CN107839500A - A kind of dynamic corrections SOC lithium battery group balance control method and system - Google Patents

Abstract

The present invention provides a lithium battery pack balance control method and system for dynamically correcting the SOC, comprising: obtaining the SOC of each single lithium battery in the lithium battery pack; calculating the range r _soc of the battery pack; comparing the range r _soc The size of the preset extreme difference threshold; select the average value of the SOC of all single lithium batteries As the target SOC for equalization, for SOC below The single lithium battery is charged and balanced, and the SOC is higher than The single lithium battery is discharged and balanced, and dSOC is the balanced control band. The invention has the advantages of high modularization degree, fast equalization speed, improved battery use efficiency and prolonged battery service life.

Description

A lithium battery pack balancing control method and system for dynamically correcting SOC

technical field

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

Background technique

Affected by environmental pollution issues and policies and regulations, new energy vehicles represented by hybrid and pure electric vehicles have received more and more attention. Due to its high energy density, low self-discharge rate, long life and no pollution, lithium batteries have gradually replaced lead-acid batteries as the main power battery energy in the field of electric vehicles.

When using lithium batteries in electric vehicles, in order to meet the requirements for load capacity and battery life, lithium battery packs must meet certain capacity and voltage requirements. For this reason, single lithium batteries are often connected in parallel to form battery cells to solve the problem of insufficient capacity of a single battery; at the same time, a higher voltage is obtained by connecting individual lithium battery cells in series to form a battery pack. In the lithium battery pack used in series, it is often affected by the inconsistency between individual batteries or battery cells. The inconsistency of lithium batteries can be defined as the inconsistency of important parameters such as voltage, capacity, internal resistance and self-discharge rate among single lithium batteries of the same specification and type. The existence of inconsistency problems in the battery pack will limit its available power, especially at the end of high-current discharge, the battery voltage with large internal resistance drops too fast, which will cause damage to the battery pack, so it is necessary to adjust the discharge current of the battery pack limit, thereby limiting its output power, resulting in a decrease in available power. At the same time, with the increase of battery charging and discharging times, long-term high-current discharge, the aging degree of each single battery is different, resulting in the inconsistency of each battery.

In order to deal with the inconsistency in lithium battery packs, equalization technology is usually integrated in the battery management system BMS. Equalization technology generally refers to the capacity utilization rate, output power and service life of the battery pack to avoid or reduce the inconsistency of the battery pack. The introduction of specialized technical means due to the adverse effects of this aspect. At present, equalization technology has become a key technology in the battery management system BMS. Efficient equalization measures can improve the effective use capacity of the entire battery and prolong the service life of the battery. The current research on equalization technology is mainly carried out from two aspects: equalization control strategy and equalization circuit topology design. The research on the balance strategy of battery packs focuses on establishing the evaluation index of the inconsistency of individual cells in the battery pack, and based on this, an effective balance control method is proposed; while the topology design of the balance circuit focuses on high efficiency, simple control structure, Design and improvement of relatively low-cost equalization circuit structures.

In terms of equalization strategy, there have been in-depth studies on the use of voltage as the balance variable in the prior art, and the use of battery state of charge (State of Charge, SOC) as the balance variable is still a research hotspot, and SOC is used as the balance variable. Variables can achieve a better balance effect, and the system is easy to control, which can better reflect the real state of the battery pack. However, there are technical problems in the prior art that the accuracy of SOC estimation is not high and not accurate in real time.

In terms of equalization circuit topology, passive equalization technology has been applied in actual products due to its mature technology and simple structure. However, passive equalization is not suitable for pure electric vehicles that pursue energy utilization. As for the active equalization technology, there are still the following problems:

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

(2) The balance based on external voltage has been extensively studied in the existing commonly used equalization technology, but due to the difference in the capacity of the single battery, the external characteristics of the charging and discharging voltage of each single battery are inconsistent, especially in the single battery In the later stage of charging, the voltage of the single battery rises rapidly, which makes the balance criterion unstable when using the external voltage of the battery as the criterion of the consistency of the battery pack. At the same time, the study also found that this method has no obvious effect on increasing the available capacity of the battery pack before and after equalization.

(3) The practicability needs to be improved, the circuit design is relatively complex and the integration is not enough, and the modular expansion cannot be conveniently carried out with the increase of the number of battery cells in series in the battery pack, etc.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned deficiencies in the prior art, and provide a lithium battery pack balance control method and system for dynamically correcting SOC, which realizes the balance control of lithium-ion battery packs in series, with a high degree of modularization and balanced control. Fast speed, improved battery efficiency and extended battery life.

The lithium battery pack equalization control method of dynamically correcting SOC that the present invention proposes is characterized in that described method comprises the steps:

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

Step 22, calculate the range r _soc of the battery pack;

Step 23, comparing the size of the range r _soc with the preset range threshold, when the range r _soc is greater than the preset range threshold, enter step 24, when the range r _soc is less than or equal to the preset range threshold When the preset extreme difference threshold is mentioned above, go to step 25;

Step 24, select the average value of the SOC of all single lithium batteries As the target SOC for equalization, for SOC below The single lithium battery is charged and balanced, and the SOC is higher than The single lithium battery is discharged and balanced, and dSOC is the balanced control band;

Step 25, end.

Preferably, step 21 specifically includes:

Step 11, judge whether the battery is in working condition, if yes, go to step 12, if not, go to step 17;

Step 12, use formula 2 to calculate the SOC _i of the current state, wherein, SOC _i is the SOC of the current state of the battery, SOC ₀ is the initial SOC when the battery starts to work, C _N is the rated capacity of the battery, I is the battery current, η For charging and discharging efficiency, when charging, η is a negative number, and when discharging, η is a positive number;

Step 13, judge whether the battery is in working condition, if yes, then return to step 12, if not, then enter step 14;

Step 14, measuring the open circuit voltage OCV ₁ , looking up the correspondence table between SOC and OCV, and obtaining the SOC _{1 corresponding to the open circuit voltage OCV 1 ;}

Step 15, calculate the difference e between SOC _i and SOC ₁ , when the absolute value of e is greater than the preset error threshold, enter step 16, when the absolute value of e is less than or equal to the preset error threshold, output SOC _i ; enter Step 11;

Step 16, calculate the corrected SOC, output the corrected SOC correction, and update the correspondence table between SOC and OCV at the same time; enter step 11;

Step 17, measure the open circuit voltage OCV ₂ , look up the correspondence table between SOC and OCV, obtain the SOC ₂ corresponding to the open circuit voltage OCV ₂ , and output the SOC ₂ ; go to step 11.

Preferably, before step 11, an interpolation method is used to establish an initial correspondence table between SOC and OCV.

Preferably, step 16 specifically includes:

Use Formula 5 to calculate the corrected S _n (i+1), output S _n (i+1), and update the corresponding S _n (i) in the correspondence table between SOC and OCV as S _n (i+1);

S _n (i+1)=S _n (i)-F(n, e) (0≤n≤50) (Formula 5)

Among them, S _n (i) represents the value in the table after the current i-th update, S _n (i+1) represents the value in the table after the i+1 update, F (n, e) is a correction factor, the The coefficient is a function related to n and e, which is expressed by formula six;

F(n, e)=a*e*n (Formula 6)

Among them, a is an adjustable constant indicating the correction rate, n is the number of interpolation points, and e is the difference between SOC _i and SOC ₁ .

Preferably, the values of a are different in different segments of the SOC.

Preferably, in step 24, when the battery is performing discharge balance, the battery that is about to enter the discharge cut-off is charged to keep its SOC consistent with the SOC of other batteries, regardless of whether the battery is in the cut-off band; When , for the battery that is about to enter the charge cut-off, start the equalization circuit to make the SOC of the battery fluctuate around, so that all the batteries can reach the state of SOC=1 at the same time.

