CN106026260B - A kind of series winding battery pack SOC estimation method with equalizing circuit - Google Patents

Abstract

The invention relates to a method for estimating the SOC of a battery pack connected in series with an equalizing circuit. In the method, the SOC of the battery cells connected in series is obtained by performing adaptive extended Kalman filter SOC estimation on the battery cells with the lowest or highest voltage during the charging and discharging phase. In order to eliminate the influence of noise on the measurement signal and analyze the voltage and current signals with unsteady drastic changes, wavelet transform is added before the adaptive extended Kalman filter for signal noise reduction and analysis. Considering that each battery parameter and the Open Circuit Voltage-State of Charge (OCV-SOC) curve change with temperature and SOC, this method can update the parameters online to improve the accuracy of SOC estimation. This estimation method can accurately estimate the SOC of the battery pack while ensuring the safe operation of the series battery pack, and is suitable for active and passive equalization. At the same time, the method can effectively eliminate the influence of noise, and effectively analyze voltage and current signals.

Description

A method for estimating the SOC of series-connected battery packs with equalization circuit

technical field

The invention discloses a method for estimating the SOC of a series battery pack with an equalizing circuit, and belongs to the technical field of electric vehicle power battery management systems.

Background technique

In recent years, new energy vehicles such as electric vehicles are developing rapidly, and power batteries, as the heart of electric vehicles, require efficient and reliable management, so as to ensure the safety and economy of vehicles and prolong the service life of batteries.

The state of charge of the battery refers to the ratio of the remaining charge capacity of the battery to the rated capacity of the battery, which can reflect the cruising range of the electric vehicle after a certain conversion. At the same time, accurate estimation of the battery SOC can prevent the battery from overcharging and overdischarging during charging and discharging, thereby ensuring the safe use of the battery and prolonging the service life of the battery. At present, SOC estimation methods mainly include the following: 1) open circuit voltage method; 2) ampere-hour integration method; 3) extended Kalman filter method and its improved algorithm; 4) neural network method; 5) fuzzy logic method, etc. Among them, the open circuit voltage method requires a long period of standing to obtain the open circuit voltage, and then look up the table to obtain the SOC. Therefore, this method is not suitable for real-time applications of electric vehicles. The ampere-hour integral method is simple and easy to implement. It is currently the SOC estimation method adopted by many electric vehicle manufacturers. However, this algorithm is greatly affected by the initial value and has cumulative errors. Intelligent algorithms such as neural network method and fuzzy logic method can obtain high-precision SOC estimates, but this type of algorithm requires a large amount of experimental data as support. At the same time, their algorithms are relatively complicated and difficult to apply to the current electric vehicle occasions. . The extended Kalman filter method and many improved algorithms are based on the battery equivalent circuit model or other models, which can perform optimal estimation of the battery state. At the same time, this type of algorithm can also eliminate the estimation error caused by inaccurate initial values.

Due to the difference of battery cells, the state of each battery cell will be different during the charging and discharging process. Therefore, the existing advanced battery management systems include equalization circuits, and the equalization strategies adopted include active equalization and passive equalization. At the end of charging and discharging, the voltage of some battery cells will first reach the upper or lower limit of the voltage. In order to ensure the safety and efficient use of the battery, it is necessary to turn on the equalization circuit for battery equalization during the charging and discharging process of the battery. At the same time, in the process of charging and discharging, the balance and charge and discharge control of the battery are determined by the battery cells with the cell voltages V _max and V _min . The traditional SOC estimation method is based on the characteristics of the battery pack. However, the SOC estimated by this algorithm ignores the differences of the battery cells, and also ignores the characteristics of the battery cells that actually determine the charging and discharging and balancing of the battery. Therefore, in order to ensure the safe use of the battery, and to obtain an accurate SOC estimation value of the battery pack, it is necessary to estimate the SOC of the battery cells with the battery voltage V _max and V _min during the charging and discharging process. Also need to consider the difference of the equalization circuit and the influence of the equalization current.

The actual electromagnetic environment of electric vehicles is very complex, which will bring problems such as electromagnetic interference. Therefore, many electrical signals collected by the battery management system (Battery Management System, BMS) contain a lot of noise, and these noises will bring measurement error. If the measured voltage, current and other signals are not noise-reduced, errors in SOC estimation will result. On the other hand, under actual working conditions, the current and voltage changes of electric vehicles are random and dramatic. Therefore, a method that can effectively analyze this type of signal is also needed. As a new mathematical analysis tool, wavelet transform can effectively analyze non-stationary signals, and at the same time, through certain methods, it can also achieve signal filtering and noise reduction.

During the use of electric vehicles, the state of the battery cells changes at any time, such as temperature, SOC, etc., will continue to change, and these changes will cause changes in the parameters of the battery cells. If these changes are not taken into account when building a battery model, estimation errors will result in SOC estimation. In order to eliminate the error caused by the state change, it is necessary to correct the battery cell parameters at all times, so that the battery model is consistent with the state.

Contents of the invention

Aiming at the defects and deficiencies of the prior art, the present invention proposes a method for estimating the state of charge (State of Charge, SOC) of a series battery pack with an equalizing circuit. It can accurately estimate the SOC of series battery packs while ensuring the safe operation of all battery cells, and is suitable for passive and active equalization; at the same time, it can realize non-stationary signal analysis and noise reduction; when considering battery parameter changes In some cases, AEKF estimates the SOC of the battery pack can improve the accuracy of the algorithm, and realize the self-adaptation of the algorithm and the correction of the initial error.

