CN105140981A - Lithium battery active equalization control method - Google Patents

Abstract

The invention discloses a lithium battery active equalization control method, and relates to the technical field of lithium batteries. The control method comprises the following steps: (1) a system is initialized, and a set value is preset; (2) the voltage and current of batteries are detected; (3) a digital-to-analog conversion module in a main control unit reads data detected by a voltage detection module and a current detection module and converts the data; (4) the main control unit performs UKF operation on the data after conversion and estimates SOC, and calculates the average SOC value after estimation is completed; and (5) the main control unit calculates the difference between the SOC of each single battery and the average SOC value, and if the difference is greater than the set value, the main control unit controls the duty ratio and opens a lithium battery active equalization module to actively equalize the SOC of the single battery until the difference between the SOC of the single battery and the average SOC value is smaller than the set value. By adopting the method, the problem that SOC estimation through the existing lithium battery equalization scheme is inaccurate can be solved.

Description

Lithium battery active equalization control method

technical field

The invention relates to the technical field of lithium batteries; in particular, an active balancing control method for lithium batteries based on SOC balancing of lithium batteries.

Background technique

With the development of lithium battery technology and more and more attention paid to energy conservation and environmental protection, the application fields of lithium batteries are becoming wider and wider, such as portable electronic products, electric vehicles and solar power generation systems and other new energy fields. Lithium-ion battery is the battery with the highest energy ratio so far. Its specific weight reaches 130 hours per kilogram, and the discharge voltage of a single battery is 3.6 volts, which is three times higher than that of nickel-cadmium batteries. In addition, lithium-ion batteries have small thermal effects, no memory effects, and higher charging efficiency than nickel-cadmium and nickel-hydrogen batteries. However, because its discharge voltage is not high, many sets of batteries need to be used in series in many applications to achieve sufficient output voltage and output power. Due to the inconsistency between the individual cells in the battery pack, after continuous charge and discharge cycles, the state of charge of each individual cell will be seriously unbalanced, which is manifested by the increasing voltage divergence between the individual cells. , which will cause permanent damage to the battery.

The state of charge of the battery, English is StateofCharge, referred to as SOC, as an important parameter when the electric vehicle is running, its accurate estimation can provide the necessary data support for the battery management system and the remaining mileage prediction, and then effectively prevent the battery from overcharging and over-discharging. Extend battery life and reduce operating costs of electric vehicles. However, SOC is not a physical quantity that can be directly measured. The battery itself is a closed electrochemical reaction. When electric vehicles are running, they present strong nonlinearity accompanied by drastic changes in current, which makes it difficult to estimate SOC. Scholars at home and abroad have conducted a lot of research on the SOC estimation of lithium batteries and proposed a variety of scientific methods for SOC estimation. Among them, the discharge test method can obtain a more accurate SOC estimate, but it must interrupt the ongoing work of the battery and cannot be applied to a real vehicle; although the current integration method can estimate the SOC of the battery in real time, it cannot automatically determine the initial value, and the error will increase over time. Further increase will lead to inaccurate estimation due to the accumulation of errors; the open circuit voltage method can only be accurately estimated when the battery current is zero, and the battery needs to be left standing for a long enough time, so it cannot be estimated in real time; the neural network method requires a large amount of training data And suitable training algorithm, and susceptible to interference, not suitable for working conditions with severe current changes.

Contents of the invention

The object of the present invention is to provide a lithium battery active equalization control method, which can solve the problem of inaccurate SOC estimation existing in the existing lithium battery equalization scheme.

In order to solve the above problems, the technical solution adopted by the present invention is: the lithium battery active balance control method is realized by using a lithium battery active balance control device, and the lithium battery active balance control device includes a DC/DC power supply module and a main control unit , the DC/DC power supply module is connected to the main control unit; the main control unit is respectively connected to the lithium battery active equalization module, the voltage detection module, the current detection module and the display module;

Its realization method is: 1), system initialization; preset setting value;

2) Detecting the voltage and current of the battery through the voltage detection module and the current detection module;

3) The digital/analog conversion module in the main control unit reads and converts the data detected by the voltage detection module and the current detection module;

4), the main control unit performs UKF calculation on the converted data and estimates SOC; after the estimation is completed, calculates the average value of SOC;

5), the main control unit calculates the difference between the SOC of each single battery and the SOC average value; if the difference is greater than the set value, the duty cycle is controlled and the active balancing module of the lithium battery is turned on Actively balance the SOC of the single battery until the difference between the SOC of the single battery and the average SOC is less than a set value.

