CN108090244A - A kind of parallel type lithium ion battery system modeling method - Google Patents

Abstract

The invention discloses a modeling method for a parallel lithium-ion battery system. The battery system is formed by connecting N branch battery strings in parallel, and each battery string is formed by connecting M lithium-ion monomers in series. The method is as follows: according to the known lithium-ion battery single model, using the operating characteristics of the parallel circuit and the screening method, the basic model of the battery system is established; the current of each branch is detected, and 1/N of the total output current of the basic model is used as The input of the SOC corrector; the SOC corrector is composed of N proportional-integral regulators and a weighter. The SOC compensation value of the battery system is obtained through the SOC corrector, and then superimposed with the SOC output by the battery system model to obtain the corrected SOC. In this way, the basic model of the battery system is updated, and an accurate battery system model is obtained through such a cycle. The parallel lithium-ion battery system model established by the invention can more accurately predict the discharge operating characteristics of the parallel battery system, and its modeling method is applicable to the parallel connection of battery cells or modules.

Description

A Modeling Method for Parallel Li-ion Battery System

technical field

The invention belongs to the technical field of design and control of a MW-level battery energy storage system in a smart grid, and relates to a modeling method for a parallel lithium-ion battery system.

Background technique

With the large-scale and rapid rise of new energy power generation (such as wind power and photovoltaics), battery energy storage systems have become one of the effective ways for new energy power generation to be integrated into the grid. At the same time, lithium-ion batteries have become the main carrier of battery energy storage systems due to their high specific energy, long cycle life, low self-discharge, and no memory effect. However, the rated capacity and rated voltage of lithium-ion battery cells are not high, and multiple battery cells need to be connected in parallel to form a battery system, that is, a parallel battery system, in order to increase the capacity and output current of the battery system. At the same time, due to the inconsistency of the battery cells in the battery system, it is more difficult to accurately characterize the working characteristics of the battery system, which seriously restricts the development and application of the battery system. Therefore, it is very important to establish an accurate parallel battery system model to accurately predict its operating characteristics for its design, control and engineering applications.

At present, the research and patents on battery modeling at home and abroad are mostly focused on battery cells, and there are not many documents on battery system modeling. A public document (CN105116338A) discloses a parallel battery system based on SOC compensator. method, the modeling method is: according to the known performance parameters of lithium-ion battery cells, using the parallel circuit operating characteristics and screening method to establish a battery system mean value model, and then using the actual value and mean value model branch of each branch battery current The current is compared to design a SOC compensator, so as to determine the mathematical relationship between the performance parameters of each battery cell and the performance parameters of the battery system, and establish a parallel battery system model. This model takes into account the impact of battery inconsistency in the battery system on the accuracy of the battery model, and improves the accuracy of the battery system model to a certain extent, but there are still the following problems: First, its SOC compensator is actually composed of a proportional P regulator After weighting, because of the inherent steady-state error when using the proportional regulator, the accuracy of the battery SOC compensation value ΔSOC _m generated by each branch must be limited; the second is to calculate the parameter mean model input amount SOC ^* , is directly obtained by adding the sum of each compensation value ΔSOC _m to the SOC _I output by the SOC module. Therefore, although the input quantity SOC ^* includes the sum of the compensation values ΔSOC _m of each branch, it does not take into account the The inconsistency of the battery cells in each branch causes the current variation range of each branch to be different, so that the compensation value ΔSOC _m of each branch is also different, so the accuracy of the compensation value SOC ^* is also limited; the third is to calculate the mean value of the parameters When the model input SOC ^* , because the SOC module is used to obtain SOC _I , on the one hand, the additional modules will increase the system cost, and on the other hand, because it is calculated by the ampere-hour method, there must be cumulative errors, and the initial battery power must be known, etc. Inherent disadvantages lead to limited SOC accuracy. Therefore, it is necessary to further improve the SOC compensator to improve the accuracy of the battery system model.

Contents of the invention

The problem to be solved by the present invention is to provide a method for modeling a parallel lithium-ion battery system. On the one hand, it solves the problem of battery performance parameters (such as voltage, Current, etc.) and charging and discharging characteristics are difficult to be accurately measured and estimated. On the other hand, it also improves the steady-state error existing in the relevant disclosed SOC compensator and does not consider the degree of inconsistency of the battery cells in each branch. As well as the problem of increased system cost, the purpose of accurately predicting the terminal voltage and SOC of the parallel battery system is achieved.

