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Jul 2 2020

AGM vs.Gel Batteries: What's the Difference in Design?

I Overview

Valve regulated lead acid battery (VRLA battery) is generally divided into gel battery and AGM battery

Figure 1. Classification of VRLA BatteryFigure 1. Classification of VRLA Battery

Gel battery is a valve regulated lead acid battery made by gel technology which not only refers to whether the battery contains gel electrolyte, but also includes the battery design ideas, structural characteristics, manufacturing technology and other technical measures to ensure the corresponding performance of the battery.

Gel Battery

Figure 2. Gel Battery

Similarly, for AGM battery, it also refers to valve regulated lead acid batteries manufactured by AGM technology. To hold the sulfuric acid in the battery with AGM separator is only one of the technical features of AGM battery.

Figure 3. AGM Battery

Because the two technologies are completely different, there is a great difference in performance between gel battery and AGM battery. In order to better understand the performance difference between gel battery and AGM battery, this article deeply discusses their differences in terms of battery design.

Sealed Lead Acid Battery Recovery

(The man in the vedio shows how to clean, open, refill, desulfate and test a totally dead sealed lead acid battery.)

Catalog

I Overview

II AGM vs. Gel Batteries: Electrolyte Fixation Technology

III AGM vs. Gel Batteries: Electrolyte

IV AGM vs. Gel Batteries: Polar Group

4.1 AGM Battery has Excellent LargeCurrent Discharge Performance

4.2 Make Full Use of Active Substances

4.3 Conducive to the Transmission of Oxygen

4.4 Prevent the Battery from Entering the Life Decline Period Rrematurely

V AGM vs. Gel Batteries: Oxygen Cycle

VI AGM vs. Gel Batteries: Cost

VII Conclusion

VIII Quiz

II AGM vs. Gel Batteries: Electrolyte Fixation Technology

Because both gel battery and AGM battery adopt cathodic absorption maintenance-free technology based on internal oxygen cycle, there is no essential difference in maintenance-free technology, only in the way of fixing electrolyte.

For the AGM battery, the AGM manufacturing technology is adopted, and the electrolyte in the battery, which is dilute sulfuric acid, can be held in the glass separator. The electrolyte is fixed by making use of the porosity of the glass separator, which has a strong adsorption, allowing the electrolyte to become immobilized. Its principle is similar to the principle of water absorbing sponge.

Figure 4. Water Absorbing Sponge

Figure 4. Water Absorbing Sponge

Because dilute sulfuric acid is made of pure sulfuric acid and water, the density of pure sulfuric acid is 1.84g/cm3, and the density of pure water is 1.0g/cm3. In the backup battery, the storage battery is kept stationary for a long time. Due to the effect of gravity, the dilute sulfuric acid electrolyte will stratify, that is, the sulfuric acid density at the bottom is high, while the sulfuric acid density at the top is low. In high-type batteries, this delamination phenomenon is particularly evident. Therefore, in conventional batteries, the height of the battery generally does not exceed 400 mm.

Figure 5. Delamination of Electrolyte

Figure 5. Delamination of Electrolyte

The layering of the electrolyte will make the active material on the top of the electrode plate unable to release the capacity it should have because of insufficient acid, and it will be overcharged during charging. And the bottom will be difficult to charge because the sulfuric acid concentration is too high. At the same time, due to the delamination of the acid, concentration polarization back-EMF will also be generated in the upper and lower parts of the electrode plate, which ultimately reduces the operating voltage and capacity of the battery. Furthermore, excessively high sulfuric acid at the bottom will also accelerate the corrosion of the bottom grid and the sulfation of the plates, thereby shortening the battery life.

For gel battery, it adopts gel manufacturing technology, and the electrolyte in the battery is fixed in the silicon gel. The fixation of the electrolyte is due to the silicone space network structure formed by the polymerization of gel gel particles to effectively fix the sulfuric acid electrolyte. The principle is similar to the use of jelly to fix the sulfuric acid electrolyte. In the space network, the silica gel is the skeleton supporting the entire network, and the sulfate ion can move freely at a certain level, which can ensure the smooth progress of the battery chemical reaction, that is, the smooth charge and discharge of the battery.

Figure 6. Gel Battery Electrolyte

Figure 6. Gel Battery Electrolyte

Because the spatial network structure of silica gel is rich in a large number of silicon oxidation bonds, it can form hydrogen bonds with hydrogen in the sulfuric acid molecule. Due to this weak chemical action, the silica gel easily adsorbs and releases sulfuric acid molecules. Even if the electrolyte is not moved for a long time, due to the existence of this effect, it can basically offset the effect of gravity, so that the diluted sulfuric acid electrolyte is evenly distributed up and down, and it is not easy to cause delamination.