Among the above technical solutions, a more specific solution may also be: the lithium battery active equalization module includes a voltage stabilization processing circuit and an isolated DC/DC circuit; the input terminal of the voltage stabilization processing circuit receives The voltage signal, its first output terminal is connected to the input terminal of the isolated DC/DC circuit, and its second output terminal is connected to the second input terminal of the inverter; the output terminal of the isolated DC/DC circuit is connected to the high-speed The second input end of the optocoupler isolation circuit is connected, the output end of the high-speed optocoupler isolation circuit is connected to the first input end of the inverter, and the output end of the inverter is connected to the input end of the switch circuit, The output end of the switch circuit is connected to the input end of the energy storage inductance circuit, and the output end of the energy storage inductance circuit is connected to the series lithium battery pack; the first input end of the high-speed optocoupler isolation circuit receives the main control unit control signal.

Further: the main control unit is a TMS320F28335 digital signal processor; the voltage stabilization processing circuit is a 12-volt voltage stabilization processing circuit; the isolated DC/DC circuit is a 12-volt to 5-volt isolated DC/DC circuit.

Further: the inverter is a 74HC04 inverter.

Due to the adoption of the above technical solution, compared with the prior art, the present invention has the following beneficial effects: since the present invention is provided with a lithium battery active equalization module, a buck-boost energy storage circuit is provided in the lithium battery active equalization module, As a carrier of energy transmission, it can transfer energy from a battery with a high state of charge to an inductor, and then from the inductor to a battery with a low state of charge to achieve energy flow.

The present invention applies the Uncented Kalman Filter (UKF for short) to the SOC estimation of the lithium battery, so that the estimation can achieve higher accuracy.

Description of drawings

Figure 1 is a schematic block diagram of the present invention.

Fig. 2 is a schematic block diagram of a lithium battery active balancing module.

Fig. 3 is a schematic diagram of the software flow of the present invention.

Figure 4 is a Thevenin model diagram.

Detailed ways

Below in conjunction with accompanying drawing and embodiment the present invention is described in further detail:

The lithium battery active balancing control device shown in Figure 1 and Figure 2 includes a DC/DC power supply module 1 and a main control unit 2, and the DC/DC power supply module 1 is connected to the main control unit 2; the main control unit 2 is respectively connected to the lithium battery active The balance module 3, the voltage detection module 5, the current detection module 6 and the display module 4 are connected.

Among them, the detection of the voltage and current of the lithium battery is carried out by the main control unit 2 controlling the voltage detection module 5 and the current detection module 6; the operation of the Uncented Kalman Filter (UKF for short) algorithm is completed by the main control unit 2; The control of the lithium battery is completed by the main control unit 2 controlling the active balancing module 3 of the lithium battery;

The lithium battery active equalization module 3 includes a voltage stabilization processing circuit 3-1 and an isolated DC/DC circuit 3-2; the input terminal of the voltage stabilization processing circuit 3-1 receives the voltage signal transmitted from the series lithium battery pack, and its first output end is connected with the input end of the isolated DC/DC circuit 3-2, and its second output end is connected with the second input end of the inverter 3-4; the output end of the isolated DC/DC circuit 3-2 is isolated from the high-speed optocoupler The second input terminal of the circuit 3-3 is connected, the output terminal of the high-speed optocoupler isolation circuit 3-3 is connected with the first input terminal of the inverter 3-4, and the output terminal of the inverter 3-4 is connected with the switch circuit 3-4. The input terminal of 5 is connected, the output terminal of the switch circuit 3-5 is connected with the input terminal of the energy storage inductance circuit 3-6, and the output terminal of the energy storage inductance circuit 3-6 is connected with the lithium battery pack in series; the high-speed optocoupler isolation circuit 3 The first input end of -3 receives the control signal of the main control unit 2 .

The lithium battery active equalization control method shown in Figure 3 is: 1), system initialization; preset set value;

2) Detect the voltage and current of the battery through the voltage detection module 5 and the current detection module 6;

3), the digital/analog conversion module in the main control unit 2 reads the data detected by the voltage detection module 5 and the current detection module 6 and performs conversion;

4), the main control unit 2 performs UKF calculation on the converted data and estimates SOC; calculates the mean value of SOC after the estimation is completed;

5), the main control unit 2 calculates the difference between the SOC of each single battery and the average value of SOC; if the difference is greater than the set value, it controls the duty cycle and turns on the lithium battery active equalization module 3 for the SOC of the single battery Perform active equalization until the difference between the SOC of the single battery and the average SOC is less than the set value.