The object of the invention is to realize through the following technical solutions:

The present invention provides a parallel lithium-ion battery system. The system is composed of N branch battery strings, and each branch battery string is formed by connecting M lithium-ion battery cells in series. Its structure diagram is shown in Figure 1 .

A parallel lithium-ion battery system modeling method is as follows: According to the known lithium-ion battery single model (1), using the parallel circuit operating characteristics and screening method, a basic model of the battery system (2) is established, and then each branch circuit is detected Battery current, each branch battery string current and 1/N of the total output current of the basic model of the battery system are used as the input of the State of Charge (SOC) calibrator (3), and the SOC calibrator consists of N proportional-integral (PI) regulator and a weighting device, the SOC compensation value (ΔSOC _i ) of N branch battery strings is obtained through N PI regulators, and the SOC compensation value of each branch battery string is weighted to obtain the SOC of the battery system The compensation value (ΔSOC _b ) is superimposed with the SOC _b output by the basic model of the battery system to obtain the corrected SOC _r , thereby updating the performance parameters of the battery system, and then updating the basic model of the battery system. In this way, an accurate battery system model is obtained. Figure 2 is a structural diagram of a parallel lithium-ion battery system model.

The equivalent circuit model of the battery cell is a second-order equivalent circuit model, and its circuit diagram is shown in Figure 3. The main circuit of the model consists of two RC parallel circuits, a controlled voltage source U ₀ (SOC) and a battery internal resistance R, etc. Composition, its mathematical expression is: U(t)＝U ₀ [SOC(t)]-I(t)[R(t)+R _s (t)/R _s (t)jωC _s (t)+R _l (t)/R _l (t)jωC _l (t)], where U ₀ (SOC) is the open circuit terminal voltage of the battery cell, R(t) is the internal resistance of the battery cell, R _s (t), R _l (t), C _s (t), and C _l (t) are the resistance and capacitance describing the transient response characteristics of the battery cell, respectively. The above performance parameters are related to SOC, and the definition of SOC is: Among them, SOC ₀ is the initial value of the SOC of the battery cell, which is generally a constant between 0 and 1; _Qu (t) is the unusable capacity of the battery cell, and Q ₀ is the rated capacity of the battery cell. The calculations of U ₀ (SOC), R _s (t), R _l (t) and C _s (t), C _l (t) are as follows:

Among them, a ₀ ~ a ₅ , c ₀ ~ c ₂ , d ₀ ~ d ₂ , e ₀ ~ e ₂ , f ₀ ~ f ₂ , b ₀ ~ b ₅ are all model coefficients, which can be obtained by fitting the battery measurement data have to.

The basic model of the parallel battery system established is a second-order equivalent circuit model, and its circuit diagram is shown in Figure 4. The battery model expression obtained by Kirchhoff's law KVC is: U(t)= _Ub0 [SOC(t)]- _Ib (t) _Zb (t). The basic model for determining the performance parameters of each battery cell and the battery system by using the operating characteristics of the parallel circuit and the screening method is determined as follows: the open-circuit terminal voltage of the battery system in the basic model is calculated as follows: U _b0 (SOC) = MU ₀ (SOC), Among them, U ₀ (SOC) is the open-circuit terminal voltage of the battery cell; the impedance of the battery system in the basic model is calculated as follows: Among them, R _b (t) is the internal resistance of the battery system, R _bs (t), R _bl (t) and C _bs (t), C _bl (t) are the resistance and capacitance describing the transient response characteristics of the battery system, respectively. The calculations of R _bs (t), R _bl (t) and C _bs (t), C _bl (t) in the basic model are as follows:

It is characterized in that the design of the SOC corrector is as follows: the SOC corrector is composed of N PI regulators and a weighter, and the two inputs of each PI regulator are respectively the i-th branch battery string output current I _i and 1/N of the total current I _b output by the battery model; the SOC compensation value ΔSOC _i of N branch battery strings is obtained after passing through each PI regulator, namely In the formula, k _P is a proportional constant, k _I is a proportional constant, s is an integral factor, and i is a natural number greater than 1; then the battery system SOC compensation value ΔSOC _b is obtained after passing through the weighter, namely In the formula, _ki is the weighting coefficient.

Finally, add the compensation value ΔSOC _b to the SOC _b output by the basic model of the battery system, and use it as the new input SOC _r of the basic model to update the performance parameters of the battery system, and then update the basic model of the battery system. system model.