The balance control system of the lithium battery pack that realizes the dynamic correction SOC of the above method proposed by the present invention, the control system is respectively a superior system and a subordinate system, wherein the superior system includes a PC and a main control MCU, and the subordinate system includes a plurality of sub-subordinate systems, each sub-subordinate system The system is used for balancing control of a small battery pack, and each sub-subsystem includes a secondary MCU and a balancing module, and is characterized in that:

The main control MCU is used to collect the data fed back by each secondary MCU in the lower-level system, transmit the data to the upper-level PC, and forward the commands sent by the PC to the corresponding secondary MCU;

PC, used to receive data sent by the main control MCU, and to send commands to the main control MCU;

The secondary MCU is used to collect the voltage, current and temperature data of each battery in the small battery pack; feed back the collected data to the main control MCU; calculate the SOC of each single battery; judge whether it is necessary to perform Balance; when balance is required, the balance module of the small battery pack is controlled to balance the batteries that need to be balanced;

The balancing module is used to balance the batteries that need to be balanced according to the control of the secondary MCU in the small battery pack.

Preferably, the equalization module adopts a bidirectional flyback transformer.

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

Preferably, for a single battery with a relatively high SOC that needs to be balanced, the corresponding secondary transformer switch of the battery is turned on, and all other switches including the primary switch are turned off, and current flows through the secondary winding of the bidirectional flyback transformer. When the electric energy is stored in the secondary winding in the form of magnetic energy; after the SOC in the battery drops to meet the requirements, the secondary transformer switch is turned off, the primary switch is turned on, the energy is transferred from the secondary winding to the primary winding, and the magnetic energy is converted into Electric energy, so that the excess energy is transferred to other batteries in the battery pack; for a single battery with a low SOC that needs to be balanced, the corresponding primary switch is turned on, all secondary switches are disconnected, and the primary winding of the bidirectional flyback transformer There is current passing through the winding, at this time, the electric energy is stored in the primary winding in the form of magnetic energy on the primary side; when enough electric energy is charged, the primary switch is turned off, the secondary switch corresponding to the lowest SOC is turned on, and the secondary winding is turned on. The primary winding, the energy is transferred from the primary winding to the secondary winding, the magnetic energy is converted back to electric energy and charged into the battery, the SOC of the single battery rises, and the overall SOC of the battery pack returns to a consistent value.

Description of drawings

In order to illustrate the technical solutions of the embodiments of the present invention more clearly, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. Those of ordinary skill in the art can also obtain other drawings based on these drawings without any creative effort.

Figure 1 is a diagram of the experimental results of lithium battery aging phenomenon;

Fig. 2 is the flowchart of improved SOC estimation algorithm;

Fig. 3 is a functional block diagram of the improved SOC estimation algorithm verification platform;

Fig. 4 is an improved SOC estimation algorithm test result diagram;

Fig. 5 is a judgment process based on the SOC balancing strategy;

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

Figure 7 is an equivalent circuit model of a flyback transformer;

Fig. 8 is an equivalent magnetic circuit diagram of a flyback transformer;

Fig. 9 is a current waveform diagram under two working modes;

Fig. 10 is the overall block diagram of balanced control system;

Fig. 11 is the execution flowchart of main control MCU;

Fig. 12 is the execution flowchart of secondary MCU;

Fig. 13 is a schematic diagram of the power supply circuit;

Fig. 14 is a schematic diagram of a single cell gating switch circuit;

Fig. 15 is a schematic diagram of a voltage inversion circuit;

Fig. 16 is a schematic diagram of a voltage-frequency conversion circuit;

Fig. 17 is a schematic diagram of a voltage polarity inversion circuit;

Figure 18 is a schematic diagram of the connection of two voltage regulators;

Fig. 19 is a schematic diagram of one active equalization circuit.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings.

1. Improved SOC estimation algorithm

Under the existing technical conditions, the inconsistency of lithium battery packs mainly occurs during the production process. At the same time, the working environment of lithium battery packs on electric vehicles is generally harsh, leading to increased inconsistency. This inconsistency is usually manifested through the parameters of battery voltage, capacity and internal resistance, which has a great impact on the life and performance of the battery pack. How to evaluate the consistency state of the lithium battery pack is a prerequisite before performing equalization control on the battery pack. The quantitative evaluation of consistency provides important data support for the balance and maintenance of the battery pack.

Electric vehicles characterize the mileage of the car by estimating the SOC of the battery pack. As the state parameter of battery pack capacity, SOC reflects the remaining capacity state of the battery pack, which is numerically defined as the ratio of the remaining capacity of the battery to the total capacity of the battery calibration:

In Formula 1, Q _R is the remaining charge capacity in the battery, and C is the nominal (rated) charge capacity of the battery.

From the definition of SOC given above, SOC represents the ratio between the remaining capacity of the battery and the available capacity of the battery. If SOC is used as the evaluation basis for inconsistency, it can avoid the existence of the available capacity of each single battery. The goal is to allow all single cells to reach the cut-off voltage of charging and discharging at the same time, so as to ensure that the available capacity of the battery pack is utilized as much as possible. Ensuring the SOC consistency of all single cells is actually equivalent to ensuring the consistency of the depth of discharge of all batteries, thus avoiding the performance degradation or even scrapping of the entire battery pack due to excessive aging of a single cell in the battery pack question. The difference in SOC can reflect the inconsistency of the battery, and the estimated value of SOC can also be obtained in real time during the operation of the battery pack, so the balancing strategy based on SOC can suppress the inconsistency of the battery pack in real time.

When using SOC estimation, the biggest difficulty is the accuracy and real-time performance of SOC estimation. At the initial stage of lithium battery discharge, the SOC of the battery changes very little. If it cannot be estimated accurately, the error will be too large due to the accumulation of SOC estimation errors at the end of battery discharge. At this time, if it is re-balanced, it will pose a considerable challenge to the balance control circuit, and at the same time the effect of the balance will be greatly reduced. In addition, the real-time estimation of SOC requires a certain amount of calculation. When the number of batteries in the battery pack is large, in order to ensure the real-time performance of the SOC, it is necessary for the balance system to use an MCU with certain calculation capabilities.

There is no way to directly measure the remaining power inside the battery. The remaining power of the battery can only be estimated by the amount of data that can be measured by the battery, such as voltage, current and internal resistance. However, the chemical properties inside the battery are very complex, so there will be no linear or other simple functional relationship between these data and SOC, and the current estimation methods for SOC will always have more or less defects. It also makes the SOC state estimation of the battery pack an important and difficult point in the battery management system. The SOC estimation method in the prior art is as follows:

(1) Anshi method

The ampere-time method is the most commonly used method for estimating battery SOC parameters. The core idea of this method can be expressed by formula 2:

Among them, SOC _i is the SOC of the current state of the battery; C _N is the rated capacity of the battery; I is the battery current; η is the charge and discharge efficiency. By calculating the integral of the current with respect to time, the amount of power lost over a period of time can be obtained. Cooperating with the known initial SOC ₀ , the SOC at a certain moment can be obtained. The principle and implementation of this method are very simple, but it has the following defects: firstly, the current measurement accuracy is high, if the current measurement is inaccurate, it will lead to SOC calculation errors, and the long-term accumulation will gradually increase the error; secondly, for the initial The estimation of SOC ₀ also needs to be obtained by a certain method; finally, when the temperature is high or the current fluctuation is large, the error of this method is relatively large. Therefore, in order to obtain a reliable SOC estimate, a high-performance current sensor is required to obtain accurate current values, and sufficient data is required for estimating the initial state.

(2) Open circuit voltage method

The electromotive force of the battery can be considered to be composed of three parts, including the open-circuit voltage (OCV) of the battery, the ohmic voltage of the battery, and the polarization voltage of the battery. When the battery is in a non-working state, and after a long period of standing, the ohmic voltage and polarization voltage of the battery will drop to 0. At this time, the open circuit voltage OCV of the battery is equal to the terminal voltage of the battery, which is the electromotive force of the battery, so The SOC can be estimated according to the relationship curve between OCV-SOC. In real time, there is a fairly good linear relationship between OCV-SOC on lead-acid batteries, and this method can be used to estimate SOC more accurately. But for lithium batteries, the linear relationship between the two is not so obvious, so it is necessary to establish a more complicated relationship comparison table.