In this embodiment, the main control unit 2 is a TMS320F28335 digital signal processor; the voltage stabilization processing circuit 3-1 is a 12-volt voltage stabilization processing circuit; the isolated DC/DC circuit 3-2 is a 12-volt to 5-volt isolated DC/DC circuit ; Inverter 3-4 is a 74HC04 inverter.

The 12V voltage stabilization processing circuit 3-1 is to stabilize the terminal voltage of the lithium battery pack in series to provide power for the 74HC04 inverter 3-4; the 12V to 5V isolated DC/DC circuit 3-2 is to use 12V voltage stabilization processing The voltage output by the circuit 3-1 is converted to provide an isolated output voltage to provide power to the high-speed optocoupler isolation circuit 3-3, and to provide a reference potential for the S terminal of the MOSFET in the switch circuit 3-5; the high-speed optocoupler isolation Circuit 3-3 isolates and outputs the duty cycle of PWM output by main control unit 2, provides driving voltage for MOSFET, and plays the role of isolating and protecting main control unit 2 at the same time; the function of 74HC04 inverter 3-4 is: because The output of the high-speed optocoupler isolation circuit 3-3 and the output of the PWM duty cycle of the main control unit 2 are reversed, and the positive output can be obtained through the 74HC04 inverter 4, so that the PWM duty cycle of the main control unit 2 It is synchronous with the opening of the MOSFET; the switching circuit 3-5 is composed of MOSFET and diode, and the flow direction and magnitude of the current can be controlled by controlling the switch of the MOSFET, wherein the diode acts as a reverse cut-off to avoid energy reverse flow; The function of the energy storage inductance circuit 3-6 is: the energy storage inductance circuit 3-6 is used as the carrier of energy transmission, and the energy is transferred from the battery with a high state of charge to the inductance, and then transferred from the inductance to the battery with a low state of charge. In order to realize the flow of energy, the energy storage inductor is connected in series with the transient bidirectional suppression diode to avoid the impact of the instantaneous pulse on the circuit.

The present invention is applicable to the balanced control of series lithium battery packs. When the present invention is connected to series lithium battery packs, the voltage and current of each battery in the series series lithium battery packs can be collected, and the SOC of each battery pack can be estimated in real time. Based on the SOC, the energy transfer active equalization is carried out. When the difference between the SOC of a battery cell in the series lithium battery pack and the average SOC of the series lithium battery pack reaches 2%, the equalization system is started, and the SOC in the series lithium battery pack The energy of the high battery is transferred to the battery with low SOC, until the SOC difference of each battery in the series lithium battery pack is within 2%, the average SOC of the series lithium battery pack and the maximum voltage difference of the single cells of the series lithium battery pack pass The display module 4 displays.

The accuracy of the battery model is a prerequisite for battery SOC estimation. Thevenin model is the most representative equivalent circuit model, which considers the polarization phenomenon in the internal chemical reaction of the battery, and can better reflect the dynamic and static characteristics of the battery; at the same time, it considers the internal resistance affected by temperature, current and charge and discharge state. It can accurately simulate the charging and discharging behavior of the battery, and the structure is relatively simple, which is more suitable for the modeling and simulation of power batteries. The model is shown in Figure 4.

Uoc in Figure 4 is the open circuit voltage; is the ohmic internal resistance of the battery; and are the polarization internal resistance and polarization capacitance respectively, and U is the terminal voltage of the battery.

will SOC and voltage on As the state variable of the system, the battery operating current For system input, battery operating voltage For the system output, a discrete state-space model is built.

The state equation of the model is:

(1)

In the formula, T is the sampling period, is the SOC value at sampling point k, is the sampling point of The voltage estimate on the, is the time constant, is the rated capacity of the battery, is the discharge efficiency, is Gaussian white noise.

The output observation equation is:

(2)

In the formula, there is a one-to-one correspondence between the open circuit voltage Uoc and the SOC, using represents the relationship, which can be obtained experimentally, is Gaussian white noise, and r0 is the ohmic internal resistance of the battery.

The experiment selects a single lithium iron phosphate battery. Through the HPPC test, the lithium battery is discharged with a short-term constant current pulse, and the recovery process of the battery terminal voltage is recorded. According to this curve, it is determined and , and determine the time constant of the circuit by recording the time, and finally determine the value of the capacitance. The parameter values of Thevenin circuit model can be obtained by using nonlinear least squares, as shown in Table 1.

