A brief comparison between GEL batteries and AGM sealed lead-acid batteries

As the use of solar energy becomes more and more widespread, the requirements for the use of reserve power　　in photovoltaic off-grid systems are becoming higher and higher. At present, the use of colloidal batteries has become the mainstream in photovoltaic application systems. The following comparative analysis is made between colloidal batteries and AGM batteries.

　　There are two types of valve-regulated sealed lead-acid batteries (VRLA) today, which use two different methods, namely, glass fiber separator (AGM) and silicone gel (Gel) to "fix" sulfuric acid electrolyte. They both use the cathode absorption principle to seal the battery, but the channels provided for the oxygen precipitated from the anode to reach the cathode are different, so the performance of the two batteries has their own advantages.

　　1. A brief historical review

　　Lead-acid batteries have been the most widely used chemical power source in military and civilian fields since their introduction. Since they use sulfuric acid electrolyte, acid will flow out during transportation and acid mist will precipitate during charging, causing damage to the environment and equipment. People tried to "fix" the electrolyte and "seal" the battery, so lead-acid batteries using colloidal electrolyte came into being.

　　The colloidal electrolyte used in the early colloidal lead-acid batteries was made of water glass, which was then directly added to the dry lead-acid batteries. Although this achieved the purpose of "fixing" the electrolyte or reducing the precipitation of acid mist, it made the battery capacity about 20% lower than the original battery capacity when using free electrolyte, so it was not accepted by people.

　　my country also started the initial research and development of colloidal batteries in the 1950s, but basically stopped by the end of the 1960s. However, from the late 1970s to the 1980s, some people outside the battery industry used the media to tout their invention of solid electrolyte lead-acid batteries, claiming that they doubled the battery capacity and life. This kind of soap bubble-like "invention" that cannot withstand the test of facts not only failed to improve the performance of lead-acid batteries, but also ruined the reputation of colloidal batteries.

　　Almost at the same time as the development of colloidal batteries, cathode absorption sealed lead-acid batteries using glass fiber diaphragms were born. They not only eliminated acid mist from lead-acid batteries, but also showed the advantages of small internal resistance and good large current discharge characteristics. Therefore, they were rapidly promoted and applied in the national economy, especially in the occasions where fixed lead-acid batteries were originally used, so people forgot about colloidal lead-acid batteries.

　　In the 1980s, the colloidal sealed lead-acid battery products of the German Sunshine Company entered the Chinese market. The results of years of use have shown that its performance is indeed different from the previous colloidal lead-acid batteries. This has forced people to re-examine colloidal lead-acid batteries.

　　This article will compare two types of valve-regulated sealed lead-acid batteries based on their development, production and use results in recent years, for reference by colleagues who are selecting batteries.

　　2. Working Principle of Battery

　　Whether it is a valve-regulated sealed lead-acid battery using a glass fiber diaphragm (hereinafter referred to as an AGM sealed lead-acid battery) or a valve-regulated sealed lead-acid battery using a colloidal electrolyte (hereinafter referred to as a colloidal sealed lead-acid battery), they all use the cathode absorption principle to make the battery sealed.

　　When the battery is charged, oxygen will be released at the positive electrode and hydrogen will be released at the negative electrode. Oxygen release at the positive electrode starts when the positive electrode is charged to 70%. The released oxygen reaches the negative electrode and reacts with the negative electrode as follows to achieve cathode absorption. Hydrogen release at the negative electrode starts when the battery is charged to 90%, and the reduction of oxygen at the negative electrode and the increase in the negative electrode's own hydrogen overpotential avoid a large amount of hydrogen release reaction.

　　For AGM sealed lead-acid batteries, although the AGM diaphragm retains most of the battery's electrolyte, 10% of the diaphragm pores must be kept from entering the electrolyte. The oxygen generated by the positive electrode reaches the negative electrode through these pores and is absorbed by the negative electrode.

　　For colloidal sealed lead-acid batteries, the silica gel in the battery is a three-dimensional porous network structure with SiQ particles as the skeleton, which encloses the electrolyte inside. After the silica sol injected into the battery turns into gel, the skeleton will further shrink, causing cracks in the gel to penetrate between the positive and negative plates, providing a channel for the oxygen released from the positive electrode to reach the negative electrode.

　　From this, we can see that the sealing working principles of the two batteries are the same. The difference lies in the way the electrolyte is "fixed" and the way oxygen is provided to reach the negative electrode channel.

