Basic knowledge of super capacitor

Supercapacitors are physical batteries. The physical battery does not undergo chemical changes when storing and releasing energy.

(1) Structural principle
A capacitor is a charge storage device. When the voltage of the power supply is applied to both ends of the capacitor, the charge of the power supply is stored in the capacitor. Using this characteristic of capacitors to store energy on electric vehicles can provide the electrical energy required by the vehicle during driving.

Supercapacitors, also known as electrochemical capacitors and electric double-layer capacitors, are a new type of energy storage device that can be quickly charged/discharged under large currents, providing large instantaneous charging/discharging power, long cycle life, operating voltage and temperature Wide range.

The energy storage method of supercapacitors is different from traditional capacitors. Traditional capacitors are composed of electrodes and dielectrics. The dielectric between the electrodes produces a polarization effect under the action of an electric field to store energy; while electrochemical capacitors do not have a medium, which relies on the electrolyte to contact the electrodes. The unique electric double layer structure formed on the interface stores energy. The capacity of electrochemical capacitors is much larger than traditional capacitors, reaching 103~104F.

German physicist Helmholz (Helmholz) found that when the conductor electrode is inserted into the electrolyte, the conductor electrode is in contact with the electrolyte, due to the warehouse force and the intermolecular force ( Van der Waals force) or interatomic force (covalent force), the electrostatic charge on the surface will attract some irregularly distributed ions with heterogeneous charges from the solution, making them on the solution side of the electrode/electrolyte solution interface , A certain distance from the electrode is not in a row, forming an interface layer with the same amount of charge and the remaining charge on the electrode surface but opposite signs, thereby forming two charge layers on the electrode and the other in the solution, called double electric Layer. Since there is a barrier on the interface, the two layers of charges cannot cross the boundary to neutralize each other. The electric double layer structure will form a plate capacitor.

The structure of a super capacitor (referred to as a super capacitor) is shown in Figure 1. The porous electrode uses activated carbon powder, activated carbon or activated carbon fiber, and the electrolyte uses organic electrolyte! Porous activated carbon has a large surface area, which adsorbs charges in the electrolyte, so it has a large capacitance and can store a large amount of electrostatic energy. The charge/discharge process of electric double-layer supercapacitors is always a physical process without chemical reactions, so the performance is stable, which is different from batteries that utilize chemical reactions.

Basic knowledge of super capacitor
Figure 1 The structure of a super capacitor

At present, electric double layer supercapacitors mainly include carbon electrode double layer capacitors, metal oxide electrode double layer capacitors and organic polymer electrode double layer capacitors. However, due to the high price of metal oxide (ruthenium oxide) electrode capacitors, secondary pollution and other factors, it is currently mainly used in the military field; and the organic polymer technology is not yet mature, so carbon electrode super capacitors are widely used in electric vehicles. .

The area of ​​the carbon electrode supercapacitor is based on porous carbon material. The porous structure of the material allows its surface area to reach 2000m2/g, and a larger surface area can be achieved through some measures. The distance that the carbon electrode supercapacitor charges are separated is determined by the size of the electrolyte ions attracted to the charged electrode, which is smaller than the distance that can be vacated by traditional capacitor film materials. This large surface area coupled with a very small charge separation distance makes supercapacitors have a huge electrostatic capacity compared to traditional capacitors. Although the energy density is lower than that of batteries, this energy storage method has the characteristics of fast charging/fast discharging, and can be used in short-term high peak current situations that are difficult to solve with traditional batteries. Figure 2 shows the appearance of the supercapacitor produced by Maxwel.

Basic knowledge of super capacitor
Figure 2 The appearance of the supercapacitor produced by Maxwell

The electric double layer capacitor is essentially an electrostatic energy storage capacitor. The surface area of ​​the activated carbon electrode material that has been developed can reach 2000m2/g, the capacitance per unit mass can reach 100F/g, and the internal resistance of the capacitor can still be Maintained at a very low level, and activated carbon materials also have the advantages of low cost and mature technology, making this type of supercapacitor the most widely used in automobiles.

In addition to being used in power drive systems, supercapacitors also have a wide range of applications in the field of auto parts. For example, the 42V electronic control system (steering setting, braking device, air conditioning, high-fidelity audio, power seat, etc.) used in the future car design, if the long-life super capacitor is used, the required power can often change and the subsystem performance can be greatly changed. In addition, it can also reduce the wiring used for electric braking, electric steering and other subsystems in the car, and at the same time reduce the power consumption of the car subsystem on the battery, and extend the battery life.