Concrete technical scheme of the present invention is as follows:

A method for estimating the SOC of a battery pack connected in series with an equalizing circuit, comprising the steps of:

(1) Establish an improved battery equivalent circuit model;

(2) Test the parameters of OCV, ohmic internal resistance, polarization resistance and polarization capacitance in the battery equivalent circuit model under different temperatures and different SOCs for the optional battery cells in the battery pack, and then perform polynomial simulation The relationship between the parameters of battery cell OCV, ohmic internal resistance, polarization resistance and polarization capacitance and SOC at different temperatures is obtained by combining them;

(3) Initialize the SOC initial value of this charge and discharge according to the SOC final value recorded by the battery management system at the end of the last charge and discharge process of the series battery pack with an equalization circuit, and judge the charge and discharge state;

(4) When discharging, select the battery cell with the lowest terminal voltage, that is, the terminal voltage is V _min as the battery pack SOC estimation object; when charging, select the battery cell with the highest terminal voltage, that is, the terminal voltage is V _max as the battery pack SOC estimation object ; During the non-charging and non-discharging stage during the charging and discharging process, the battery cell selected at the previous moment is still selected as the battery pack SOC estimation object;

(5) Read the terminal voltage, bus current, balance current and temperature of the selected battery cell;

(6) Carry out wavelet decomposition, noise reduction and reconstruction to the terminal voltage, bus current and balanced current read in step (5), and obtain the terminal voltage, bus current and balanced current after noise reduction;

(7) According to the temperature read in step (5), respectively select the relational expressions of battery cell OCV, ohmic internal resistance, polarization resistance, polarization capacitance and SOC in the battery equivalent circuit model at the current temperature; and then according to the above Calculate the estimated SOC value obtained by an estimation cycle, and calculate the value of the OCV, ohmic internal resistance, polarization resistance, and polarization capacitance of the battery cell under the current temperature and SOC state; for the first cycle, from the previous charge and discharge process Calculate the value of the battery cell OCV, ohmic internal resistance, polarization resistance, and polarization capacitance under the current temperature and SOC state from the final SOC value recorded by the battery management system at the end;

(8) The noise-reduced battery cell terminal voltage, bus current, and balance current obtained in step (6), as well as the OCV, ohmic internal resistance, polarization resistance, and polarity at the current temperature and SOC obtained in step (7). The capacitor is used to estimate the SOC of the battery pack with the equalization circuit through AEKF to obtain the estimated value of the SOC at the current moment;

(9) Judging whether charging or discharging is over, stop estimating when it is over, and the battery management system records the estimated SOC value estimated in step (8) as the final SOC value; if it is not over, continue with steps (4)-(9), wherein, step (8) The obtained estimated SOC value is used in the step (7) of the next estimation cycle to estimate various parameters in the battery equivalent circuit model.

Furthermore, before performing SOC estimation, it is necessary to determine the balance judgment basis and balance strategy. The battery balance judgment basis is the value of the voltage value of all battery cells in the battery pack and the set threshold value of the battery cell voltage difference, which specifically includes:

a. According to the requirements of actual balance, set the appropriate threshold value of the battery cell voltage difference;

b. If the BMS adopts passive equalization, discharge the battery cells whose voltage and V _min difference is greater than the threshold, and not balance the cells whose voltage and V _min difference is smaller than the threshold;

c. If the BMS adopts active equalization, discharge the battery cells whose voltage and V _min difference is greater than the threshold, and charge the battery cells whose voltage and V _min difference is smaller than the threshold;

d. The sign of the end of equalization is that the difference between the voltage of all battery cells and V _min is less than the threshold.

Further, in step (1), an improved battery equivalent circuit model is established, which specifically includes the following parts:

a. Variable voltage source: used to simulate the OCV value that changes with SOC and temperature;

b. A series resistor: used to simulate the ohmic internal resistance of the battery;

c. Two parallel RC links: used to simulate the polarization characteristics of the battery, which are polarization resistance and polarization capacitance;

d. Three links connected in series constitute the equivalent circuit model of the battery.

Further, step (2) specifically includes the following steps:

(2-1) Perform OCV tests of battery cells at different temperatures and different SOCs, and perform polynomial fitting on the OCV-SOC numerical curves at different temperatures to obtain a set of relationship expressions between OCV and SOC at different temperatures;

(2-2) Test the ohmic internal resistance, two polarization resistances, and two polarization capacitances of the battery cells at different temperatures and different SOCs. Polynomial fitting is carried out on the numerical curves of the polarization resistance and the two polarization capacitances, and the relationship expressions between the ohmic internal resistance of the battery cell, the two polarization resistances, the two polarization capacitances and the SOC at different temperatures are respectively obtained.

Further, step (3) specifically includes the following steps:

(3-1) According to the final SOC value recorded by the battery management system at the end of the last charging and discharging process of the series battery pack with a balancing circuit, initialize it to the initial SOC value of this charging and discharging;

(3-2) Judging whether the battery pack is in a charging or discharging state based on the positive or negative value of the bus current value of the battery pack received by the BMS. If the current is positive, the battery pack is in a discharging state; if the current is negative, the battery pack is in a charging state;

Further, step (4) specifically includes the following steps:

(4-1) If the battery pack is in the discharge state, in order to ensure that all battery cells in the series battery pack are within the safe operating voltage range, the battery cell whose terminal voltage is V _min is selected as the battery pack SOC estimation object ;

(4-2) If the battery pack is in the charging state, in order to ensure that all battery cells in the series battery pack are within the safe operating voltage range, select the battery cell whose terminal voltage is V _max as the battery pack SOC estimation object ;

(4-3) If the battery pack is not being charged and discharged, and the battery pack was in the discharge state at the last moment, the battery cell whose terminal voltage is V _min is still selected as the battery pack SOC estimation object; If the battery pack is in the charging state, the battery cell whose terminal voltage is V _max is still selected as the battery pack SOC estimation object.

Further, step (5) specifically includes the following steps:

(5-1) For the discharge state, if the battery pack adopts passive equalization, read the load current of the battery cell whose terminal voltage is V _min , that is, the bus current, as well as the terminal voltage and temperature of the battery cell, at this time V _min The battery cell has no equalization current; if the battery pack adopts active equalization, the terminal voltage is read as V _min , the load current of the battery cell, that is, the bus current, and the equalization current, terminal voltage and temperature;

(5-2) For the state of charge, regardless of whether the battery pack is passively balanced or actively balanced, it is necessary to read the charging current of the V _max battery cell, that is, the bus current, as well as the balanced current, terminal voltage and temperature;

(5-3) For the non-charging and non-discharging state during the charging and discharging process, regardless of whether the battery pack adopts passive balancing or active balancing, it is necessary to read the balancing current, terminal voltage and Temperature, at this time the battery cell has no bus current.