　　3. Main differences in battery structure and process

　　AGM sealed lead-acid batteries use pure sulfuric acid aqueous solution as electrolyte, with a density of 1.29-1.3lg/cm3. In addition to a portion of electrolyte absorbed inside the plate, most of it exists in the glass fiber membrane. In order to provide a channel for oxygen precipitated from the positive electrode to the negative electrode, the diaphragm must maintain 10% of the pores not occupied by the electrolyte, that is, the lean liquid design. In order to make the plate fully contact the electrolyte, the pole group adopts a tight assembly method.

　　In addition, in order to ensure that the battery has a sufficient life, the plate should be designed to be thicker, and the positive grid alloy should use the Pb'-q2w-Srr--A1 quaternary alloy.

　　The electrolyte of the colloidal sealed lead-acid battery is made of silica sol and sulfuric acid. The concentration of sulfuric acid solution is lower than that of AGM battery, usually 1.26-1.28g/cm3. The amount of electrolyte is 20% more than that of AGM battery, which is equivalent to that of flooded battery. This electrolyte exists in a colloidal state, filling the diaphragm and between the positive and negative electrodes. The sulfuric acid electrolyte is surrounded by gel and will not flow out of the battery.

　　Since this battery adopts a flooded non-tight assembly structure, the positive grid material can be low-antimony alloy or tubular battery positive plate. At the same time, in order to increase the battery capacity without reducing the battery life, the plate can be made thinner. The internal space of the battery slot can also be expanded.

　　4. Battery discharge capacity

　　The discharge capacity of early colloidal batteries was only about 80% of that of flooded batteries. This was because the colloidal electrolyte with poor performance was directly poured into the unmodified flooded battery, resulting in a large internal resistance of the battery and difficulty in ion migration in the electrolyte.

　　Recent research shows that by improving the colloidal electrolyte formula, controlling the size of the colloidal particles, adding hydrophilic polymer additives, reducing the concentration of the colloidal solution to increase permeability and affinity for the plates, using a vacuum filling process, replacing the rubber separator with a composite separator or AGM separator, and improving the battery's liquid absorption; eliminating the battery's sedimentation tank and moderately increasing the content of active substances in the plate area, the discharge capacity of the colloidal sealed battery can reach or approach the level of an open lead-acid battery.

　　AGM sealed lead-acid batteries have less electrolyte, thicker plates, and lower active material utilization than open-type batteries, so the discharge capacity of the battery is about 10% lower than that of open-type batteries. Compared with today's colloid sealed batteries, its discharge capacity is smaller.

　　5. Battery internal resistance and high current discharge capability

　　The internal resistance of a lead-acid battery is composed of ohmic internal resistance, concentration polarization internal resistance, and electrochemical polarization internal resistance. The former includes plates, lead parts, electrolyte, and inter-electrode resistance. The glass fiber separator used in AGM sealed lead-acid batteries has a porosity of 90%, sulfuric acid is adsorbed inside it, and the battery is tightly assembled, so the diffusion and electromigration of ions in the separator are less hindered, so AGM sealed lead-acid batteries have low internal resistance characteristics and strong high current rapid discharge capabilities.

　　The electrolyte of the colloidal sealed lead-acid battery is silica gel. Although the diffusion speed of ions in the gel is close to that in the aqueous solution, the migration and diffusion of ions are affected by the gel structure. The more curved the diffusion path of ions in the gel, the narrower the pores in the structure, and the greater the obstacles. Therefore, the internal resistance of the colloidal sealed lead-acid battery is greater than that of the AGM sealed lead-acid battery.

　　However, the test results show that the high current discharge performance of the colloidal sealed lead-acid battery is still very good, fully meeting the requirements of the relevant standards for the high current discharge performance of sealed batteries. This may be because the concentration of acid and other related ions in the liquid layer inside the porous electrode and near the plate plays a key role in high current discharge.

　　6. Thermal runaway

　　Thermal runaway means that in the late stage of charging (or floating charge state), the charging voltage is not adjusted in time, causing the charging current and temperature of the battery to have a cumulative mutual reinforcement effect. At this time, the temperature of the battery rises sharply, causing the battery slot to expand and deform, the water loss rate to increase, and even battery damage.

　　The above phenomenon is a very destructive phenomenon that occurs when AGM sealed lead-acid batteries are used improperly. This is because AGM sealed lead-acid batteries use a lean liquid tight assembly design, and 10% of the pores in the separator must be kept to prevent the electrolyte from entering, so the thermal conductivity inside the battery is poor and the heat capacity is small. When charging, the oxygen generated by the positive electrode reaches the negative electrode and reacts with the negative electrode lead to generate heat. If it is not conducted away in time, the battery temperature will rise; if the charging voltage is not reduced in time, the charging current will increase, the oxygen evolution rate will increase, and in turn the battery temperature will rise. This vicious cycle will cause thermal runaway.

