A
Authority: CN
Inventors: 王宜怀; 陈建明; 徐达; 彭楠楠
Original assignee: Suzhou University
Current assignee: Ningbo Fei Chida Electronic Technology Development Co.,Ltd.
Priority date: 2017-07-11
Filing date: 2017-07-11
Publication date: 2020-01-14
Anticipated expiration: 2037-07-11
Also published as: CN107839500A

Abstract

The invention provides a lithium battery pack balance control method and system for dynamically correcting SOC, which comprises the following steps: obtaining the SOC of each single lithium battery in the lithium battery pack; calculating the range r of the battery_soc(ii) a Comparing the range r_socThe size of the preset range threshold value is obtained; selecting the mean value of SOC of all the single lithium batteries

As target SOC for equalization, lower than SOCThe single lithium battery is charged evenly, and the SOC is higher than that of the single lithium batteryThe single lithium battery performs discharge equalization, wherein dSOC is an equalization control band. The invention has the advantages of high modularization degree, high equalization speed, improved battery use efficiency and prolonged battery service life.

Description

Lithium battery pack balance control method and system for dynamically correcting SOC

Technical Field

The invention belongs to the field of new energy automobile batteries, and particularly relates to a lithium battery pack balance control method and system for dynamically correcting SOC.

Background

Under the influence of environmental pollution problems and policy and regulation, new energy vehicles represented by hybrid electric vehicles and pure electric vehicles are receiving more and more attention. The lithium battery gradually replaces a lead-acid battery in the field of electric automobiles to become a main power battery energy source due to the advantages of high energy density, low self-discharge rate, long service life and no pollution.

When lithium batteries are used in electric vehicles, the lithium battery pack must meet certain capacity and voltage requirements in order to meet the requirements for load capacity and endurance. Therefore, the single lithium batteries are often connected in parallel to form a battery unit so as to solve the problem of insufficient capacity of the single battery; and simultaneously, a higher voltage is obtained by connecting single lithium battery units in series to form a battery pack. In lithium batteries used in series in groups, there is often a risk of inconsistencies between individual cells or battery cells. The inconsistency of lithium batteries can be defined as inconsistency of important parameter characteristics such as voltage, capacity, internal resistance and self-discharge rate among single lithium batteries with the same specification and the same model. The available power of the battery pack is limited due to the existence of the inconsistency problem, and particularly at the end stage of heavy current discharge, the battery pack is damaged due to the fact that the voltage of the battery with high internal resistance drops too fast, so that the discharge current of the battery pack needs to be limited, the output power of the battery pack is limited, and the available power is reduced. Meanwhile, when large current is discharged for a long time along with the increase of the charging and discharging times of the battery, the aging degree of each single battery is different, so that the inconsistency degree of each battery is intensified.

In order to deal with the inconsistency problem in the lithium battery pack, an equalization technology is generally integrated in the battery management system BMS, and the equalization technology generally refers to a special technical means introduced to avoid or reduce adverse effects on the capacity utilization rate, the output power, the service life and the like of the battery pack caused by the inconsistency problem of the battery pack. The current equalization technology becomes a key technology in a battery management system BMS, the effective use capacity of the whole battery can be improved through efficient equalization measures, and the service life of the battery is prolonged. The current research on the equalization technology is mainly developed from two aspects of an equalization control strategy and an equalization circuit topological structure design. The research on the battery pack balancing strategy focuses on establishing evaluation indexes of the inconsistency of the single batteries in the battery pack, and an effective balancing control method is provided based on the evaluation indexes; the topology design of the equalization circuit focuses on the design and improvement of the equalization circuit structure with high efficiency, simple control structure and relatively low cost.

In the aspect of an equalization strategy, in the prior art, a relatively deep research has been made on the adoption of voltage as an equalization variable, but the adoption of a State of Charge (SOC) as an equalization variable is still a research hotspot, so that a better equalization effect can be obtained by taking the SOC as the equalization variable, a system is also easy to control, and the real State of a battery pack is better reflected. However, the conventional technology has the technical problems that the SOC estimation precision is not high and the real-time accuracy is not achieved.

In the aspect of an equalization circuit topology, a passive equalization technology is applied to actual products due to the reasons of mature technology, simple structure and the like, but the passive equalization is not suitable in the field of pure electric vehicles pursuing energy utilization rate. For the aspect of active equalization technology, the following problems still exist:

(1) the equalization time is long, which is a common problem in the existing equalization systems, and the equalization time of most equalization systems is more than one hour, and some equalization systems even reach several hours.

(2) The existing common equalization technology based on external voltage has been widely researched, but due to the existence of the capacity difference of the single batteries, the external characteristics of the charging and discharging voltages of the single batteries are inconsistent, particularly the voltage of the single batteries rises rapidly in the later charging period of the single batteries, so that the equalization criterion is unstable when the external voltage of the batteries is used as the criterion of the consistency of the battery pack. Meanwhile, research also finds that the method has no obvious effect on increasing the available capacity of the battery pack before and after equalization.

(3) The practicability is yet to be improved, the circuit design is relatively complex, the integration is not enough, and the modular expansion can not be conveniently carried out along with the increase of the number of the serial battery sections of the battery pack.

Disclosure of Invention

The invention aims to overcome the defects in the prior art, and provides the lithium battery pack balance control method and the lithium battery pack balance control system for dynamically correcting the SOC, so that the balance control of the series lithium ion battery pack is realized, the modularization degree is high, the balance speed is high, the battery use efficiency is improved, and the service life of the battery is prolonged.

The invention provides a lithium battery pack balance control method for dynamically correcting SOC, which is characterized by comprising the following steps:

step 21, obtaining the SOC of each single lithium battery in the lithium battery pack;

step 22, calculating the range T of the battery pack_soc；

Step 23, comparing the range r_socAnd a preset range thresholdWhen the range r is large, the range r is small_socIf the difference is larger than the preset range threshold, the step 24 is entered, and when the range r is larger than the preset range threshold_socWhen the difference is less than or equal to the preset range threshold, entering step 25;

step 24, selecting the mean value of the SOC of all the single lithium batteries

As target SOC for equalization, lower than SOCThe single lithium battery is charged evenly, and the SOC is higher than that of the single lithium battery

The single lithium battery performs discharge equalization, wherein dSOC is an equalization control band;

and step 25, ending.

Preferably, step 21 specifically includes:

step 11, judging whether the battery is in a working state, if so, entering step 12, and if not, entering step 17;

step 12, calculating the SOC of the current state by using a formula two_iWherein, SOC_iIs the SOC, SOC of the current state of the battery₀Is the initial SOC, C of the battery at the beginning of the working state_NIs the rated capacity of the battery, I is the battery current, η is the charge-discharge efficiency, η is a negative number when charging, η is a positive number when discharging;

step 13, judging whether the battery is in a working state, if so, returning to step 12, and if not, entering step 14;

step 14, measuring the open circuit voltage OCV₁And searching a corresponding relation table of SOC and OCV to obtain the open-circuit voltage OCV₁Corresponding SOC₁；

Step 15, calculating SOC_iAnd SOC₁When the absolute value of e is greater than the preset error threshold, the step 16 is entered, and when the absolute value of e is less than or equal to the preset error threshold, the SOC is output_i(ii) a Entering a step 11;

step 16, calculating the corrected SOC, outputting the corrected SOC correction, and updating a corresponding relation table of the SOC and the OCV; entering a step 11;

step 17, measuring the open-circuit voltage OCV₂And searching a corresponding relation table of SOC and OCV to obtain the open-circuit voltage OCV₂Corresponding SOC₂Output SOC₂(ii) a Step 11 is entered.

Preferably, prior to step 11, a correspondence table between the initial SOC and OCV is established by interpolation.