Further, step (6) specifically includes the following steps:

(6-1) Establish an 8th-order or higher or lower ²ⁿ -order wavelet transform matrix WTM, where the 8th-order matrix is in the form

(6-2) Multiply the terminal voltage, bus current and balanced current signal sequence NS of the noisy battery cell read in step (5) to the left by the wavelet transform matrix respectively, and the terminal voltage, bus current and balanced current can be obtained respectively The wavelet decomposition coefficient matrix CFD is expressed as follows:

CFD＝WTM·NS (2)

(6-3) Calculate the threshold value that needs to be set for noise reduction of battery cell terminal voltage, bus current and balanced current signal. The threshold value calculation expression is as follows:

σ is the standard deviation of the detail coefficients of the coefficient matrix CFD obtained by wavelet decomposition in step (6-2), N _d is the length of the coefficient matrix CFD obtained by wavelet decomposition, and δ _d is the threshold to be set for noise reduction.

(6-4) Perform soft threshold noise reduction on the coefficients in the coefficient matrix CFD obtained by decomposing the battery cell terminal voltage signal, and perform hard threshold reduction on the coefficients in the coefficient matrix CFD obtained by decomposing the battery cell bus current and balanced current signals noise. The soft threshold is expressed as follows:

The hard threshold is expressed as follows:

cD _k is the coefficient in the coefficient matrix CFD obtained by wavelet decomposition.

After denoising the coefficients, a new wavelet decomposition coefficient matrix DC after denoising is obtained.

(6-5) By multiplying the new wavelet decomposition coefficient matrix DC to the left by the inverse matrix IWTM of the wavelet transformation matrix, the original noise-containing battery cell terminal voltage, bus current and balanced current signals can be restored to the noise-eliminated battery cell The bulk terminal voltage, bus current and balanced current signal DS are expressed as follows:

DS = IWTM DC (6)

Here WTM uses an 8-order (2 ³ ) matrix, which can establish a lower or higher-order (2 ⁿ ) wavelet transform matrix. The matrix form can be derived by analogy to an 8-order matrix, but the corresponding signal sequence contains the signal The number also changes. When the WTM is 8-level, the battery cell terminal voltage, bus current and equalization current signal sequence all need to read 8 consecutive signals. After one wavelet transform denoising, 8 continuous signals after denoising can be obtained. Therefore, in the first 8 measurements of the algorithm, the wavelet transform noise reduction may not be performed first. When the number of signal readings of battery cell terminal voltage, bus current, and balance current exceeds 8, each time the SOC is estimated, the signal read this time and the signals read for the previous 7 times are used to form a signal sequence for noise reduction , to obtain 8 consecutive signals after noise reduction, and the signal value of this measurement signal after noise reduction is the last value of the sequence.

Further, step (7) specifically includes the following steps:

(7-1) According to the temperature detection value, select the corresponding OCV-SOC relationship obtained by fitting at the current temperature, and then estimate the SOC estimated value obtained from the previous estimation cycle or the final SOC value recorded in the last charge and discharge process, Obtain the OCV value of the battery cell under the current temperature and SOC state;

(7-2) According to the temperature detection value, select the ohmic internal resistance of the battery cell fitted at the current temperature, as well as the relational expressions of each polarization resistance and polarization capacitance, and then estimate the SOC estimated value based on the previous estimation cycle Or the final value of SOC recorded in the last charging and discharging process, to obtain the ohmic internal resistance of the battery cell under the current temperature and SOC state, as well as the values of each polarization resistance and polarization capacitance.

Further, step (8) specifically includes the following steps:

(8-1) According to the improved battery equivalent circuit model, select the SOC in the current state and the voltage of two RC parallel links as state variables, and the bus current and balanced current of the battery cell after noise reduction as input variables. The estimated value of the cell terminal voltage is used as the output variable to establish the state space of the battery:

is the state equation, is the measurement equation for calculating the estimated value of the terminal voltage of the battery cell; the subscript and superscript k represent the previous moment, and k+1 represents the current moment; SOC, U _p1 and U _p2 are battery state variables, and U _p1 and U _p2 are two Δt is the signal sampling time; η is the Coulomb efficiency; C _norm is the rated capacity of the battery cell; R _p1 and R _p2 are the two polarization resistances of the battery cell; C _p1 and C _p2 are the battery cell Two polarized capacitances of the body; τ _p1 and τ _p2 are the time constants of the two RC parallel links, where τ _p1 = R _p1 C _p1 , τ _p2 = R _p2 C _p2 ; U is the terminal voltage of the battery cell; U _OCV is the open _circuit voltage _of the battery cell; I _C is the bus current of the battery cell; resistance;

(8-2) The battery state space is updated and established according to the parameters of the battery cells in the current state obtained in the above step (7), and then the noise-reduced battery cell bus current, balanced current, Terminal voltage, step (8) further carries out AEKF estimation, the steps are as follows:

In the formula:

The subscript k represents the previous moment, and k+1 represents the current moment; Q and R are process noise covariance and measurement noise covariance respectively, and A and C are two Jacobian matrices required for calculation, is the state estimate, is the state covariance estimate, L is the Kalman gain, is the optimal estimated value of the state, y is the measured value of the terminal voltage of the battery cell, is the optimal estimate of the state covariance, and I is the identity matrix;

In order to achieve self-adaptation, the covariance matrix needs to be updated once in each cycle. The update process is as follows:

Among them, the subscript k and i represent time k and time i, d is the error between the voltage measurement value and the estimated value, W is the window length, and C _d is the estimated variance of the innovation;

After the current cycle estimation is completed, the estimated value of the SOC at the current moment can be obtained.

Further, step (9) specifically includes the following steps:

(9-1) BMS judges whether charging or discharging is over;

(9-2) If charging or discharging ends, the battery management system records the estimated SOC value estimated in step (8) as the final SOC value;

(9-3) If charging or discharging is not over, continue to steps (4)-(9), wherein, the SOC estimation value obtained in step (8) is used in the step (7) of the next estimation cycle to estimate the battery equivalent Individual parameters in the circuit model.