　　For open lead-acid batteries, since there is no phenomenon of cathode absorbing oxygen, and the amount of electrolyte is relatively large, the battery is easy to dissipate heat, and the heat capacity is also large, so there will be no thermal runaway. The amount of electrolyte used in colloidal sealed lead-acid batteries is equivalent to that of open lead-acid batteries. The area around the pole group and between the pole group and the tank is filled with gel electrolyte, which has a large heat capacity and heat dissipation, and will not cause heat accumulation.

　　The colloidal sealed lead-acid batteries of Germany's Sunshine Company have entered the Chinese market for more than ten years. Several agents said that they have not heard any reports from users about thermal runaway of the batteries.

　　7. Service life

　　There are many factors that affect the service life of valve-regulated sealed lead-acid batteries, including factors related to battery design and manufacturing, as well as factors related to user use and maintenance conditions. As for the former, the corrosion resistance of the positive grid and the water loss rate of the battery are the two most important factors. Due to the increase in the thickness of the positive grid and the use of Pb-Ca-Sn--Al quaternary corrosion-resistant alloy, the battery life can reach 10 to 15 years based on the grid corrosion rate. However, judging from the battery use results, the water loss rate has become the most critical factor affecting the service life of sealed batteries.

　　For AGM sealed lead-acid batteries, due to the lean liquid design, the battery capacity is extremely sensitive to the amount of electrolyte. If the battery loses 10% of water, the capacity will decrease by 20%; if the battery loses 25% of water, the battery life will end. However, the colloidal sealed lead-acid battery adopts a rich liquid design, and the electrolyte density is lower than that of the AGM sealed lead-acid battery, which reduces the corrosion rate of the grid alloy; the electrolyte volume is also 15% to 20% more than the latter, and the sensitivity to water loss is lower. These measures are all conducive to extending the service life of the battery. According to the information provided by the German Sunshine Company, the amount of water contained in the colloidal electrolyte is enough to keep the battery running for 12 to 14 years. In the first year of the battery being put into operation, the water loss is 4%-5%, and then it decreases year by year. After 4 years, the total water loss is only 2%. After 10 years of floating charge operation at 2.27V/cell, the OP2V sealed battery still has 90% of its capacity. According to the feedback from some domestic postal and telecommunications departments, although the price of Sunshine's colloidal sealed lead-acid batteries is higher, their service life is longer than that of domestic AGM sealed lead-acid batteries.

　　8. Oxygen recombination efficiency

　　Recombination efficiency refers to the ratio of oxygen generated by the positive electrode to be absorbed and recombined by the negative electrode during charging. Factors such as charging current, battery temperature, negative electrode characteristics and the speed at which oxygen reaches the negative electrode will affect the gas recombination efficiency of sealed batteries.

　　According to the product manual of the colloidal sealed lead-acid battery provided by the German Sunshine Company, the oxygen recombination efficiency of the colloidal sealed lead-acid battery product is low in the early stage of use, 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 the battery. The water loss rate of the colloidal sealed lead-acid battery is relatively large in the first year of operation, reaching 4% to 5%, and then gradually decreases. The main reason for the above characteristics seems to be that the colloidal electrolyte has no or very few cracks inside in the early stage of formation, and does not provide enough channels for the oxygen precipitated from the positive electrode. As the colloid gradually shrinks, more and more channels will be formed, then the oxygen recombination efficiency will inevitably increase gradually, and water loss will inevitably decrease.

　　There are unsaturated gaps in the AGM sealed lead-acid battery diaphragm, which provides a large number of oxygen channels, so its oxygen recombination efficiency is very high, and new batteries can reach more than 98%.

　　9. Choose genuine colloid sealed lead-acid batteries

　　The above-mentioned characteristics of the colloidal sealed lead-acid battery are the properties of the new generation of colloidal sealed lead-acid batteries at home and abroad. The colloidal electrolyte used in this battery is different from the colloidal electrolyte used in the early colloidal batteries in terms of performance. The latter is made of ordinary water glass or silica sol that is generally available on the market. In addition, the structure and material selection of the new generation of colloidal sealed lead-acid batteries are also different from those of ordinary lead-acid batteries.

　　From the current domestic and international technology development level, it is not difficult to make a colloidal lead-acid battery, but it is not easy to make a good colloidal sealed lead-acid battery. The technical tricks are what no manufacturer is willing to disclose. Users must be careful when choosing a colloidal sealed lead-acid battery.