CN101237154A - A power battery-supercapacitor hybrid power system for electric vehicles - Google Patents

Abstract

The invention relates to a power battery-supercapacitor hybrid power system for electric vehicles, belonging to the technical field of electric vehicles. Including power battery pack, motor controller and 24V storage battery; step-up DC/DC converter, bidirectional DC/DC converter and supercapacitor pack; among them, the low-voltage end of the step-up DC/DC converter and the output end of the power battery pack The high voltage end of the boost DC/DC converter is connected to the motor controller; at the same time, the high voltage end of the supercapacitor bank and the bidirectional DC/DC converter are directly connected to the high voltage end of the boost DC/DC converter, and the bidirectional DC The low-voltage terminal of the /DC converter is connected to the 24V battery. The power battery of the present invention has gentle output current, small peak current, high discharge efficiency, and long service life; when the SOC of the battery pack is low, the hybrid system can still guarantee normal power output capability; the supercapacitor directly absorbs the braking feedback energy, and the energy High conversion efficiency.

Description

A power battery-supercapacitor hybrid power system for electric vehicles

technical field

The invention belongs to the technical field of electric vehicles, and in particular relates to a power battery-supercapacitor hybrid power system for electric vehicles.

Background technique

Electric vehicles have the characteristics of environmental protection, energy saving and high energy conversion efficiency, and have been paid more and more attention. The power battery pack is the core part of the electric vehicle. Improving the energy efficiency of the power battery pack during the discharge process, the utilization rate of the battery pack's energy storage, and prolonging the service life of the power battery pack are one of the key technologies for the development of the electric vehicle power system.

Restricted by the working principle and manufacturing level of power batteries, the discharge efficiency of advanced power battery packs (such as nickel-metal hydride battery packs, lithium-ion battery packs, etc.) for electric vehicles is not satisfactory. The experimental research results show that the energy efficiency of a certain type of 100Ah lithium-ion power battery pack is 86% when it is discharged at a constant current of 100A, and the energy efficiency is only 69% when it is discharged at a constant current of 200A. At the same time, the research results also show that the state of charge (SOC) of the battery pack also has a significant impact on the discharge efficiency. With the decrease of the SOC level of the battery pack, the discharge efficiency of the battery pack drops significantly; in addition, large Current discharge will shorten the service life of the battery pack, thereby increasing the cost of EV use, and this effect is more pronounced at lower battery pack SOC levels. Therefore, the ideal use condition of the power battery pack should be continuous small current discharge. However, when the power battery pack directly drives the electric vehicle, this discharge condition will affect the power performance of the electric vehicle. The power system structure of the currently commonly used pure electric vehicles is shown in Figure 1. It is mainly composed of a power battery pack, a step-down DC/DC converter, a motor controller and a 24V battery. The connection relationship is: the output end of the power battery pack At the same time, it is connected with the high-voltage end of the step-down DC/DC converter and the motor controller, and the low-voltage end of the step-down DC/DC converter is connected with the 24V battery. This power system uses a power battery pack as the only energy source. Due to the characteristics of frequent acceleration of the vehicle under urban working conditions, the power battery pack is frequently discharged with high current, which reduces the energy efficiency and service life of the battery pack, thereby affecting the energy efficiency and service life of the vehicle.

On the other hand, when electric vehicles are running in urban areas, due to the characteristics of urban working conditions, the vehicles are frequently in the process of acceleration, deceleration and idling. In the state of high current discharge, however, during the deceleration and coasting process of the vehicle, the output power of the power system is very small or even zero. At this time, the discharge current of the power battery pack is small or does not discharge. The statistical results of the vehicle's urban working conditions show that the zero output time of the power system caused by vehicle sliding, deceleration and idling is about 30% of the total running time of the vehicle. Small.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art and provide a power battery-supercapacitor hybrid power system for electric vehicles. In this power system, the discharge current of the battery pack is basically stable near the average value of the current. The discharge current is small and changes smoothly, which improves the discharge efficiency and service life of the battery pack. At the same time, the charging and discharging process of the supercapacitor is adaptively controlled through the output U-I characteristic of the step-up DC/DC converter.