Preferably, step 16 specifically includes:

calculating the corrected S by adopting a formula V_n(i +1), output S_n(i +1), and updating corresponding S in the corresponding relation table of SOC and OCV_n(i) Is S_n(i+1)；

S_n(i+1)＝S_n(i) -F (n, e) (0. ltoreq. n. ltoreq.50) (equation five)

Wherein S is_n(i) Representing the value in the table after the current i-th update, S_n(i +1) represents the values in the table after the i +1 th update, and F (n, e) is a correction coefficient which is a function related to n and e and expressed by formula six;

f (n, e) ═ a × e × n (formula six)

Where a is an adjustable constant representing the correction rate, n is the number of points interpolated, e is the SOC_iAnd SOC₁The difference between them.

Preferably, a has different values in different sections of the SOC.

Preferably, in step 24, when the battery is subjected to discharge equalization, the battery which is about to enter into discharge cutoff is charged so that the SOC of the battery is consistent with the SOC of other batteries, regardless of whether the battery is in a cutoff band; when the batteries are subjected to charge equalization, for the battery which is about to enter the charge cutoff, the equalization circuit is started to enable the SOC of the battery to fluctuate nearby, so that all the batteries can finally reach the state that the SOC is 1 at the same time.

The invention provides a lithium battery pack equalization control system for dynamically correcting SOC (system on chip) for realizing the method, which is characterized in that the control system is respectively an upper system and a lower system, wherein the upper system comprises a PC (personal computer) and a master control MCU (microprogrammed control unit), the lower system comprises a plurality of sub lower systems, each sub lower system is used for performing equalization control on a small battery pack, each sub lower system comprises a secondary MCU and an equalization module, and the system is characterized in that:

the master control MCU is used for collecting data fed back by each secondary MCU in a subordinate system, transmitting the data to the superior PC and forwarding a command sent by the PC to the corresponding secondary MCU;

the PC is used for receiving the data sent by the master control MCU and sending a command to the master control MCU;

the secondary MCU is used for acquiring voltage, current and temperature data of each battery in the small battery pack; feeding back the acquired data to a main control MCU; calculating the SOC of each single battery; judging whether balancing is needed or not according to the SOC; when balancing is needed, the balancing module of the small battery pack is controlled to balance the batteries needing balancing;

and the balancing module is used for balancing the batteries needing balancing according to the control of the secondary MCU in the small battery pack.

Preferably, the balancing module adopts a bidirectional flyback transformer.

Preferably, the bidirectional flyback transformer adopts an LTC3300-1 chip.

Preferably, for a single battery with a higher SOC and requiring equalization, the battery is turned on corresponding to the secondary switch, all other switches including the primary switch are turned off, current passes through the secondary winding of the bidirectional flyback transformer, and at this time, electric energy is stored in the secondary winding in the form of magnetic energy; after the SOC in the battery is reduced to meet the requirement, the secondary variable switch is disconnected, the primary switch is switched on, energy is transferred from the secondary winding to the primary winding, and magnetic energy is converted into electric energy, so that redundant energy is transferred into other batteries in the battery pack; for the single batteries with lower SOC and needing to be balanced, the switches corresponding to the primary side are turned on, all secondary switches are turned off, current passes through the primary winding of the bidirectional flyback transformer, and at the moment, electric energy is stored into the primary winding in a magnetic energy mode on the primary side; after enough electric energy is charged, the primary switch is disconnected, the secondary switch corresponding to the lowest SOC is opened, the secondary winding is conducted, energy is transferred from the primary winding to the secondary winding, the magnetic energy is converted back to the electric energy to be charged into the battery, the SOC of the single battery rises back, and the whole SOC of the battery pack returns to the same value.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

FIG. 1 is a graph showing the results of an aging test of a lithium battery;

FIG. 2 is a flow chart of an improved SOC estimation algorithm;

FIG. 3 is a functional block diagram of an improved SOC estimation algorithm verification platform;

FIG. 4 is a graph of improved SOC estimation algorithm test results;

FIG. 5 is a flow of determination based on SOC balancing strategies;

FIG. 6 is a schematic diagram of a bidirectional flyback transformer equalization circuit;

FIG. 7 is an equivalent circuit model of a flyback transformer;

fig. 8 is an equivalent magnetic circuit diagram of the flyback transformer;

FIG. 9 is a current waveform diagram for two modes of operation;

FIG. 10 is an overall block diagram of the equalization control system;

FIG. 11 is a flow chart of the master MCU execution;

FIG. 12 is a flow chart of the execution of a secondary MCU;

FIG. 13 is a schematic diagram of a power circuit;

FIG. 14 is a schematic diagram of a cell gating switching circuit;

FIG. 15 is a schematic diagram of a voltage reversing circuit;

FIG. 16 is a schematic diagram of a voltage to frequency conversion circuit;

FIG. 17 is a schematic diagram of a voltage polarity reversing circuit;

FIG. 18 is a schematic diagram of two voltage connector connections;

fig. 19 is a schematic diagram of an active equalization circuit.

Detailed Description

The present invention is described in further detail below with reference to the attached drawings.

Improved SOC estimation algorithm

Under the existing technical conditions, the inconsistency of the lithium battery pack is mainly generated in the production process. Meanwhile, the working environment of the lithium battery pack on the electric automobile is generally severe, which causes the inconsistency to be aggravated. This inconsistency is typically manifested by the parameters of the voltage, capacity and internal resistance of the battery, and has a significant impact on the life and performance of the battery. Before the balancing control is carried out on the battery pack, how to evaluate the consistency state of the lithium battery pack is a prerequisite, and quantitative evaluation of consistency provides important data support for balancing and maintaining of the battery pack.

The electric automobile is characterized by estimating the SOC of a battery pack to represent the driving range of the automobile. The SOC is used as a state parameter of the battery pack capacity, which reflects the state of the remaining capacity of the battery pack, and is numerically defined as the ratio of the remaining capacity of the battery to the calibrated total capacity of the battery:

in formula one, Q_RIs the remaining charge capacity in the battery, and C is the nominal (nominal) charge capacity of the battery.

From the above-mentioned SOC definition, the SOC represents the ratio between the remaining capacity of the battery and the available capacity of the battery, and if the SOC is used as the evaluation criterion of the inconsistency, the difference in the available capacity between the individual batteries can be avoided, and the goal is to allow all the individual batteries to reach the cut-off voltage of charging and discharging at the same time, so as to ensure the maximum use of the available capacity of the battery pack. The SOC consistency of all the single batteries is guaranteed, namely, the consistency of the discharge depths of all the batteries is guaranteed, so that the problem that the performance of the whole battery pack is reduced and even the whole battery pack is scrapped due to the fact that a certain single battery in the battery pack is too large in aging degree is solved. The difference of the SOC can reflect the inconsistency of the battery, and meanwhile, the SOC estimated value can be obtained in real time in the running process of the battery pack, so the SOC-based equalization strategy can inhibit the inconsistency of the battery pack in real time.

When using SOC estimation, the greatest difficulty is the accuracy and real-time for SOC estimation. At the initial stage of discharging of the lithium battery, the SOC of the battery has small change, and if the SOC cannot be accurately estimated, the SOC estimation error is accumulated at the final stage of discharging of the battery, so that the error is overlarge. The challenge for the equalization control circuit is considerable if the equalization is performed again, and the equalization effect is also compromised. In addition, a certain amount of calculation is needed for real-time estimation of the SOC, and when the number of batteries in the battery pack is large, in order to ensure the real-time performance of the SOC, the equalization system needs to be capable of adopting an MCU with a certain calculation capability.

The remaining capacity of the battery is directly measured without any means, and the remaining capacity of the battery can be estimated only by the data amount that can be measured by the battery, such as voltage, current, internal resistance and the like. However, the chemical characteristics inside the battery are very complex, so that the data and the SOC do not present a linear or other simple functional relationship, and the currently proposed SOC estimation method always has more or less defects, which also makes the SOC state estimation of the battery pack a key and difficult point in the battery management system. The SOC estimation method in the prior art is as follows:

(1) method of setting time

The ampere-hour method is the most commonly used method for estimating the SOC parameter of the battery, and the core idea of the method can be expressed by a formula II:

wherein the SOC_iIs the SOC of the current state of the battery; c_NIs the rated capacity of the battery; i is the battery current; η is the charge-discharge efficiency. By calculating the integral of the current over time, the amount of power lost over time can be obtained. Fitting a known initial SOC₀The SOC at a certain time can be obtained. The principle and implementation of the method are simple, but the method has the following defects: firstly, the requirement on the current measurement precision is high, if the current measurement is inaccurate, SOC calculation errors can be caused, the SOC calculation errors are accumulated for a long time, and the errors are gradually amplified; second, for the initial SOC₀Estimation also requires acquisition of a certain method; finally, when the temperature is high or the current fluctuation is large, the error of the method is large. Therefore, in order to obtain a reliable SOC estimation value, it is necessary to use a high-performance current sensor to obtain an accurate current value, and sufficient data is required for estimating the initial state.

