An absorbent glass mat (AGM) battery, also known as a starved electrolyte sealed lead-acid (SESLA) battery, is a type of valve regulated lead-acid (VRLA) battery. Unlike conventional flooded lead-acid batteries, AGM batteries utilize an absorbent glass-fiber mat between the plates to separate the lead plates and hold the electrolyte in place.

History and Development of AGM Battery Technology

The development of AGM battery technology dates back to the 1970s when battery manufacturers began experimenting with alternative separators to contain the liquid electrolyte found in conventional flooded lead-acid batteries. Early battery separators like cotton or felt material had issues like acid stratification and spillage of electrolyte if tipped over. Various absorbent materials like glass wool, cotton-cellulose, and synthetic polymers were tested as separators in the development of sealed lead-acid batteries.

By the late 1980s, manufacturers settled on utilizing an Absorbent Glass Mat Battery as the most effective separator material. The non-woven mat of micro-fine borosilicate glass fibers helped evenly distribute and immobilize the electrolyte without any fluid movement inside the battery. This led to maintenance-free operation without risk of acid spillage. The glass mat also improved resistance to charging and discharging, increasing the cycle life of AGM batteries.

Construction and Design Features of AGM Batteries

AGM batteries typically have a cylindrical or rectangular shape with terminal posts on the top. The inner components consist of alternately stacked lead acid plates and glass mat separators that hold just enough electrolyte to serve the plates. Polymer/fibreglass encasing at the sides seal the cell and prevents electrolyte leakage even when inverted or immersed in water.

The glass mat separators are made of an absorbent microglass fibre sheet impregnated with electrolyte. The mat allows oxygen and hydrogen gases formed during charge/discharge to recombine internally without venting. The immobilized electrolyte is 'starved' and held stationary between the lead plates allowing rapid charge acceptance.

Applications of AGM Battery Technology

Some common applications where AGM batteries are utilized include:

- Motorcycles and Powersports: AGM batteries provide steadier power delivery and higher vibration/shock tolerance ideal for motorcycles, ATVs, personal watercraft, etc.

- Renewable Energy Systems: Often used as the backup battery bank for off-grid solar and wind power installations due to deep-discharge tolerance.

-UPS Systems: Uninterruptible power supply applications value AGM batteries for reliable backup power during power outages or surges.

- Marine and Recreational Vehicles: Immobilized electrolyte makes AGM batteries spill-proof and suitable for use in rocking boat environments.

- Electric Vehicles: Developing as alternatives to traditional flooded batteries in electric cars, golf carts, forklifts for maintenance-free use.

- Telecom Installations: Necessitate robust backup power for telecom towers, data centers in remote off-grid sites.

Advantages of AGM batteries

There are several advantages of Absorbent Glass Mat Battery over conventional flooded lead-acid batteries:

- Maintenance-free operation: No additions of water like in wet batteries. Glass mat immobilizes and recombines gases internally.

- High discharge and recharge capability: Can withstand repeated deep discharges up to 80% depth of discharge and fast recharging.

- Superior cycle life: AGM batteries typically last 2-3 times longer than flooded lead-acid often exceeding 1000 cycles.

- Immune to spillage or leakage: Absorbed electrolyte is stationary even when inverted or immersed in water.

- Low self-discharge rate: Self-discharges at a rate of around 5% per month at 20°C allowing long shelf-life if not in use.

-Tolerant to varying temperatures: Can operate and provide reliable starting power from -40°C up to 55°C.

- Lower maintenance and installation costs: No water refills or acid stratification checks needed. Easier installation in varied orientations.

-Higher cyclic efficiency and charge acceptance: Thinner plates and immobile electrolyte enhances charge acceptance over flooded lead acid.

Some Drawbacks of AGM batteries

However, AGM batteries also have some limitations compared to flooded lead-acid batteries:

- Higher upfront cost: AGM batteries have a 15-30% higher initial cost per amp-hour compared to conventional flooded batteries.

- Heavier weight: Glass mat and gel electrodes add weight and mass compared to liquid electrolyte batteries reducing gravimetric energy density.

-Lower CCA ratings: Thinner plates result in 10-15% lower cold cranking ampere ratings during starting in extreme cold conditions.

-Intolerant to high overcharge voltages: Overcharging AGM batteries, especially for prolonged periods, can cause premature failure through electrolyte boil-off.

-Specialized charging techniques required: Need intelligent/advanced chargers calibrated for AGM battery charging voltages and temperature compensation.

Despite a marginally higher cost, AGM batteries have established themselves as reliable maintenance-free alternatives to flooded lead-acid batteries across diverse applications. Immobilizing the electrolyte using absorbent glass mat technology eliminates issues like electrolyte spillage while enhancing cycle life, discharge capability and vibration tolerance. Ongoing developments aim to further drive down the costs through higher energy densities and recyclability of AGM batteries.

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