The present invention proposes a power battery-supercapacitor hybrid power system for electric vehicles, which is characterized in that the system includes: a power battery pack, a step-up DC/DC converter, a motor controller, a bidirectional DC/DC converter, Supercapacitor pack and 24V storage battery; the connection relationship is: the low-voltage end of the boost DC/DC converter is connected to the output end of the power battery pack, and the high-voltage end of the boost DC/DC converter is connected to the motor controller; The high-voltage end of the voltage DC/DC converter is directly connected to the high-voltage end of the supercapacitor bank and the bidirectional DC/DC converter, and the low-voltage end of the bidirectional DC/DC converter is connected to the 24V battery.

The working principle of the present invention: before the power system is started, due to the self-discharge characteristic, the terminal voltage of the super capacitor may be low. When the voltage is significantly lower than the no-load voltage of the battery pack, the step-up DC/DC converter cannot be started directly Charge the supercapacitor, otherwise, it will cause a large charging current impact and damage the boost converter, power battery and supercapacitor. At this time, the 24V battery needs to precharge the supercapacitor through a bidirectional DC/DC converter until the terminal voltage of the supercapacitor is equal to or close to the no-load voltage of the power battery pack. Then, the boost DC/DC converter enters the working state, and the power battery pack continues to charge the supercapacitor bank through the boost DC/DC converter until the terminal voltage of the supercapacitor bank reaches the empty space of the boost DC/DC converter. load the output voltage.

After the power system is started, the power battery pack provides electric energy for the motor controller through the step-up DC/DC converter. Since the boost converter is designed with a reduced U-I output characteristic, when the vehicle demand power increases, the output voltage of the boost converter decreases with the increase of the discharge current of the power battery pack, resulting in the discharge of the supercapacitor pack. The supercapacitor pack and the power battery pack together provide electric energy for the motor controller. Moreover, the discharge current of the supercapacitor pack is proportional to the rate of change of the discharge current of the power battery pack, which suppresses the further increase of the discharge current of the power battery pack; when the vehicle demand power decreases, the discharge current of the power battery pack decreases. Small, the output voltage of the boost converter rises, and the supercapacitor is charged to store energy for the next vehicle acceleration process. Therefore, under the action of the U-I output characteristics of the boost DC/DC converter, the discharge process of the power battery pack and the charging and discharging process of the supercapacitor pack are adaptively controlled, so that the discharge current of the power battery pack is stabilized near the average current.

Features and effects of the present invention:

The invention adapts to the requirement of large fluctuations in the driving power of electric vehicles. Due to the self-adaptive charging and discharging process of the supercapacitor group, the peak discharge current of the power battery group is reduced, and the discharge current of the power battery group changes smoothly, which is basically near the average current. The energy efficiency and service life of the battery pack are improved. Due to the high charging and discharging efficiency of the supercapacitor bank (about 99%) and long service life (the number of cycles can reach hundreds of thousands of times), the energy efficiency and service life of the entire power system are improved.

Under urban working conditions, the power battery pack of the present invention is basically in a lower discharge current working state, therefore, the power battery pack can be discharged to a lower SOC level, thereby making more effective use of the energy storage of the power battery pack .

The supercapacitor bank of the present invention is directly connected to the DC bus, can directly absorb the electric energy fed back by the drive motor during the braking process, and has high energy conversion efficiency.

Description of drawings

Figure 1 is a structural diagram of an existing electric vehicle power system.

Fig. 2 is a structural block diagram of the hybrid power system of the present invention.

Fig. 3 is a structural diagram of the main circuit of the step-up DC/DC converter of the present invention.

Fig. 4 is a block diagram of the control circuit of the step-up DC/DC converter of the present invention.

Figure 5 shows the energy flow diagram of the power system when the 24V battery precharges the supercapacitor bank before the start of the hybrid power system.

Figure 6 shows the energy flow diagram of the power system when the power battery pack charges the supercapacitor pack through the step-up DC/DC converter when the hybrid power system is started.