Compared with the prior art, the present invention has the following advantages:

1. The present invention can ensure that all battery cells of the battery pack are within the safe working range, prolong the service life of the battery, and realize accurate estimation of the SOC of the series-connected battery pack;

2. The present invention is suitable for passive equalization and active equalization;

3. The wavelet transform method proposed by the present invention can realize the analysis and noise reduction of non-stationary signals, thereby obtaining information such as accurate voltage and current;

4. The present invention considers the influence of changes in temperature, SOC and other conditions on battery parameters, and corrects the model in real time to make the model more accurate;

5. The adaptive extended Kalman filter (Adaptive Extended Kalman Filter, AEKF) proposed by the present invention can realize the self-adaptation of the algorithm, has improved the precision of SOC estimation, meanwhile, also has the correction function to initial error;

6. The algorithm of the present invention can be implemented by programming in embedded chips such as ARM.

Description of drawings

The accompanying drawings are used to provide a further understanding of the present invention, and constitute a part of the description, and are used to explain the present invention together with the examples of the present invention, and do not constitute a limitation to the present invention.

Fig. 1 is an improved PNGV battery equivalent circuit model diagram;

Fig. 2 is the HPPC battery test current diagram;

Fig. 3 is a HPPC battery test voltage response diagram;

Fig. 4 is a schematic diagram of passive equalization in a discharge state;

Figure 5 is a schematic diagram of active equalization in the discharge state;

Fig. 6 is a schematic diagram of passive equalization of charging state;

Fig. 7 is a schematic diagram of active equalization of charging state;

Fig. 8 is a schematic diagram of wavelet decomposition, noise reduction and reconstruction of the present invention;

FIG. 9 is a flow chart of SOC estimation for a series battery pack with an equalization circuit according to the present invention.

Detailed ways

In order to enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be described in detail below in conjunction with the accompanying drawings.

A kind of method for estimating the SOC of a series battery pack with an equalizing circuit of the present invention is as follows:

First, before SOC estimation, it is necessary to determine the balance judgment basis and balance strategy. The battery balance judgment basis is the value of all battery cell voltages in the battery pack and the threshold value of the battery cell voltage difference set, including:

a. According to the requirements of actual balance, set the appropriate threshold value of the battery cell voltage difference;

b. If the BMS adopts passive equalization, discharge the battery cells whose voltage and V _min difference is greater than the threshold, and not balance the cells whose voltage and V _min difference is smaller than the threshold;

c. If the BMS adopts active equalization, discharge the battery cells whose voltage and V _min difference is greater than the threshold, and charge the battery cells whose voltage and V _min difference is smaller than the threshold;

d. The sign of the end of equalization is that the difference between the voltage of all battery cells and V _min is less than the threshold.

A method for estimating the SOC of a battery pack connected in series with an equalizing circuit, specifically comprising the steps of:

Step 1: Establish an improved battery equivalent circuit model, as shown in Figure 1. The model specifically includes the following parts:

a. Variable voltage source: used to simulate the OCV value that changes with SOC and temperature;

b. A series resistor: used to simulate the ohmic internal resistance of the battery;

c. Two parallel RC links: used to simulate the polarization characteristics of the battery, which are polarization resistance and polarization capacitance;

d. Three links connected in series constitute the equivalent circuit model of the battery.

Step 2: Test the parameters of OCV, ohmic internal resistance, polarization resistance and polarization capacitance in the battery equivalent circuit model under different temperatures and different SOCs for the optional battery cells in the battery pack, and then perform polynomial simulation Combined to obtain the relationship between the parameters of battery cell OCV, ohmic internal resistance, polarization resistance and polarization capacitance and SOC at different temperatures, specifically including the following steps:

(2-1) The OCV test of the battery cell is carried out at different temperatures and different SOCs, and the OCV values under different states can be obtained. Further, the OCV-SOC numerical curves at different temperatures are polynomially fitted, so that it can be obtained A set of OCV-SOC relational expressions at different temperatures, and then the OCV value in the current state can be obtained according to the estimated value of temperature and SOC;

(2-2) Perform HPPC tests at different temperatures and different SOCs, as shown in Figure 2, the test process is: 1) The battery cell is left to stand to a steady state; 2) A 40s pulse discharge experiment with a current of 5C is performed; 3 ) The battery cell was left standing for 200s; 4) The 40s pulse charging experiment with a current of 5C was carried out; 5) The battery was left standing for 200s. During the test, the BMS records the terminal voltage value of the battery cell, and the terminal voltage response curve is shown in Figure 3;

(2-3) The battery cell ohmic internal resistance value can be obtained from the battery cell terminal voltage mutation value at time t ₁ and t ₂ , and the terminal voltage response curves of t ₁ -t ₂ and t ₂ -t ₃ can be obtained in Perform curve fitting in Matlab to obtain the values of the polarization resistance and polarization capacitance of the battery cell.

(2-4) Perform polynomial fitting on the ohmic internal resistance, two polarization resistances, and two polarization capacitance numerical curves of the battery cell at different temperatures and different SOCs, and the ohmic internal resistance of the battery cell at different temperatures can be obtained respectively 1. The relationship between the two polarization resistances and two polarization capacitances of the battery cell and the SOC, and then the values of the parameters of the battery cell in the current state can be obtained according to the temperature and the estimated value of the SOC.

