Supercapacitors improve vehicle starting performance-EEWORLD

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1 Problems with batteries The battery is a key electrical component in the car, and its performance directly affects the start of the car. Today's cars are all started by starting the motor without exception. During the starting process, especially at the moment of starting, since the starting motor speed is zero, no induced potential is generated, so the starting current is: 1=E/Rm+Rs+Rl, where: E is the no-load terminal voltage of the battery, RM is the armature resistance of the starting motor, RB is the internal resistance of the battery, and RL is the line resistance. Since RM, RB, and RL are all very low, the starting current is very large. For example, when a 12V, 45Ah battery is used to start a car equipped with a 1.9-liter diesel engine, the battery voltage drops from 12.6V to about 3.6V at the moment of starting! The battery voltage waveform during the starting process is shown in Figure 1; the instantaneous current at the start reaches 550A, which is about the 12C discharge rate of the battery! The battery current waveform during the starting process is shown in Figure 2, (the current/voltage conversion ratio of the current sensor: 100A/V). Although the car battery is a special battery for starting and can be discharged at a high rate, it can be seen from Figure 1 that the battery performance becomes very poor when discharged at a high rate of more than 10 times, and such a high rate discharge also causes obvious damage to the battery. The drastic change in voltage during the starting process is also a strong electromagnetic interference, which can cause the electrical equipment to "power off", forcing the electrical equipment to power on again after the generator starting process is completed. The computer is very likely to crash during this process. Therefore, whether from the perspective of improving the electromagnetic environment of automotive electrical equipment or improving the starting performance of the car and the performance of the battery and extending the service life, it is necessary to improve the performance of the automotive power supply during the starting process.

The solution to the problem is to increase the capacity of the battery, but it needs to increase a lot, which increases the volume. This is not a good solution. Connecting a supercapacitor in parallel with a battery can solve this problem well.

2 Principle and characteristics of supercapacitors 2.1 Principle of supercapacitors Supercapacitors are extremely large capacitors with a capacitance of thousands of farads. According to the principle of capacitors, the capacitance depends on the distance between electrodes and the surface area of electrodes. In order to obtain such a large capacitance, supercapacitors minimize the distance between electrodes and increase the surface area of electrodes as much as possible. For this purpose, the double-layer principle and activated carbon porous electrodes are used. The structure of the supercapacitor is shown in Figure 3. When the double-layer dielectric applies voltage to the two electrodes of the capacitor, a charge opposite to the charge carried by the electrode is generated on the dielectric interface close to the electrode and is bound to the dielectric interface, forming two electrodes of the actual capacitor, as shown in Figure 4. Obviously, the distance between the two electrodes is very small, only a few nanometers, and the activated carbon porous electrode can obtain a large electrode surface area, which can reach 200m2/gram. Therefore, supercapacitors with this structure have a large capacitance and can store a large amount of electrostatic energy. In terms of energy storage, this characteristic of supercapacitors is between traditional capacitors and batteries. When the potential between the two plates is lower than the redox electrode potential of the electrolyte, the charge on the electrolyte interface will not leave the electrolyte, and the supercapacitor is in a normal working state (usually below 3V). If the voltage across the capacitor exceeds the redox electrode potential of the electrolyte, the electrolyte will decompose and become an abnormal state. As the supercapacitor discharges, the charge on the positive and negative plates is discharged by the external circuit, and the charge response on the electrolyte interface decreases. From this, it can be seen that the charging and discharging process of the supercapacitor is always a physical process without chemical reactions. Therefore, the performance is stable, which is different from batteries that use chemical reactions.

        2.2 Main features

        Although the energy density of supercapacitors is 5% or less of that of batteries, this energy storage method can be applied to the shortcomings of traditional batteries and short-term high peak currents. Compared with batteries, this supercapacitor has the following advantages:

        large capacitance. Supercapacitors use activated carbon powder and activated carbon fiber as polarizable electrodes, which greatly increases the area in contact with the electrolyte. According to the calculation formula of capacitance, the larger the surface area of the two plates, the greater the capacitance. Therefore, the capacity of a general double-layer capacitor can easily exceed 1F. Its appearance has suddenly increased the capacity range of ordinary capacitors by 3 to 4 orders of magnitude. At present, the maximum capacitance of a single supercapacitor can reach 5000F.

        The charge and discharge life is very long, up to 500,000 times, or 90,000 hours, while the charge and discharge life of batteries is difficult to exceed 1,000 times; it can provide a very high discharge current, such as the rated discharge current of a 2700F supercapacitor is not less than 950A, and the peak discharge current can reach 1680A. General batteries usually cannot have such a high discharge current. The service life of some batteries with high discharge current will be greatly shortened under such a high discharge current. It

        can be quickly charged within tens of seconds to minutes, while it would be extremely dangerous or almost impossible to fully charge a battery in such a short time.

        It can work normally in a wide temperature range (-40℃～+70℃), while batteries are difficult to work in high temperature, especially low temperature environment; the materials used in supercapacitors are safe and non-toxic, while lead-acid batteries and nickel-cadmium batteries are toxic; moreover, supercapacitors can be used in parallel arbitrarily to increase the capacity, and can also be used in series after taking voltage equalization measures.

3 Combination of supercapacitors and batteries to improve the starting performance of automobiles         3.1 Improvement of electrical performance

        The voltage waveform of the starting process when the supercapacitor is connected in parallel with the battery is shown in Figure 5, and the current waveform is shown in Figure 6. Compared with Figures 1 and 2, the voltage drop at the start-up moment is increased from 3.2V when only the battery is used to 7.2V; the starting current is increased from 560A to 1200A; the power output power at the start-up moment is increased from 2kW to 8.7kW; the steady voltage during the starting process is increased from 7V to 9.4V; the steady current during the starting process is increased from 280A to 440A; the steady power output power during the starting process is increased from 2.44kW to 4.12kW.

        3.2 Improvement of starting performance

        The parallel application of supercapacitors and batteries can improve the starting performance of locomotives. The supercapacitor (450F/16.2V) and the 12V, 45Ah battery are connected in parallel to start a car equipped with a 1.9-liter diesel engine. It starts smoothly at 10 degrees Celsius. Although in this case, the battery can also be started without the supercapacitor, the speed and performance of the starting motor are very good when the supercapacitor is connected in parallel with the battery. Due to the increase in the output power of the power supply, the starting speed increases from 300rpm when only the battery is used to 450rpm; especially in improving the starting performance of the car in cold weather (higher starting torque), supercapacitors are very meaningful. At minus 20 degrees Celsius, due to the greatly reduced performance of the battery, it is likely that it will not start normally or it will take multiple starts to succeed, while when the supercapacitor is connected in parallel with the battery, only one ignition is required. Its advantages are very obvious.

        3.3 Improvement of battery application status

        When the supercapacitor is connected in parallel with the battery, the equivalent series resistance (ESR) of the supercapacitor is much lower than the internal resistance of the battery. Therefore, at the moment of starting, 800A of the 1200A starting current is provided by the supercapacitor, and the battery only provides 400A. This is significantly lower than the 560A of the battery alone, which effectively reduces the polarization of the battery plate, prevents the increase of the battery internal resistance, and improves the stable voltage during the starting process. The most important thing is that the reduction of the polarization of the battery plate is not only conducive to extending the service life of the battery, but also can eliminate the impact of frequent starting on the battery life.

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