Fig. 7 is a diagram of the energy flow of the power system when the supercapacitor bank is in the discharge state during the vehicle acceleration process of the hybrid power system of the present invention.

Fig. 8 is a change diagram of the output current and voltage operating point of the step-up DC/DC converter during the vehicle acceleration process of the hybrid power system of the present invention

FIG. 9 is a diagram of the energy flow of the power system when the supercapacitor pack is in the charging state during the deceleration of the vehicle or the idling of the power system in the hybrid power system of the present invention.

Fig. 10 is a change diagram of the output current and voltage operating point of the step-up DC/DC converter during the deceleration of the vehicle or the idling process of the power system of the hybrid power system of the present invention.

Fig. 11 is a power system energy flow diagram when the hybrid power system of the present invention is in the braking energy feedback state.

Detailed ways

Referring to the accompanying drawings, specific embodiments of the present invention will be further described.

The structure of the power battery-supercapacitor hybrid system proposed by the present invention is shown in Figure 2. The system includes: a power battery pack, a step-up DC/DC converter, a motor controller, a bidirectional DC/DC converter, and a supercapacitor pack and 24V battery; the connection relationship is: the low-voltage end of the boost DC/DC converter is connected to the output end of the power battery pack, and the high-voltage end of the boost DC/DC converter is connected to the motor controller; at the same time, the boost DC/DC The high-voltage end of the DC converter is directly connected to the high-voltage end of the supercapacitor bank and the bidirectional DC/DC converter, and the low-voltage end of the bidirectional DC/DC converter is connected to the 24V battery.

The embodiment of each component of this system is described as follows respectively:

The power battery pack uses a 100Ah lithium manganese oxide power battery. The nominal voltage of the battery cells is 3.6V. A total of 50 cells are used in series. The working voltage range of the battery pack is 150-210V.

1. The step-up DC/DC converter is composed of a main circuit and a control circuit that drives the main circuit. The main circuit adopts a BOOST main circuit structure with high energy conversion efficiency. While the control circuit adopts the voltage negative feedback control strategy, the current negative feedback regulation is added, so that the step-up DC/DC converter has a reduced U-I output characteristic. The specific composition and structure of the embodiments of each circuit are described as follows:

The structure of the main circuit is shown in Figure 3, which is composed of a switch tube T, a boost inductor L and a power diode D connected to it. The switch tube T adopts a 300A/600V IGBT power module, the boost inductor L adopts a microcrystalline magnetic core, and the design inductance is 60uH, and the power diode D adopts a fast recovery diode with a rated working index of 300A/600V. Its working process is: during the conduction period of the switch tube T, the input voltage U ₁ passes through L and T to form a current loop, and the electric energy is stored in the inductor L; during the cut-off period of the switch tube T, the input voltage U ₁ passes through L and D to output the voltage U _2. Due to the energy storage function of the inductor L, the output voltage U ₂ is greater than the input voltage U ₁ . Therefore, the lower input voltage U ₁ can be transformed into a higher output voltage U ₂ through the alternate turn-on and turn-off of the switch tube. The value of the output voltage U ₂ is determined by the duty cycle of the driving voltage U _dri at both ends of G and S of the switch tube T. The driving voltage _Udri is output by the control circuit of the step-up DC/DC converter.

The control circuit of the step-up DC/DC converter is shown in Figure 4, which is composed of a voltage outer loop circuit and a current inner loop circuit. The voltage outer loop circuit is mainly composed of operational amplifier N2C, Hall current sensor N1 and follower N2B; the current inner loop circuit is mainly composed of operational amplifier N2C, PWM regulator N3 and driver chip N4. The main circuit adopts a double closed-loop control structure of voltage outer loop and current inner loop, which can make the step-up DC/DC converter have constant current maximum current protection characteristics and high working safety.

