What Is an AGM Battery

Shine

Administrator
Staff member
An AGM battery is sealed lead-acid (SLA) battery that has its electrolyte soaked in a porous absorbed glass mat (AGM) separator. The absorbed glass mat construction allows the electrolyte to be immobilized, which means the batteries of this type can be operated in any orientation without fear of acid spill. The highly porous seperator system in conjunction with a sealed design drive recombination of hydrogen and oxygen into water within the battery, rather than venting the gases outside the container as in flooded lead acid batteries. There's very little water loss from the electrolysis of the electrolyte during charging. The need for routine maintenance of electrolyte is therefore eliminated.

Electrochemistry

A lead acid battery is an assembly of a lead dioxide (PbO2) positive plate, sulfuric acid (H2SO4) electrolyte, separator, and a lead (Pb) negative plate. During discharge the sulfuric acid electrolyte separates into hydrogen ions and sulfate ions. At the negative plate bonds with sulfate ion sulfate ions to produce lead sulfate (PbSO4) and electrons (e⁻). At the positive plate lead dioxide reacts with sulfate and hydrogen ion, and electrons created at the negative plate to produce lead sulfate and water. The process is then reversed during recharge. When a charging voltage is applied, lead sulfate and water are electrochemically converted to lead at the negative plate, lead oxide at both the positive plate and sulfuric acid. By electrolysis, oxygen is ripped form water molecules at the positive plate and hydrogen is formed on the negative electrode in a stoichiometrical relation of 1 : 2.

Recombinant Technology

In flooded lead acid batteries, both oxygen and hydrogen generated during charging escape as gas bubbles because of the low solubility of both gases in the electrolyte. As result there's is a net loss of water from the cell. The electrolyte will eventually dry out if the battery’s water is not replenished. This means the battery has to be topped up with water regularly to remain effective. The battery must be vented to prevent buildup of internal pressure. Flooded batteries therefore must be installed upright to prevent leakage or spillage of the liquid electrolyte. These issues not only increase maintenance costs but also present restrictions when integrating lead-acid batteries into mobile systems and vehicles. Acid stratification is another issue in flooded cells. Higher concentration acid collects at the bottom of battery cell because of gravimetric effects, resulting in corrosion of the plates at the bottom side.

AGM batteries are developed to address the inconveniences of using flooded batteries. The oxygen transfer from the positive plate in these batteries takes place through a porous separator consisting of a sponge-like mass of matted glass fibers. The oxygen recombines with the hydrogen being generated by the negative plate to form water. The water content of the electrolyte thus remains unchanged throughout the charging process. The oxygen recombination efficiency of AGM batteries is typically 99%. The remaining 1 % oxygen will be vented through a one-way, pressure relief valve. The valves only let gas out and will open only when internal pressure exceeds the predetermined level of the valve. The AGM battery is also referred to as valve regulated lead-acid (VRLA) battery because it utilizes a one-way, pressure-relief valve system. The pressure relief mechanism functions to keep the cell pressurized, safely release excessive pressure and gas due to the overcharge or gassing reaction, and protect the cell from being contaminated by the atmosphere.

Absorbent Glass Mat

The absorbent glass mat has a critical role in battery performance. The mat which is sandwiched between plates fulfills two functions as electrolyte reservoir and as separator. The AGM enables ion transport but prevents direct contact between the electrodes. However, a critical characteristic of the AGM is that it drives oxygen recombination reactions between the positive and negative plates. An AGM is woven using glass fibers in different thicknesses and lengths, which can be reinforced by plastic fibers. The oxygen transport is made through the free pores in the separator. The quantity and size of the pores are predefined by the level of saturation of the separator with 95 to 97 %. The AGM separator must provide permeability, porosity, mechanical strength, electrical resistance, and ionic conductivity for maximum performance and longevity.

Advantages

AGM batteries are by far the most advantageous of the three types of lead-acid battery construction. AGM has very low internal electrical resistance thanks to the use of electrolyte holding fibers and microporous composition. The lower the internal resistance, the lower the losses while charging and discharging, especially at higher currents. Low internal resistance allows AGM lead-acid batteries to optimize current flow between plates and deliver all available energy without a significant voltage drop. This ability makes AGM batteries advantageous in deep-cycle applications where continuous power over longer periods of time is required. AGM batteries have higher specific power and power density than flooded and gel lead-acid batteries. They are capable of delivering power peaks necessary for engine cranking and supporting large electrical loads. The VRLA technology eliminates the need for electrolyte maintenance and enables installation in virtually any position. Spillproof design allows the batteries to be used near sensitive electronic equipment. Another benefit of AGM lead-acid batteries is their resistance to vibration because highly compressible mats provide padded protection.

Downfalls

A drawback of AGM rechargeable batteries is that they are more susceptible to thermal runaway than other types of lead acid batteries. The higher the charging voltage, the higher the float current and the more heat is generated by the battery due to increased rate of oxygen recombination. When heat is generated within the electrochemical cell at a rate greater than that at which it can be dissipated, the AGM battery is at a significantly elevated risk of thermal runaways. The battery charger must have a specific AGM setting to avoid overcharge damage and support temperature-compensated charging.
View attachment 116
 
Top