The obvious disadvantage of the open circuit voltage method is that the battery needs to be fully rested before the measurement, which takes several hours or even more than ten hours, which makes the measurement difficult; secondly, the length of the resting time is also difficult to determine; As the battery ages, the correspondence between the open circuit voltage OCV and the SOC will also change. These reasons make it impossible to use the open circuit voltage method to obtain SOC online in actual use. The open-circuit voltage method can be combined with the ampere-hour method as a method for obtaining the initial SOC value in the ampere-hour method. However, the relationship between the open circuit voltage and SOC will change as the battery ages, and it is impossible to always re-measure the relationship between the two in actual use, so a method that can dynamically correct the corresponding relationship between the open circuit voltage OCV and SOC is needed algorithm.

(3) load voltage method

The principle of the load voltage method is consistent with that of the open circuit voltage method, and it is proposed to overcome the shortcoming that the open circuit voltage method cannot estimate the battery SOC online. Its principle is as follows: If the internal resistance r and the working current I of the battery can be obtained, the balanced electromotive force EMF of the battery can be calculated according to the following formula by measuring the voltage U across the load R.

EMF＝U+I*r (Formula 3)

From the analysis of the open circuit voltage method, we can know that the corresponding relationship between EMF and SOC is the relationship between OCV and SOC in the open circuit voltage method, so the SOC of the battery can be obtained after knowing the EMF.

Theoretically speaking, this method overcomes the shortcoming that the open circuit voltage method cannot measure SOC in real time, but there are still obvious defects in this method in actual use: first, there are many factors that affect the internal resistance r of the battery, and the internal resistance of the battery It is inherently inconsistent, and the internal resistance of each single battery may vary greatly, so it is difficult to accurately obtain the internal resistance of the battery; secondly, the method itself is based on the open circuit voltage method, so the open circuit The problems faced by the voltage method cannot be avoided by this method. The load voltage method is rarely used to obtain SOC online in the field of electric vehicles, but it is often used as the basis for judging the battery charge and discharge cut-off.

(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 studies have shown that there is a close relationship between the AC impedance or DC internal resistance of the lithium battery and the SOC of the battery. If such a relationship can be determined through some battery samples , then the SOC of the battery can be obtained by detecting the internal resistance of the battery.

The internal resistance of the battery can be divided into AC impedance and DC internal resistance. AC impedance reflects the battery's ability to resist AC, which can be measured by an AC impedance meter. Similarly, the DC internal resistance indicates the battery's ability to resist DC, and this value can be obtained by detecting the voltage and current changes in a short period of time.

However, as mentioned in the previous analysis of the load voltage method, the relationship between the internal resistance of the battery and the SOC is very complicated. It is not only affected by the SOC, but also affected by multiple factors such as temperature and battery health. The relationship between resistance and SOC is difficult to really determine. At the same time, the internal resistance of the battery is often very small, only at the milliohm level, which requires very high measurement accuracy, and measurement errors have a great impact on the results. Therefore, this method is rarely used in the actual application of electric vehicles.

(5) Neural network method

The neural network method is to estimate the battery SOC through a large number of samples for data training on the premise of building a good network model. The battery is a highly nonlinear system, and the neural network method has the basic characteristics of nonlinearity, which can better simulate the nonlinear dynamic characteristics of the battery. Therefore, the neural network method has a better effect in estimating SOC. The neural network method needs to train a large amount of sample data to estimate SOC, and the training sampling data and training methods will affect the estimation results. Another defect is that the neural network method requires a large amount of resources, which puts forward more challenges for the design of the battery management system. High requirements often require the use of higher performance control chips, which greatly increases the cost.

(6) Kalman filter method

The Kalman filter method regards the battery system as a nonlinear dynamic system, in which the SOC of the battery is only one state in the system, and the battery model is established accordingly, and the state equation and observation equation are listed according to the model, and the extended Kalman filter is used Method to estimate battery SOC. The basic idea of this method is to make an optimal estimate on the minimum variance of the state of the dynamic system. It solves the problems of inaccurate estimation of SOC initial value and cumulative error in the ampere-time method. If the battery model can be established accurately, the Kalman filter method will be able to accurately estimate the SOC of the battery. However, this method also has the following problems: first, the accuracy of estimating the battery SOC depends on the accuracy of the battery model; Just improved.

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 filter method. The above methods have their own unique characteristics in different use environments and for different power batteries, but they also have different defects and deficiencies. Table 1 summarizes the advantages and disadvantages of common SOC estimation methods.

Table 1 Summary of common SOC estimation methods

To sum up, the open-circuit voltage method in the commonly used SOC algorithm needs to estimate the SOC in the non-working state of the battery, so it is difficult to meet the requirements of the online estimation of the SOC of the electric vehicle power, and it is suitable to estimate the SOC in combination with other methods. The ampere-time method has the problems of dependence on the initial value and increasing cumulative errors, and cannot cope with the self-discharge problem of the battery. The Kalman filter method solves the problem of inaccurate initial SOC estimation and cumulative error in the ampere-hour method. At the same time, its main problem is that it is highly dependent on the battery model. Only when an accurate battery model is established can it be compared. Accurately estimate the SOC of the battery; and because a large number of matrix operations are used in the Kalman filter method, it requires a higher speed of the system processor. The main problem of the neural network method for estimating SOC is that it needs to train a large number of sample data, so the estimation error will be affected by the training sampling data and training methods. Another defect is that the neural network method requires a large amount of resources. For the design of the battery management system put forward higher requirements.

In actual use, lithium batteries will age as the number of cycles increases. The aging phenomenon of lithium battery is manifested in the change of internal parameters of the battery, the most important is the attenuation of battery capacity. Such attenuation can be seen from the relationship between the open-circuit voltage and SOC of lithium batteries with different cycle times. In the present invention, the lithium iron phosphate battery of 3400mAh has been tested between OCV and SOC after 500 cycles of the new battery. The results of the experiment are shown in Figure 1.

It can be seen from Figure 1 that there is a certain correspondence between the open circuit voltage OCV and SOC of the lithium battery, but at the same time, the gradual aging of the lithium battery will cause the relationship between the open circuit voltage OCV and the battery SOC to gradually change. With the increase of charge and discharge cycles, the battery gradually ages, and the capacity of the battery continues to decrease. It can also be seen from Figure 1 that at the same open circuit voltage OCV, the OCV-SOC curves of new batteries and aging batteries correspond to different SOCs. For this reason, when using the open circuit voltage as the basic estimation algorithm of SOC, it is necessary to add a certain correction algorithm to deal with the change of the relationship between OCV and SOC.

Considering the influence of lithium battery aging factors, and because the embedded chip used in the lithium battery management system usually has relatively weak computing performance, the improved SOC estimation algorithm proposed in the present invention combines the ampere-hour method and the open-circuit voltage method. A certain dynamic correction algorithm is developed to keep the SOC estimation value within an acceptable error range to deal with the influence of lithium battery aging.

As mentioned above, the ampere-hour method is a commonly used method, and its source of error lies in the estimation of the initial SOC and the measurement accuracy of the current during use. The measurement accuracy of the current can be improved by replacing the high-precision measurement device, and the high-precision initial SOC can be measured by the open-circuit voltage method. However, due to the aging effect of the battery, the correspondence between the open circuit voltage and the SOC of the battery changes dynamically, so the core of the improved SOC estimation algorithm proposed by the present invention is to dynamically maintain the correspondence table between SOC and OCV. During battery use, if the difference between the measured value of the battery SOC and the value in the correspondence table is greater than a preset error threshold, the correspondence table is corrected.

In the present invention, the charging and discharging process of the battery is divided into three stages: before charging and discharging, during charging and discharging and after charging and discharging. Since the open circuit voltage measurement requires the battery to stand for a long time, the open circuit voltage method is only suitable for estimating the SOC when the battery is not in use, that is, before charging and discharging or after charging and discharging. The SOC of these two stages is obtained by looking up the correspondence table between SOC and OCV. When charging and discharging, the SOC cannot be obtained directly, but it can be obtained by calculating the discharged or charged power value through the ampere-hour method, and then matching the SOC before charging and discharging. After charging and discharging, the SOC after charging and discharging can be calculated by the ampere-hour method. At the same time, after standing still, a theoretical SOC can also be obtained by looking up the table through the open circuit voltage method. There will be a deviation between the calculated SOC and the theoretical SOC, and if the difference is greater than the preset error threshold, the corresponding relationship table between SOC and OCV will be updated.

