CN104122447B - Online estimation method for direct current resistance of power battery of electric vehicle - Google Patents

Abstract

The invention relates to an online estimation method for direct current resistance of a power battery of a plug-in hybrid electric vehicle or a pure electric vehicle. The online estimation method includes that in the process of charging through a vehicle-mounted charger, charging to appointed equally spaced SOC points in sequence in a constant-current charging mode, and then using a constant-voltage charging mode; when the current is reduced to an equalizing current designed by a monomer equalizer, casually opening and closing the equalizer of each monomer in sequence, using the equalizer to generate a composite pulse excitation on each monomer, recording the change curve of the terminal voltage thereof changed along with the time, substituting into a corresponding mathematical model of direct current resistance of the power battery to obtain the direct current resistance and open-circuit voltage of the battery, and calculating the direct current resistance and open-circuit voltage of a power battery pack according to the direct current resistance and open-circuit voltage of the battery. The online estimation method for the direct current resistance of the power battery of the electric vehicle is simple and reliable and has high practicability and performability.

Description

An online estimation method for DC impedance of electric vehicle power battery pack

technical field

The invention relates to an online estimation method for the DC impedance of a power battery pack of an electric vehicle, and belongs to the field of electric energy management of the power battery of an electric vehicle.

Background technique

The power battery is an important part of the electric vehicle. During its use, accurate control of the state parameters of the power battery plays a vital role in improving the performance and safety of the electric vehicle. At present, some state parameters of the power battery such as state of charge (state of charge, SOC), state of health (state of health, SOH) and power state (state of power, SOP) cannot be obtained directly through sensors or other devices, and need to contact the power battery The electrical characteristic parameters are estimated in real time by using certain mathematical methods.

The most important electrical characteristic parameters that are difficult to obtain directly for power battery packs are the DC internal resistance and open circuit voltage of the battery pack. It is of great significance to study the method of online prediction of the DC internal resistance of the power battery pack: 1) it helps to establish an accurate model of the electrical characteristics of the power battery; 2) it helps to improve the accuracy of battery state estimation; 3) it helps Improve the performance and safety of electric vehicles.

At present, the DC internal resistance of the power battery pack is generally obtained by offline testing of the battery using battery testing equipment, and it is treated as a constant in the later state estimation. However, with the increase of battery life and the frequent acceleration, deceleration, start-stop and other complex operating conditions in the actual operation of electric vehicles, the DC internal resistance of the power battery pack will change to a certain extent. Considering the time-varying nature of the parameters, the estimation accuracy of the corresponding state parameters is gradually reduced. At home and abroad, there is still no method that can accurately predict the DC internal resistance and open circuit voltage of power battery packs online.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned deficiencies in the prior art and provide an online estimation method for the DC impedance of the power battery pack of an electric vehicle, which can realize the purpose of online estimation of the internal parameters of the DC impedance of the battery pack.

The technical solution adopted to realize the object of the present invention is: an online estimation method for DC impedance of a power battery pack of an electric vehicle, comprising the following steps:

Charge the power battery through the on-board charger, and charge with constant voltage and small current at preset equidistant SOC points;

Use the opening and closing actions of the equalizer assigned to each battery cell to generate constant current pulses in two directions, positive and negative;

Collect the voltage signal of each battery cell, and perform curve fitting on the data of the relationship between the voltage of each cell and time, and obtain the fitted curve;

Perform parameter identification on the obtained fitting curve, and calculate the DC impedance and open circuit voltage parameters of each battery cell;

According to the connection form of the battery, mathematical operation is performed on the obtained battery cell parameters, so as to obtain the DC impedance and open circuit voltage of the entire power battery pack.

Update the DC impedance data estimated online to the BMS (Battery Management System) controller, and provide online updated data for the subsequent BMS on the battery state of charge (SOC) and state of health (SOH).

In the above technical solution, the on-board charger first charges the power battery in a constant current charging mode, and calculates the charging power by the ampere-hour integration method; and whenever the power battery is charged to a preset equal interval SOC point, the The charging mode is changed to constant voltage and low current charging mode.

In the above technical solution, the equalizer should use a bidirectional buck-boost circuit to generate constant current pulses with positive and negative directions.

In the above technical solution, the time for the buck-boost circuit to generate two pulse currents and the rest time for the intermediate equalizer should be consistent with the pulse curve required by the mixed pulse power characteristic test.

In the above technical solution, the BMS collection unit collects the voltage of the battery cells, and uses the CAN network to transmit the data to the BMS main control unit.