Step 3: Initialize the initial SOC value of this charging and discharging according to the final SOC value recorded by the battery management system at the end of the last charging and discharging process of the series battery pack with a balance circuit, and judge the charging and discharging status, specifically including the following steps:

(3-1) According to the final SOC value recorded by the battery management system at the end of the last charging and discharging process of the series battery pack with a balancing circuit, initialize it to the initial SOC value of this charging and discharging;

(3-2) Judging whether the battery pack is in a charging or discharging state based on the positive or negative value of the bus current value of the battery pack received by the BMS. If the current is positive, the battery pack is in a discharging state; if the current is negative, the battery pack is in a charging state;

Step 4: When discharging, select the battery cell with the lowest terminal voltage, that is, the terminal voltage is V _min as the battery pack SOC estimation object; when charging, select the battery cell with the highest terminal voltage, that is, the terminal voltage is V _max as the battery pack SOC estimation object ; In the non-charging and non-discharging stage during the charging and discharging process, the battery cell selected at the previous moment is still selected as the battery pack SOC estimation object, which specifically includes the following steps:

(4-1) If the battery pack is in the discharge state, in order to ensure that all battery cells in the series battery pack are within the safe operating voltage range, the battery cell whose terminal voltage is V _min is selected as the battery pack SOC estimation object ;

(4-2) If the battery pack is in the charging state, in order to ensure that all battery cells in the series battery pack are within the safe operating voltage range, select the battery cell whose terminal voltage is V _max as the battery pack SOC estimation object ;

(4-3) If the battery pack is not being charged and discharged, and the battery pack was in the discharge state at the last moment, the battery cell whose terminal voltage is V _min is still selected as the battery pack SOC estimation object; If the battery pack is in the charging state, the battery cell whose terminal voltage is V _max is still selected as the battery pack SOC estimation object.

Step 5: Read the terminal voltage, bus current, balance current and temperature of the selected battery cell, specifically including the following steps:

(5-1) Figure 4 is a schematic diagram of passive equalization in the discharge state. In the figure, it is assumed that the n-1th battery cell is a V _min battery cell, and the difference between the voltage of the 1st and nth battery cells and V _min is greater than the threshold , needs equalization; while the difference between cell voltage and V _min of cells 2 and n-1 is less than the threshold, no equalization is required. For passive equalization of the discharge state, it is necessary to read the load current of the V _min battery cell, that is, the bus current, as well as the terminal voltage and temperature of the battery cell. At this time, the V _min battery cell has no equalization current;

(5-2) Figure 5 is a schematic diagram of active equalization of the discharge state. In the figure, it is assumed that the n-1 battery cell is a V _min battery cell, and the difference between the battery cell voltage and V _min of the first and n cells is greater than the threshold , discharge balance is required; although the difference between the battery cell voltage and V _min of cells 2 and n-1 is less than the threshold, charge balance is required. For the active equalization of the discharge state, it is necessary to read the load current of the V _min battery cell, that is, the bus current, and the charging equalization current, terminal voltage and temperature of the battery cell;

(5-3) Figure 6 is a schematic diagram of passive equalization of the charging state. In the figure, it is assumed that the first battery cell is a V _max battery cell, and the difference between the voltage of the first and n battery cells and V _min is greater than the threshold. Equalization; while the difference between the cell voltage of cells 2 and n-1 and V _min is less than the threshold, no equalization is required. For passive equalization of the charging state, it is necessary to read the load current of the V _max battery cell, that is, the bus current, and the discharge equalization current, terminal voltage and temperature of the battery cell;

(5-4) Figure 7 is a schematic diagram of the active equalization of the charging state. In the figure, it is assumed that the first battery cell is a V _max battery cell, and the difference between the voltage of the first and n battery cells and V _min is greater than the threshold value, and it is necessary to Discharge equalization; although the difference between the cell voltage of cells 2 and n-1 and V _min is less than the threshold, charge equalization is required. For the active equalization of the state of charge, it is necessary to read the load current of the V _max battery cell, that is, the bus current, and the discharge equalization current, terminal voltage and temperature of the battery cell;

(5-5) For the non-charging and non-discharging state during the charging and discharging process, regardless of whether the battery pack adopts passive balancing or active balancing, it is necessary to read the balancing current, terminal voltage and Temperature, at this time the battery cell has no bus current.

Step 6: Perform wavelet decomposition, noise reduction and reconstruction on the terminal voltage, bus current and balanced current read in step 5 to obtain the denoised terminal voltage, bus current and balanced current. The execution process is shown in Figure 8. Specifically include the following steps:

(6-1) Establish an 8th-order or higher or lower ²ⁿ -order wavelet transform matrix WTM, where the 8th-order matrix is in the form

(6-2) Multiply the terminal voltage, bus current and balanced current signal sequence NS of the noise-containing battery cell read in step 5 to the left by the wavelet transformation matrix respectively, and the wavelet decomposition of the terminal voltage, bus current and balanced current can be obtained respectively The coefficient matrix CFD is expressed as follows:

CFD＝WTM·NS (2)

(6-3) Calculate the threshold value that needs to be set for noise reduction of battery cell terminal voltage, bus current and balanced current signal. The threshold value calculation expression is as follows:

σ is the standard deviation of the detail coefficients of the coefficient matrix CFD obtained by wavelet decomposition in step (6-2), N _d is the length of the coefficient matrix CFD obtained by wavelet decomposition, and δ _d is the threshold to be set for noise reduction.

(6-4) Perform soft threshold noise reduction on the coefficients in the coefficient matrix CFD obtained by decomposing the battery cell terminal voltage signal, and perform hard threshold reduction on the coefficients in the coefficient matrix CFD obtained by decomposing the battery cell bus current and balanced current signals noise. The soft threshold is expressed as follows:

The hard threshold is expressed as follows:

cD _k is the coefficient in the coefficient matrix CFD obtained by wavelet decomposition.

After denoising the coefficients, a new wavelet decomposition coefficient matrix DC after denoising is obtained.

(6-5) By multiplying the new wavelet decomposition coefficient matrix DC to the left by the inverse matrix IWTM of the wavelet transformation matrix, the original noise-containing battery cell terminal voltage, bus current and balanced current signals can be restored to the noise-eliminated battery cell The bulk terminal voltage, bus current and balanced current signal DS are expressed as follows:

DS = IWTM DC (6)

Here WTM uses an 8-order (2 ³ ) matrix, which can establish a lower or higher-order (2 ⁿ ) wavelet transform matrix. The matrix form can be derived by analogy to an 8-order matrix, but the corresponding signal sequence contains the signal The number also changes. When the WTM is 8-level, the battery cell terminal voltage, bus current and equalization current signal sequence all need to read 8 consecutive signals. After one wavelet transform denoising, 8 continuous signals after denoising can be obtained. Therefore, in the first 8 measurements of the algorithm, the wavelet transform noise reduction may not be performed first. When the number of signal readings of battery cell terminal voltage, bus current, and balance current exceeds 8, each time the SOC is estimated, the signal read this time and the signals read for the previous 7 times are used to form a signal sequence for noise reduction , to obtain 8 consecutive signals after noise reduction, and the signal value of this measurement signal after noise reduction is the last value of the sequence.