Compared with the public document (CN105116338A), the present invention has the following beneficial technical effects: one is to replace the P regulator with the PI regulator, which eliminates the steady-state error; Add them together to calculate the battery system SOC compensation value ΔSOC _b ; the third is to ignore the SOC module and directly replace it with the SOC estimated value SOC _b output by the current system model. Therefore, during the entire discharge process, the equivalent circuit model of the battery system proposed by the present invention can more accurately predict the change of the terminal voltage of the battery system, indicating that the model has higher precision.

Description of drawings

Figure 1 is a schematic structural diagram of a parallel lithium-ion battery system;

Figure 2 is a structural diagram of a parallel lithium-ion battery system model;

Fig. 3 is the equivalent circuit model diagram of a lithium-ion battery cell;

Fig. 4 is an equivalent circuit model diagram of a parallel lithium-ion battery system;

Figure 5-1 to Figure 5-2 show the pulse discharge characteristics of batteries under different SOC ₀ conditions, where Figure 5-1 shows the current change of each branch, and Figure 5-2 shows the comparison of the parallel battery system terminal voltage before and after correction.

Detailed ways

The present invention will be further described in detail below in conjunction with specific examples, which are for explanation of the present invention rather than limitation.

1. Parallel battery system and battery cell model

1.1 Parallel battery system

The parallel battery system is formed by connecting two battery strings in parallel. Each battery string is composed of one battery cell. The rated voltage of each battery cell is 3.7V, the rated capacity is 860mAh, and the discharge cut-off voltage is 3V.

1.2 Equivalent circuit model of battery cell

The equivalent circuit model of a battery cell is a second-order equivalent circuit model. The main circuit of the model is composed of two RC parallel circuits, a controlled voltage source U ₀ (SOC) and a battery internal resistance R. The mathematical model expression is: U( t)＝U ₀ [SOC(t)]-I(t)[R(t)+R _s (t)/R _s (t)jωC _s (t)+R _l (t)/R _l (t) jωC _l (t)], where the calculations of U ₀ (SOC), R _s (t), R _l (t) and C _s (t), C _l (t) are as follows: Among them, the values of a ₀ ~ a ₅ are -0.915, -40.867, 3.632, 0.537, -0.499, 0.522 respectively, the values of c ₀ ~ c ₂ are 0.1063, -62.49, 0.0437 respectively, and the values of d ₀ ~ d ₂ are respectively are -200, -138, 300, the values of e ₀ ~ e ₂ are 0.0712, -61.4, 0.0288 respectively, the values of f ₀ ~ f ₂ are -3083, -180, 5088 respectively, and the values of b ₀ ~ b ₅ are respectively are -0.1463, -30.27, 0.1037, 0.0584, 0.1747, 0.1288.

2. Equivalent circuit model of parallel lithium-ion battery system

The basic model of the parallel lithium-ion battery system established is a second-order equivalent circuit model. According to Kirchhoff's law KVC, the battery model expression is: U(t)= _Ub0 [SOC(t)]- _Ib (t) _Zb (t). The basic model for determining the performance parameters of each battery cell and the battery system by using the operating characteristics of the parallel circuit and the screening method is determined as follows: the open-circuit terminal voltage of the battery system in the basic model is calculated as follows: Among them, U _0i (SOC) represents the open-circuit terminal voltage of the i-th battery cell; the impedance of the battery system in the basic model is calculated as follows: Among them, the calculations of R _bs (t), R _bl (t) and C _bs (t), C _bl (t) are as follows:

The design of the SOC corrector is as follows: the SOC corrector is composed of two PI regulators and a weighter, and the two inputs of each PI regulator are respectively the i-th branch battery string output current I _i and the battery model output 1/2 of the total current I _b ; after passing through each PI regulator, the SOC compensation value ΔSOC _i of the two branch battery strings is obtained, namely In the formula, k _P is a proportional constant, k _I is a proportional constant, s is an integral factor, and i is a natural number greater than 1; then the battery system SOC compensation value ΔSOC _b is obtained after passing through the weighter, namely In the formula, _ki is the weighting coefficient.

Finally, add the compensation value ΔSOC _b to the SOC _b output by the basic model of the battery system, and use it as the new input SOC _r of the basic model to update the performance parameters of the battery system, and then update the basic model of the battery system.

