Comparison of gel battery and AGM

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 also getting higher and higher. At present, the use of colloidal batteries in photovoltaic application systems has become the mainstream. Now make the following comparative analysis of gel battery and AGM battery.

Nowadays, there are two types of valve-regulated sealed lead batteries (VRLA), namely, glass fiber separator (AGM) and silicone gel (Gel) are used to "fix" sulfuric acid electrolyte in two different ways. They all 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 its own merits.  

1. A brief review of history 

Lead-acid batteries have been the most widely used chemical power source in the military and civilian fields since their inception until now. Since it uses sulfuric acid electrolyte, acid will flow out during transportation, and acid mist will precipitate out during charging, which will cause damage to the environment and equipment. People try to "fix" the electrolyte and "seal" the battery. So lead-acid batteries using colloidal electrolyte came into being.  

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

Almost at the same time as the development of colloidal batteries, the cathode-absorption sealed lead-acid battery with glass fiber membrane was born. It not only eliminated the acid mist of the lead-acid battery, but also showed the advantages of small internal resistance and good high-current discharge characteristics. Therefore, in the national economy, especially where fixed lead storage batteries were originally used, they have been rapidly promoted and applied, so people put the colloidal lead storage batteries behind.  

In the 1980s, the gel-sealed lead storage battery products of the German Sunshine Company entered the Chinese market, and the results of years of use have shown that its performance is indeed different from the previous colloidal lead storage batteries. This forces people to re-understand colloidal lead batteries.  
This article will compare the development, production and use effects of two valve-regulated sealed lead batteries in recent years for the reference of colleagues who choose batteries.  

Second, the working principle of the battery  

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

When the battery is charged, the positive electrode will release oxygen and the negative electrode will release hydrogen. Oxygen evolution on the positive electrode starts when the positive electrode charge reaches 70%. The precipitated oxygen reaches the negative electrode and reacts with the negative electrode to achieve the purpose of cathode absorption. The hydrogen evolution of the negative electrode must 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 sealed lead batteries, although most of the electrolyte of the battery is kept in the AGM diaphragm, it is necessary to prevent the electrolyte from entering 10% of the pores of the diaphragm. The oxygen generated by the positive electrode reaches the negative electrode through this part of the pores and is absorbed by the negative electrode.  

For colloidal sealed lead batteries, the silicon gel in the battery is a three-dimensional porous network structure composed of SiQ particles as the skeleton, which encapsulates the electrolyte inside. After the silica sol poured into the battery turns into a gel, the skeleton will shrink further, causing cracks 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 batteries is the same, and the difference lies in the way the electrolyte is "fixed" and the way in which oxygen is supplied to the negative electrode channel is different.  

Three, the main difference in battery structure and technology  

AGM sealed lead storage battery uses pure sulfuric acid aqueous solution as electrolyte, and its density is 1.29-1.3lg/cm3. Except for a part of the electrolyte absorbed inside the plate, most of it exists in the glass fiber membrane. In order to provide a channel for the oxygen precipitated from the positive electrode to the negative electrode, it is necessary to keep 10% of the pores of the diaphragm not occupied by the electrolyte, that is, the lean design. In order to make the electrode plate fully contact the electrolyte, the electrode group adopts a tight assembly method.  

In addition, in order to ensure sufficient battery life, the electrode plates should be designed to be thicker, and the positive grid alloy uses Pb’-q2w-Srr--A1 quaternary alloy.  

The electrolyte of the colloidal sealed lead storage battery is made up of silica sol and sulfuric acid. The concentration of the sulfuric acid solution is lower than that of the AGM battery, usually 1.26 to 1.28g/cm3. The amount of electrolyte is 20% more than that of AGM batteries, which is equivalent to flooded 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 gel and will not flow out of the battery.  

Since this battery adopts a liquid-rich non-tight assembly structure, the positive grid material can be a low antimony alloy, or a tubular battery positive plate can be used. 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 compartment can also be enlarged.  

4. Battery discharge capacity 

The discharge capacity of the initial gel battery is only about 80% of the flooded battery. This is because the poor performance of the gel electrolyte is directly poured into the unmodified flooded battery. The internal resistance of the battery is relatively large, and the electrolyte Caused by difficulty in ion migration.  

Recent research work has shown that the formulation of colloidal electrolyte is improved, the size of colloidal particles is controlled, and hydrophilic polymer additives are added to reduce the concentration of colloidal liquid to increase permeability and affinity to the electrode plate. Vacuum filling technology is used and composite spacers are used. Plates or AGM separators replace rubber separators to improve battery liquid absorption; eliminate the battery’s sedimentation tank and appropriately increase the active material content of the plate area. As a result, the discharge capacity of the gel-sealed battery can reach or approach that of an open-type lead battery. Level.  

