There are two types of VRLA batteries: one is VRLA battery using ultra-fine glass fiber membrane (AGM); the other is VRLA battery using colloidal electrolyte (GFL) (abbreviated as GFL-VRLA battery). They all use the principle of cathode absorption to seal the lead-acid battery. Therefore, there must be about 10% of the diaphragm gap in the diaphragm of AGM-VRLA battery. For GFL-VRLA battery, after the injected silica sol becomes a gel, the skeleton will further shrink, and the viscosity of the silica sol should be controlled at 10mPa /s or so, so that cracks appear in the gel to penetrate between the positive and negative plates. The voids or cracks provide a channel for the oxygen evolved from the positive plate to reach the negative electrode. In the production of AGM-VRLA batteries, too much electrolyte perfusion is not conducive to the recombination of oxygen at the cathode, and too little electrolyte perfusion will cause the internal resistance of AGM-VRLA batteries to increase; in the production of GFL-VRLA batteries, if If the viscosity of the silica sol is too high, that is, the amount of silicon solution added is too large, which will cause excessive cracks in the gel and increase the internal resistance of the GFL-VRLA battery. On the contrary, it is not conducive to the recombination of oxygen at the cathode. Therefore, VRLA batteries have very strict production process requirements.
The colloidal electrolyte used in the early GFL-VRLA batteries was made of water glass, and then directly added to the dry state ordinary lead-acid battery. Although this achieves the purpose of “fixing” the electrolyte or reducing the precipitation of acid mist, the capacity of the GFL-VRLA battery is about 20% lower than that of the ordinary lead-acid battery using liquid electrolyte, so it is not accepted by people. .
In the 1950s, China carried out the research and development of GFL-VRLA batteries. In the process of developing GFL-VRLA batteries, a cathode absorption battery with glass fiber diaphragm was born, which not only eliminated acid fog from ordinary lead-acid batteries, but also It also has the advantages of small internal resistance and good discharge characteristics at large currents. Therefore, in the national economy, especially in the occasions where ordinary lead-acid batteries were used, they have been rapidly promoted and applied. During this period, the development of GFL-VRLA batteries in China was stagnant.
In the 1980s, the GFL-VRLA battery products of the German Sunshine Company entered the Chinese market. The use effect over the years shows that its performance is better than the early GFL-VRLA battery, which makes the GFL-VRLA battery enter a new development. stage. Want to learn more about batteries? go now.
- Main differences in structure and process of VRLA battery
Whether it is AGM-VRLA battery or GFL-VRLA battery, they all use the principle of cathode absorption to seal the battery. When the VRLA battery is charged, the positive electrode will evolve oxygen and the negative electrode will evolve hydrogen. The oxygen evolution of the positive electrode starts when the charge of the positive electrode reaches 70%. The precipitated oxygen reaches the negative electrode, and the following reaction occurs with the negative electrode to achieve the purpose of absorption by the negative electrode.
The hydrogen evolution of the negative electrode should start when the charge reaches 90%, coupled with the reduction of oxygen on the negative electrode and the increase of the hydrogen overpotential of the negative electrode itself, so as to avoid a large number of hydrogen evolution reactions. For AGM-VRLA batteries, in AGM-VRLA, although most of the electrolyte of lead-acid batteries is maintained, 10% of the pores of the diaphragm must not enter the electrolyte, that is, the lean liquid design, the oxygen generated by the positive electrode. It is through this part of the pores that it reaches the negative electrode and is absorbed by the negative electrode.
For the GFL-VRLA battery, the three-dimensional porous network structure is composed of SiO2 particles as the skeleton in the GFL-VRLA battery, which hides the electrolyte. After the silica sol poured into the GFL-VRLA battery becomes a gel, the skeleton will further shrink, so that cracks appear in the gel to penetrate between the positive and negative plates, providing a channel for the oxygen precipitated from the positive electrode to reach the negative electrode.
It can be seen that the sealing working principle of the two VRLA batteries is the same, the difference lies in the way of “fixing” the electrolyte and the way of supplying oxygen to the negative channel.
