Design of supercapacitor energy storage module to replace batteries

introduction

Electric energy is an indispensable and important resource in contemporary society, and the quality of energy storage equipment directly affects the full application of power equipment. In recent years, with the development and use of portable devices, uninterruptible power supply systems and electric vehicles, the use of batteries has increased. Rechargeable batteries, especially lead-acid batteries, are widely used in all walks of life due to their excellent characteristics such as low price, stable performance and no memory function. However, batteries are restricted by their inherent conditions, and there are problems such as poor cycle life, poor high and low temperature performance, sensitive charging and discharging process, difficulty in deep discharge performance and capacity recovery, and environmental pollution. Traditional batteries are increasingly unable to meet people's requirements for energy storage systems.

Supercapacitors are a new type of power storage device that has only been mass-produced in recent years. They are also called electrochemical capacitors. They have the advantages of high discharge power of electrostatic capacitors and large charge storage capacity like batteries [1, 2]. The capacity of a single cell has now reached the level of 10,000 farads. At the same time, supercapacitors also have the advantages of long cycle life, high power density, fast charge and discharge speed, good high temperature performance, flexible capacity configuration, environmental friendliness and maintenance-free. Since the American Becker published the first patent on supercapacitors in 1957, the application scope of supercapacitors has become increasingly wide: research has been conducted in the application directions of DC electrified railway power supply and UPS, and 50kVA and 80kVA experimental prototypes have been developed [3]; supercapacitors are used in combination with batteries as auxiliary power sources to promote energy recovery in automobiles and improve energy utilization [4], and supercapacitor hybrid vehicles have emerged [5]. With the improvement of supercapacitor performance, it is expected to partially replace traditional batteries in low-power electronic devices, new energy utilization and some other fields.

This article introduces a supercapacitor module designed based on supercapacitors to replace 12V batteries. Through calculation and analysis, the module's combined structure, optimal charging current range, charging time, and total output energy are obtained. The module has the advantages of long life, no pollution, high power and energy density, and has a good development and application prospect.

1. Design of supercapacitor energy storage module

Since the discharge of the supercapacitor is incomplete, there is a minimum operating voltage, so the energy of a single supercapacitor is, where C is the single capacitance of the supercapacitor and is the voltage value when the single supercapacitor is fully charged.

Supercapacitors have limited storage energy and low withstand voltage. They need to be expanded through corresponding series-parallel connection methods to expand the use range of supercapacitors. The minimum operating voltage of supercapacitors can be increased through corresponding DC-DC chips. Assume that supercapacitors are composed of m series and n parallel groups. The energy output of each supercapacitor is

(1)

Among them, is the minimum starting voltage of the chip. Therefore, the total energy output of the supercapacitor array is, is the total energy of the supercapacitor.

This article uses SU2400P-0027V-1RA supercapacitors, which have high power ratio, energy ratio and low equivalent series resistance (ESR (DC) = 1mΩ). In order to form a supercapacitor module to replace the 12V battery, we use 8 2400F/2.7V capacitors to form a module, with 4 supercapacitor monomers connected in series and two groups connected in parallel, as shown in Figure 1.

The characteristics of supercapacitors, such as power density, energy density, energy storage efficiency, and cycle life, depend on the materials, structure, and process inside the device. Connecting the devices in parallel or in series will not affect their characteristics[6].

(2)

Among them, is the number of series devices and is the number of parallel branches.

The equivalent capacitance of the supercapacitor bank is:

(3)

Therefore, the equivalent internal resistance and equivalent capacitance of the supercapacitor array are:

Compare the capacity of the supercapacitor module with the capacity parameters of the battery.

(4)

The capacity of the supercapacitor array corresponding to the ampere-hours of the battery is obtained as , where Umin is the minimum starting voltage of the corresponding chip.

3. Design of related circuits

The overall structure of the circuit is shown in Figure 3. It includes a charging circuit, a supercapacitor energy storage module, and a working discharge circuit. Its design flow chart is shown in Figure 2.

Figure 2 Circuit design process

3.1 Charging Circuit

The supercapacitor is equivalent to an ideal capacitor C, which is connected in series with a resistor with a smaller resistance (equivalent series impedance, ) and in parallel with a resistor with a larger resistance (equivalent parallel impedance, ), as shown in Figure 3 [7].

Supercapacitors can be charged with large currents, but due to the existence of equivalent series resistance, the charging efficiency of supercapacitors will be reduced to a certain extent when charged with excessively large currents. Therefore, it is necessary to consider the impact of charging current on the working efficiency of supercapacitors.

When constant current charging is used, as shown in Figure 3, Is is the constant current charging current value, then

(5)

u(t) represents the terminal voltage of the supercapacitor, which is the capacitance voltage determined by the charge stored in the supercapacitor.

(6)

Where =0V is the initial voltage of the supercapacitor, which represents the voltage drop on the equivalent series resistance Res.