(2) Open circuit voltage method

The electromotive force of the battery may be considered to be composed of three parts including an open-circuit voltage (OCV) of the battery, an ohmic voltage of the battery, and a polarization voltage of the battery. When the battery is in a non-working state and is kept still for a long time, the ohmic voltage and the polarization voltage of the battery can both drop to 0, the open-circuit voltage OCV of the battery is equal to the terminal voltage of the battery, namely the electromotive force of the battery, so that the SOC can be estimated according to the relationship curve between the OCV and the SOC. In real time, the OCV-SOC on the lead-acid battery has quite good linear relation, and the SOC can be estimated more accurately by the method. However, for lithium batteries, the linear relationship between the two is not so obvious, so that a relatively complex relationship comparison table needs to be established.

The open circuit voltage method has the obvious disadvantages that the battery needs to be fully kept still before measurement, and the time length needs several hours or even more than ten hours, which causes difficulty in measurement; secondly, the standing time is also a place which is difficult to determine; meanwhile, as the battery ages, the correspondence relationship between the open-circuit voltage OCV and the SOC also changes. These reasons make it impossible to obtain SOC online using the open circuit voltage method in practical use. The open circuit voltage method can be combined with an ampere-hour method and used as an initial SOC value obtaining method in the ampere-hour method. However, the relationship between the open-circuit voltage and the SOC changes with the aging of the battery, and it is not always possible to re-measure the relationship between the open-circuit voltage and the SOC in actual use, so an algorithm that can dynamically correct the correspondence between the open-circuit voltage OCV and the SOC is required.

(3) Load voltage method

The principle of the load voltage method is consistent with the open circuit voltage method, and the load voltage method is provided for overcoming the defect that the open circuit voltage method cannot estimate the SOC of the battery on line. The principle is as follows: if the internal resistance R and the operating current I of the battery can be obtained, the balanced electromotive force EMF of the battery can be calculated by measuring the voltage U across the load R according to the following formula.

EMF ═ U + I r (formula three)

The analysis of the open-circuit voltage method shows that the corresponding relation between EMF and SOC is the relation between OCV and SOC in the open-circuit voltage method, so the SOC of the battery can be obtained correspondingly after the EMF is known.

Theoretically, the method does overcome the defect that the open-circuit voltage method cannot measure the SOC in real time, but in practical use, the method has the obvious defects: firstly, the factors influencing the internal resistance r of the battery are very many, the internal resistance of the battery is inconsistent, and the internal resistance of each single battery can have great difference, so that the accurate acquisition of the internal resistance of the battery is difficult; secondly, the method itself is based on the open circuit voltage method, so the problem faced by the open circuit voltage method is that the method cannot be avoided. The load voltage method is rarely used for obtaining the SOC on line in the field of electric vehicles, and is often used as a criterion for determining the charge and discharge cutoff of the battery.

(4) Internal resistance method

The basic idea of the internal resistance method is consistent with the open-circuit voltage method, a large number of experimental researches show that the alternating current impedance or the direct current internal resistance of the lithium battery has a close relationship with the SOC of the battery, and if the relationship can be determined by some battery samples, the SOC of the battery can be obtained by detecting the internal resistance of the battery.

The internal resistance of the battery may be divided into ac impedance and dc internal resistance. The ac impedance reflects the resistance of the battery to ac current, and can be measured by an ac impedance meter. Similarly, the internal resistance of the direct current indicates the reactive capability of the battery to the direct current, and the value can be obtained by detecting the change value of the voltage and the change value of the current in a short time.

However, in the previous analysis of the load voltage method, it is also mentioned that the relationship between the internal resistance of the battery and the SOC is very complicated, and it is not only influenced by the SOC but also influenced by a plurality of factors such as the temperature and the state of health of the battery, so that the relationship between the internal resistance and the SOC is difficult to be actually determined. Meanwhile, the internal resistance of the battery is usually very small and only has a milliohm level, so that the requirement on the measurement precision is very high, and the measurement error has a very large influence on the result. Therefore, the method is rarely used in practical application of electric vehicles.

(5) Neural network method

The neural network method is to estimate the SOC of the battery by performing data training through a large number of samples on the premise of building a network model. The battery is a highly nonlinear system, the neural network method has nonlinear basic characteristics and can well simulate the nonlinear dynamic characteristics of the battery, and therefore the neural network method has a good effect of estimating the SOC. The neural network method for estimating the SOC needs to train a large amount of sample data, wherein the training sample data and the training method both influence the estimation result, and the other defect is that the neural network method needs a large amount of resources, higher requirements are provided for the design of a battery management system, a control chip with higher performance is often needed, and the cost is greatly improved.

(6) Kalman filtering method

The Kalman filtering method is to take a battery system as a nonlinear dynamic system, wherein the SOC of the battery is only one state in the system, a battery model is correspondingly established, a state equation and an observation equation are listed according to the model, and the SOC of the battery is estimated by adopting an extended Kalman filtering method. The basic idea of this method is to make an optimal estimate of the state of the dynamic system with minimal variance. The method solves the problems of inaccurate estimation of the initial SOC value and accumulative error in an ampere-hour method. If the battery model can be accurately established, the kalman filtering method can accurately estimate the SOC of the battery. However, this method also has several problems: firstly, the accuracy of estimating the SOC of the battery depends on the accuracy of a battery model, and secondly, the requirement on the running speed of a system processor is improved due to the fact that a large number of matrix operations are applied in a Kalman filtering method.

By summarizing the existing SOC estimation methods, the following are commonly used: ampere-hour method, open-circuit voltage method, internal resistance method, neural network method and Kalman filtering method. The above methods have respective unique characteristics under different use environments and for different power batteries, but have different defects and shortcomings, and table 1 summarizes the advantages and disadvantages of the common SOC estimation method.

TABLE 1 summary of common SOC estimation methods

In summary, the open-circuit voltage method in the conventional SOC algorithm needs to estimate the SOC when the battery is not in operation, so that the requirement of online SOC estimation of the electric vehicle power is difficult to meet, and the method is suitable for being combined with other methods to estimate the SOC. The ampere-hour method has the problems of dependence on an initial value and larger accumulated error and cannot deal with the self-discharge of the battery. The Kalman filtering method solves the problems of inaccurate estimation of an initial SOC value and accumulated errors in an ampere-hour method, and meanwhile, has the main problems that the battery model has strong dependence, and the SOC of the battery can be estimated more accurately only by establishing an accurate battery model; and because a large amount of matrix operations are used in the Kalman filtering method, the requirement on the speed of a system processor is high. The main problem of the neural network method for estimating the SOC is that a large amount of sample data needs to be trained, so that estimation errors are affected by the training of the sample data and the training method, and another defect is that the neural network method needs a large amount of resources, and higher requirements are put forward on the design of a battery management system.

In practical use, the lithium battery can be aged along with the increase of the cycle number. The aging phenomenon of lithium batteries is represented by the change of internal parameters of the batteries, and most importantly, the attenuation of the battery capacity. The attenuation can be seen by comparing the relationship between the open-circuit voltage and the SOC of the lithium battery with different cycle times, the relationship between the OCV and the SOC of the lithium iron phosphate battery of 3400mAh when the battery is new and after 500 cycles is tested in the invention, and the experimental result is shown in figure 1.