AGM-VRLA battery uses pure sulfuric acid aqueous solution as electrolyte, and its density is 1.29~1.31g/cm”. Except for a part of electrolyte absorbed inside the plate, most of it exists in the glass fiber membrane. In order to make the plate fully In contact with the electrolyte, the pole group is tightly assembled. In addition, in order to ensure that the VRLA battery has sufficient life, the pole plate should be designed to be thicker, and the positive grid alloy is Pb-Ca-Sn-A1 quaternary alloy.
The electrolyte of GFL-VRLA battery is composed of silica sol and sulfuric acid. The concentration of sulfuric acid solution is lower than that of AGM-VRLA battery, usually 1.26~1.28g/cm’. The amount of electrolyte is 20% more than that of AGM-VRLA batteries, which is equivalent to that of ordinary lead-acid batteries. This electrolyte exists in a colloidal state and is filled in the separator and between the positive and negative electrodes. The sulfuric acid electrolyte is surrounded by the gel and will not flow out of the battery.
Since the GFL-VRLA battery adopts a flooded non-tight assembly structure, the material of the positive grid can be made of low brocade alloy or a tubular positive plate. At the same time, in order to increase the capacity of the GFL-VRLA battery without reducing the life of the GFL-VRLA battery, the plate can be made thinner. The internal space of the GFL-VRLA battery compartment can also be expanded.
- Discharge capacity of VRLA battery
The discharge capacity of the early GFL-VRLA battery is only about 80% of that of the ordinary lead-acid battery. This is because the colloidal electrolyte with poor performance is directly poured into the unmodified ordinary lead-acid battery, and the internal resistance of the GFL-VRLA battery is larger, which is caused by the difficulty of ion migration in the electrolyte. Recent research work has shown that by improving the formulation of colloidal electrolyte, controlling the size of the colloidal particles, incorporating hydrophilic polymer additives, reducing the concentration of the colloidal liquid, improving the permeability and the affinity of the electrode plate, the vacuum filling process is adopted, and the The separator or AGM replaces the rubber separator to improve the liquid absorption of the GFL-VRLA battery; cancel the precipitation tank of the GFL-VRLA battery, and appropriately increase the content of the active material in the plate area, as a result, the discharge capacity of the GFL-VRLA battery can reach or Close to the level of ordinary lead-acid batteries.
AGM-VRLA batteries have less electrolyte, thicker plates, and lower active material utilization than ordinary lead-acid batteries, so the discharge capacity of AGM-VRLA batteries is about 10% lower than that of ordinary lead-acid batteries.
- VRLA battery internal resistance and high current discharge capacity
The glass fiber separator used in the AGM-VRLA battery has a porosity of 90%, and sulfuric acid is adsorbed in it, and the AGM-VRLA battery adopts a tight assembly form, and the diffusion and electromigration of ions in the separator are very little hindered, so AGM- VRLA battery has the characteristics of low internal resistance and strong ability to discharge high current quickly.
The electrolyte of GFL-VRLA battery is silica gel. Although the diffusion speed of ions in the gel is close to the diffusion speed in the aqueous solution, the migration and diffusion of ions are affected by the structure of the gel. The more tortuous the pathway, the narrower the pores in the structure, and the more hindered it is. Therefore, the internal resistance of GFL-VRLA battery is larger than that of AGM-VRLA battery.
However, the test results show that the high-current discharge performance of the GFL-VRLA battery is still very good, which fully meets the requirements of the relevant standards for the high-current discharge performance of the battery. This is because the concentration of acid and other related ions inside the porous electrode and in the liquid layer near the electrode plate plays a key role in high current discharge.
- Thermal runaway
Thermal runaway refers to the fact that the VRLA battery does not adjust the charging voltage in time at the later stage of charging (or in the floating state), so that the charging current and temperature of the VRLA battery have a cumulative mutual enhancement effect. At this time, the temperature of the VRLA battery rises sharply. As a result, the VRLA battery shell will expand and deform, the water loss rate will increase, and even the valve-regulated sealed lead-acid battery will be damaged.
