Development of long-life LL-S valve-regulated lead-acid battery for energy storage

1 Introduction

In recent years, people have paid more and more attention to issues such as global warming. The Kyoto Protocol adopted at the third conference of the parties to the Framework Convention on Climate Change specifically stipulates the reduction targets for greenhouse gases, mainly carbon dioxide.

The domestic electricity demand in Japan needs to be further energy-efficient in the industrial field, while the electricity consumption for people's livelihood is increasing. Overall, it is an increasing trend. Therefore, it is necessary to take a thorough solution to achieve the reduction target. In addition, the demand for electricity is concentrated in the daytime, but overall, the demand for electricity is reduced. The difference in electricity demand is not large when the day and night are combined. The surplus electricity at night is stored in batteries, and the electricity in the energy storage system can be used during the peak electricity consumption during the day. This can reduce the power generation during the day. This method can effectively reduce the normalization of day and night electricity loads and carbon dioxide emissions. Consumers can use the low-priced nighttime electricity in a controlled manner during the peak hours of daytime electricity consumption, which can reduce the electricity bill standard.

Currently, the battery power storage system is mainly based on lead-acid batteries, but there are also systems such as sodium-sulfur batteries, redox batteries, and lithium-ion batteries. However, there are various issues that need to be further developed in terms of life performance, cost, installation space, etc.

Valve-regulated lead-acid batteries have the advantages of low price, simple use, good reliability and high safety, but long-life batteries for practical power storage have yet to be developed. In addition, in recent years, solar power generation, wind power generation and other projects combined with batteries have attracted much attention. Therefore, the demand for long-cycle life valve-regulated fixed lead-acid batteries will increase.

The company currently has a battery (LL series) with a cycle life of 3,000 times, which needs to be further improved for use as an energy storage system. The development of a long-life valve-regulated lead-acid battery for power storage has been approved by the New Energy and Industrial Technology Development Organization (NEDO) and the Industrial Technology Practical Development Expenses Subsidy Project from 2001 to 2003, and a battery with a cycle life of 4,500 times has been developed. The following report is the specific content of the development.

2 Development goals

The development goals are as follows:

(1) 2V/1000Ah, 2V/1500Ah large capacity single battery;

(2) Cycle life: 4500 times (25°C environment, 70% discharge);

(3) Battery charge and discharge efficiency is 87%.

3. Extending battery life

The structure of the valve-regulated fixed lead-acid battery is shown in Figure 1. The battery tank is filled with positive plates, negative plates, a pole group consisting of a glass fiber liquid-limiting separator, a dilute sulfuric acid electrolyte, and an active material that maintains a porous body.

The main purpose of fixed lead-acid batteries is emergency power supply and UPS and other backup power supplies. However, compared with this product, the original LL model battery (3000 cycles) has corresponding improvements in the high density of positive active materials and negative electrode additives, and adopts a limited liquid separator suitable for horizontal structure and horizontal use, so that the charging conditions are optimized, etc., which improves the cycle life performance. The life test results of the battery with a life of 3000 cycles are shown in Figure 2. The results of the dissection study of the battery that has reached the end of its life are listed in Table 1.

The results of the dissection of the battery that reached 3000 times in the cycle life test showed that the main reasons affecting the life were corrosion and deformation of the positive grid, mudification of the positive active material, and sulfation of the negative plate. In order to improve the cycle life performance, the above-listed items that affect the life performance were focused on for improvement.

3.1 Positive Plate

In order to improve the durability of the positive grid, it is necessary to inhibit grid corrosion and reduce corrosion deformation. The selection of grid alloy should not only reduce corrosion, but also make corrosion uniform and reduce deformation. The Pb-Ca-Sn alloy with the best Ca and Sn addition ratio should be selected. In order to reduce the corrosion deformation of the positive grid, the grid design should be determined after the simulation test of grid corrosion deformation.

The actual grid corrosion deformation is mostly manifested in the expansion of the surface layer. Therefore, in order to conduct a simulation test close to the actual situation, the grid is heated from the outside, and the temperature distribution in the grid is formed by the heat conduction to the inside of the grid. The expansion caused by the increase in grid temperature is regarded as the expansion of lead corrosion, and then the corrosion deformation simulation test is carried out. The grid temperature distribution during dissection is shown in Figure 3. The cross-sectional condition of the grid, the changes in thick and thin ribs are simulated, and the results are shown in Figure 4. Through the test, it is known that the increase in the weight of the grid is limited to a minimum, and compared with the traditional grid, this structural design reduces the corrosion deformation of the grid.

The cycle life test and grid corrosion of the conventional 3000-cycle battery and the new battery were compared, and the results are shown in Figure 5. The corrosion of the grid of the new battery is about 65% of that of the conventional battery, and the expected life of the grid is more than 1.5 times that of the conventional battery, and the cycle life performance has achieved 4500 times. The volume change caused by charging weakens the binding force between the positive electrode active materials and destroys the conductive grid (softening and mudding of the active material). These increase the density of the active material, strengthen the binding points between the active material particles, and effectively improve the firmness of the active material.

In order to evaluate the active material, a small-sized battery for evaluation test was used with a grid collector with little corrosion deformation. The grid was filled with high-density active material and the battery cycle life test was carried out. The results are shown in Figure 6. The figure confirms that the charge and discharge cycle life performance is more than 4500 times.

In summary, the combination of a grid collector with little corrosion deformation and a high-density active material can extend the cycle life of the positive plate to more than 4,500 times.