As can be seen from fig. 1, the lithium battery exhibits a certain correspondence between the open-circuit voltage OCV and the SOC, but at the same time, the gradual aging of the lithium battery causes a gradual change in the relationship between the open-circuit voltage OCV and the battery SOC. The capacity of the battery is continuously reduced as the battery ages gradually with the increase of charge-discharge cycles. It can also be seen from fig. 1 that at the same open circuit voltage OCV, different SOCs are mapped on the OCV-SOC curves for the new and aged cells. Therefore, when the open circuit voltage is used as the basic estimation algorithm of the SOC, a certain correction algorithm needs to be added to cope with the change of the relationship between the OCV and the SOC.

Considering the influence of the aging factor of the lithium battery, and meanwhile, because the embedded chip adopted by the lithium battery management system is generally weak in calculation performance, the improved SOC estimation algorithm provided by the invention integrates an ampere-hour method and an open-circuit voltage method, and a certain dynamic correction algorithm is added to keep the SOC estimation value within an acceptable error range so as to deal with the influence of the aging of the lithium battery.

As described above, ampere-hour is a commonly used method, and the error sources thereof are the estimation of the initial SOC and the measurement accuracy of the current during use. The measurement accuracy of the current is improved by replacing a high-accuracy measurement device, and the high-accuracy initial SOC can be measured by an open-circuit voltage method. However, due to the aging effect of the battery, the corresponding relationship between the open-circuit voltage and the SOC of the battery is dynamically changed, so the core of the improved SOC estimation algorithm provided by the invention is to dynamically maintain the corresponding relationship table of the SOC and the OCV. And in the use process of the battery, if the difference between the measured value of the SOC of the battery and the value in the corresponding relation table is larger than a preset error threshold value, correcting the corresponding relation table.

The charging and discharging process of the battery is divided into three stages: before, during and after charging and discharging. Since the open-circuit voltage measurement requires the battery to be left standing for a long time, the open-circuit voltage method is only suitable for estimating the SOC of the battery in a non-use state, i.e., at a stage before or after charging and discharging. The SOC of the two stages is obtained by looking up the correspondence table of SOC and OCV. During charging and discharging, the SOC cannot be directly obtained, but the SOC can be obtained by calculating the discharged or charged electric quantity value by an ampere-hour method and matching with the SOC before charging and discharging. After the charge and discharge are finished, the SOC after the charge and discharge can be calculated by an ampere-hour method. And simultaneously, after standing, looking up a table by an open-circuit voltage method to obtain a theoretical SOC. And if the difference value is larger than a preset error threshold value, updating a corresponding relation table of the SOC and the OCV.

A flow chart of the improved SOC estimation algorithm of the present invention is shown in fig. 2. The method specifically comprises the following steps:

For example, when the battery is discharged and the open-circuit voltage thereof can be measured before discharging, the SOC at this time can be known by looking up the corresponding relationship table of SOC and OCV, which is denoted as S1. When the battery starts to discharge, the consumed electric quantity of the current is recorded, and finally the total discharged electric quantity is recorded as Sc. Through calculation, the electric quantity at the last moment can be known, and S2 is S1-Sc. When the discharge is finished, the theoretical SOC is obtained by measuring open-circuit voltage and comparing the data in the table, and is marked as S3. Error value Sb is defined as the difference between S3 and S2, i.e., Sb — S3-S2. And when the Sb exceeds a preset error threshold, starting a correction process, and correcting data in a corresponding relation table of the SOC and the OCV.

When the algorithm is started, an initial SOC-OCV correspondence table needs to be established before step 11. In order to obtain such a table, taking into account space and computational power constraints, interpolation is chosen to obtain such a table. 20 points, each 5% SOC, can be selected to measure the value of the open circuit voltage. If it is necessary to be denser, 50 dots or more may be selected depending on the requirements and the available memory of the chip.

The correspondence table of the corrected SOC and OCV in step 16 can be completed by equations four and five.

e＝SOC_Table-SOC_Cal(formula four)

S_n(i+1)＝S_n(i) -F (n, e) (0. ltoreq. n. ltoreq.50) (equation five)

SOC in equation four_TableThe middle is the original SOC in the table, and the SOC_CalThen it is the calculated SOC and finally an e is used to represent the difference between the two. Formula V S_n(i) Representing the value, S, in the table currently updated for the ith time_n(i +1) is the value in the updated (i +1) th sub-table. F (n, e) is the correction factor, which is a function related to n and e. The invention adopts the expression of a linear function, namely:

f (n, e) ═ a × e × n (formula six)

Where a is an adjustable constant representing the correction rate, it can be seen from fig. 1 that the voltage correspondence at different segments of SOC is different, so the effect of aging is different at different SOCs. Therefore, the correction rate should not be the same in different segments, i.e. a should take different values in different segments. n is the number of points interpolated. The functional form of f (n) may be changed if desired, and the updated functional form may be modified depending on different battery characteristics.

In order to verify the validity and practicability of the corrected SOC estimation algorithm provided by the invention, whether a corresponding relation table of SOC and OCV obtained by calculation after the battery is charged and discharged for many times is consistent with a current real corresponding table of the battery or not needs to be verified, and the error is in what range. For this reason, the invention designs an experimental platform for verifying the validity of the SOC estimation algorithm, and the schematic block diagram of the platform is shown in fig. 3. The invention takes a 18650 lithium battery monomer with the rated capacity of 3400mAh and the rated voltage of 3.7V as a test object. In order to see obvious attenuation in the experiment, the number of charge and discharge cycles of the battery needs to be increased, and the cycle is repeated for 500 times in the experiment. Meanwhile, in order to avoid the influence of the temperature on the test result, the temperature is controlled to be 25 ℃ in the test. In order to reliably return the battery to the stationary state, the battery needs to be left stationary for 30 minutes or more when the open circuit voltage is measured. Meanwhile, in order to reduce the influence of the voltage drop of the internal resistance during discharging, the battery uses a low-speed discharging mode during discharging, so that the voltage curve can be directly used for estimating the open-circuit voltage curve.

The results of the test are shown in fig. 4. It can be seen from fig. 4 that there is a large difference between the two curves for the fresh cell and the aged cell. If the correction algorithm is not added, the relationship between the voltage and the capacity of a new battery is still used after the battery ages, which causes the error of the SOC estimation value to become larger and larger along with the continuous aging of the battery, and is very unfavorable for prolonging the service life of the battery. After the correction method is added, the calculated curve has quite good conformity with the curve of an aged battery, which shows that the algorithm in the invention can dynamically correct the corresponding relation between the voltage and the capacity along with the aging of the battery. This can greatly help reduce the error of battery aging on SOC estimation.

Second, equilibrium control strategy

As described above, the SOC estimation algorithm is one of the core techniques of the equalization strategy in the equalization technology, and after the modified SOC estimation algorithm is determined, the equalization strategy adopted in the present invention is described below. The design of the equalization strategy and the topology of the equalization circuit is the two most important key points in the equalization control system, the topology of the equalization circuit needs to be matched with a reasonable equalization strategy to exert a good equalization effect, and the equalization strategy and the topology of the equalization circuit are two parts which supplement each other in the equalization control system. The invention adopts the balancing circuit based on the bidirectional flyback transformer, and simultaneously designs a set of balancing strategy for the circuit.

According to the analysis and research, in order to obtain a good balancing effect and better achieve the design goal of prolonging the service life of the battery pack, the SOC is used as a variable of the balancing control, and a quantization standard is used in a balancing control system to start or stop a balancing function. To evaluate and identify the inconsistent state of the battery, the present invention analyzes the variance δ_soc ²Sum and difference r_socThe expressions of two different quantization indexes are consistent with those used in statistical mathematics, and are as follows:

r_socmax (soc (i)) -min (soc (i))), i ═ 1.. n (formula nine)

Analyzing from the aspect of statistics and mathematics, the dispersion degree of the SOC values among the single batteries in the battery pack is represented by variance, namely the smaller the variance is, the smaller the dispersion degree of the SOC of the single batteries in the battery pack relative to the average battery is, the smaller the difference of the SOC values in the battery pack is; conversely, if the variance is larger, it indicates that the higher the degree of dispersion of each unit cell from the average SOC value in the assembled battery is, the larger the difference in SOC value between the unit cells is. So if the variance is used as a quantification criterion for evaluating the consistency of the lithium battery pack, good results can be theoretically obtained. However, the calculation amount of the variance or the standard deviation is quite large, and considering that the embedded chip used in the equalization control system is often not strong in calculation performance, and the evaluation of the consistency of the battery pack needs to be made frequently or even in real time, the variance is not suitable as the quantification standard of the consistency evaluation of the system.