Step 7: According to the temperature read in step 5, respectively select the relational expressions of battery cell OCV, ohmic internal resistance, polarization resistance, polarization capacitance and SOC in the battery equivalent circuit model at the current temperature; and then estimate according to the previous one Calculate the SOC estimated value obtained by cycle estimation or the SOC final value recorded in the last charging and discharging process, and calculate the values of battery cell OCV, ohmic internal resistance, polarization resistance, and polarization capacitance under the current temperature and SOC state, including the following step:

(7-1) According to the temperature detection value, select the corresponding OCV-SOC relationship obtained by fitting at the current temperature, and then estimate the SOC estimated value obtained from the previous estimation cycle or the final SOC value recorded in the last charge and discharge process, Obtain the OCV value of the battery cell under the current temperature and SOC state;

(7-2) According to the temperature detection value, select the ohmic internal resistance of the battery cell fitted at the current temperature, as well as the relational expressions of each polarization resistance and polarization capacitance, and then estimate the SOC estimated value based on the previous estimation cycle Or the final value of SOC recorded in the last charging and discharging process to obtain the current temperature and SOC

The ohmic internal resistance of the battery cell in the state, and the values of each polarization resistance and polarization capacitance.

Step 8: The noise-reduced battery cell terminal voltage, bus current, and balanced current obtained in step 6, as well as the current temperature and OCV, ohmic internal resistance, polarization resistance, and polarization capacitance at the SOC obtained in step 7, are passed through AEKF estimates the SOC of the battery pack with a balancing circuit, and obtains the estimated value of the SOC at the current moment, which specifically includes the following steps:

(8-1) According to the improved battery equivalent circuit model, select the SOC in the current state and the voltage of two RC parallel links as state variables, and the bus current and balanced current of the battery cell after noise reduction as input variables. The estimated value of the cell terminal voltage is used as the output variable to establish the state space of the battery:

is the state equation, is the measurement equation for calculating the estimated value of the terminal voltage of the battery cell; the subscript and superscript k represent the previous moment, and k+1 represents the current moment; SOC, U _p1 and U _p2 are battery state variables, and U _p1 and U _p2 are two Δt is the signal sampling time; η is the Coulomb efficiency; C _norm is the rated capacity of the battery cell; R _p1 and R _p2 are the two polarization resistances of the battery cell; C _p1 and C _p2 are the battery cell Two polarized capacitances of the body; τ _p1 and τ _p2 are the time constants of the two RC parallel links, where τ _p1 = R _p1 C _p1 , τ _p2 = R _p2 C _p2 ; U is the terminal voltage of the battery cell; U _OCV is the open _circuit voltage _of the battery cell; I _C is the bus current of the battery cell; resistance;

(8-2) Update the established battery state space according to the parameters of the battery cell in the current state obtained in the above step 7, and then obtain the noise-reduced bus current, balanced current, and terminal voltage of the battery cell obtained in step 6, step Eight further AEKF estimation, the steps are as follows:

In the formula:

The subscript k represents the previous moment, and k+1 represents the current moment; Q and R are process noise covariance and measurement noise covariance respectively, and A and C are two Jacobian matrices required for calculation, is the state estimate, is the state covariance estimate, L is the Kalman gain, is the optimal estimated value of the state, y is the measured value of the terminal voltage of the battery cell, is the optimal estimate of the state covariance, and I is the identity matrix;

In order to achieve self-adaptation, the covariance matrix needs to be updated once in each cycle. The update process is as follows:

Among them, the subscript k and i represent time k and time i, d is the error between the voltage measurement value and the estimated value, W is the window length, and C _d is the estimated variance of the innovation;

After the current cycle estimation is completed, the estimated value of the SOC at the current moment can be obtained.

Step nine specifically includes the following steps:

(9-1) BMS judges whether charging or discharging is over;

(9-2) If charging or discharging ends, the battery management system records the estimated SOC value estimated in step 8 as the final SOC value;

(9-3) If the charging or discharging is not finished, continue to step 4 to step 9, wherein the estimated SOC value obtained in step 8 is used in step 7 of the next estimation cycle to estimate various parameters in the battery equivalent circuit model.

Fig. 9 is a flow chart of estimating the SOC of a battery pack in series with an equalizing circuit, as can be seen from the figure, the present invention can perform SOC on a battery pack with an equalizing circuit under the conditions of charging, discharging or not charging and discharging. estimate. During the estimation process, battery parameters that change due to changes in battery state are continuously adjusted. At the same time, by using the wavelet transform method, the analysis and filtering of non-stationary noise-containing signals can be realized.

To sum up, in order to realize the accurate estimation of the SOC of the series battery pack with the equalization circuit, the present invention proposes a method for estimating the SOC of the series battery pack with the equalization circuit. Its main advantages are: 1) Under the condition of ensuring the safe operation of all battery cells, it can realize the accurate estimation of the SOC of the series battery pack; 2) It is suitable for the situation of passive equalization and active equalization; 3) It can realize the analysis and reduction of non-stationary signals. 4) Considering the influence of changes in temperature and SOC on battery parameters, the model is more accurate; 5) The proposed AEKF can realize the self-adaptation of the algorithm and improve the SOC The accuracy of the estimate, meanwhile, also has the effect of correcting the initial error.

The above embodiments are only used to help understand the method and core idea of the present invention. For those of ordinary skill in the art, on the premise of not departing from the principle of the present invention, other equivalent application schemes that can be naturally associated with the above description and examples , and several improvements and modifications to the present invention are included within the protection scope of the present invention.