The flowchart of the improved SOC estimation algorithm described in the present invention is shown in FIG. 2 . Specifically:

Step 11, judge whether the battery is in working condition, if yes, go to step 12, if not, go to step 17;

Step 12, use formula 2 to calculate the SOC _i of the current state, wherein, SOC _i is the SOC of the current state of the battery, SOC ₀ is the initial SOC when the battery starts to work, C _N is the rated capacity of the battery, I is the battery current, η For charging and discharging efficiency, when charging, η is a negative number, and when discharging, η is a positive number;

Step 13, judge whether the battery is in working condition, if yes, then return to step 12, if not, then enter step 14;

Step 14, measuring the open circuit voltage OCV ₁ , looking up the correspondence table between SOC and OCV, and obtaining the SOC _{1 corresponding to the open circuit voltage OCV 1 ;}

Step 15, calculate the difference e between SOC _i and SOC ₁ , when the absolute value of e is greater than the preset error threshold, enter step 16, when the absolute value of e is less than or equal to the preset error threshold, output SOC _i ; enter Step 11;

Step 16, calculate the corrected SOC, output the corrected SOC correction, and update the correspondence table between SOC and OCV at the same time; enter step 11;

Step 17, measure the open circuit voltage OCV ₂ , look up the correspondence table between SOC and OCV, obtain the SOC ₂ corresponding to the open circuit voltage OCV ₂ , and output the SOC ₂ ; go to step 11.

For example, if the battery is discharged, its open-circuit voltage can be measured before discharging, then by looking up the corresponding relationship table between SOC and OCV, the SOC at this time can be known, which is recorded as S1. When the battery starts to discharge, record the current to calculate the consumed power, and finally get the total discharged power, denoted as Sc. Through calculation, the power at the last moment can be known, S2=S1-Sc. When the discharge is over, the theoretical SOC is obtained by measuring the open circuit voltage and comparing the data in the table, which is recorded as S3. The error value Sb is defined as the difference between S3 and S2, ie Sb=S3-S2. When Sb exceeds the preset error threshold, a correction process is started to correct the data in the correspondence table between SOC and OCV.

When starting to use the algorithm, before step 11, it is necessary to establish an initial correspondence table between SOC and OCV. In order to obtain such a table, the limitation of space and computing power is also considered, so the interpolation method is chosen to obtain such a table. You can choose 20 points, that is, each 5% SOC to measure the value of the open circuit voltage. If you need to be denser, you can also choose 50 points or more, depending on the requirements and the available memory of the chip.

The corresponding relationship table between the corrected SOC and OCV in step 16 can be completed through formulas 4 and 5.

e＝SOC _Table -SOC _Cal (Formula 4)

S _n (i+1)=S _n (i)-F(n, e) (0≤n≤50) (Formula 5)

The SOC _Table in Formula 4 is the original SOC in the table, and the SOC _Cal is the calculated SOC, and finally an e is used to represent the difference between the two. In Formula 5, S _n (i) represents the value in the table after the current i-th update, and S _n (i+1) is the value in the table after the i+1 update. F(n, e) is the correction coefficient, which is a function related to n and e. What adopted in the present invention is the representation of a function that is:

F(n, e)=a*e*n (Formula 6)

Among them, a is an adjustable constant, indicating the correction rate. It can be seen from Figure 1 that the voltage correspondences in different segments of the SOC are different, so the impact of aging is different under different SOCs. Therefore, the correction rates in different segments should be different, that is, a should take different values in different segments. n is the number of points to interpolate. If necessary, the functional form of F(n) can also be changed, and the updated functional form can be modified according to different battery characteristics.

In order to verify the effectiveness and practicability of the revised SOC estimation algorithm proposed by the present invention, it is necessary to verify whether the corresponding relationship table between SOC and OCV calculated by the battery after charging and discharging for many times is consistent with the current real corresponding table of the battery, and the error is within what range. For this reason, the present invention has designed an experimental platform to be used for verifying the validity of this SOC estimation algorithm, and the principle block diagram of this platform is as Fig. 3. The present invention is with rated capacity 3400mAh, and rated voltage is the 18650 lithium battery monomer of 3.7V as test object. In order to see obvious attenuation in the experiment, the number of charge and discharge cycles of the battery needs to be more. In this experiment, 500 cycles were cycled. At the same time, in order to avoid the influence of temperature on the test results, the temperature was controlled at 25 degrees Celsius during the test. In order to restore the battery to the resting state, it is necessary to let the battery rest for more than 30 minutes when measuring the open circuit voltage. At the same time, in order to reduce the impact of the voltage drop of the internal resistance during discharge, the battery uses a low-speed discharge mode during discharge, so the curve of the voltage can be directly used to estimate the curve of the open circuit voltage.

The results of the test are shown in Figure 4. It can be seen from Figure 4 that there is a large gap between the two curves of the new battery and the aged battery. If the correction algorithm is not added, the relationship between the voltage and capacity of the new battery will still be used after the battery ages, which will cause the error in the estimated SOC value to increase as the battery ages, which is very unfavorable for extending the battery life. service life. After the correction method is added, the calculated curve has a fairly good agreement with the curve of the aging battery, which shows that with the aging of the battery, the algorithm in the present invention can dynamically correct the corresponding relationship between voltage and capacity. This can greatly help reduce the error in SOC estimation caused by battery aging.

2. Balance control strategy

As mentioned above, the estimation algorithm of SOC is one of core technologies of the equalization strategy in the equalization technology, after determining the SOC estimation algorithm of above-mentioned revision, the equalization strategy that the present invention adopts is introduced below. The balance strategy and the balance circuit topology design are the two most important key points in the balance control system. The balance circuit topology needs to cooperate with a reasonable balance strategy to achieve a good balance effect. The two are two complementary parts of the balance control system. In the present invention, an equalization circuit based on a bidirectional flyback transformer is adopted, and a set of equalization strategies is designed for the circuit at the same time.

According to the above analysis and research, in order to obtain a good balance effect and better realize the design goal of extending the life of the battery pack in the present invention, SOC is used as a variable of balance control in the present invention, and quantitative standards need to be used in the balance control system to enable or disable the balance function. In order to evaluate and identify the inconsistent state of the battery pack, the present invention analyzes two different quantitative indicators of the variance δ _soc ² and the range r _soc , the expressions of the two are consistent with those used in statistical mathematics, specifically as follows:

r _SOC ＝max(SOC(i))-min(SOC(i)), i=1...n (Formula 9)

From the perspective of statistical mathematics, the degree of dispersion of the SOC values of the individual cells in the battery pack is expressed by the variance, that is, the smaller the variance, the smaller the dispersion of the individual cells in the battery pack relative to the average battery SOC. The smaller the difference of SOC value in the battery pack; on the contrary, if the variance is larger, it means that the dispersion degree of each single battery in the battery pack relative to the average SOC value is higher, and the difference of SOC value of each single battery is greater. Therefore, if variance is used as the quantitative standard for evaluating the consistency of lithium battery packs, theoretically good results can be obtained. However, the amount of calculation of the variance or standard deviation is quite large. Considering that the embedded chips used in the balance control system are often not powerful in calculation performance, and the evaluation of the consistency of the battery pack needs to be done often or even in real time, so the Variance is not suitable as a quantitative standard for system consistency evaluation.

The extreme difference indicates the difference between the maximum SOC and the minimum SOC in the battery pack. When the value is small, it means that the difference between the maximum SOC and the minimum SOC in the battery pack is small, that is to say, the SOC of each single battery Distributed in a small range, it can explain the consistency of the battery pack is better. When the extreme value is large, there is a large gap between the maximum SOC and the minimum SOC of the battery pack, and the SOC of the battery pack may be distributed in a relatively wide range, indicating that the consistency of the battery pack may be poor. At the same time, it is not necessary to take into account the SOC conditions of all single cells in the battery pack when calculating the extreme difference. It is only necessary to find the maximum and minimum SOC values, which greatly reduces the amount of calculation.

From the above analysis, it can be known that the variance and range of SOC can reflect the consistency of the battery pack, so it is appropriate to use the variance and range as the evaluation criteria for quantifying the consistency of the battery pack. But at the same time, considering the computing power of the balanced control system, the use of range can greatly reduce the calculation of the system and speed up the response speed of the system, so the range is more suitable for use in the present invention than the variance.