3.2 Negative plate

The negative active material of the lead-acid battery, due to repeated charge and discharge cycles, causes the active material to be sulfated and the active material grains to be coarsened, resulting in a decrease in capacity. In order to prevent this phenomenon, additives such as graphite, lignin, and barium sulfate are added to the negative active material. These additives are added in certain types and different proportions, which have a great impact on the discharge performance, charge acceptance performance, and life performance of the negative plate.

Adding graphite to lead sulfate can improve the conductivity of the lead sulfate surface, and it is believed that the reaction occurs on the graphite surface. It is well known that ultrafine grain graphite and the increase in the amount of addition can improve the charging performance. The physical properties of graphite vary greatly depending on the type, so it can be believed that different types of graphite have different effects.

In order to compare the effects of different types of graphite on the negative electrode charging performance, six types of graphite used to evaluate the test small battery are listed in Table 2. After the battery has been charged for 1000 times, the results of the study on the relationship between the lead sulfate remaining in the negative plate and the type of graphite are shown in Figure 7. The accumulation of lead sulfate varies with the type of graphite. Compared with the traditional graphite A, graphites D and E have less lead sulfate content and better negative electrode charging performance. The data in Table 2 cannot determine the relationship between the physical properties of graphite and the negative electrode charging performance. The amount and type of functional groups on the surface of graphite grains are different, and the effects are also different. The function of graphite is still under discussion.

4 Structural characteristics of new batteries

4.1 Single Cell

The improvement contents and purpose of the LL-S battery with a long life of 4500 times are listed in Table 3. The battery structure, horizontal placement of the usage conditions, liquid limitation, and charge and discharge control of power management still follow the technology of LL type batteries.

The comparison between the new battery and the traditional product is listed in Table 4, and the appearance photo of the product is shown in Figure 8. Compared with the traditional battery with a cycle life of 3000 times, the battery has the same size and the weight has increased by about 8%. The expected life of the battery has increased to 4500 times, which is 1.5 times the original. The battery slot material is made of polypropylene resin with excellent drug resistance and moisture permeability. The terminal is bolt-type. The battery cover is designed with an insert-molded lead alloy pole sleeve, which is welded to the terminal part and sealed with epoxy glue to prevent electrolyte leakage. The recommended charging method of this battery is shown in Figure 9. The charging voltage of the single cell is set to 2.42V, and multi-stage low-current charging and charging in a controlled manner to prevent overcharging are used.

The discharge characteristics of the new battery at each hour rate are shown in Figure 10. The discharge performance is the same as that of the conventional LL type battery. The charge and discharge efficiency of the recommended charging mode after 70% discharge is shown in Figure 11. The efficiency of the new battery when charging and discharging is about 87%.

4.2 Battery Pack

The battery pack for power storage uses multiple single cells, generally in a frame unit. The LL1500-S battery pack using this method is shown in Figure 12. In this case, a temperature difference will occur between the cells depending on the position of the cells. One of the reasons that affect the battery life performance is the increase in positive grid corrosion and excessive temperature. The relationship between the temperature of the cycle life test and the corrosion of the positive grid is shown in Figure 13.

Reducing the temperature rise of each battery pack and the temperature difference between batteries is the key control part when using the battery pack. In order to suppress the temperature rise of the battery during charging and discharging of the battery pack and balance the temperature between batteries, the simulation test of heat conduction and heat flow analysis and the measured data of the simulated frame battery pack were studied, and the results are listed in Table 5. It is well known that the frame structure with gaps between batteries is for battery ventilation, which can reduce the charging power of the first stage of multi-stage charging as shown in Figure 9, and can effectively suppress the temperature rise and achieve temperature balance when used in shallow discharge. The charging power and discharge depth are related to the operating conditions of the system, so the frame structure with gaps between batteries should be improved. By improving the use of the frame and air flow, the battery temperature rise during cyclic use can be controlled at about 7°C, and the average temperature between batteries can be reduced by 8°C.

The battery pack (12 cells per block, 4 cells per layer, 3 layers in total) with 1500Ah batteries was charged and discharged, and the comparison with the conventional frame is shown in Figure 14. The battery temperature rose by an average of 6°C, the temperature difference between the batteries decreased by about 4°C, and the air flow between the batteries was optimized, with significant effects.

5 Practical test of power reserve system

At present, the company has three power reserve devices that are already undergoing practical tests on the actual scale of system peak operation. The structure of the energy storage system in the practical test is listed in Table 6. The maximum power of units 1 and 2 is 100 kW, and unit 3 is a charging and discharging system with a maximum power of 400 kW. The device with the longest battery working period is unit 1 shown in Figure 15, and Figure 16 shows the voltage condition at the end of discharge in the practical test. So far, the battery group has been continuously operated for 2 years and 6 months, with good results, and there has been no phenomenon of battery voltage drop and increased deviation. The same effect was achieved for units 2 and 3, and the test proved that there is no problem in the practical use of the power reserve system.

6 Summary

The LL-S valve-regulated fixed lead-acid battery with long cycle life developed by the company has the following characteristics:

(1) The positive electrode plate is made of corrosion-resistant alloy, grid with small corrosion deformation and high-density active material. The negative electrode plate is improved by graphite additives to improve charging performance. At the same time, the improvement of the sealing part further improves the reliability of the battery. The new battery pack achieves 4500 cycle life performance (25°C, 70% discharge capacity) and up to 87% charge and discharge efficiency.

(2) Develop and manufacture a frame component that is compatible with the battery pack and can reduce the temperature rise of the battery and control the temperature difference between the batteries during use.

(3) After 2 years and 6 months of practical operation tests of the power storage system, it was confirmed that the new battery has reliable performance and operates well.