Furthermore, the average pore size of the gel itself is about 100 times smaller than the average pore size of the AGM separator, and the comparative area of the gel itself is much larger than the specific surface area of the AGM separator. The small pores or micropores can be better keep sulfuric acid electrolyte. In the gel battery, since the electrolyte is not layered, the active materials on the upper and lower parts of the inner electrode plate of the battery can be fully utilized, so the battery has a long life span and can also be manufactured as a high-type battery.

Figure 7 Gel Battery ElectrolyteFigure 7. NO Delamination of Electrolyte

III AGM vs. Gel Batteries: Electrolyte

The AGM battery uses an AGM separator to fix the sulfuric acid electrolyte. In order to make the oxygen generated in the positive electrode in the later stage of charging easy to pass through the separator to the negative electrode and be absorbed by the negative electrode, a lean liquid design must be adopted to ensure the smooth progress of the internal oxygen cycle. The so-called lean electrolyte design takes the sponge's liquid absorption as an example.

Under normal conditions, a sponge can be 100g of water. The actual design is only to allow the sponge to absorb 80~90g of water. This design is a lean electrolyte design. Therefore, the amount of sulfuric acid electrolyte in the AGM battery is relatively small. In lead-acid batteries, the sulfuric acid electrolyte is involved in the electrochemical reaction of the battery. In order to ensure the discharge performance of the battery (the amount of sulfuric acid needs to be sufficient), it can only be achieved by increasing the sulfuric acid concentration. Therefore, AGM batteries generally use higher density/concentration sulfuric acid.

Due to the small amount of electrolyte in the AGM battery, the high concentration of sulfuric acid has a series of adverse effects on the battery itself. Since the amount of electrolyte in the AGM battery is relatively small and the heat capacity of the battery is also small, the AGM battery is sensitive to temperature. For batteries with the same capacity that can be fully charged, if the same amount of electricity is charged, the temperature rise of the AGM battery is significantly greater than that of the gel battery.

For AGM batteries, due to the use of higher concentration of sulfuric acid, the grid corrosion is faster, and it is easier to produce inert lead sulfate, which makes the battery's charge acceptance worse, and the battery is more difficult to charge. That is, AGM batteries are more prone to early capacity decay. With the extension of battery life, the AGM battery continuously loses water due to overcharging, and the sulfuric acid concentration in the battery rises slowly, which is more serious. In batteries, the sulfuric acid saturation of the AGM separator is usually 95% to 85%.

Compared with a fully saturated separator, when the saturation is 85%, the effective internal resistance of the battery increases by 90%. When the sulfuric acid saturation of the separator in the battery is less than 85%, the battery life will quickly end due to excessive tail current, lack of acid and excessive internal resistance. Therefore, for AGM batteries, the battery loses 10% of its water, and the life of the battery is reduced by more than 50%.

Figure 8. Influence of AGM Battery Lean Liquid Design on Battery Performance

Figure 8. Influence of AGM Battery Lean Liquid Design on Battery Performance

The development of the gel battery itself is based on the improvement of the flooded battery. Because the sulfuric acid electrolyte is fixed by silicon gel, the gas transmission inside the gel battery is completed through the channel formed by the cracks generated by the gel cracking. The amount of electrolyte does not affect the gas transmission channel. Therefore, there is no strict limit on the amount of electrolyte, and the liquid-rich design is usually adopted to ensure that the battery has better performance. Therefore, the amount of electrolyte in the gel battery is relatively large.

For large-density batteries, the amount of rich liquid is about 20%, and for medium-density batteries, the amount of rich liquid is about 15%. Gel batteries generally use a lower concentration of acid than AGM batteries. At a lower acid density, the corrosion rate of the grid is lower, and the battery's charge acceptance is also significantly improved, thereby extending the battery's life span. Secondly, the gel battery has more electrolyte. The more electrolyte, the greater the heat capacity of the battery, so the gel battery is not very sensitive to temperature. The high temperature has relatively little effect on the performance and life span of the gel battery.

 In addition, due to the high voltage or maintenance operations such as equalizing or overcharging the battery during long-term use, the battery may lose water. The gel battery has more electrolyte and a small amount of water loss, which has little effect on the life of the battery. Therefore, the life span is longer and the battery stability is better.

How a lead-acid battery works

(The man in this vedio explains the essential principles of a lead-acid battery. )

IV AGM vs. Gel Batteries: Polar Group

In AGM batteries, the assembly compression ratio of the pole group has a very important impact on the battery performance and battery life.

Appropriately increasing the pole group assembly pressure of the battery has the following benefits:

4.1 AGM Battery has Excellent LargeCurrent Discharge Performance

The AGM battery adopts a tight assembly structure, so that the distance between the positive and negative plates is smaller, and the distance of ion conduction in the battery is shorter. Thereby reducing the internal resistance of the battery. Furthermore, with the tight assembly structure, the electrode plate and the separator maintain good contact, and the contact resistance between the separator and the electrode plate is also reduced. These make AGM batteries more conducive to large current discharge.