In the SOC balancing strategy, the purpose of battery pack balancing is mainly achieved by reducing the SOC difference between batteries. The SOC of each single lithium battery is used as the main control object, and the battery is reduced by charging and discharging the single battery. The difference in SOC between. The equalization process used in the present invention is as follows: when equalization is started, the SOCs of all batteries are measured, and one of them is selected as the target SOC for equalization. Usually, however, the mean is chosen As the target SOC for equalization, using the average value as the target can improve the efficiency of equalization and give full play to the advantages of charge-discharge equalization. When the present invention uses SOC as an equalization control means, an equalization control band (dSOC) is set to prevent equalization fluctuations. In the present invention, 1% is used as the cut-off band to control dSOC. followed by SOC higher than The single lithium battery is discharged to equalize, and the same for the lower than battery pack for charge equalization. This process can be illustrated by a flowchart, as shown in FIG. 5 . Specifically:

Step 21, obtaining the SOC of each single lithium battery in the lithium battery pack, specifically obtaining the SOC of each single lithium battery through the above-mentioned improved SOC estimation algorithm;

Step 22, calculating the range r _soc of the battery pack, that is, calculating the difference r _soc between the maximum SOC and the minimum SOC of each single lithium battery in the battery pack;

Step 23, comparing the size of the range r _soc with the preset range threshold, when the range r _soc is greater than the preset range threshold, enter step 24, when the range r _soc is less than or equal to the preset range threshold When the preset extreme difference threshold is mentioned above, go to step 25;

Step 24, select the average value of the SOC of all single lithium batteries As the target SOC for equalization, for SOC below The single lithium battery is charged and balanced, and the SOC is higher than The single lithium battery carries out discharge equalization, wherein, dSOC is the balance control band, adopts 1% as cut-off band control dSOC among the present invention;

Step 25, end.

In this process, some special nodes need to be paid attention to. When the battery is in the discharge state, the battery with poor health will be fully discharged in advance to reach the cut-off voltage, while the battery with good health will have a part of the remaining power, which leads to the capacity of the battery pack not being fully used. In order to solve this problem, but also protect the life of each battery so that it cannot be in an over-discharge state, the solution in the present invention is to charge the battery that is about to enter the end of discharge to make its SOC consistent with the SOC of other batteries , regardless of whether the cell is in the cut-off band or not. In this way, the batteries in the entire battery pack can reach the state of SOC=0 at the same time, and the capacity of the battery pack can be fully utilized.

Similarly, when the battery is in the charging state, the battery with poor health will enter the full charge state and reach the cut-off voltage in advance, so the capacity of the same battery pack is wasted, so in order to prevent these batteries from entering the cut-off voltage in advance, the present invention The countermeasure taken in is to start the equalization circuit to make the SOC of the battery fluctuate around the SOC of the battery when it is detected that a battery is about to enter the charging cutoff, so that all the batteries can reach the state of SOC=1 at the same time.

3. Balance circuit

The present invention selects a bidirectional flyback transformer equalization circuit. The main reason is that the equalization circuit has a large current and a fast equalization speed, and can realize two-way equalization. A schematic diagram of a typical bidirectional flyback transformer equalization circuit is shown in Figure 6.

The essence of flyback transformer equalization is to realize the two-way transfer of energy between battery cells through the mutual conversion of electric energy and magnetic energy. When a single battery in the battery pack has more energy than other batteries, the flyback transformer is used as the energy transfer medium to transfer the excess energy of the battery to the entire battery pack; and when the energy of a certain battery in the battery pack is compared When there are few other batteries, the flyback transformer is also used as the energy transfer medium to input the energy of the entire battery pack to the single battery to prevent the battery energy from being too low and causing damage to the battery. This structure has two directions of equalization, as follows.

(1) Balance from single cell to battery pack (top balance)

After the balance control system detects the SOC of all single batteries, for a single battery with a high SOC that needs to be balanced, the corresponding secondary transformer switch of the battery is turned on, and all other switches including the primary switch are turned off. The flyback transformer There is current passing through the secondary winding of the battery, and the electric energy is stored in the secondary winding in the form of magnetic energy at this time; after the SOC in the battery drops to the required value, the secondary variable switch is turned off and the primary switch is turned on, so that the energy will be transferred from the secondary The primary winding is transmitted to the primary winding, and the magnetic energy is converted into electrical energy and transmitted to the entire battery pack, thus controlling the battery with the highest SOC and transferring excess energy to other batteries in the battery pack.

(2) Balance from the battery pack to the single battery (bottom balance)

After the balance control system detects the SOC of all single cells, for the low SOC of the single cells that need to be balanced, the corresponding primary switch is turned on, and all secondary switches are turned off, and there will be current in the primary winding of the flyback transformer Through, at this time, the electric energy is stored in the primary winding in the form of magnetic energy on the primary side; when enough electric energy is charged, the primary switch is turned off, and the secondary switch corresponding to the lowest SOC is turned on, and the secondary winding is turned on , so that the energy is transferred from the primary winding to the secondary winding, and the magnetic energy is converted back to electric energy and charged into the battery. Through this process, the SOC of the single battery can be recovered, and the overall SOC of the battery pack also returns to a relatively consistent value.

The circuit and magnetic circuit analysis of the flyback transformer can wait until the equivalent circuit model shown in Figure 7 and the equivalent magnetic circuit diagram shown in Figure 8.

When the MOS switch is turned on, it can be seen from the left side of Figure 8 that the reluctance R _M in the iron core is connected in parallel with the reluctance R _S of the leakage flux, and the magnetic equipotentiality is N ₁ i ₁ , and the magnetic flux Φ _M generated on R _M passes through After passing through the iron core and disconnecting the MOS tube, the current i ₂ flowing in the secondary winding generates a magnetic flux Φ _M . It can be seen in the right part of FIG. 8 that N ₁ i ₁ =N ₂ i ₂ again at the moment of conversion. If N ₁ : N ₂ =1:1, the equivalent circuit model in Figure 7 is converted from the equivalent magnetic circuit model of the flyback transformer, and the primary and secondary leakage inductance L _S in the figure are the same. Due to the air gap in the flyback transformer, its iron core has a small inductance value, and the leakage inductance L _S in the flyback transformer cannot be ignored. After the MOS tube is turned on, i ₀ flows through the primary L _S and L _M , and no current flows through the secondary at this time. When the MOS tube is turned off, all the energy of L _S and part of the energy of L _M in the leakage inductance of the input side are consumed in the absorption network (clamp circuit: in the flyback transformer, it can reduce the voltage stress on the switch tube), and The remaining part of the energy in _LM and all the energy in the secondary _LS are output through the secondary.

In Figure 6, when the MOS transistor S is turned on, the input voltage _Ui is applied to both ends of the primary winding of the transformer. According to Lenz's law, it can be seen that the secondary winding generates a positive and negative induced electromotive force at this time, and the diode D2 cannot be turned _on . Therefore current cannot flow in the secondary circuit. At this time, the primary winding of the transformer is equivalent to an inductor. Assuming that the inductance of the primary winding is Lp and the inductance of the secondary winding is Ls, the current flowing through the primary winding during the conduction period of the MOS transistor is:

At t=t _on , the primary winding current reaches its maximum value:

When the MOS tube is turned off, according to Lenz’s law, it can be known that the voltage polarity of the secondary winding turns positive and negative, at this time, the diode D2 is turned _on , the magnetic energy stored in the transformer is converted into electric energy, and there is current in the secondary winding Flowing, the current is:

When t=t _off , the current of the secondary winding reaches the minimum value I _smin . When I _smin =0, the energy stored in the magnetic field during the conduction period of the MOS transistor is completely released. This process is called the discontinuous operation mode of the flyback transformer; when I _smin >0, the energy stored in the magnetic field during the conduction period of the MOS transistor is completely released. The energy in the transformer is not completely released, and this process is called the continuous working mode of the flyback transformer. The current waveforms of the above two working modes are the same as shown in FIG. 9 .