Table 1 RC parameter list R ₀ R ₁ C ₁ 0.032 0.004 580.1316

Based on the Unscented Transform (UT for short), UKF realizes the approximation of the state distribution through UT. This method avoids the analytical derivation of nonlinear functions, and considers the problem of probability propagation. The mean and variance The approximation accuracy of is improved to at least second order, improving the accuracy of SOC estimation. Combined with the battery model (1) (2), the UKF is applied to the SOC estimation process as follows:

(1) System initialization

Filter initial value: (3)

in, is the matrix formed by the initial value of SOC and the initial value of polarization voltage, is the initial value of the state covariance.

(2) Generate sigma points

(4)

(5)

(6)

(7)

in, is a small positive number, usually taken as =1; n is the number of system state variables, for a single battery, , is a proportionality coefficient, is the state variable of the corresponding sigma point.

(3) Determine the weighting coefficient

(8)

,(9)

(10)

in, is the second-order proportionality coefficient, usually taken as =1; Used to combine prior information, for a Gaussian white noise system, take =2, is the weighting coefficient of the corresponding sigma point.

(4) Time update (UT)

(11)

(12)

(13)

in is the ith column of the square root of the matrix, is the weight, is the process noise variance, is the state update value obtained by unscented transformation, The state covariance update value obtained for the unscented transformation.

(5) Measurement update

(14)

(15)

(16)

(17)

(18)

in is the measured system output, , is the state covariance, is the system output matrix, For the measurement noise variance, is the updated state covariance of the measure.

UKF generates Sigma points, performs UT on Sigma points, uses nonlinear state equations and observation equations, avoids linear errors, improves estimation accuracy, and avoids calculating partial derivatives of the matrix, reducing the amount of calculation.

Claims (5)

1. A lithium battery active equalization control method is characterized in that: the method is realized by adopting a lithium battery active equalization control device, wherein the lithium battery active equalization control device comprises a DC/DC power supply module and a main control unit, and the DC/DC power supply module is connected with the main control unit; the main control unit is respectively connected with the lithium battery active equalization module, the voltage detection module, the current detection module and the display module;

the realization method comprises the following steps: 1) initializing a system; presetting a set value;

2) detecting the voltage and the current of the battery through the voltage detection module and the current detection module;

3) the digital/analog conversion module in the main control unit reads the data detected by the voltage detection module and the current detection module and converts the data;

4) the master control unit performs UKF operation on the converted data and estimates the SOC; calculating the SOC mean value after estimation is finished;

5) the main control unit calculates the difference between the SOC of each single battery and the SOC mean value; if the difference is larger than a set value, controlling the duty ratio and starting the lithium battery active balancing module to actively balance the SOC of the single battery until the difference between the SOC of the single battery and the SOC mean value is smaller than the set value.

2. The active equalization control method for lithium batteries according to claim 1, wherein: the lithium battery active equalization module comprises a voltage stabilization processing circuit and an isolation DC/DC circuit; the input end of the voltage stabilizing processing circuit receives a voltage signal transmitted from a series lithium battery pack, the first output end of the voltage stabilizing processing circuit is connected with the input end of the isolation DC/DC circuit, and the second output end of the voltage stabilizing processing circuit is connected with the second input end of the phase inverter; the output end of the isolation DC/DC circuit is connected with the second input end of the high-speed optical coupling isolation circuit, the output end of the high-speed optical coupling isolation circuit is connected with the first input end of the phase inverter, the output end of the phase inverter is connected with the input end of the switch circuit, the output end of the switch circuit is connected with the input end of the energy storage inductance circuit, and the output end of the energy storage inductance circuit is connected with the series lithium battery pack; and a first input end of the high-speed optical coupling isolation circuit receives a control signal of the main control unit.

3. The active equalization control method for lithium batteries according to claim 1 or 2, characterized in that: the main control unit is a TMS320F28335 digital signal processor; the voltage stabilization processing circuit is a 12-volt voltage stabilization processing circuit; the isolation DC/DC circuit is a 12-volt to 5-volt isolation DC/DC circuit.

4. The active equalization control method for lithium batteries according to claim 1 or 2, characterized in that: the inverter is a 74HC04 inverter.

5. The active equalization control method for lithium batteries according to claim 3, wherein: the inverter is a 74HC04 inverter.