4.2 Make Full Use of Active Substances

Because the AGM separator not only plays the role of isolating the positive and negative plates, but also plays the role of storing and maintaining the electrolyte. Adopting a tight assembly structure, the polar plate is close to the AGM separator, which allows the electrolyte to impregnate the entire polar plate, so that the active material is fully utilized, and the use capacity of the battery is increased.

4.3 Conducive to the Transmission of Oxygen

With a tight assembly structure, the polar plate is in close contact with the AGM separator, which is conducive to the smooth diffusion of oxygen through the separator to the negative electrode. Because the polar plate is under a large pressure, the AGM separator is compressed, and the micropores in the AGM separator perpendicular to the direction of the separator become larger, making oxygen easily penetrate the separator from the positive electrode to the negative electrode.On the other hand, because the AGM separator is compressed, the pores parallel to the direction of the separator plate become smaller, thereby suppressing the escape of oxygen generated in the positive electrode along the plane direction of the separator.

4.4 Prevent the Battery from Entering the Life Decline Period Rrematurely

To prevent the battery from entering the life decline period prematurely, AGM separator has a strong liquid absorption performance and a high porosity. It is a separator made of hydrophilic glass fibers and does not contain a binder. The separator itself has poor strength. The material of the AGM separator is shown in Figure 9 below.Figure 9. Glass Fibers 

Figure 9. Glass Fibers 

Put the AGM separator in boiling water for 1 hour, the AGM separator may disintegrate into pure glass fiber. If the AGM battery is loosely assembled, the escape of oxygen generated at the end of the battery charge may change the microstructure of the AGM separator, making the battery prone to early capacity decay.

The tight assembly structure can effectively suppress the softening and shedding of the positive electrode active material, thereby greatly extending the life span of the AGM battery. Therefore, the AGM battery must adopt a tight assembly design. This structure not only makes the oxygen circulation in the battery more smoothly, but also ensures that the AGM battery has excellent large current discharge performance and better life span.

In a gel battery, the viscosity of the gel electrolyte is much greater than that of dilute sulfuric acid, so if the assembly is too tight, it is not conducive to the gel electrolyte entering the pole group. Therefore, the assembly of the pole group is relatively loose. The separator used is usually a microporous plastic separator containing ribs to facilitate the gel electrolyte to enter the pole group and the inside of the separator. The separator of the gel battery mainly serves to isolate the positive and negative plates.

The gel battery itself is developed on the basis of the rich liquid battery, and basically maintains the characteristics of the original rich liquid battery, and there is no strict requirement for the assembly pressure of the battery pole group. In the gel battery, due to the loose requirements for the assembly of the pole group, the distance between the positive and negative plates is relatively large, and the ion conduction distance in the battery is long, so the internal resistance of the gel battery is usually large, which is more suitable for medium current and small. When the current is discharged, the large current performance of the battery is relatively poor.

V AGM vs. Gel Batteries: Oxygen Cycle

In VRLA batteries, oxygen is transferred from the positive electrode to the negative electrode, where it is compounded. According to the principle of oxygen circulation, the recombination of oxygen at the negative electrode mainly occurs at this three-phase interface.

There are two ways of oxygen transmission: one is vertical transmission, that is, the oxygen generated by the positive electrode first moves to the periphery of the pole group, and then reaches the negative electrode plate. The second is horizontal transmission, that is, the oxygen generated by the positive plate directly penetrates the separator to reach the negative electrode.

In VRLA batteries, the gas channel that generates oxygen circulation not only occurs in the separator between the positive and negative plates, but also occurs in the outer space of the pole group. In AGM sealed batteries, the gas generated from the positive electrode needs to grow from small bubbles to larger bubbles at the end of charging or during the float charging process. As the bubbles continue to grow, they expand into the AGM separator, and the sulfuric acid electrolyte in the large pores in the separator is discharged to form a gas channel, and oxygen is transferred from the positive electrode to the negative electrode.

Figure 10. VRLA Battery

Figure 10. VRLA Battery

Therefore, only when the oxygen pressure generated by the positive electrode reaches a certain level, a number of oxygen channels will be formed in the AGM separator. After the gas is transferred to the negative electrode, it is absorbed by the negative electrode, and the discharged sulfuric acid liquid will reoccupy the gas channel in the AGM separator until the bubbles generated on the surface of the positive electrode grow again to form a gas channel. Therefore, in the AGM battery, the gas passage from the positive electrode through the separator to the negative electrode may be unstable or discontinuous.