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

5. Battery internal resistance and high current discharge capacity 

The internal resistance of lead-acid batteries is composed of ohmic internal resistance, concentration polarization internal resistance, and electrochemical polarization internal resistance. The former includes plates, lead parts, electrolyte, and barrier resistors. The glass fiber separator used in the AGM sealed lead battery has a porosity of 90%, and sulfuric acid is adsorbed in it. The battery adopts a tightly assembled form. The diffusion and electromigration of ions in the separator are very small. Therefore, the AGM sealed lead battery has Low internal resistance, high-current rapid discharge capability is strong.  

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

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

Six, thermal runaway  

Thermal runaway refers to the fact that the charging voltage of the battery is not adjusted in time in the late stage of charging (or in the floating state), so that the charging current and temperature of the battery have a cumulative mutual enhancement. At this time, the temperature of the battery rises sharply, resulting in The battery slot swells and deforms, the rate of water loss increases, and even the battery is damaged.  

The above phenomenon is a very destructive phenomenon caused by improper use of AGM sealed lead batteries. This is because the AGM sealed lead battery adopts a lean-liquid tight-fitting design, and 10% of the pores in the separator must be kept away from the electrolyte. Therefore, the internal thermal conductivity of 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 and the rate of oxygen evolution will increase. Large, which in turn increases the battery temperature. If such a vicious circle continues, it will cause thermal runaway.  

For open-type lead-acid batteries, since there is no phenomenon of oxygen absorption by the cathode, and the electrolyte volume is relatively large, the battery heat dissipation is easy, and the heat capacity is also large, of course, there will be no thermal runaway phenomenon. The amount of electrolyte used in the gel-sealed lead storage battery is equivalent to that of the open-ended lead storage battery. The area around the pole group and between the tank body is filled with gel electrolyte, which has large heat capacity and heat dissipation, and does not produce heat accumulation.  

The gel-sealed lead storage battery of the German Sunshine Company has entered the Chinese market for more than ten years, and several agents have said that they have not heard users report that the battery is thermally out of control.

Seven, service life  

There are many factors that affect the service life of VRLA batteries, including battery design and manufacturing factors, and user use and maintenance conditions. As far as the former is concerned, the corrosion resistance of the positive grid and the water loss rate of the battery are the two most important factors. Due to the increased thickness of the positive grid and the use of Pb-Ca-Sn-Al quaternary corrosion-resistant alloy, the service life of the battery can be up to 10 to 15 years based on the calculation of the corrosion rate of the grid. However, judging from the results of battery use, the rate of water loss has become the most critical factor affecting the service life of sealed batteries.  

For AGM sealed lead 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, its capacity will be reduced by 20%; if it loses 25% of its water, the battery's life will end. However, the colloidal sealed lead battery adopts a flooded design, and the electrolyte density is lower than that of the AGM sealed lead battery, which reduces the corrosion rate of the grid alloy; the amount of electrolyte is also 15%-20% more than the latter, and it is sensitive to water loss. Lower. These measures are conducive to prolonging 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 sufficient to run the battery for 12 to 14 years. In the first year of operation of the battery, water loss was 4%-5%, and then decreased year by year. After 4 years, the total water loss was only 2%. The OP2V sealed battery has a capacity of 90% after 10 years of floating charge operation at 2.27V/cell. Judging from the reflections of some domestic post and telecommunications departments, although Sunshine’s colloidal sealed lead storage battery has a higher price, its service life is longer than that of the domestically produced AGM sealed lead storage battery.  

8. Oxygen recombination efficiency  

The recombination efficiency refers to the rate at which oxygen generated by the positive electrode is 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, all affect the gas recombination efficiency of a sealed battery.  

According to the introduction of the colloidal sealed lead storage battery product manual provided by the German Sunshine Company, the oxygen recombination efficiency of the colloidal sealed lead storage battery product is low at the beginning 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 gel-sealed lead storage battery in the first year of operation is relatively large, reaching 4% to 5%, and then it will gradually decrease. The main reason for the above-mentioned characteristics seems to be that in the early stage of formation of the gel electrolyte, there are no or very few cracks inside, which does not provide sufficient channels for the oxygen precipitated in the positive electrode. With the gradual shrinkage of the colloid, more and more channels will be formed, so the recombination efficiency of oxygen will inevitably increase gradually, and the water loss will inevitably decrease.  

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

Nine, choose genuine colloidal sealed lead batteries

Some of the characteristics of the gel-sealed lead-acid batteries mentioned above are only the characteristics of the new generation of gel-sealed lead-acid batteries at home and abroad. The gel electrolyte used in this battery is different in performance from the gel electrolyte used in the early gel battery, which is made of ordinary water glass, or is made of common commercially available silica sol. In addition, the structure and material selection of a new generation of gel-sealed lead storage batteries are also different from ordinary lead storage batteries.