CN104122447A - Online estimation method for direct current resistance of power battery of electric vehicle - Google Patents

Abstract

The invention relates to a method for plug-in hybrid electric vehicles and pure electric vehicles. During the charging process of using the on-board charger, the constant current charging is carried out sequentially to designated SOC points at equal intervals, and then the constant voltage charging is performed. mode, when the current decreases to the equalization current designed by the monomer equalizer, temporarily turn on and off the equalizer configured for each monomer in turn, use the equalizer to generate a composite pulse excitation for each monomer, and record its terminal voltage as it changes. The time change curve is substituted into the corresponding DC internal resistance mathematical model of the power battery to calculate the DC internal resistance and open circuit voltage of the single battery, and then calculate the DC internal resistance and open circuit voltage of the power battery pack accordingly. The method of the invention is simple and reliable, and has high practicability and practicability.

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 power batteries 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 during 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 DC impedance of the battery can be estimated during the process.

2. The estimation of open circuit voltage (OCV) is based on the arithmetic mean value of the voltage value of 40s after discharge and the voltage value of 40s after charging in 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 ways

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 similar hybrid pulse power characterization (HPPC) curve, as shown in Figure 3 .

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:

${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; oc</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 6</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Time constant τ:

$\mathrm{<span\; class="notranslate">\; \&tau;</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; In</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Capacitance Cb:

${\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; AmpSec</span>}<span\; class="notranslate">\; *</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; oc</span>}}}{\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; *</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 100</span>}<span\; class="notranslate">\; \%</span>\mathrm{<span\; class="notranslate">\; SOC</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; \%</span>\mathrm{<span\; class="notranslate">\; SOC</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

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:

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; pp</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; I</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

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 (7)

1. an estimation on line method for electric automobile power battery group DC impedance and open-circuit voltage, is characterized in that, comprises the following steps:

By Vehicular charger to power battery charging, at default uniformly-spaced SOC point place constant voltage low current charge;

Unlatching, the closing motion of the balanced device of each battery cell distributed in utilization, produces the constant-current pulse of positive and negative 2 directions;

Gather the voltage signal of each battery cell, the variation relation data line curve to each monomer voltage to the time, obtains mixed pulses power characteristic;

The curve of the matching obtaining is carried out to parameter identification, calculate DC impedance and the open-circuit voltage parameter of each battery cell;

According to the type of attachment of battery, the battery cell parameter obtaining is performed mathematical calculations, thereby draw whole power battery pack DC impedance and open-circuit voltage.

2. the estimation on line method of electric automobile power battery group DC impedance according to claim 1, it is characterized in that: first described Vehicular charger charges to electrokinetic cell with constant current charging mode, and calculate charge capacity with ampere-hour integral method, in the time that electrokinetic cell is charged to default uniformly-spaced SOC point, make charge mode into constant voltage low current charge pattern.

3. the estimation on line method of electric automobile power battery group DC impedance according to claim 1, is characterized in that: described balanced device should adopt two-way buck-boost circuit to produce the constant-current pulse with positive and negative 2 directions.

4. the estimation on line method of electric automobile power battery group DC impedance according to claim 3, is characterized in that: time and the standing time of middle balanced device of 2 pulse currents of described buck-boost circuit generation should be consistent with the pulse curve of mixed pulses power characteristic test request.

5. the estimation on line method of electric automobile power battery group DC impedance according to claim 1, is characterized in that: gather the voltage of battery cell by BMS collecting unit, and utilize CAN network that data are sent to BMS main control unit.

6. the estimation on line method of electric automobile power battery group DC impedance according to claim 1, is characterized in that: the voltage arithmetic mean value after the voltage using pulsed discharge and after leaving standstill and pulse charge also leave standstill is as battery open-circuit voltage at that time.

7. the estimation on line method of electric automobile power battery group DC impedance according to claim 1, is characterized in that BMS utilizes following formula to estimate DC impedance and the open-circuit voltage of each battery cell:

Open-circuit voltage:

$U_{\mathrm{oc}}=\frac{1}{2}(U_4+U_6)---\left(1\right)$

Timeconstantτ:

$\mathrm{\&tau;}=-\frac{(t_4-t_3)}{\mathrm{In}(1-\frac{U_4-U_3}{U_1-U_3})}---\left(2\right)$

Capacitor C b:

$C_b=\frac{\mathrm{AmpSec}*U_{\mathrm{oc}}}{\frac{1}{2}*(U_{100\%\mathrm{SOC}}^2-U_{0\%\mathrm{SOC}}^2)}---\left(3\right)$

In formula, AmpSec is rated capacity, and Uoc is open-circuit voltage;

Ohmic internal resistance:

$R_0=\frac{U_1-U_1^{\&prime;}}{I}---\left(4\right)$

Polarization resistance:

$R_{\mathrm{pp}}=\frac{U_4-U_3}{I}---\left(5\right)$

Then, according to battery cell type of attachment, draw DC impedance and the open-circuit voltage of whole electric battery by mathematical operation, concrete computation process is as follows:

The total open-circuit voltage of electric battery is:

Total ohmic internal resistance and polarization resistance are respectively:

Total polarization capacity is:

In above formula, t ₁for starting pulsed discharge zero hour, t ₂for electric discharge cut-off time, t ₃for leaving standstill the zero hour, t ₄for leaving standstill the finish time, t ₅for pulse charge cut-off time; The corresponding t that gets ₁moment final voltage value is U ₁, the value after voltage dip is U ₁', t2 moment magnitude of voltage is U ₂, t ₃moment magnitude of voltage is U ₃, t ₄moment magnitude of voltage is U ₄, t ₅moment magnitude of voltage is U ₅, after charging leaves standstill, magnitude of voltage is U ₆.