3. Model simulation results and effect comparison

In order to verify the accuracy of the invented model, the battery system model (before correction) proposed by the present invention is compared with the battery system model (after correction) established by the public document (CN105116338A). The simulation test is a pulse working condition, and the pulse The change of load current during discharge is as follows: work at 0.8A constant current for 600s at the beginning, after standing still for 600s, then work at 0.8A constant current for 600s, and so on. At the initial moment, the initial capacities of the three battery cells are not equal, that is, the initial SOCs are not the same, which are 1, 0.95, and 0.9, respectively.

Figure 5 shows the pulse discharge characteristics of batteries under different conditions of SOC ₀ , where Figure 5-1 shows the current change of each branch, and Figure 5-2 shows the comparison before and after correction of the terminal voltage of the parallel battery system. It can be seen from Figure 5-1 that when the SOC ₀ of the battery cells in the circuit strings of branch 1 and branch 2 are 1 and 0.9 respectively, at the initial stage of discharge, the discharge current of branch 1 is larger than that of branch 2. As the discharge process proceeds, the discharge currents of the two branches are almost equal in the end. It can be seen from Figure 5-2 that during the entire discharge process, the corrected simulation results (solid line) are closer to the reference value than the uncorrected simulation results (dotted line), which shows that the established battery model can more accurately predict the parallel lithium battery. Operating characteristics of ion battery systems.

Finally, it should be noted that the combination of the above embodiments only illustrates the technical solution of the present invention rather than limiting it. Those of ordinary skill in the art should understand that those skilled in the art can modify or equivalently replace the specific embodiments of the present invention, but these modifications or changes are within the protection scope of the pending claims.

Claims (3)

1. The parallel lithium ion battery system modeling method is characterized by specifically comprising the following steps of:

step (1): establishing a basic model of a parallel battery system by utilizing the working characteristics of a parallel circuit and a screening method according to a known lithium ion battery monomer model;

step (2): the SOC corrector is formed by N proportional-integral PI regulators and a weighting device;

and (3): detecting the current of each branch battery string, and taking the current of each branch battery string and 1/N of the total current output by the basic model of the battery system as the input of the SOC corrector;

and (4): obtaining SOC compensation values delta SOC of the battery strings of the N branch circuits through N PI regulators in the SOC corrector_i；

And (5): SOC compensation value delta SOC of each branch battery string_iObtaining the SOC compensation value delta SOC of the battery system after weighting_b；

And (6): compensating the SOC of the battery system by the SOC compensation value delta SOC_bSOC from output of basic model of battery system_bSuperposing to obtain corrected SOC_r；

And (7): according to the corrected SOC_rAnd updating the performance parameters of the battery system, further updating the basic model of the battery system, and repeating the steps to obtain an accurate battery system model.

2. The modeling method of the parallel lithium ion battery system according to claim 1, wherein the basic model of the parallel lithium ion battery system is a controlled voltage source U comprising 2 RC parallel circuits_b0(SOC) and internal resistance R of battery_bThe second-order equivalent circuit model of which kirchhoff's law KVC expression is U (t) ═ U_b0[SOC(t)]-I_b(t)Z_b(t) open end voltage calculation formula is U_b0(SOC)＝MU₀(SOC) in which U₀(SOC) is cell open terminal voltage; the impedance is calculated by the formulaWherein R is_b(t) is the internal resistance of the battery system, R_bs(t)、R_bl(t) and C_bs(t)、C_bl(t) resistance and capacitance describing transient response characteristics of the battery system respectively; r_bs(t)、R_bl(t) and C_bs(t)、C_blThe formula for calculation of (t) is: wherein, c₀～c₂、d₀～d₂、e₀～e₂、f₀～f₂all are model coefficients, which are obtained by fitting the battery measurement data;SOC₀the initial value of SOC of the battery cell is a constant of 0-1, Q_u(t) is the cell's unusable capacity, Q₀The rated capacity of the battery cell.

3. The modeling method of a parallel lithium ion battery system according to claim 2, wherein the SOC corrector is designed as follows: the SOC corrector is composed of N PI regulators and a weighting device, wherein 2 inputs of each PI regulator are respectively the output current I of the ith branch battery string_iAnd total output flow I of battery model_b1/N of (1); obtaining SOC compensation values delta SOC of the battery strings of the N branches after passing through each PI regulator_iI.e. byIn the formula, k_P、k_IIs a proportionality constant, s is an integral factor, and i is a natural number greater than 1; obtaining the SOC compensation value delta SOC of the battery system after the weighting device_bI.e. byIn the formula, k_iAre weighting coefficients.