Application of variable amplitude pulse charging technology in DC power supply device

Abstract: Aiming at the use and maintenance issues of battery packs in DC power systems widely used in various industries, a new charging method - variable amplitude pulse charging method - is proposed, and its software and hardware implementation is briefly described.

Keywords: variable amplitude pulse charging; DC power supply device; battery pack; verification discharge

introduction

In recent years, with the continuous advancement of technology, the degree of comprehensive automation of substations has also been gradually improved. As a device that provides reliable DC power to substation control circuits, signal circuits, emergency lighting circuits, relay protection devices, automatic devices, remote control devices (RTU) and inverter power supplies, the importance of DC power supply devices has become increasingly prominent. As the main component of the DC power supply device, the battery pack becomes the only DC power provider when the power grid fails. Therefore, it is particularly important to do a good job in the daily maintenance of the battery pack to ensure that the capacity of the battery pack is sufficient and to maintain the consistency of the voltage and internal resistance of a single battery.

At present, the main method for maintaining battery packs is regular check discharge, while the charging method mostly adopts the traditional three-stage charging: constant current voltage limiting - constant voltage current limiting - trickle charging. However, this charging method is not very suitable for charging VRLA battery packs with multiple batteries in series, and it is easy to cause the following faults to the battery: First, if the battery is undercharged for a long time, a large amount of large and poorly active PbSO4 crystals and poorly active PbO2 under their coating will be deposited on the positive and negative electrodes. It is manifested as the battery voltage quickly rises to the controlled termination voltage during charging, and quickly drops to the discharge termination voltage during discharge, and the battery cannot discharge electricity. Second, the frequent equalization charge and long-term floating charge cause the battery to precipitate a large amount of gas, resulting in accelerated water loss in the electrolyte and increased internal resistance. Third, the difference in performance of each battery in the battery pack is very obvious after long-term use. At present, 50% of battery failures are caused by the above reasons.

Therefore, developing a new charging mode suitable for VRLA batteries to increase the service life of the battery pack, reduce user costs, save resources, and improve the stability of the DC power supply device has become a common requirement of current equipment manufacturers and users.

1 Technical Principle

Since the lead-acid battery was invented by G. Plante in 1859, the industry has lacked theoretical research on the battery life problem from the perspective of charging management, resulting in the continued use of the traditional methods of constant voltage current limiting and balanced charging. Practice has proved that it is difficult to balance the internal resistance of batteries in battery packs with a large number of single cells by using the traditional balanced charging method, which will seriously affect the life. The reason is: due to the different polarization voltage drops of each single cell (Note: The polarization voltage drop consists of three parts, namely ohmic polarization, concentration polarization and electrochemical polarization), and the more single cells there are, the greater the relative difference. When the charging capacity of the battery pack reaches 90%, the voltage of each single cell will be significantly different, and the voltage difference of some single cells may even exceed 150mV. If the battery pack continues to be charged to the set termination total voltage, the single cells with large polarization will be seriously overcharged, and the single cells with small polarization will be undercharged. If the cycle continues, the performance of the battery pack will deteriorate quickly, the electrolyte of the overcharged single cell will dry up, the capacity will decline, the plates of the undercharged single cell will be sulfated, and the battery will fail.

In response to the above problems, we have developed a new variable amplitude pulse equalization charging technology: first use a large current constant current to charge to about 70% of the rated capacity (at this time, the battery produces very little polarization, and the electrical performance of each single battery is basically the same), and then start pulse charging. During pulse charging, the positive pulse current is determined by the difference between the battery pack voltage and the output voltage set by the charging power supply, that is, the positive pulse current is proportional to the above voltage difference. The depolarization pulse (negative pulse) current changes very little from beginning to end, so that as the battery pack voltage increases (polarization increases), the ratio of positive and negative pulses becomes smaller and smaller during the charging process, that is, the depolarization effect increases, thereby achieving the effect of suppressing polarization voltage and balancing internal resistance. The variable amplitude pulse charging mode starts from the structural characteristics of the positive plate of the VRLA battery, and studies a charging mode that can keep the positive plate with high capacity, high charging and discharging efficiency and good mechanical properties during the battery cycle, so that the battery pack is sufficient but not overcharged, and the gassing rate is controlled within the allowable range.

The charging curve and current waveform of the variable amplitude pulse charging technology are shown in Figure 1 and Figure 2 respectively. As shown in Figure 2, by controlling the ratio between the charging pulse power and the depolarization pulse power, the actual charging curve can be fitted with the optimal charging efficiency curve shown in Figure 1, thereby improving the charging efficiency, reducing the amount of gas evolution, and avoiding the increase of the electrolyte temperature.

The variable amplitude pulse equalization charging technology we proposed refers to the optimal charging efficiency curve of the battery described by the Mass law to design the charging process, that is, charging with a large constant current in the area with the highest charging efficiency. When the charging capacity reaches the rated capacity or 60-70% of the capacity used in the previous week, intermittent depolarization pulses are added, and charging pulses and depolarization pulses are periodically applied to the battery in succession to appropriately reduce the average charging current and reduce polarization.

Our company has obtained the invention patent for the above technology: "Charging method of valve-regulated sealed lead-acid battery", patent number: ZL 2004 1 0079386.6.

