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AGM battery construction and characteristics

AGM battery, what is it and why do you need it?

What is an AGM battery?

AGM stands for an absorbed glass mat or absorbent glass mat. An AGM battery has positive and negative plates and separators just like regular flooded lead-acid batteries, but they have an absorbent glass mat between the plates that absorbs and holds the liquid electrolyte. Think of the glass mat like a sponge that holds the electrolyte. Because the electrolyte is held so close to the plates, an AGM battery offers distinct advantages over an SLI battery.

Differences between an AGM battery and a standard flooded lead-acid car battery

Standard starting, lighting, and ignition (SLI) lead-acid batteries are also referred to as “flooded lead-acid,” “spillable lead-acid,” or simply as a “wet cell.” The battery electrolyte is a mixture of 35% sulphuric acid (H2SO4) and 65% water (H2O). The battery may have a vent cap above each cell or the battery may be a maintenance-free style with no vent caps.

A lead-acid battery off-gasses hydrogen and oxygen when it’s being discharged and recharged. The vent caps allow the gasses to be released into the atmosphere. Unfortunately, as the battery loses these gasses, it also loses water. Plus, a certain portion of the water evaporates due to underhood heat. So a typical lead-acid battery with vent caps must be checked on a regular basis and the water level topped off with distilled water.

Maintenance free flooded lead acid battery

A maintenance-free battery, on the other hand, doesn’t have vent caps. Instead, the baffles in the cover of the battery allow the oxygen and hydrogen gasses to condense and “recombine” with the electrolyte. However, a maintenance-free battery isn’t totally sealed and can vent hydrogen and oxygen into the atmosphere. So a maintenance-free battery can still lose water. When that happens, the plates are exposed to air, which kills the battery.

AGM battery construction differences compared to a flooded battery

An AGM battery also contains positive and negative plates like a flooded lead-acid battery. But glass mat separators replace the standard plastic separators. The glass mats absorb and hold the electrolyte close to the plates. Plus each cell in an AGM battery is tightly packed so there’s optimal between the electrolyte and the plates. The high pack pressure reduces  internal resistance, reduces plate shedding, and thus increases battery life.

In addition, an AGM battery off-gasses less than a typical flooded lead-acid battery. It doesn’t have vent caps and it isn’t fully vented to the atmosphere. Instead, it is referred to as a fully recombinant battery, which means it converts a larger portion of the gasses back into the electrolyte.

AGM batteries contain a valve regulated vent that opens when the internal pressure reaches approximately 3-7-psi. This pressure build occurs when an AGM battery is charged at too high a voltage or too high a current. The vent valve is a one-way check valve, so overcharging an AGM can cause permanent loss of electrolyte.

AGM batteries are NOT gel cells

Many people refer to cylindrical spiral-wound (Optima brand) AGM batteries as gel cells. They are not. Gel cell batteries are never used in the automotive industry for starting applications.

Not all AGM batteries have cylindrical cells

AGM batteries can be made with flat plate technology or wound cylindrical cells.

AGM battery

Cylindrical cell versus flat plate technology

What are the advantages of an AGM battery over a flooded lead-acid battery?

An AGM car battery is designed to do three things better than a standard SLI battery:

  1. Deliver powerful burst of power for starting the engine
  2. Run the vehicle’s electronics for longer periods when the engine isn’t running. In other words, an AGM battery is designed to handle deeper discharge cycles and recover faster than an SLI battery, making it better suited for start/stop systems.
  3. Last longer than an SLI battery

How does an AGM battery do that?

An AGM battery has a lower internal resistance so it can create more power

The glass mat and tight packing allows for easier electron flow during power demands and recharging. In addition, the internal plate chemistry and bus bars provide lower internal resistance which translates into more power from a smaller battery. In many cases, an AGM car battery will be smaller and deliver more power than a comparable flooded lead-acid SLI battery. An AGM battery provides up to 35% more cranking power than an SLI battery.

AGM batteries have a lower self-discharge rate

All lead-acid batteries self-discharge over time. AGM batteries have a lower self-discharge rate than SLI batteries, so you can leave them longer without starting the vehicle, and stores can leave them on the shelf longer without having to charge them.

An AGM battery is more resistant to sulfation and degradation

All car batteries are susceptible to sulfation, but AGM batteries don’t sulfate as easily as flooded SLI batteries. What causes battery sulfation?

AGM batteries lose less electrolyte

AGM batteries have an internal valve that regulates how much hydrogen gas is allowed to escape during charging. Batteries with these valves are referred to as valve-regulated, lead-acid batteries (VRLA). By limiting the amount of hydrogen and H2o lost, AGM batteries are more maintenance-free. You do not have to add water to an AGM battery

AGM batteries recharge faster than a flooded lead-acid battery

Due to their lower internal resistance, AGM batteries recharge faster and at a lower voltage, which lowers the heat, which extends battery life. In fact, an AGM battery accepts a charge up to five times faster than a comparably sized SLI battery.

AGM batteries are more resistant to vibration

Because AGM batteries don’t have battery acid sloshing around and because the cells are tightly packed, they don’t shed as much plate material as SLI batteries.

©, 2020 Rick Muscoplat


Posted on by Rick Muscoplat


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