In the above technical solution, the arithmetic mean value of the voltage after pulse discharge and rest and the voltage after pulse charge and rest is taken as the open circuit voltage of the battery at that time.

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

1. Utilize the on-board charger and single equalizer to simulate the hybrid pulse power characterization (HPPC) test online, which is different from the traditional battery dynamic characteristic test method. The present invention does not need to disassemble the on-board power battery. The DC impedance of the battery can be estimated during the process.

2. The estimation of the open circuit voltage (OCV) is based on the arithmetic mean value of the 40s voltage value after discharge and the 40s voltage value after charging in the hybrid pulse power characterization (HPPC) test. Its value, which is different from the general traditional method of obtaining the open circuit voltage of the battery after working for a long time, has high engineering operability.

Description of drawings

Fig. 1 is a flow chart of the online estimation method of DC impedance of electric vehicle power battery pack according to the present invention.

Figure 2 shows the bidirectional buck-boost circuit topology adopted by the equalizer.

Fig. 3 is the change curve of the battery cell current during the opening and closing process of the equalizer.

FIG. 4 is a curve of battery cell terminal voltage changing with time collected during the opening and closing process of the equalizer.

FIG. 5 is a graph showing the relationship between the open-circuit voltage and the SOC variation obtained by parameter identification based on the fitting curve.

FIG. 6 is a graph showing the relationship between the time constant and the SOC variation obtained through parameter identification from the fitting curve.

FIG. 7 is a graph showing the relationship between polarization capacitance and SOC variation obtained by parameter identification based on the fitting curve.

FIG. 8 is a graph showing the relationship between ohmic internal resistance and SOC variation obtained by parameter identification based on the fitting curve.

FIG. 9 is a graph showing the relationship between polarization internal resistance and SOC variation obtained by parameter identification based on the fitting curve.

Figure 10 is a schematic diagram of the adopted PNGV equivalent circuit model.

detailed description

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

This embodiment adopts a lithium-ion power battery pack composed of a lithium iron phosphate lithium-ion monomer produced by a certain company, and the purpose is to obtain the open circuit voltage, ohmic internal resistance, and electrical characteristic parameters of the polarization impedance in its first-order PNGV equivalent circuit. The schematic diagram of the PNGV equivalent circuit model is shown in Figure 10.

As shown in Figure 1, the online estimation method of the DC impedance of the electric vehicle power battery pack of the present invention comprises the following steps:

S100. The vehicle-mounted charger performs constant current charging on the power battery, and replaces the power battery at equidistant SOC points (this embodiment takes 5% equidistant points, for example: 5%, 10%, 15%, ..., 100%). Constant voltage and small current trickle charging, the voltage value at this time is the same as the voltage value at each equal interval SOC point (5%, 10%, 15%, ..., 100%) calibrated off-line on the battery test equipment.

S200 , generating constant current pulses in two directions, positive and negative, by using the opening and closing actions of the equalizers assigned to each battery cell.

The topology of the equalization circuit of the battery pack in this embodiment is shown in FIG. 2 , and the loop is a typical bidirectional buck-boost circuit composed of inductors and field effect transistors. Stage 1: Turn on the switch tube Q1, the energy is stored in the inductor L1 from the battery B1, close Q1, and turn on Q2, at this time, the energy stored in the inductor L1 is transferred to the adjacent battery B2, and the constant current is controlled to 0.1C, and the duration 10s, thus generating a 0.1C pulse current from batteries B1 to B2. Stage 2: After holding for 40s, since the topological structure has been designed as a symmetrical structure, the same principle is used to generate a 0.1C reverse pulse, and the pulse energy transmitted from B1 to B2 is returned to B1. During the whole process, the on-board charger charges the battery pack with a small current trickle in constant voltage mode to make up for the energy consumption caused by opening and closing the equalizer, so it can be considered that the SOC at this time remains constant.

S300. Record the time-varying curve of the terminal voltage at each SOC point, and use a curve fitting tool to perform least square fitting on the data to obtain a hybrid pulse power characterization (HPPC) curve, as shown in FIG. 3 Show.

S400, use the following formula to carry out parameter identification to the obtained HPPC fitting curve, each voltage value and time point are as shown in accompanying drawing 3, accompanying drawing 4, obtain each parameter value of DC impedance at each SOC point, obtain open circuit voltage, time The constant, polarization capacitance, ohmic internal resistance and polarization internal resistance are shown in Figures 5 to 8, respectively.

Open circuit voltage:

Time constant τ:

Capacitance Cb:

In the formula, AmpSec is the rated capacity, and Uoc is the open circuit voltage.