The range difference represents the difference between the maximum SOC and the minimum SOC of the battery pack, and when the difference is smaller, the difference between the maximum SOC and the minimum SOC of the battery pack is smaller, that is, the SOC of each single battery is distributed in a smaller range, which indicates that the uniformity of the battery pack is better. When the extreme value is larger, the difference between the maximum SOC and the minimum SOC of the battery pack is large, the SOC of the battery pack may be distributed in a wider range, and the consistency of the battery pack may be poor. Meanwhile, the SOC conditions of all the single batteries of the battery pack are not required to be considered during calculation, and only the maximum SOC value and the minimum SOC value are required to be found, so that the calculation amount is greatly reduced.

As can be seen from the above analysis, since the variance and the range of the SOC reflect the state of the uniformity of the battery pack, it is appropriate to use both the variance and the range as evaluation criteria for quantifying the uniformity of the battery pack. But simultaneously, considering the computing power of the balance control system, the use of the range can greatly reduce the computation of the system and accelerate the response speed of the system, so that the range is more suitable for the use in the invention compared with the variance.

In the SOC-based equalization strategy, the purpose of equalizing the battery pack is mainly realized by reducing the SOC difference among the batteries, and the SOC difference among the batteries is reduced by taking the SOC of each single lithium battery as a main control object and charging and discharging the single batteries. The equalization process used in the present invention is as follows: the SOC of all the cells is measured at the start of equalization, and one of the measured SOC is selected as a target SOC for equalization. But generally the mean value will be chosen

The use of the mean value as the target SOC for equalization can improve the efficiency of equalization and fully exhibit the advantages of charge/discharge equalization. When the SOC is used as an equalization control means, an equalization control band (dSOC) is arranged to prevent the fluctuation of the equalization, and 1% is adopted as a cut-off band to control the dSOC in the invention. Then for SOC higher than

The single lithium battery is subjected to discharge equalization, and the discharge is lower than the discharge equalization

The charging equalization is performed on the battery pack. This process can be illustrated by a flow chart, as shown in fig. 5. The method specifically comprises the following steps:

step 21, obtaining the SOC of each single lithium battery in the lithium battery pack, and specifically obtaining the SOC of each single lithium battery through the improved SOC estimation algorithm;

step 22, calculating the pole difference r of the battery pack_socThat is, the difference r between the maximum SOC and the minimum SOC in each lithium cell of the battery pack is calculated_soc；

Step 23, comparing the range r_socAnd the size of a preset range threshold value is obtained when the range r is equal to_socIf the difference is greater than the preset range threshold, go to step 24, if soThe above range of r_socWhen the difference is less than or equal to the preset range threshold, entering step 25;

The single lithium battery performs discharge equalization, wherein dSOC is an equalization control band, and 1% is used as a cut-off band to control dSOC in the invention;

and step 25, ending.

Some special nodes are noted in this process. When the battery is in a discharging state, the poor battery can enter into the discharging state in advance to reach a cut-off voltage, and the good battery can remain a part of electric quantity, so that the capacity of the battery pack cannot be fully used. In order to solve the problem and simultaneously protect the service life of each battery from being in an over-discharge state, the method for solving the problem is to charge the battery which is about to enter into the discharge cutoff state so that the SOC of the battery is consistent with the SOC of other batteries, regardless of whether the battery is in a cutoff band or not. In this way, the batteries in the entire battery pack can reach a state where the SOC is 0 at the same time, and the capacity of the battery pack can be fully utilized.

Similarly, when the battery is in a charging state, the poor battery can enter a full-charge state in advance and reach a cut-off voltage, so that the capacity of the battery pack is wasted, and therefore in order to prevent the battery from entering the cut-off voltage in advance, the countermeasure adopted in the invention is to start an equalizing circuit to enable the SOC of the battery to fluctuate near the SOC of the battery when the battery is detected to be about to enter the charge cut-off state, so that all the batteries can finally reach the state of SOC 1 at the same time.

Three, equalizing circuit

The invention selects a bidirectional flyback transformer equalizing circuit. The main reasons are that the equalizing circuit has large current and high equalizing speed, and can realize bidirectional equalization. A schematic diagram of a typical bidirectional flyback transformer equalization circuit is illustrated in fig. 6.

The balance of the flyback transformer is essentially that energy is transmitted in two directions among the single batteries through mutual conversion of electric energy and magnetic energy. When a certain single battery of the battery pack has more energy compared with other batteries, the flyback transformer is used as an energy transfer medium to transfer redundant energy of the battery to the whole battery pack; when the energy of a certain battery of the battery pack is less than that of other batteries, the flyback transformer is also used as an energy transfer medium, and the energy of the whole battery pack is input to the single battery, so that the battery is prevented from being damaged due to the fact that the energy of the battery is too low. This structure has a two-directional equalization pattern, as follows.

(1) Cell to battery equalization (top equalization)

After the balance control system detects the SOC of all the single batteries, for the single batteries with higher SOC and needing balance, the battery is opened corresponding to the secondary variable switch, all other switches including the primary switch are disconnected, current passes through a secondary winding of the flyback transformer, and at the moment, electric energy is stored in the secondary winding in a magnetic energy mode; after the SOC in the battery is reduced to a required value, the secondary variable switch is disconnected, the primary switch is switched on, so that energy can be transferred from the secondary winding to the primary winding, the magnetic energy is converted into electric energy and transferred to the whole battery pack, the battery with the highest SOC is controlled, and meanwhile, redundant energy is transferred to other batteries in the battery pack.

(2) Battery to cell equalization (bottom equalization)

After the balance control system detects the SOC of all the single batteries, for the single batteries with lower SOC and needing balance, the corresponding primary switch is opened, all the secondary switches are disconnected, current can pass through the primary winding of the flyback transformer, and at the moment, electric energy is stored into the primary winding in a magnetic energy mode on the primary side; after enough electric energy is charged, the primary switch is disconnected, the secondary switch corresponding to the lowest SOC is opened, the secondary winding is conducted, so that the energy is transferred from the primary winding to the secondary winding, the magnetic energy is converted back to the electric energy to be charged into the battery, the SOC of the single battery can be raised to some extent in the process, and the whole SOC of the battery pack is recovered to a more consistent value.

The circuit and magnetic circuit analysis of the flyback transformer can be equal to the equivalent circuit model shown in fig. 7 and the equivalent magnetic circuit diagram shown in fig. 8.

When the MOS switch tube is turned on, the magnetic resistance R in the core is seen from the left side of FIG. 8_MMagnetic resistance R with leakage flux_SParallel connection with magnetic equilibrium of N₁i₁At R_MMagnetic flux phi generated thereby_MThe current i flows through the secondary winding after the MOS tube is disconnected after passing through the iron core₂Generating a magnetic flux phi_M. In the right-hand part of FIG. 8, it can be seen that N is again present at the instant of conversion₁i₁＝N₂i₂. If N is present₁：N₂1: 1, the equivalent circuit model in fig. 7 is converted from the equivalent magnetic circuit model of the flyback transformer, and the leakage inductance L of the primary and the secondary in the figure_SThe sizes are the same. The iron core of the flyback transformer has a smaller inductance value due to the existence of the air gap, and the leakage inductance L in the flyback transformer_SCannot be ignored. After MOS tube is turned on, i₀Flows through the primary L_SAnd L_MAt which time no current flows through the secondary. L in leakage inductance at input side when MOS tube is turned off_SAll energy of (2) and L_MIs dissipated in the absorption network (clamping circuit: having a voltage stress reducing switch tube in flyback transformer), and L_MA part of the remaining energy and the secondary L_SAll of the energy in (b) is output through the secondary.