The magnetic flux in the transformer core must return to its original position at the end of each cycle. This principle is called the magnetic flux reset principle. In continuous operation mode, there is residual magnetism. In theory, the magnetic flux at the end of each cycle should be maintained. can be restored to the initial value, but due to the iron loss of the magnetism, the coil winding also has copper loss, which causes the temperature to rise during use, causing the initial value of the magnetic flux to shift, which cannot be reset, and finally leads to the change of the magnetic flux into nonlinearity area, the inductance decreases, the current value increases, the magnetic core is easy to reach saturation, the transformer cannot work normally, and the circuit is greatly unsafe. The transformer is smaller in continuous operation mode and allows larger primary and secondary currents. Therefore, the flyback transformer used in the present invention works in a discontinuous working mode.

The primary side of the flyback transformer used in the present invention is connected to a battery pack in which 6 single lithium iron phosphate batteries are connected in series, each secondary side is connected to each single battery, and the rated voltage of each single battery is 3.6V , so 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 10KHz.

(1) Maximum duty cycle

Generally, the output efficiency of the transformer increases as the duty ratio increases, but when the duty ratio exceeds 50%, the circuit will oscillate. Although this phenomenon can be improved by adding a harmonic compensation module in the circuit, if the appropriate components are not selected and arranged reasonably, the harmonic compensation module in the circuit may not be able to play a role at this time, causing the circuit to fail. The state remains unstable at duty cycles greater than 50%. Therefore, the maximum duty cycle of the transformer is generally between 40% and 50%, but the maximum duty cycle of the present invention is finally selected as 45%. The actual duty cycle used has to be obtained through simulation.

(2) Turns ratio

The turns 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.

At the beginning of the design, the primary and secondary turns of the transformer cannot be directly known. In the present invention, the transformation ratio N of the transformer is directly determined according to the reflected voltage of the transformer shown in Formula 13 as:

In the above formula, the reflected voltage V _OR indicates that when the current flows through the secondary winding, an opposite voltage is formed on the primary winding, V _o indicates the output voltage, and V _f is the voltage drop of the MOS tube. V _OR can be calculated by the following formula.

N=5.2 can be obtained through formula 14 and formula 15, and N=5.

Experiments have proved that when the duty cycle is greater than 40%, the equalization current is too large, which does not meet the hardware conditions, and when the duty cycle is 20%, the current of the secondary side is less than 5A, which is inconsistent with the goal of the present invention, so the top equalization The duty cycle should be between 25% and 35%. At the same time, when the duty cycle is greater than 35%, the transformer works in continuous mode, so it is not suitable, so the duty cycle should be selected between 20% and 30% in the bottom balance. In summary, choosing a duty cycle of 25% or 30% is a more appropriate range for top balance and bottom balance.

4. Balance control system

The present invention adopts the latest K64 chip from NXP as the control chip on the MCU side, the latest embedded chip with powerful performance, which provides a good performance guarantee for the development of the SOC estimation algorithm and equalization strategy of the present invention. At the same time, a voltage measurement module, a current measurement module, a temperature measurement module and an equalization module are designed with the chip as the core. Among them, the equalization module uses the LTC3300-1 chip specially designed for the active equalization circuit of bidirectional transformers. With the advantage of the integrated chip, the equalization module has further improved the circuit complexity and cost control.

Lithium battery packs often need to combine hundreds of single lithium-ion batteries when in use. Considering the flexibility of battery pack distribution, the entire battery pack needs to be divided into many small battery packs and installed in different battery boxes. At the same time, in order to expand the capacity , the new battery can be conveniently expanded into the battery pack in the future, and the present invention adopts a modular expansion design. The overall design block diagram is shown in Figure 10.

The whole system is designed to be divided into upper and lower levels. The secondary MCU implements the function of monitoring the battery of this group, including: collecting data such as voltage, current and temperature of each battery in this group; feeding back the collected data to the main control MCU; calculating the SOC of each single battery; According to the SOC, it is judged whether balancing is required; when balancing is required, the balancing module of the group is controlled to balance the batteries to be balanced. The main control MCU is responsible for collecting the data fed back by the lower-level MCU, and at the same time transmits the data to the upper-level PC to facilitate data collection and debugging of the entire battery pack. The main 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 Figure 11, and the execution flow of the secondary MCU is shown in Figure 12.

Adopting such a hierarchical and modular design has the following advantages:

(1) Improve the scalability of the system. Lithium battery packs need to be combined with different numbers of batteries when in use. For example, car manufacturers often classify the price of the same model based on different mileage for sales. Different mileage needs to integrate different quantities in the battery pack. If different balance control systems are used in the same model for this purpose, it will not only increase the initial research and development costs, but also require more energy and cost for the maintenance of different balance control systems. Therefore, the use of hierarchical design can enhance the scalability of the system.

(2) Improve the real-time performance of the system. The MCU in each small battery group only needs to manage the batteries in the group, which greatly reduces the calculation amount of each lower-level MCU and enhances the real-time performance of the balance control system.

(3) Improve system compatibility. If it is necessary to use small battery packs from different manufacturers in the same large battery pack, it is only necessary to re-adjust or design the equalization module of the group, avoiding large-scale modifications to the entire system.

(4) Increase the reliability of the balance control system. The modular design can avoid the breakdown of the whole system. When the balance module in a small battery pack fails, other small battery packs can still work normally. This has not only protected the batteries in other small battery packs, but also avoided more serious accidents.

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

power circuit

The function of the power supply module is to provide the power supply for the normal operation of the secondary MCU. The circuit schematic diagram of the power supply module designed by the present invention is shown in Figure 13. The normal operating voltage of the main control chip K64 of the secondary MCU is between 1.71V and 3.6V, and the power supply of the chip is guaranteed to be around 3.3V during normal use, and the equalizing system of the present invention also needs to use the voltage of 5V. Since the secondary MCU and the balancing module are all set in the small battery pack, they can directly take power from the small battery pack. A small battery pack is usually made up of 6 single cells, and the working voltage of each battery is between 3.0V and 4.2V, so the terminal voltage of the small battery pack is between 18V and 25.2V. This system adopts the LM2576 voltage conversion chip of NI Company. The LM2576 chip can accept voltage input from 7 to 40V, the output voltage is 5V, and can drive a load of 3A. Components can complete a good voltage output. Figure 13 also includes a 5V to 3.3V circuit, because the normal operating voltage of K64 is 3.3V, so it is necessary to use an ASM1117-3.3V regulated power supply module to convert 5V to 3.3V and output it to K64 for power supply. Two power indicator lights added to the circuit are used to indicate whether the two power supplies are working normally.

Voltage Acquisition Circuit

The accurate voltage acquisition circuit is not only related to the normal use and monitoring of the battery pack, but also a necessary guarantee for accurate balance judgment of the battery. Therefore, when connecting lithium batteries in series to form a battery pack, it is necessary to accurately measure the voltage of each cell in the battery pack.

The battery voltage acquisition methods commonly used in series lithium battery packs include common mode measurement method and differential mode measurement method. Among them, the common mode measurement is to measure the voltage of the battery pack by means of proportional attenuation of precision resistors relative to the same reference level, and then subtract them sequentially to obtain the voltage of each single cell. The advantage of this method is that the circuit is simple, but the measurement accuracy of this method depends on the voltage divider resistance, which is easily affected by temperature and produces serious cumulative errors. Therefore, this method is only suitable for a small number of batteries in series and low requirements for measurement accuracy. occasions. It is not suitable for occasions that rely on accurate SOC calculations with voltage.

In the present invention, the method of differential mode measurement is selected, and the method selects each battery cell sequentially for measurement through a certain method. This method is suitable for occasions where there are a large number of batteries connected in series and high precision is required. Another advantage of using this differential mode measurement method is that when the acquisition of a certain channel fails, it does not affect the normal operation of other channels. In addition, compared with the acquisition method using integrated chips, this sub-channel measurement method only needs to repair the corresponding faulty channel when a fault occurs, instead of replacing the entire chip, which is of great benefit to reducing maintenance costs in the later period of.

(1) Single battery selection circuit

The way of differential mode measurement needs to be able to gate each single cell. In the present invention, MOSFET PS7241-2A is used as the single cell gate switch for voltage collection. PS7241 series devices are composed of light-emitting diode (input side) and normally open contact MOS tube (output side). Each PS7241-2A contains two independent strobe switches. This device is characterized by low operating current, high withstand voltage and very fast response speed. The schematic diagram of the cell gating switch circuit is shown in Figure 14.