Ohmic internal resistance:

Polarization internal resistance:

S500. According to the connection form of battery cells, the total open circuit voltage of the battery pack is:

The total ohmic internal resistance and polarization internal resistance are:

The total polarization capacitance is:

In the above _formula , _t1 is the start time of pulse discharge, _t2 is the end time of discharge, _t3 is the start time of rest, t4 is the end time of rest, and t5 is the end time of pulse charging _; the corresponding time is _{t1 The} final voltage value is U ₁ , the value after the voltage slump is U ₁ ', the voltage value at t2 is U ₂ , the voltage value at t3 is U ₃ , the voltage value at _t4 is U ₄ , and the voltage value at _t5 is U ₅ , the voltage value after charging and resting is U ₆ .

Finally, the above-mentioned online estimated DC impedance is updated to the BMS controller to provide online updated data for the subsequent BMS on the state of charge and health of the battery.

Claims (6)

1. an online estimation method of DC impedance and open circuit voltage of an electric vehicle power battery pack, is characterized in that, comprises the following steps: Charge the power battery through the on-board charger, and charge with constant voltage and small current at preset equidistant SOC points; Use the opening and closing actions of the equalizer assigned to each battery cell to generate constant current pulses in two directions, positive and negative; Collect the voltage signal of each battery cell, and perform curve fitting on the data of the relationship between the voltage of each cell and the time change, and obtain the mixed pulse power characteristic curve; Perform parameter identification on the obtained fitted curve, and calculate the DC impedance and open circuit voltage parameters of each battery cell; According to the connection form of the battery, mathematical operations are performed on the obtained battery cell parameters, so as to obtain the DC impedance and open circuit voltage of the entire power battery pack, specifically, BMS uses the following formula to estimate the DC resistance and open circuit voltage of each battery cell: Open circuit voltage: ${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 6</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$ Time constant τ: $\mathrm{<span\; class="notranslate">\; \&tau;</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; I</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$ Capacitance Cb: ${\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; A</span>}\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; p</span>}\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; e</span>}\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; *</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; c</span>}}}{\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; *</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 100</span>}\mathrm{<span\; class="notranslate">\; \%</span>}\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; C</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; \%</span>}\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; C</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$ In the formula, AmpSec is the rated capacity, and Uoc is the open circuit voltage; Ohmic internal resistance: ${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$ Polarization internal resistance: ${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}\mathrm{<span\; class="notranslate">\; p</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; I</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$ Then, according to the connection form of the battery cells, the DC impedance and open circuit voltage of the entire battery pack are obtained through mathematical operations. The specific calculation process is as follows: The total open circuit voltage of the battery pack is: The total ohmic internal resistance and polarization internal resistance are: The total polarization capacitance is: In the above _formula , _t1 is the start time of pulse discharge, _t2 is the end time of discharge, _t3 is the start time of rest, t4 is the end time of rest, and t5 is the end time of pulse charging _; the corresponding time is _{t1 The} final voltage value is U ₁ , the value after the voltage slump is U ₁ ', the voltage value at t2 is U ₂ , the voltage value at t3 is U ₃ , the voltage value at _t4 is U ₄ , and the voltage value at _t5 is U ₅ , the voltage value after charging and resting is U ₆ . 2. The online estimation method of the DC impedance and open circuit voltage of the electric vehicle power battery pack according to claim 1, characterized in that: the on-board charger first charges the power battery in a constant current charging mode, and integrates it in ampere-hours When the power battery is charged to the preset equidistant SOC point, the charging mode is changed to the constant voltage and small current charging mode. 3. The online estimation method of DC impedance and open circuit voltage of electric vehicle power battery pack according to claim 1, characterized in that: the equalizer should use a bidirectional buck-boost circuit to generate a constant current with positive and negative directions pulse. 4. The online estimation method of DC impedance and open circuit voltage of electric vehicle power battery pack according to claim 3, characterized in that: the time for the buck-boost circuit to generate two pulse currents and the time for the middle equalizer to stand still should be It is consistent with the pulse curve required by the mixed pulse power characteristic test. 5. The online estimation method of DC impedance and open circuit voltage of electric vehicle power battery pack according to claim 1, characterized in that: the voltage of the battery cell is collected through the BMS acquisition unit, and the data is transmitted to the BMS main control by using the CAN network unit. 6. The online estimation method of DC impedance and open circuit voltage of electric vehicle power battery pack according to claim 1, characterized in that: the arithmetic mean value of the voltage after pulse discharge and rest and the voltage after pulse charge and rest is used as The open circuit voltage of the battery at that time.