In fig. 6, when the MOS transistor S is turned on, the input voltage U_iLoaded to two ends of primary winding of transformer, and the diode D can be used for knowing that the secondary winding generates lower positive and upper negative induced electromotive force according to Lenz's law₂And therefore, current cannot flow through the secondary circuit. The primary winding of the transformer now behaves as an inductor. Suppose it is firstThe inductance of the secondary winding is Lp, the inductance of the secondary winding is Ls, and the current flowing through the primary winding during the conduction period of the MOS transistor is:

at t ═ t_onThe primary winding current reaches a maximum value:

when the MOS tube is closed, the voltage polarity of the secondary winding is converted into positive and negative according to Lenz law, and at the moment, the diode D₂Is conducted, the magnetic energy stored in the transformer is converted into electric energy, and current flows through the secondary winding, wherein the current is as follows:

when t is equal to t_offWhen the current of the secondary winding reaches a minimum value I_smin. When I is_sminWhen the voltage is equal to 0, the energy in the magnetic field stored during the conduction period of the MOS tube is completely released, and the process is called an intermittent operation mode of the flyback transformer; when I is_smin>At 0, the energy stored in the magnetic field during the conduction of the MOS transistor is not completely released, and this process is called a continuous operation mode of the flyback transformer. The current waveforms for the two modes of operation are as shown in fig. 9.

The magnetic flux in the magnetic core of the transformer must return to the original position when each period is finished, the principle is called a magnetic flux reset principle, residual magnetism exists in a continuous working mode, the magnetic flux can be theoretically kept to be recovered to an initial value when each period is finished, but the magnetic property has iron loss, and copper loss also exists in a coil winding, so that the temperature is increased in the using process, the initial value magnetic flux is deviated, the reset cannot be realized, the magnetic flux changes to enter a nonlinear area, the inductance is reduced, the current value is increased, the magnetic core is easy to reach a saturated state, the transformer cannot normally work, the circuit is greatly unsafe, meanwhile, the volume of the flyback transformer in the continuous working mode is large, the volume of the transformer in the intermittent working mode is small, and large primary and secondary currents are allowed. Therefore, the flyback transformer adopted by the invention works in an intermittent working mode.

According to the invention, the primary side of the flyback transformer is connected with the battery pack formed by connecting 6 single lithium iron phosphate batteries in series, each secondary side is connected with each single battery, and the rated voltage of each single battery is 3.6V, so that the working voltage of the primary side of the transformer is about 18-24V, the working voltage range of the secondary side is 4.2-3.0, the working efficiency of the transformer is designed to be 80%, and the working frequency is 10 KHz.

(1) Maximum duty cycle

In general, the output efficiency of the transformer increases with an increase in the duty ratio, but when the duty ratio exceeds 50%, the circuit oscillates. Although this phenomenon can be improved by adding a harmonic compensation module to the circuit, if no suitable component is selected and reasonably arranged, the harmonic compensation module in the circuit may not function, so that the operating state of the circuit may still be unstable when the duty ratio is greater than 50%. Therefore, the maximum duty cycle of the transformer is generally between 40% and 50%, and the maximum duty cycle of the invention is finally selected to be 45%. The actual duty cycle used is also obtained by simulation.

(2) Turns ratio

The turn ratio N of the transformer is the ratio of the number of turns Np of the primary coil of the transformer to the number of turns Ns of the secondary coil of the transformer.

The number of primary and secondary turns of the transformer cannot be directly known at the beginning of design. In the invention, the transformation ratio N of the transformer is directly determined according to the reflected voltage of the transformer shown in a formula thirteen:

in the above formula, the reflected voltage V_ORIndicating that an opposite voltage, V, is developed across the primary winding when current flows across the secondary winding_oRepresenting the output voltage, V_fIs the voltage drop of the MOS transistor. V_ORThe calculation is as follows.

N is 5.2, and N is 5, which can be calculated by formula fourteen and formula fifteen.

Experiments prove that when the duty ratio is more than 40%, the equalizing current is too large and does not meet the hardware condition, and when the duty ratio is 20%, the current of the secondary side is less than 5A and does not meet the aim of the invention, so that the duty ratio is between 25% and 35% when top equalization is carried out. Meanwhile, when the duty ratio is more than 35%, the transformer operates in a continuous mode, so it is not suitable, and therefore, the bottom equalization is that the duty ratio should be selected between 20% and 30%. In sum, selecting a duty cycle of 25% or 30% is a suitable range for both top and bottom equalization.

Four, balance control system

The invention adopts the K64 chip newly promoted by Emizpu as the control chip of the MCU terminal, and the latest embedded chip with strong performance provides good performance guarantee for the expansion of the SOC estimation algorithm and the equalization strategy. Meanwhile, a voltage measuring module, a current measuring module, a temperature measuring module and a balancing module are designed by taking the chip as a core. The balancing module uses an LTC3300-1 chip specially designed for the active balancing circuit of the bidirectional transformer, and the circuit complexity and cost control of the balancing module are further improved by virtue of the advantages of an integrated chip.

The lithium battery pack is usually required to be combined with hundreds of single lithium ion batteries when in use, the whole battery pack needs to be divided into a plurality of small battery packs to be respectively arranged in different battery boxes in consideration of the distribution flexibility of the battery pack, and meanwhile, in order to expand the capacity, a new battery can be conveniently expanded into the battery pack in the follow-up process. The overall design block diagram is shown in fig. 10.

The whole system is divided into an upper stage and a lower stage in design. The secondary MCU realizes the function of monitoring the battery group, and comprises: collecting data such as voltage, current, temperature and the like of each battery in the battery pack; feeding back the acquired data to a main control MCU; calculating the SOC of each single battery; judging whether balancing is needed or not according to the SOC; and when balancing is needed, the balancing modules of the group are controlled to balance the batteries needing balancing. The master control MCU is responsible for collecting data fed back by the lower MCU and transmitting the data to the upper PC at the same time so as to conveniently acquire and debug the data of the whole battery pack. The master control MCU can also forward the command sent by the PC to the corresponding battery pack. The execution flow of the master MCU is shown in fig. 11, and the execution flow of the secondary MCU is shown in fig. 12.

The use of such a hierarchical and modular design has the following advantages:

(1) and the expandability of the system is improved. The lithium battery pack needs to be combined with batteries of different quantities when in use, for example, in order to meet the requirement of sales of automobile manufacturers, the price of the same automobile type is often graded according to different endurance mileage, different endurance mileage needs to integrate different quantities of lithium batteries in the battery pack, and if different balance control systems are adopted in the same automobile type, the early research and development cost is increased, and more energy and cost are required to be invested in the maintenance of different balance control systems. Therefore, the system expansibility can be enhanced by adopting a hierarchical design.

(2) The real-time performance of the system is improved. The MCU in each small battery pack only needs to manage the battery in the battery pack, so that the calculation amount of each subordinate MCU is greatly reduced, and the real-time performance of the balance control system is enhanced.

(3) And the compatibility of the system is improved. If small battery packs of different manufacturers are needed to be adopted in the same large battery pack, only the equalization modules of the small battery pack need to be debugged or designed again, and large-scale modification of the whole system is avoided.

(4) The reliability of the balance control system is increased. The modular design can avoid the paralysis of the whole system, and when the balancing module in a certain small battery pack breaks down, other small battery packs can still work normally. Thus, the batteries in other small battery packs are protected, and meanwhile, more serious accidents can be avoided.