The total energy consumed during charging is

(7)

The energy stored in a supercapacitor is

(8)

According to the energy conservation formula, the equation holds true. Ideally, the constant current charging efficiency of the supercapacitor is expressed as:

(9)

Matlab was used to simulate the charging current and working efficiency of the supercapacitor, and origin software was used to process the results. The results are as follows:

Figure 4 Relationship between charging current and charging efficiency η

As shown in Figure 4, the supercapacitor monomer maintains a relatively high charging efficiency when the charging current is 3A~8A. After that, as the current intensity increases, the power lost in the corresponding resistor also increases, and the charging efficiency gradually decreases.

According to the above results, we use L4970A chip to form a related charging circuit to charge the supercapacitor.

As shown in FIG5 , the circuit can provide a constant current charging current of 10A, and its output voltage is determined by resistors R7 and R9.

L4970A is the second generation of monolithic switching regulator launched by ST. It has the characteristics of large output current, wide input voltage range, high switching frequency, and high charging efficiency. The 220V AC power outputs a 35V DC voltage after rectification and filtering, and then passes through the circuit shown in Figure 5. As shown in the figure, C1 and C2 are input filter capacitors, C3 and C4 are filter capacitors at the driver start-up end and Vref end respectively. R1 and R2 form a resistor divider at the reset input end, C5 is a soft start capacitor, and C6 is a reset delay capacitor. C8 and R3 form a frequency compensation network for the error amplifier, and C7 is used for high-frequency compensation. R4 and C9 are timing resistors and timing capacitors respectively. C10 is a bootstrap capacitor. The freewheeling diode VD uses a Schottky diode of type MBR2080 (20A/80V). C11 and R5 form an absorption network, and R6 is the collector resistor of the internal transistor at the reset output end. C12~C14 are output filter capacitors. Three identical 220μF/40V electrolytic capacitors are connected in parallel to reduce their equivalent inductance.

The output voltage of the L4970A chip is set to 10.8V, and its output resistance R7 is determined by the following formula:, where R9=4.7K, let Uo=10.8V, then R7=5.25K, and take the nominal value 5.1K.

The charging time of the supercapacitor is based on the formula, where C is the rated capacity of the supercapacitor, dv is the voltage change of the supercapacitor, I is the charging current of the supercapacitor, and t is the charging time. Therefore, the charging time of the supercapacitor array is (when the charging current is 10A)

3.2 Regulated Output Circuit

Since the output voltage of the replaced battery module is 12V, and the voltage of the supercapacitor is 10.8V, and as the supercapacitor continues to discharge, the voltage at both ends will continue to decrease. When the supercapacitor releases 50% of the stored energy, its terminal voltage will drop to 70% of the initial voltage. Therefore, a corresponding boost control circuit is required to avoid affecting the normal operation of the load due to the reduction of the supercapacitor array voltage and improve the utilization rate of supercapacitor energy storage.

Figure 6 Voltage regulated output circuit

We use the boost type dc/dc chip MAX668 from MAXIM. MAX668 has a wide input and output voltage range, it can boost the input voltage of 3~12V to 12V output, at the same time, because it uses a current detection voltage as low as 100mV and MAXIM's unique idle mode, the conversion efficiency is as high as over 90%, with a maximum current output capability of 1A. The boost circuit is shown in Figure 6.

MAX668 is a fixed frequency, current feedback PWM controller. It uses a bipolar CMOS multi-input comparator inside. It can process the output error signal, current detection signal and slope compensation signal at the same time. Since the traditional error amplifier is omitted, the phase shift caused by error amplification is suppressed. MAX668 can drive various types of N-channel MOSFETs. The FDS6680 is selected here. Since the chip works at a high frequency of more than 100 kHz, the diode D1 should be a Schottky diode that can be turned off at high speed. The MBR5340T3 is selected in this article.

The supercapacitors are connected in series with 4 and in parallel with 2. The energy output of each supercapacitor is

Among them, is the minimum startup voltage of the chip.

Therefore, the total energy output of the supercapacitor array is, and the capacity of the supercapacitor array is

The capacity of this supercapacitor replacement module is 10Ah, and the maximum output current is 1A. To expand its application range, you only need to change the number of supercapacitors in series and parallel and the corresponding chips.

IV. Conclusion

Due to the limitation of capacity, the role of capacitors has been limited to filtering, coupling, resonance and other aspects. With the development of supercapacitors, their application scope has been continuously broadened. This paper introduces a supercapacitor energy storage module that replaces batteries. By rationally designing the charging and voltage stabilization circuits, the energy output of the module can reach 59200J, with good stability and high conversion efficiency. The relationship between the current and efficiency of the charging circuit in this paper is calculated by matlab software, and the optimal charging current range is determined. With the improvement of supercapacitor withstand voltage, the expansion of capacity and the reduction of price, the corresponding low-power energy storage module has a good application prospect.