Resistors R1-R4 in Figure 14 are current-limiting voltage-dividing resistors, which are used to limit the magnitude of the current in the circuit during the measurement process. When you need to measure the voltage of a single battery, you only need to pull up the corresponding pins 1 and 3 on the PS7241 through K64. output on two pins. Take the circuit in Figure 14 as an example, when it is necessary to measure the voltage of bat1, pull up pins 1 and 3 of PS1, at this time, the output from pins 6 and 8 is the voltage at both ends of bat1, and pin 6 outputs the positive pole of the battery. Pin 8 is the negative pole of the battery. When the battery bat2 needs to be measured, the 3 pins of PS1 and the 1 pins of PS2 are strobed, the output from the 6 pins of PS1 and the 8 pins of PS2 is the voltage at both ends of bat2, and the 8 pins of PS2 are the battery bat2 Positive pole, pin 6 of PS1 is the negative pole of battery bat1.

(2) Voltage inversion circuit

It has been pointed out in the above analysis that when measuring bat1 and bat2, the voltages on the corresponding pins of CAP_1 and CAP_2 are opposite. In fact, the voltages of all odd-numbered batteries and even-numbered batteries are opposite when measured. Therefore the present invention has used two PS7241 more, has designed the voltage reversal circuit as shown in Figure 15. Through this circuit, the measurement of odd-numbered batteries and even-numbered batteries can output the same voltage direction, which is convenient for the subsequent AD circuit to measure the voltage value of each single battery. The reason why the relay is not used is because the response of the relay is not as fast as that of the PS7241, and the voltage drop caused by the relay will also affect the accuracy of the result.

When testing odd-numbered batteries, CAP2 is battery positive and CAP1 is battery negative. At this time, it is necessary to control K_AD_1 to be low voltage and K_AD_2 to be high voltage, that is, pin 8 of PS5 and PS6 in the figure is turned on, and pin 6 is not. Pass. When testing even-numbered batteries, CAP1 is battery positive and CAP2 is battery negative. At this time, control K_AD_1 to set high level and K_AD_2 to set low level. At this time, pin 6 of PS5 and PS6 is turned on, but pin 8 is not turned on. Through this circuit, AD_P can always be connected to the positive pole of the battery, while AD_N can always be connected to the negative pole of the battery.

(3) Voltage-frequency conversion circuit

The voltage-frequency conversion circuit (VFC) can convert the input voltage signal into a frequency signal output. The frequency signal output by the VFC circuit is linearly proportional to the input voltage signal. That is to say, the higher the voltage, the faster the output frequency. . VFC circuits are widely used in various circuits, including signal frequency modulation, phase modulation, AD conversion circuits, etc. The VFC circuit has the advantages of strong anti-interference ability, convenient isolation, stable performance, high sensitivity and small nonlinear error. At the same time, when the analog signal is digitized, the resolution and accuracy of the VFC circuit are higher than that of the AD conversion circuit, and the cost of the VFC circuit is usually lower under the same precision premise.

The voltage-frequency conversion circuit designed in the present invention consists of two parts. The first part is the operational amplifier circuit, and the core of this part is an OP07C operational amplifier. OP07C has the characteristics of low noise and non-chopping zero stability. For most usage scenarios, OP07C does not require external components for offset zeroing and frequency calibration. In addition, OP07C features low bias current, high open-loop gain and wide operating temperature range. In order to ensure the stability and anti-interference ability of the VFC circuit, the present invention sets the magnification of the operational amplifier to 2 times.

The second part of the voltage-frequency conversion circuit is the VFC circuit, and the core of this part is the AD7740 chip. AD7740 is a low-cost, very small voltage-to-frequency conversion chip. This chip can work between 3.0V to 3.6V or 4.75V to 5.25V, and the working current can be as low as 0.9mA. AD7740 supports a very wide operating temperature range, relies on few external original components, and has precise voltage conversion frequency. The chip integrates a 2.5V reference reference, and also supports the use of externally input VDD as a reference voltage. The chip also has a synchronous clock input pin - CLKIN, which can support up to 1MHz frequency input. In the present invention, the output clock of K64 is used as the synchronous clock of AD7740, which reduces unnecessary original devices and reduces circuit complexity.

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

Finally, the voltage-frequency conversion circuit is given as shown in Figure 16.

It is worth noting that OP07C needs positive and negative voltage power supply, so a reverse polarity circuit is needed to convert +5V voltage into -5V voltage, which is used to provide negative power supply for OP07C. In the present invention, the MAX660 charge pump reverse polarity switch integrated regulator is used to realize this function, and the circuit diagram is shown in Figure 17.

Current Acquisition Circuit

Accurate current measurement is an indispensable condition when using the ampere-hour method to estimate SOC, and a Hall current sensor is used to measure the current in the present invention. The principle of the Hall current sensor is that when the primary current flows through a long wire, a magnetic field is generated around the wire, and the magnitude of the magnetic field is proportional to the magnitude of the current. The Er element is used to measure and amplify the output, and its output voltage can reflect the magnitude of the primary current.

The advantage of the Hall current sensor is that it has a wide measurement range, can measure current and voltage with arbitrary waveforms, and can even faithfully reflect transient peak current and voltage signals. The response speed of the Hall current sensor is extremely fast, and can reach the response speed of us level. At the same time, the accuracy of the Hall current sensor is very high, which can achieve a measurement accuracy better than 1%, and the measurement thread level is also good, and it can work without failure for a long time, and can usually guarantee continuous work for several hours. In addition, the Hall element can be made to be small in size and convenient to use.

The measuring range of the Hall current sensor selected by the present invention 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; when the current in the circuit is 100A, the output is 5V. For the sampling circuit that adopts OP07C, the output of the Hall current sensor that the present invention adopts is in the normal operating range.

Temperature Acquisition Circuit

The temperature acquisition circuit is used to collect the temperature value of each small battery pack. The temperature has a great influence on the operation of the lithium battery. On the one hand, the real-time temperature value can ensure the accuracy of the estimation of the lithium battery SOC, on the other hand Excessively high temperatures are also essential for the operational safety of the system.

The present invention adopts the temperature acquisition circuit designed based on the NTC thermistor NTC10KB3950K to realize the temperature acquisition of the small battery pack. The circuit has the characteristics of high measurement accuracy, simple structure and good stability. The accuracy of NTC10KB3950K can reach 1%. At 0 degrees, the resistance is 32.5K, and the corresponding voltage is 0.29V. At 85 degrees, the resistance is 1.063K, and the corresponding voltage is 3.26V. The calculation between resistance and voltage is given by:

Then according to the voltage and temperature correspondence table of NTC10K-3950, the temperature value in the small battery pack can be found.

Balance circuit

The balance control circuit is one of the cores of the present invention. The present invention designs a transformer equalization circuit based on the LTC3300-1 chip. The LTC3300-1 is a controller IC with fault protection for transformer-based bidirectional active balancing of multicell battery packs. The device integrates all required gate drive circuitry, high-accuracy battery sensing, fault detection circuitry, and a watchdog with a built-in timer. Each LTC3300-1 can balance up to six series-connected Li-Ion cells with a 36V input common-mode voltage. Charge from any selected cell can be efficiently transferred back and forth between itself and 12 or more adjacent cells. The SPI interface of the LTC3300-1 can complete the series connection with multiple LTC3300-1 devices without optocoupler isolation, so as to realize the charge balance of each battery in the long string of series-connected batteries. LTC3300-1s connected in series can run independently simultaneously, thus allowing all cells in the stack to be managed independently and simultaneously.

The equalizer circuit corresponding to each LTC3300-1 operates independently. The primary side of the transformer is connected to each single battery through a MOS tube, and the secondary side of the transformer is connected to the entire battery pack through a MOS. The LTC3300-1 supports two transformer balancing methods. One is that each transformer has its own transformer primary and secondary; the other is that all transformers have their own primary but share a secondary connected to the battery pack. The schematic diagram of the connection method of the two transformers on the LTC3300-1 is shown in Figure 18;

In Figure 18(a), the balanced transformer for each single battery has independent primary and secondary sides, the primary side is connected to the single battery through a MOS tube, and the secondary side is connected to the battery pack through a MOS tube ; In Figure 18(b), each equalizing transformer has only a single primary side, which is connected to the battery cell through a MOS tube, and the secondary side is shared by all transformers. Considering the module design goal of the whole system and the perspective of easier maintenance, the present invention adopts all the forms in Fig. 18(a).