Figure 1 Typical curve of VRLA battery pack using variable amplitude pulse charging

Figure 2 Schematic diagram of the change of current pulse amplitude in each charging stage of VRLA battery pack

2 Hardware Principle

2.1 Variable amplitude pulse generator controller

The electrical principle block diagram of variable amplitude pulse generation and control is shown in the figure

Figure 3 Electrical block diagram of variable amplitude pulse generation and control

The AC/DC module is a high-frequency switching power supply. Q1 and Q2 are turned on and off in turn according to the set requirements under the control of the MCU, which can meet the requirement that the charging output current is a variable amplitude pulse. This unit device has a reasonable design of the drive circuit, protection circuit and device combination, which can ensure high reliability of continuous operation under high current and high voltage conditions and millisecond working conditions.

2.2 Centralized Controller

The corresponding hardware structure is designed according to the requirements of the variable amplitude pulse charging mode. The main controller uses a fully integrated mixed signal MCU chip to form a chip system that can work independently. The MCU can effectively manage analog and digital peripherals, and complete data exchange and control between the monitoring hardware digital interface and the variable amplitude pulse high-frequency switching power supply module. At the same time, non-volatile ferroelectric memory FRAM is used to save key process and operating parameters. Important process parameters can be saved for 10 years, and the number of parameter modifications can reach 1012. Through the implementation of monitoring hardware, a reasonable charging model that perfectly combines power electronic conversion technology with embedded system control technology and electrochemical variable amplitude pulse charging technology is innovatively established. The hardware schematic diagram of the model is shown in Figure 4. Through the carefully designed PC comprehensive management software, the data exchange and control between the PC and the charging unit are completed, and the synchronous sampling and control of various parameters are realized. A complete charging data analysis and management database is established on the computer terminal to achieve the purpose of continuously improving the variable amplitude pulse balanced charging technology and establishing a more scientific and reasonable charging model.

3 Software Flowchart

According to the use specification of the DC power supply device and the optimal charging curve of the lead-acid battery group, the corresponding software is implanted in the DC power supply monitoring device, and the software has obtained a national software copyright certificate. The software flow chart is shown in Figure 5.

Figure 5 Flowchart of variable amplitude pulse charge and discharge machine monitoring software

4 Experimental results and analysis

In order to prove the good effect of variable amplitude pulse charging technology on battery charging, we specially selected a group of 9 CGB12V/65Ah batteries used in DC system for charging experiment. Due to long-term over-discharge with load, the battery lost water seriously, the capacity was emptied, and the plate was severely sulfided, resulting in the opening voltage of a single battery between 0.74V and 0.86V, and it could not be charged or discharged.

Check the inside of the battery: pry open the upper cover of the battery, open the rubber cap, there is a small amount of acid in the rubber cap, the separator inside the battery shows signs of drying up, and the acid in the rubber cap is weakly acidic when tested with a wide range pH test paper.

High-voltage hydrotherapy battery activation: Add 1.05g/mL dilute sulfuric acid (50mL/cell) to the battery, connect two 12V/12Ah batteries in series to charge a CGB12V/65Ah battery, the charging voltage is as high as 25V, the loop current increases slowly, and after about 45 minutes the loop current increases to about 5A. At this point, the battery is considered to have been initially activated.

The activated batteries were connected in series and first charged with a constant current of 5A. After charging, the battery pack was tested for discharge at a capacity of 5.0A. The results are shown in Table 1.

The data in Table 1 show that the battery voltage of this group of batteries was artificially high and the balance was poor due to severe plate sulfidation at the beginning of charging. In the traditional three-stage charging mode, the voltage of the battery pack quickly rose to the charging voltage limit value, resulting in the battery being unable to be further charged after the capacity reached 40Ah. In addition, the battery balance was quite different, with the maximum voltage difference reaching 0.79V.

The battery was charged by variable amplitude pulse charging method, and then discharged at 5A to detect the battery capacity. The results are shown in Table 2 and Table 3.

The data in Table 2 show that after 8 hours of variable amplitude pulse charging, the rate of battery voltage rise slowed down significantly, proving that the large crystals of lead sulfate in the plate were decreasing and the structure was gradually loosening. In the final stage of charging, the voltage balance between the individual cells gradually became consistent, and the maximum voltage difference was reduced to 0.21V. This shows that under the action of the variable amplitude pulse, the battery with a larger internal resistance in this group of batteries did not generate a larger polarization voltage as the charging progressed, and the battery with a small internal resistance also accepted the charged power very well, and there was no undercharging phenomenon. The final charging capacity reached 70Ah, and there was a certain amount of overcharge.

After the variable amplitude pulse charging was completed, the battery pack was discharged for verification, and the parameters are shown in Table 3.

From the above data, it can be seen that the battery has good consistency when discharging, and the battery discharge capacity after recharging is 96% of the rated capacity.

5 Conclusion

Based on the existing battery charging technology, the variable amplitude pulse charging technology formed by the theoretical expansion of the Mass law solves the problems of poor internal resistance balance, short service life and insufficient capacity of the battery pack during use and maintenance. It provides a solid technical guarantee for the DC power supply device to provide a stable and reliable DC power supply in the substation. This will definitely greatly improve the reliability of the operation of the DC power supply device. As a technology with independent intellectual property rights, its promotion space is also very broad.