Claims (9)

1. a kind of series battery SOC estimation method with equalizing circuit, which is characterized in that include the following steps：

(1) improved battery equivalent circuit model is established；

(2) battery cell optional in battery pack is carried out in the battery equivalent circuit model under different temperatures, difference SOC The test of OCV, ohmic internal resistance, polarization resistance and each parameter of polarization capacity, then carry out fitting of a polynomial obtain it is electric under different temperatures Pond monomer OCV, ohmic internal resistance, polarization resistance and each parameter of polarization capacity and the relational expression of SOC；

(3) SOC that battery management system records at the end of series battery last time charge and discharge process of the basis with equalizing circuit Final value initializes the SOC initial values of this charge and discharge, and judges charging and discharging state；

(4) selection terminal voltage is minimum when discharging, i.e., terminal voltage is V_minBattery cell as battery pack SOC estimate object；Charging When select terminal voltage highest, i.e., terminal voltage be V_maxBattery cell as battery pack SOC estimate object；Go out in charge and discharge process Existing not filling is not put the stage, and the battery cell that last moment selects still is selected to estimate object as battery pack SOC；

(5) terminal voltage, bus current, euqalizing current and the temperature of battery cell selected by reading；

(6) terminal voltage, bus current, euqalizing current progress wavelet decomposition, noise reduction and the reconstruct to being read in step (5), obtains Terminal voltage, bus current and euqalizing current after noise reduction；

(7) according to the temperature read in step (5), battery cell in battery equivalent circuit model is selected under Current Temperatures respectively OCV, ohmic internal resistance, polarization resistance, the relational expression of polarization capacity and SOC；The SOC obtained further according to upper one estimation cycle estimation The numerical value of battery cell OCV under Current Temperatures and SOC states, ohmic internal resistance, polarization resistance, polarization capacity is obtained in estimated value； For first time recycle, by last charge and discharge process at the end of battery management system record SOC final values be obtained Current Temperatures and The numerical value of battery cell OCV, ohmic internal resistance, polarization resistance, polarization capacity under SOC states；

(8) battery cell terminal voltage, bus current, euqalizing current and the step (7) after the noise reduction obtained by step (6) obtain OCV, ohmic internal resistance, polarization resistance and polarization capacity under the Current Temperatures and SOC that arrive, pass through adaptive extended kalman filtering The SOC of battery pack of the estimation with equalizing circuit, obtains the SOC estimated values at current time；

(9) judge whether charge or discharge terminate, end then stops estimating, battery management system recording step (8) estimation obtains SOC estimated values as SOC final values；Do not terminate, continue step (4)-(9), wherein, the SOC estimated values that step (8) obtains are used Parameters in next the step of estimating cycle (7) estimation battery equivalent circuit model.

2. the series battery SOC estimation method according to claim 1 with equalizing circuit, which is characterized in that into Before row SOC estimations, it is thus necessary to determine that balanced basis for estimation and balance policy, battery balanced basis for estimation are all electricity of battery pack The threshold value of the numerical value of pond monomer voltage and the battery cell voltage difference of setting, specifically includes：

A. according to practical balanced requirement, the threshold value of suitable battery cell voltage difference is set；

B. if battery management system is using passive equilibrium, to battery cell voltage and V_minDifference is more than the battery cell of threshold value It discharges, to battery cell voltage and V_minThe battery cell that difference is less than threshold value does not do equilibrium；

C. if battery management system uses active equalization, to battery cell voltage and V_minDifference is more than the battery cell of threshold value It discharges, to battery cell voltage and V_minThe battery cell that difference is less than threshold value charges；

D. voltage and V of the balanced mark terminated for all battery cells_minDifference is less than threshold value.

3. the series battery SOC estimation method according to claim 1 with equalizing circuit, which is characterized in that step (1) in, improved battery equivalent circuit model is established, which specifically includes such as lower part：

A. variable voltage source：For simulating with SOC and the OCV numerical value of temperature change；

B. a series resistance：For simulated battery ohmic internal resistance；

C. two RC links in parallel：It is polarization resistance and polarization capacity respectively for the polarization characteristic of simulated battery；

D. the equivalent-circuit model of three link batteries in series.

4. according to any series battery SOC estimation methods with equalizing circuit of claim 1-3, feature exists In step specifically comprises the following steps in (2)：

(2-1) carries out the OCV tests of battery cell under different temperatures, difference SOC, by the OCV-SOC numerical value under different temperatures Curve carries out fitting of a polynomial and obtains the relational expression of OCV and SOC under one group of different temperatures；

(2-2) carries out ohmic internal resistance, two polarization resistances and the two polarization electricity of battery cell under different temperatures, difference SOC The test of appearance, by battery cell ohmic internal resistance, two polarization resistances and the two polarization capacity numerical curves under different temperatures into Row fitting of a polynomial respectively obtains multigroup different temperatures lower battery cell ohmic internal resistance, two polarization resistances and two polarization electricity Hold the relational expression with SOC.

5. the series battery SOC estimation method according to claim 4 with equalizing circuit, which is characterized in that step (3) specifically comprise the following steps in：

Battery management system records at the end of the last charge and discharge process of series battery of (3-1) basis with equalizing circuit SOC final values are initialized as the SOC initial values of this charge and discharge；

The positive negative judgement battery pack for the battery pack bus current numerical value that (3-2) is received by battery management system is in charging or puts Electricity condition：If electric current is just, battery pack is in discharge condition；If electric current is negative, battery pack is in charged state.

6. the series battery SOC estimation method according to claim 5 with equalizing circuit, which is characterized in that step (4) specifically comprise the following steps in：

(4-1) is in trouble free service electricity if battery pack is in discharge condition, for all battery cells in guarantee series battery In the range of pressure, battery cell terminal voltage is selected as V_minBattery cell as battery pack SOC estimate object；

(4-2) is in trouble free service electricity if battery pack is in charged state, for all battery cells in guarantee series battery In the range of pressure, battery cell terminal voltage is selected as V_maxBattery cell as battery pack SOC estimate object；

(4-3) be not if battery pack carries out charge and discharge, and last moment battery pack is in discharge condition, then still selects battery list Body end voltage is V_minBattery cell as battery pack SOC estimate object；If last moment battery pack is in charged state, Battery cell terminal voltage is still selected as V_maxBattery cell as battery pack SOC estimate object.