LTC3300-1 can be connected to up to 6 batteries for equalization. Figure 19 shows the connection method of channel 2, and the solution of other channels is similar to this channel. In the figure, the C2 pin is connected to the positive pole of bat2. I2P and I2S are used to measure the current of the primary side and the secondary side of the transformer respectively, and G2P and G2S are used to control the on-off of the MOS tube of the primary side and the secondary side. BAT+, BAT- are linked to the positive and negative poles of the entire battery pack.

It is to be understood that the embodiments described herein can be implemented by hardware, software, firmware, middleware, microcode, or any combination thereof. For hardware implementation, the processing unit can be implemented in 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 ( FPGA), processors, controllers, microcontrollers, microprocessors, other electronic units designed to perform the functions described herein, or combinations thereof. When the embodiments are implemented in software, firmware, middleware or microcode, program code or code segments, they may be stored in a machine-readable medium such as a memory component.

It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, but that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Accordingly, the embodiments should be regarded in all points of view as exemplary and not restrictive, the scope of the invention being defined by the appended claims rather than the foregoing description, and it is therefore intended that the scope of the invention be defined by the appended claims rather than by the foregoing description. All changes within the meaning and range of equivalents of the elements are embraced in the present invention. Any reference sign in a claim should not be construed as limiting the claim concerned.

In addition, it should be understood that although this specification is described according to implementation modes, not each implementation mode only contains an independent technical solution, and this description in the specification is only for clarity, and those skilled in the art should take the specification as a whole , the technical solutions in the various embodiments can also be properly combined to form other implementations that can be understood by those skilled in the art.

Claims (10)

1. a kind of dynamic corrections SOC lithium battery group balance control method, it is characterised in that methods described comprises the following steps：

Step 21, the SOC of each single lithium battery in lithium battery group is obtained；

Step 22, the extreme difference r of battery pack is calculated_soc；

Step 23, the extreme difference r_socWith the size of default extreme difference threshold value, as the extreme difference r_socMore than the default extreme difference During threshold value, into step 24, as the extreme difference r_socDuring less than or equal to the default extreme difference threshold value, into step 25；

Step 24, the SOC of all single lithium batteries average is selectedAs the target SOC of equilibrium, SOC is less thanSingle lithium battery carry out charge balancing, SOC is higher thanSingle lithium battery discharge Weighing apparatus, wherein, dSOC is Balance route band；

Step 25, terminate.

2. dynamic corrections SOC according to claim 1 lithium battery group balance control method, it is characterised in that step 21 has Body includes：

Step 11, judge whether battery is in running order, if it is, enter step 12, if it is not, then into step 17；

Step 12, the SOC of current state is calculated using formula two_i, wherein, SOC_iIt is the SOC of battery current state, SOC₀For electricity Initial SOC, C during the start-up operation state of pond_NIt is the rated capacity of battery, I is battery current, and η is efficiency for charge-discharge, works as charging When, η is negative, and when electric discharge, η is positive number；

Step 13, judge whether battery is in running order, if it is, return to step 12, if it is not, then into step 14；

Step 14, open-circuit voltage OCV is measured₁, SOC and OCV mapping table is searched, is obtained and the open-circuit voltage OCV₁Phase Corresponding SOC₁；

Step 15, SOC is calculated_iAnd SOC₁Between difference e, when e absolute value is more than default error threshold, into step 16, When e absolute value is less than or equal to default error threshold, SOC is exported_i；Into step 11；

Step 16, revised SOC, the output revised SOC amendments are calculated, while updates SOC and OCV corresponding relation Table；Into step 11；

Step 17, open-circuit voltage OCV is measured₂, SOC and OCV mapping table is searched, is obtained and the open-circuit voltage OCV₂Phase Corresponding SOC₂, export SOC₂；Into step 11.

3. dynamic corrections SOC according to claim 2 lithium battery group balance control method, it is characterised in that：In step Before 11, initial SOC and OCV mapping table are established using interpolation method.

4. dynamic corrections SOC according to claim 2 lithium battery group balance control method, it is characterised in that：Step 16 Specifically include：

Revised S is calculated using formula five_n(i+1) S, is exported_n(i+1), and phase in SOC and OCV mapping table is updated The S answered_n(i) it is S_n(i+1)；

S_n(i+1)=S_n(i)-F (n, e) (0≤n≤50) (formula five)

Wherein, S_n(i) value after the current ith renewal of expression in table, S_n(i+1) value after the renewal of expression i+1 time in table, F (n, e) is correction factor, and the coefficient is a function related to n and e, and the function is represented using formula six；

F (n, e)=a*e*n (formula six)

Wherein, a is an adjustable constant, represents amendment speed, n is the points of interpolation, and e is SOC_iAnd SOC₁Between difference.

5. dynamic corrections SOC according to claim 4 lithium battery group balance control method, it is characterised in that：SOC's Different sections, a value are different.

6. dynamic corrections SOC according to claim 1 lithium battery group balance control method, it is characterised in that：

In step 24, when battery carries out equalization discharge, the battery that will enter electric discharge cut-off is charged, makes its SOC It is consistent with the SOC of other batteries, no matter whether the battery is in rejection zone；When battery carries out charge balancing, to i.e. The battery of charge cutoff will be entered, start equalizing circuit and the SOC of the battery is nearby fluctuated so that final all battery energy Reach SOC=1 state simultaneously.

A kind of 7. lithium battery group Balance route system for the dynamic corrections SOC for realizing the method any one of claim 1-6 System, control system difference superior system and lower system, wherein superior system include PC and main control MCU, and lower system includes More sub- lower systems, every sub- lower system are used to carry out a baby battery group Balance route, every sub- lower system bag Include a secondary MCU, a balance module, it is characterised in that：

Main control MCU, for collecting the data of each secondary MCU feedbacks in lower system, the PC of superior transmits the data, And the order for PC to be sent is forwarded to corresponding secondary MCU；

PC, ordered for receiving the data of main control MCU transmission, and for being sent to main control MCU；

Secondary MCU, for gathering the voltage of each battery, electric current and temperature data in this baby battery group；By the data of collection Feed back to main control MCU；Calculate the SOC of each cell；Judge whether to need to carry out equilibrium according to the SOC；When need into Row controls the balance module of this baby battery group balanced to needing the battery for carrying out equilibrium to carry out when balanced；

Balance module, for the control according to the secondary MCU in this baby battery group to needing to carry out balanced battery progress equilibrium.

8. dynamic corrections SOC according to claim 7 lithium battery group balance control system, it is characterised in that：It is described equal Weighing apparatus module uses two-way flyback transformer.

9. dynamic corrections SOC according to claim 8 lithium battery group balance control system, it is characterised in that：It is two-way anti- Swash formula transformer and use LTC3300-1 chips.

10. dynamic corrections SOC according to claim 8 lithium battery group balance control system, it is characterised in that：

Balanced cell is needed for SOC is higher, corresponding change switch of the battery is opened, disconnection includes primary switch and existed Other interior all switches, there is electric current by the way that now electric energy is in the form of magnetic energy in the secondary windings of two-way flyback transformer It is stored in the secondary windings；SOC drops in the battery meet the requirements after, disconnect time become switch, turn on primary switch, Energy is transferred to armature winding from secondary windings, and magnetic energy is converted into electric energy, so as to which unnecessary energy transfer has been arrived in battery pack In other batteries；

Balanced cell is needed for SOC is relatively low, corresponding primary switch is opened, disconnects all secondary switch, it is two-way There is electric current in the armature winding of flyback transformer by the way that now electric energy is deposited into armature winding in primary side in the form of magnetic energy In；After enough electric energy have been filled with, disconnect the switch of primary, open corresponding to minimum SOC and time to become switch, turn on the secondary around Group, energy are delivered to secondary windings from armature winding, and magnetic energy goes back to electric energy and has been filled into the battery, and the SOC of cell is returned Rise, battery pack entirety SOC is returned to consistent numerical value.