The connection relationship and working process of the components of the control circuit are as follows: in the voltage outer loop, the voltage given signal Ug is connected to the positive input terminal of the operational amplifier N2C (using LM324), and the voltage feedback signal _Uf is connected to the negative terminal of the operational amplifier N2C. At the input end, at the same time, the current feedback signal output by the Hall current sensor N1 (using KH-300A) passes through the follower N2B (using LM324) to obtain Ui, which is also connected to the negative input end of the operational amplifier N2C, U _g , U _f and Satisfied between Ui:

U _g - U _i = U _f

In the control circuit, _Ug is a constant. As the output current increases, Ui increases, resulting in a decrease in the output voltage and _Uf , thereby obtaining a decreased output characteristic of the converter UI. In the current inner loop, the voltage regulation signal output by the operational amplifier N2C is connected to the positive input terminal of the operational amplifier N2D (using LM324), the current feedback signal is connected to the negative input terminal of the operational amplifier N2D, and the output of the operational amplifier N2D is connected to the PWM regulation 2 terminals of the device N3, in the present invention, the PWM regulator N3 adopts SG3524, according to the voltage value of the 2 terminals, N3 outputs a pulse signal with a certain duty ratio at the 11 and 14 terminals, and this signal is connected to 2, 2 of the driver chip N4. Between the 4 terminals, the driver chip N4 adopts UCC27321, and the driver chip N4 outputs the driving pulse voltage signal Udri from terminals 6, 7 and 5, and directly adds it between the G and S terminals of the switching tube T in Figure 3 after passing through the resistor R12, so that The switching tube produces a working process of being turned on and off alternately.

In the test system of the present invention, the no-load output voltage of the step-up DC/DC converter is designed to be 360V, the falling slope of the U-I output characteristic is 1.5V/A, and the maximum output current is 100A.

The supercapacitor bank adopts the products provided by Maxwell Company of the United States. The nominal capacity of the supercapacitor monomer is 2600F. A total of 144 cells are used in series. The actual working voltage range is 200-360V, the maximum charging current is 400A, and the maximum discharging current is 600A.

The supercapacitor bank of the present invention is directly connected to the output end of the boost DC/DC converter, and automatically adjusts the charging current or the discharging current as the output voltage of the boost DC/DC converter rises or falls. At the same time, the supercapacitor bank is directly connected to the DC bus, which can directly absorb the electric energy fed back by the driving motor during the braking process, and the energy conversion efficiency is high.

The 24V storage battery of the present invention precharges the supercapacitor group through the bidirectional DC/DC converter until the terminal voltage of the supercapacitor reaches the no-load terminal voltage of the power battery group. The battery is a 190Ah/24V lead-acid battery for vehicles;

The low-voltage end of the bidirectional DC/DC converter is designed to output a constant voltage of 28V with a maximum output current of 200A; the high-voltage end is designed to output a constant current of 30A with a maximum output voltage of 200V.

The working principle of the present invention is described in detail as follows:

The system of the invention performs self-adaptive control on the discharge process of the power battery pack and the charge and discharge process of the supercapacitor pack according to the output U-I characteristics of the step-up DC/DC converter. Hereinafter, the operating principle of the system will be explained with reference to FIGS. 5 to 11 , respectively.

As shown in Figure 5, when the electric vehicle is left for a long time, due to the self-discharging effect of the supercapacitor and the effect of the balance circuit of the capacitor monomer, the voltage at both ends of the capacitor will gradually drop to about zero. At this time, if the boost DC/ The DC converter and the power battery pack charge the super capacitor pack through the boost converter, which will generate excessive charging current, causing damage to the power battery, boost converter and super capacitor. In the present invention, a 24V storage battery is first used to precharge the supercapacitor with a constant 30A current through bidirectional DC/DC. DC converter stopped working. Figure 5 shows the energy flow diagram of the power system when the 24V battery precharges the supercapacitor bank. At this time, the bidirectional DC/DC converter works in a boost state.