In terms of specific hardware circuits, the invention adopts the following design mode:

power supply circuit

The power supply module is used for providing a power supply for normal operation for the secondary MCU, and a circuit schematic diagram of the power supply module designed by the invention is shown in FIG. 13. The normal working voltage of the main control chip K64 of the secondary MCU is between 1.71V and 3.6V, the power supply of the chip is ensured to be about 3.3V in normal use, and the equalizing system of the invention also needs to use 5V voltage. Because the secondary MCU and the balancing module are arranged in the small battery pack, the small battery pack can directly take electricity. A small battery pack is usually composed of 6 cells, each of which has an operating voltage of 3.0V to 4.2V, and thus the terminal voltage of the small battery pack is 18V to 25.2V. The system adopts an LM2576 voltage conversion chip of NI company. The LM2576 chip can accept voltage input of 7-40V, output voltage is 5V, the load of driving 3A, linearity and load regulation ability are very strong, meanwhile, a frequency compensator and a fixed frequency oscillator are integrated in the LM2576, and good voltage output can be achieved by few external components. Fig. 13 also includes a circuit for converting 5V to 3.3V, since the normal operating voltage of K64 is 3.3V, it is necessary to use an ASM1117-3.3V regulated power supply module to convert 5V to 3.3V for output to K64. Two power indicator lamps are added in the circuit to indicate whether the two power supplies work normally or not.

Voltage acquisition circuit

The accurate voltage acquisition circuit is not only related to normal use and monitoring of the battery pack, but also is necessary to guarantee accurate balance judgment of the battery. Therefore, when lithium batteries are connected in series to form a battery pack for use, accurate voltage measurement needs to be performed on the voltage of each battery in the battery pack.

The cell voltage acquisition modes commonly used for the series lithium battery pack comprise a common mode measurement method and a differential mode measurement method. The common mode measurement is to measure the voltage of the battery pack by using a precise resistance equal proportion attenuation mode relative to the same reference level, and then the voltage of each single battery is obtained after subtraction in sequence. The method has the advantages of simple circuit, but the measurement accuracy of the method depends on the determination of the divider resistance, and is easy to generate serious accumulative error due to the influence of temperature, so the method is only suitable for the occasions with few series batteries and low requirement on the measurement accuracy. It is not suitable for situations that rely on voltage to calculate an accurate SOC.

The invention selects a differential mode measurement method, and the method sequentially gates each battery to carry out measurement by a certain method. The method is suitable for occasions with a large number of batteries connected in series and high precision requirements. The mode of using the differential mode measurement has the advantage that when the acquisition of a certain path fails, the normal work of other channels can not be influenced. In addition, compared with the acquisition mode using an integrated chip, the channel-dividing measurement mode only needs to repair the corresponding fault channel when a fault occurs, but does not need to replace the whole chip, and is beneficial to reducing the later maintenance cost.

(1) Single battery selection circuit

The mode of differential mode measurement needs to be capable of gating each single battery, and the MOSFET PS7241-2A is adopted as a single battery gating switch for voltage collection in the invention. The PS7241 series device consists of a light emitting diode (input side) and a normally-open contact MOS tube (output side). Each PS7241-2A comprises two mutually independent gating switches, and the device is characterized by low working current, high voltage resistance and very high reaction speed. A schematic diagram of the cell gating switching circuit is shown in fig. 14.

Resistors R1-R4 in FIG. 14 are current-limiting voltage-dividing resistors for limiting the magnitude of current in the circuit during measurement. When the voltage of a certain single battery needs to be measured, the levels of the 1 pin and the 3 pin on the corresponding PS7241 only need to be pulled high through the K64, and the voltages at the two ends of the corresponding single battery are output from the 6 pin and the 8 pin of the PS 7241. Taking the circuit in fig. 14 as an example, when the voltage of bat1 needs to be measured, pins 1 and 3 in PS1 are pulled high, and the voltage output from

pins

6 and 8 is the voltage across bat1, where pin 6 is the battery anode and pin 8 is the battery cathode. When the battery bat2 needs to be measured, the pin 3 in the PS1 and the pin 1 in the PS2 are gated, and the voltage output from the pin 6 in the PS1 and the pin 8 in the PS2 is the voltage across the bat2, wherein the pin 8 in the PS2 is the positive pole of the battery bat2, and the pin 6 in the PS1 is the negative pole of the battery bat 1.

(2) Voltage reversing circuit

In the above analysis it has been indicated that the voltages on the corresponding pins for CAP _1 and CAP _2 are opposite when measuring bat1 and bat2, and that virtually all odd-numbered cells and even-numbered cells are opposite when measuring. Therefore, the invention adopts two PS7241 blocks and designs a voltage reversal circuit as shown in FIG. 15. The circuit can ensure that the cells with the odd number and the cells with the even number are measured to output the same voltage direction, thereby facilitating the subsequent measurement of the voltage value of each single cell by the AD circuit. The reason for not using a relay is because the relay does not react as fast as PS7241, and the voltage drop caused by the relay also has an effect on the accuracy of the result.

When odd-numbered batteries are tested, the CAP2 is positive, the CAP1 is negative, and it is necessary to control K _ AD _1 to be low and K _ AD _2 to be high, i.e. pins 8 of PS5 and PS6 are turned on and pin 6 is turned off. When even-numbered batteries are tested, the CAP1 is positive, the CAP2 is negative, the K _ AD _1 is controlled to be at a high level, the K _ AD _2 is controlled to be at a low level, and the pins 6 of the PS5 and the PS6 are conducted while the pin 8 is not conducted. Through the circuit, the AD _ P can be always connected to the positive pole of the battery, and the AD _ N can be always connected to the negative pole of the battery.

(3) Voltage-frequency conversion circuit

The voltage-frequency conversion circuit (VFC) can convert an input voltage signal into a frequency signal and output the frequency signal, wherein the frequency signal output by the VFC circuit is in linear proportion to the input voltage signal, that is, the higher the voltage is, the faster the output frequency is. VFC circuits are widely used in various circuits, including signal frequency modulation, phase modulation, AD conversion circuits, and the like. The VFC circuit has the advantages of strong anti-interference capability, convenient isolation, stable performance, high sensitivity and small nonlinear error. Meanwhile, when the analog signal is digitized, the resolution and precision of the VFC circuit are higher than those of the AD conversion circuit, and the VFC circuit is usually lower in cost with the same precision.

The voltage-frequency conversion circuit designed by the invention is composed of two parts. The first part is an operational amplifier circuit, and the core of the part is an OP07C operational amplifier. OP07C has the characteristics of low noise and non-chopper stabilization. For most usage scenarios, OP07C does not require external original devices to offset nulling and frequency calibration. In addition, OP07C has the characteristics of low bias current, high open-loop gain and wide working temperature range. In order to ensure the stability and the anti-interference capability of the VFC circuit, the amplification factor of the operational amplifier is set to be 2 times.

The second part of the voltage-frequency conversion circuit is a VFC circuit, and the core of the part is an AD7740 chip. AD7740 is a low-cost and extremely small-size voltage frequency conversion chip. The chip can work between 3.0V and 3.6V or between 4.75V and 5.25V, and the working current can reach 0.9mA at the lowest. AD7740 supports a very wide operating temperature range, relies on few external original devices, and has accurate voltage conversion frequency. A2.5V reference is integrated in the chip, and the VDD which is input externally is also used as a reference voltage. The chip also has a synchronous clock input pin-CLKIN, which can support the frequency input of 1MHz at most. In the invention, the output clock of K64 is used as the synchronous clock of AD7740, thus reducing unnecessary original devices and reducing the complexity of the circuit.

When the analog voltage varies from 0V to VREF, the signal output frequency of AD7740 varies linearly between 0.1 and 0.9 times FCLKIN. The conversion formula of the voltage and the frequency is as follows:

finally, a voltage-frequency conversion circuit is provided as shown in fig. 16.

It is noted that the OP07C requires positive and negative voltages for power supply, so a reverse polarity circuit is also required to convert the +5V voltage to-5V for providing negative power to the OP 07C. The MAX660 charge pump reverse polarity switch integrated voltage regulator is used for realizing the function, and a circuit diagram is shown in figure 17.

Current acquisition circuit

Accurate current measurement is an indispensable condition for SOC estimation using the ampere-hour method, and the current measurement is performed using a hall current sensor in the present invention. The principle of the Hall current sensor is that when a primary current flows through a long lead, a magnetic field is generated around the lead, the magnitude of the magnetic field is in direct proportion to the magnitude of the current, the generated magnetic field is gathered in a magnetic ring, measurement is carried out through a Hall element in an air gap of the magnetic ring, the Hall element is amplified and output, and the output voltage of the Hall element can reflect the magnitude of the primary current.