Because traditional batteries (such as aluminum-acid batteries) have low power density, they cannot meet the requirements of frequent starting, acceleration and braking conditions of the vehicle, and too much energy is wasted during acceleration, which makes the driving range of the vehicle unable to meet the requirements. . Vehicles equipped with super capacitors can effectively solve this problem, which can provide a larger drive current to meet the driving conditions of the vehicle. It can also save battery energy, extend the driving range of the vehicle, reduce the frequent charging/discharging working state of the battery, and improve the service life of the battery.

(2) How to use
The combination of super capacitor and DC/DC (direct current/direct current) converter system is commonly used. The super capacitor and battery are connected in parallel. The capacitor does not participate in the work during normal driving; but when the vehicle is accelerating or going uphill, the capacitor provides a short-term large current through the control of the DC/DC converter, and supplies power together with the battery, and the two pass through the motor controller. Regulate, supply power to the motor to drive the vehicle. When the voltage of the capacitor is lower than the terminal voltage of the battery, the DC/DC converter steps down through the working circuit, so that the super capacitor reaches the energy saturation state. When the battery is in urgent need of energy, the capacitor energy is boosted and output to the positive and negative terminals of the battery through the control circuit.

(3) Features
The characteristics of supercapacitors are as follows.
①Fast charging speed. Charging for 10s~10min can reach more than 95% of its rated capacity.
②Long cycle life. Deep charge/discharge cycles can be used up to 500,000 times without memory effect.
③Super high current discharge capability. The energy conversion efficiency is high, the process loss is small, and the high current energy cycle efficiency is greater than or equal to 90%.
④High specific power. The specific power can reach 300~5000W/kg, which is equivalent to 5~10 times that of lead-acid batteries. The specific energy can reach 20W·h/kg.
⑤The product raw material composition, production, use, storage and dismantling process are free of pollution, which is an ideal green and environmentally friendly power source.
⑥Simple charging/discharging circuit. No charging circuit, high safety factor, maintenance-free for long-term use.
⑦Good ultra-low temperature characteristics. Wide temperature range -40~+70℃.
③Convenient detection. The remaining power can be read directly.
④The capacity range is usually 0.1~1000F.

(4) Application in electric vehicles
Supercapacitors have become one of the ideal power sources for electric vehicles due to their advantages such as high specific power, long cycle life, and short charge/discharge time. At present, countries all over the world are scrambling to study supercapacitor-related technologies and increasingly apply them to electric vehicles. The US Department of Energy first issued a statement in the “Business Times” in the 1990s, strongly recommending the development of capacitor technology and applying this technology to electric vehicles. The announcement of the Department of Energy led some companies such as Maxwell to enter the technical field of electrochemical capacitors.

Japan is a pioneer in applying super capacitors to hybrid electric vehicles. Super capacitors have been one of the important areas in the development of electric vehicle power systems in Japan in recent years.

Honda’s FCX fuel cell-supercapacitor hybrid vehicle is the world’s first commercialized fuel cell sedan. The vehicle was launched in Japan and California in the United States in 202; Nissan produced and installed a vehicle on June 24, 2002. The parallel hybrid bus with diesel engine, motor and super capacitor is 2~4 times more economical than the original traditional natural gas vehicle; the electric vehicle launched by Fuji Heavy Industries of Japan has used the lithium battery made by Hitachi Mechatronics and the storage made by Panasonic. Can be combined with the device.

The United States has also made some progress in the research of supercapacitor hybrid electric vehicles. The supercapacitors developed by Maxwell have been well applied in various types of electric vehicles. The hybrid bus developed by the Lewis Research Center of the National Aeronautics and Space Administration (NASA) uses super capacitors as the main energy storage system.

The use of supercapacitor-battery composite power supply system in pure electric vehicles and hybrid electric vehicles is considered to be one of the best ways to solve the power problems of electric vehicles in the future. With the further research and development of supercapacitors for electric vehicles, people have discovered supercapacitors. The battery composite power system is more practical in meeting performance and cost requirements, and its market prospects are broad.

Imperial College London, University of London is developing a composite of polymer resin and carbon fiber. First, nano-structured carbon fiber materials are made into thin sheets, then formed, dried, hardened, and then supercapacitors are implanted in it. It can be made into a battery module by superimposition, and made into a body panel, which is arranged on the body frame. Research shows that this new material battery can charge faster than conventional battery packs, and has better strength and applicability. It can replace body panels, thereby saving the space required for battery packs. This new type of battery panel can replace car doors, luggage compartment covers, engine hoods, car roofs, etc. If this new material is used to replace the traditional steel body, the weight of the entire car will be reduced by 15%. This can not only reduce the weight of electric vehicles, but also store more energy.