7. the series battery SOC estimation method according to claim 6 with equalizing circuit, which is characterized in that step (5) specifically comprise the following steps in：

(5-1) is for discharge condition, if battery pack reads terminal voltage as V using passive equilibrium_minBattery cell it is negative The terminal voltage and temperature of electric current, i.e. bus current and battery cell are carried, at this time V_minBattery cell does not have euqalizing current；If Battery pack uses active equalization, then reads terminal voltage as V_minThe load current of battery cell, i.e. bus current and balanced electricity Stream, terminal voltage and temperature；

(5-2) for charged state, no matter battery pack using passive balanced or active equalization, is required to read V_maxBattery cell Charging current, i.e. bus current and euqalizing current, terminal voltage and temperature；

(5-3) for the not discharge condition that do not charge in charge and discharge process, no matter battery pack is using passive balanced or active equalization, The euqalizing current of the battery cell selected in read step (4-3), terminal voltage and temperature are required to, battery cell is without mother at this time Line current.

8. the series battery SOC estimation method according to claim 7 with equalizing circuit, which is characterized in that step (6) specifically comprise the following steps in：

(6-1) establishes 8 ranks or exponent number higher, lower 2ⁿRank wavelet transform matrix WTM, wherein, 8 rank matrix forms are

Terminal voltage, bus current and the euqalizing current signal sequence of the battery cell of Noise that (6-2) will be read in step (5) NS difference premultiplication wavelet transform matrix is arranged, terminal voltage, bus current and euqalizing current coefficient of wavelet decomposition square can be respectively obtained Battle array CFD, represents as follows：

CFD=WTMNS (2)

(6-3) calculates the threshold value that battery cell terminal voltage, bus current and euqalizing current signal de-noising needs are set, threshold calculations Expression formula is as follows：

σ is the standard deviation of the detail coefficients of coefficient matrix CFD that wavelet decomposition obtains in step (6-2), N_dIt is that wavelet decomposition obtains Coefficient matrix CFD length, δ_dIt is the threshold value that noise reduction needs are set；

Coefficient in the coefficient matrix CFD that (6-4) decomposes battery cell terminal voltage signal carries out soft threshold de-noising, to electricity Coefficient in the coefficient matrix CFD that pond monomer bus current and euqalizing current signal decomposition obtain carries out hard -threshold noise reduction, soft threshold Value is expressed as follows：

Hard -threshold is expressed as follows：

cD_kIt is the coefficient in the coefficient matrix CFD that wavelet decomposition obtains；

By the noise reduction process of coefficient, the new coefficient of wavelet decomposition matrix D C after noise reduction is obtained；

(6-5) can be incited somebody to action original by the inverse matrix IWTM to new coefficient of wavelet decomposition matrix D C premultiplication wavelet transform matrix Noise battery cell terminal voltage, bus current and euqalizing current signal revert to eliminate noise battery cell end electricity Pressure, bus current and euqalizing current signal DS, are expressed as follows：

DS=IWTMDC (6)

When WTM is 8 rank, battery cell terminal voltage, bus current and euqalizing current signal sequence are required to read continuous 8 A signal；After a wavelet transformation noise reduction is carried out, continuous 8 signals after noise reduction can be obtained；Therefore, in algorithm It, can be first without wavelet transformation noise reduction when 8 times most started measure；When battery cell terminal voltage, bus current and equilibrium When the signal-obtaining number of electric current is more than 8, when carrying out SOC estimations every time, by the signal of this reading and the signal of first 7 times readings It forms signal sequence and carries out noise reduction and obtain continuous 8 signals after noise reduction, the signal value after this measuring signal noise reduction is sequence The last one numerical value of row.

9. the series battery SOC estimation method according to claim 8 with equalizing circuit, which is characterized in that step (8) specifically comprise the following steps in：

(8-1) selects the voltage of SOC and two RC parallel connection link under current state according to improved battery equivalent circuit model As state variable, the bus current and euqalizing current of the battery cell after noise reduction are as input variable, battery cell terminal voltage Estimated value establishes the state space of battery as output variable：

It is state equation,It is the measurement equation for calculating battery cell terminal voltage estimated value；Subscript and upper Target k represents last moment, and k+1 represents current time；SOC,U_p1And U_p2It is battery status variable, U_p1And U_p2Be two RC simultaneously Join the voltage of link；Δ t is the signal sampling time；η is coulombic efficiency；C_normIt is battery cell rated capacity；R_p1、R_p2It is battery Two polarization resistances of monomer；C_p1、C_p2It is two polarization capacities of battery cell；τ_p1、τ_p2Be two RC parallel connection links time it is normal Number, wherein, τ_p1=R_p1C_p1,τ_p2=R_p2C_p2；U is battery cell terminal voltage；U_OCVIt is battery cell open-circuit voltage；I_CIt is battery Monomer bus current, I_BIt is battery cell euqalizing current, plus-minus symbol depends on battery cell euqalizing current and bus current side To whether unanimously；R_OIt is battery cell ohmic internal resistance；

The more newly-established battery status of battery cell parameters under the current state that (8-2) is obtained according to above-mentioned steps (7) Battery cell bus current, euqalizing current behind space, then the noise reduction obtained by step (6), terminal voltage, step (8) are further Carry out AEKF estimations, step is as follows：

In formula：

Lower target k represents last moment, and k+1 represents current time；Q, R is process noise covariance and measurement noise association side respectively Difference, A and C are to calculate two required Jacobian matrixs,It is state estimation,It is state covariance estimated value, L is karr Graceful gain,It is state optimization estimated value, y is battery cell terminal voltage measured value,It is state covariance optimal estimation value, I It is unit matrix；

It is adaptive in order to realize, it needs to be cyclically updated a covariance matrix each, renewal process is as follows：

Wherein, lower target k and i represent k moment and i moment, and d is the error of voltage measuring value and estimated value, and W is length of window, C_d It is new breath estimation variance；

After the completion of this cycle estimation, the SOC estimated values at current time can be obtained.