The above phenomenon is a very destructive phenomenon that occurs when the AGM-VRLA battery is not used properly. This is because the AGM-VRLA battery adopts a lean liquid tight assembly design, and 10% of the pores in the separator must be kept from entering the electrolyte, so the internal thermal conductivity of the AGM-VRLA battery is poor and the heat capacity is small. When the oxygen generated by the positive electrode reaches the negative electrode and the negative electrode lead reacts, heat will be generated during charging. If it is not conducted away in time, the temperature of the AGM-VRLA battery will rise; if the charging voltage is not lowered in time, the charging current will increase. The increased oxygen velocity, in turn, increases the temperature of the AGM-VRLA battery. Such a vicious cycle will lead to thermal runaway.
The electrolyte volume of GFL-VRLA battery is equivalent to that of ordinary lead-acid battery. The surrounding of the pole group and between the battery and the tank is filled with gel electrolyte, which has a large heat capacity and heat dissipation, and will not generate heat accumulation. Combined with the operation practice of GFL-VRLA battery for more than 30 years, no thermal runaway phenomenon has been found in GFL-VRLA battery.
- Service life
There are many factors that affect the service life of VRLA batteries, including the design and manufacture of VRLA batteries, and the factors of user usage and maintenance conditions. As far as the former is concerned, the corrosion resistance of the positive grid and the water loss rate of the VRLA battery are the two most important factors. Due to the increased thickness of the positive grid and the use of Pb-Ca-Sn-A1 quaternary corrosion-resistant alloy, according to the corrosion rate of the grid, the service life of the VRLA battery can reach 10 to 15 years. However, from the results of using VRLA batteries, the water loss rate has become the most critical factor affecting the service life of VRLA batteries.
Since the AGM-VRLA battery adopts a lean liquid design, the capacity of the VRLA battery is extremely sensitive to the amount of electrolyte. VRLA battery loses 10% of water, and its capacity will decrease by 20%; if it loses 25% of water, the life of AGM-VRLA battery ends. However, the GFL-VRLA battery adopts a flooded design, and the density of the electrolyte is lower than that of the AGM-VRLA battery, which reduces the corrosion rate of the grid alloy. Sensitivity is low. These measures are beneficial to prolong the service life of GFL-VRLA battery. According to the information provided by the German Sunshine Company, the amount of water contained in the colloidal electrolyte is enough to make the GFL-VRLA battery run for 12 to 14 years. In the first year of operation of the GFL-VRLA battery, the water loss is 4% to 5%, and then decreases year by year. , After 4 years of operation, the annual water consumption is only 2%.
- Compounding efficiency
Recombination efficiency refers to the rate at which the oxygen generated by the positive electrode of the VRLA battery is absorbed and recombined by the negative electrode during charging. Factors such as charging current, VRLA battery temperature, negative electrode characteristics, and the speed at which oxygen reaches the negative electrode will all affect the gas recombination efficiency of VRLA batteries.
According to the GFL-VRLA battery product manual provided by German Sunshine Company, in the early stage of GFL-VRLA battery use, the oxygen recombination efficiency is low, but after several months of operation, the recombination efficiency can reach more than 95%. This phenomenon can also be verified from the water loss rate of GFL-VRLA batteries. The water loss rate of GFL-VRLA batteries is relatively large in the first year of operation, reaching 4% to 5%, and then gradually decreases. In the early stage of formation, the colloidal electrolyte has no or very few cracks inside, and does not provide enough channels for the oxygen evolved from the positive electrode. With the gradual shrinkage of the colloid, more and more channels will be formed, so the recombination efficiency of oxygen will gradually increase, and the water loss will inevitably decrease.
There are unsaturated voids in the diaphragm of AGM-VRLA battery, which provide a large number of oxygen channels, so its oxygen recombination efficiency is very high, and the new AGM-VRLA battery can reach more than 98%.
Read more: What is the steering mechanism of a wind turbine?