The Hall current sensor has the advantages that the measuring range is wide, the current and the voltage of any waveform can be measured, and even the peak current and the voltage signals of the transient state can be faithfully reflected. The Hall current sensor has extremely high corresponding speed and can reach the us-level reaction speed. Meanwhile, the precision of the Hall current sensor is very high, the measurement precision is better than 1%, the measurement thread degree is good, the Hall current sensor can work for a long time without faults, and continuous work for hours can be generally guaranteed. In addition, the Hall element can be small in size and convenient to use.

The measuring range of the selected Hall current sensor can reach +/-100A, and the working voltage is 5V. When the current in the circuit is zero, the voltage output is 2.5V; when the current in the single circuit is-100A, the voltage output is 0V; the output is 5V when the current in the circuit is 100A. For the sampling circuit using OP07C, the output of the hall current sensor used in the present invention is in the normal operating range.

Temperature acquisition circuit

The temperature acquisition circuit is used for gathering the temperature value of every little group battery, and the temperature has great influence to the operation of lithium cell, and real-time temperature value can guarantee the accuracy to the estimation of lithium cell SOC on the one hand, and on the other hand can be too high temperature also be indispensable to the operation safety of system.

The temperature acquisition circuit designed by the NTC-based thermistor NTC10KB3950K is adopted to realize the temperature acquisition of the small battery pack, and the circuit has the characteristics of high measurement precision, simple structure and good stability. The NTC10KB3950K has an accuracy of 1%, and the resistance at 0 degree is 32.5K, corresponding to a voltage of 0.29V, 85 degrees is 1.063K, corresponding to a voltage of 3.26V. The calculation between resistance and voltage is given by:

then, according to the NTC10K-3950 voltage and temperature corresponding relation table, the temperature value in the small battery pack can be checked.

Equalizing circuit

The equalization control circuit is one of the cores in the present invention. The invention designs a transformer equalizing circuit based on an LTC3300-1 chip. The LTC3300-1 is a controller IC with fault protection, and is suitable for bidirectional active equalization based on transformer of battery pack composed of multiple batteries. The device integrates all the required gate drive circuitry, high precision battery sensing, fault detection circuitry and a built-in timer watchdog. Each LTC3300-1 can utilize a 36V input common mode voltage to equalize up to 6 series connected lithium batteries. The charge of any selected battery can be transferred back and forth between itself and 12 or even more adjacent batteries in an efficient manner. The SPI interface of LTC3300-1 can accomplish the series connection with a plurality of LTC3300-1 devices under the condition of not adopting optical coupling isolation, thereby realizing the charge balance of each battery in the long series connection battery. The series LTCs 3300-1 can operate independently at the same time, thus allowing equal management of all cells in the battery pack independently at the same time.

Each LTC3300-1 corresponding equalizer circuit operates independently. The primary side of the transformer is connected with each single battery through an MOS tube, and the secondary side of the transformer is connected with the whole battery pack through an MOS tube. LTC3300-1 supports two transformer equalization modes. One is that each transformer has its own primary and secondary side; the other is that all transformers have their own primary side but share a secondary side that is connected to the battery pack. A schematic diagram of the connection of two transformers to the LTC3300-1 is shown in FIG. 18.

In fig. 18(a), the balancing transformers for each cell have independent primary and secondary sides, the primary side is connected to the cell via a MOS transistor, and the secondary side is connected to the battery pack via a MOS transistor; in fig. 18(b), each balancing transformer has a single primary side, and the cells are connected through MOS transistors, and the secondary side is common to all transformers. In view of the modular design goals of the positive system, and the subsequent ease of maintenance, the present invention takes all of the forms of FIG. 18 (a).

The LTC3300-1 can be accessed at most to balance 6 batteries, and FIG. 19 shows the connection of the channel 2 therein, and the solutions of other paths are similar to the path. In the figure, a pin C2 is linked to the positive pole of a bat2, I2P and I2S are respectively used for measuring the current of the primary side and the secondary side of the transformer, and G2P and G2S are used for controlling the on-off of MOS tubes of the primary side and the secondary side. BAT +, BAT-linked to the positive and negative poles of the whole battery.

It is to be understood that the embodiments described herein may be implemented by hardware, software, firmware, middleware, microcode, or any combination thereof. For a hardware implementation, the processing units may be implemented within one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. When the embodiments are implemented in software, firmware, middleware or microcode, program code or code segments, they can be stored in a machine-readable medium, such as a storage component.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.

Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.

Claims

1. A lithium battery pack balance control method for dynamically correcting SOC is characterized by comprising the following steps:

step 22, calculating the pole difference r of the battery pack_soc；

Step 23, comparing the range r_socAnd the size of a preset range threshold value is obtained when the range r is equal to_socIf the difference is larger than the preset range threshold, the step 24 is entered, and when the range r is larger than the preset range threshold_socWhen the difference is less than or equal to the preset range threshold, entering step 25;

As target SOC for equalization, lower than SOC

The single lithium battery is charged evenly, and the SOC is higher than that of the single lithium battery

step 25, ending;

the step 21 specifically includes:

2. The lithium battery pack balance control method for dynamically correcting SOC according to claim 1, wherein: before step 11, an initial SOC/OCV correspondence table is created by interpolation.

3. The lithium battery pack balance control method for dynamically correcting SOC according to claim 1, wherein: step 16 specifically comprises:

S_n(i+1)＝S_n(i) -F (n, e) (0. ltoreq. n. ltoreq.50) (equation five)

f (n, e) ═ a × e × n (formula six)

4. The lithium battery pack balance control method for dynamically correcting SOC according to claim 3, wherein: the value of a is different in different sections of the SOC.

5. The lithium battery pack balance control method for dynamically correcting SOC according to claim 1, wherein:

in step 24, when the battery is in discharge equalization, the battery which is about to enter into discharge cutoff is charged, so that the SOC of the battery is consistent with the SOC of other batteries, no matter whether the battery is in a cutoff band or not; when the batteries are subjected to charge equalization, for the battery which is about to enter the charge cutoff, the equalization circuit is started to enable the SOC of the battery to fluctuate nearby, so that all the batteries can finally reach the state that the SOC is 1 at the same time.

6. A lithium battery pack balancing control system for dynamically modifying SOC for implementing the method according to any of claims 1-5, the control system comprising a superior system and an inferior system, wherein the superior system comprises a PC and a master MCU, the inferior system comprises a plurality of sub-inferior systems, each sub-inferior system is used for balancing control of a small battery pack, each sub-inferior system comprises a secondary MCU, a balancing module, and the method comprises the steps of:

7. The system of claim 6, wherein the system further comprises: the balancing module adopts a bidirectional flyback transformer.

8. The system of claim 7, wherein the system further comprises: the bidirectional flyback transformer adopts an LTC3300-1 chip.

9. The system of claim 7, wherein the system further comprises:

for a single battery with higher SOC and needing to be balanced, the battery is opened corresponding to a secondary variable switch, all other switches including a primary switch are disconnected, current passes through a secondary winding of the bidirectional flyback transformer, and at the moment, electric energy is stored in the secondary winding in a magnetic energy mode; after the SOC in the battery is reduced to meet the requirement, the secondary variable switch is disconnected, the primary switch is switched on, energy is transferred from the secondary winding to the primary winding, and magnetic energy is converted into electric energy, so that redundant energy is transferred into other batteries in the battery pack;

for the single batteries with lower SOC and needing to be balanced, the switches corresponding to the primary side are turned on, all secondary switches are turned off, current passes through the primary winding of the bidirectional flyback transformer, and at the moment, electric energy is stored into the primary winding in a magnetic energy mode on the primary side; after enough electric energy is charged, the primary switch is disconnected, the secondary switch corresponding to the lowest SOC is opened, the secondary winding is conducted, energy is transferred from the primary winding to the secondary winding, the magnetic energy is converted back to the electric energy to be charged into the battery, the SOC of the single battery rises back, and the whole SOC of the battery pack returns to the same value.