As shown in Figure 6, the energy flow diagram of the "power battery-supercapacitor" hybrid system is given when the hybrid power system is started. The terminal voltage of the power battery pack outputs a higher voltage after passing through the step-up DC/DC converter. The capacitor pack is charged. Under the action of the output characteristics of the boost DC/DC converter, as the terminal voltage of the supercapacitor bank gradually increases, the charging current gradually decreases until the no-load voltage of the boost DC/DC converter is 360V. The working point at this time is shown as point A in FIG. 8 .

As shown in Fig. 7, the energy flow diagram of the power system when the supercapacitor bank is in the discharge state is given during the starting or accelerating process of the "power battery-supercapacitor" hybrid power system. When the vehicle starts or accelerates, the required power of the motor increases, requiring the DC bus to provide a larger current. Under the effect of the U-I characteristics of the boost DC/DC converter output (as shown in Figure 8), the boost converter As the output current increases, the voltage drops, and at the same time, the supercapacitor bank discharges; the faster the output voltage of the boost converter drops, the greater the discharge current of the supercapacitor bank, thereby inhibiting the further increase of the discharge current of the power battery.

As shown in Fig. 8, the change chart of the output current and voltage operating point of the step-up DC/DC converter is given during the vehicle start or acceleration process of the "power battery-supercapacitor" hybrid system. The working point moves from point A to point B along the external characteristic curve, the current increases and the voltage drops. During this process, the supercapacitor is discharged.

As shown in Fig. 9, the energy flow diagram of the power system when the super capacitor bank is in the charging state is given during the deceleration of the vehicle or the idling of the power system of the "power battery-super capacitor" hybrid power system. During the deceleration process of the vehicle, the required power of the motor decreases, the output current of the boost DC/DC converter decreases, and the voltage rises. At the same time, the super capacitor bank is charged; the faster the output voltage of the boost converter rises, the faster the super capacitor The larger the charging current of the battery pack, the smaller the output current of the power battery is maintained, and the smooth discharge current of the power battery pack is guaranteed.

As shown in Fig. 10, the change diagram of the output current and voltage operating point of the step-up DC/DC converter is given during the deceleration of the vehicle or the idling of the power system of the "power battery-supercapacitor" hybrid power system. The working point moves from point B to point C along the external characteristic curve, the current decreases and the voltage rises. During this process, the supercapacitor is charged.

As shown in Fig. 11, the energy flow diagram of the power system when the "power battery-supercapacitor" hybrid power system is in the braking energy feedback state is given. When the vehicle realizes the braking energy feedback during the braking process, the motor is in the power generation state, and the current generated by the motor directly charges the supercapacitor bank through the input terminal of the motor controller. During this process, the terminal voltage of the supercapacitor group rises. When the terminal voltage of the supercapacitor group is lower than the no-load voltage of the step-up DC/DC converter, the power battery can still output a small current, ensuring the discharge of the power battery group. The smoothness of the current. In the hybrid power system of the present invention, the braking energy feedback control is simple and the energy utilization rate is high.

Claims (3)

1. A power battery-supercapacitor hybrid system for electric vehicles, including a power battery pack, a motor controller and a 24V storage battery; it is characterized in that the system also includes: a step-up DC/DC converter, a bidirectional DC/DC Converter and supercapacitor bank; the connection relationship is: the low-voltage end of the boost DC/DC converter is connected to the output end of the power battery pack, and the high-voltage end of the boost DC/DC converter is connected to the motor controller; The high-voltage end of the voltage DC/DC converter is directly connected to the high-voltage end of the supercapacitor bank and the bidirectional DC/DC converter, and the low-voltage end of the bidirectional DC/DC converter is connected to the 24V battery. 2. The hybrid power system according to claim 1, wherein the step-up DC/DC converter is composed of a main circuit and a control circuit for driving the main circuit. The piezoelectric inductance L and the power diode D constitute. 3. The hybrid power system according to claim 2, wherein the control circuit adopts a double closed-loop control structure of a voltage outer loop and a current inner loop, and the voltage outer loop circuit is mainly composed of an operational amplifier N2C, a Hall current sensor N1 and follower N2B are composed; the current inner loop circuit is mainly composed of operational amplifier N2C, PWM regulator N3 and driver chip N4.

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