In the AGM battery, due to the fact that after the sulfuric acid electrolyte is added to the separator, the assembly pressure is greatly reduced, and the surface of the electrode plate and the separator is uneven, so that there is always a relatively large gap between the electrode plate and the separator. The direct transmission of oxygen from the positive electrode to the negative electrode is difficult, so in AGM batteries, a considerable amount of oxygen is transferred vertically. Since the pore size of the polar plate is smaller than that of the AGM separator, more electrolyte is retained in the polar plate.

The lead and negative electrodes on the side plates of the battery pole group are not covered with colloids, etc., and it is easy to form a three-phase interface of gas, liquid, and solid. In the AGM battery, the reaction between lead and anode and oxygen not only occurs on the surface between the positive and negative plates, but a considerable amount of oxygen circulation also occurs on the negative plates outside the pole group. The rapid recombination of oxygen on the side panel of the pole group also makes the total pressure of the gas chamber inside the battery lower than that of the gel battery. It is reported that stiffening strips on the inner wall of the battery tank can greatly increase the efficiency of oxygen circulation.

Figure 11. AGM Battery

In the study of the gel battery, it was found that in the gel battery, with the extension of the use time and the formation of micro-cracks in the colloid, the total gas chamber pressure in the gel battery is higher than the total gas chamber pressure of the AGM battery at equilibrium, vertical oxygen transmission in the direction is suppressed. In the gel battery, with the extension of use time, the silicone gel will dry crack, forming fine cracks, thereby forming a channel for oxygen to be transferred from the positive electrode to the negative electrode. In a gel battery in normal operation, since both the positive and negative plates are covered with gel, the oxygen compound reaction rate on the negative plate on the side of the pole group is extremely low.

The compound reaction of oxygen on the negative electrode mainly occurs on the negative electrode plate corresponding to the positive electrode plate. That is, horizontal transmission is mainly used. Furthermore, in the gel battery, the electrolyte saturation has an important influence on the oxygen transmission mode.

When the saturation is higher than 91.5%, the transmission mode is mainly vertical transmission; when the saturation is lower than 91.5%, mainly horizontal transmission. In the oxygen cycle of the gel battery, the transmission of oxygen in the vertical direction is slower than in the AGM battery; Compared with the AGM battery, the gel battery is more conducive to the horizontal transmission of oxygen. In the gel battery, since it is a gas channel formed by dry cracking of the gel, the gas transmission channel is basically stable.

VI AGM vs. Gel Batteries: Cost

The cost of gel batteries is higher than that of AGM batteries in terms of equipment, separators, and materials:

First, the cost of equippment. In the production of gel batteries, not only the equipment for glue distribution but also the special equipment for glue filling is needed. Due to the use of gel electrolyte, the gelation phenomenon may occur due to the high viscosity of the gel electrolyte, so the production process control requirements are completely different from the production process of the AGM battery, the process is more complicated, and the technology is difficult to master.

Second, the cost of separators. The gel battery uses a microporous plastic separator, which requires high porosity, good strength, and thin thickness. The cost of a high-quality microporous separator is relatively high. For AGM batteries, the separator is made of ultra-fine glass fiber, and the manufacturing process is relatively simple and the cost is low.

Third, the cost of materials. The design margin of the electrolyte of the gel battery is generally larger. Not only does it increase the silicone gel material, it also requires more sulfuric acid electrolyte. The total weight of sulfuric acid electrolyte and silica gel is heavier and the material cost is higher.

Therefore, the overall cost of the gel battery is high. As for AGM battery, the production process is relatively simple, the process is better controlled, and the battery is lighter, so the cost is lower than gel battery.

Figure 12. AGM vs. Gel Batteries: Cost

Figure 12. AGM vs. Gel Batteries: Cost

VII Conclusion

From the foregoing comparative analysis, it can be seen that AGM batteries and gel batteries are two types of batteries manufactured using different design ideas.

For AGM batteries, AGM separators are used to fix the electrolyte, which is generally designed with a lean electrolyte and a tight assembly structure, which makes the battery lighter and has better large current discharge performance. Due to the low amount of acid, high-density sulfuric acid is required. The life span of AGM batteries is relatively short, and the manufacturing cost is relatively low.

For the gel battery, it adopts a rich-liquid design, the amount of sulfuric acid electrolyte used is large, the density of sulfuric acid is relatively rare, the battery performance is more stable, the life span is longer, and the manufacturing cost is higher.

VIII Quiz

Q: Is AGM the same as gel battery?

Figure 13. Quiz

A: AGM (Absorbed Glass Mat) and gel batteries are both examples of VRLA (Valve Regulated Lead-Acid) batteries. They are also known as SLA (Sealed Lead-Acid) batteries. ... In common parlance, the term gel battery is used to indicate both AGM and Gel batteries.

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