First, the composition of AGM batteries looks likes this:

Fiberglass mats
Lead plates
Electrolyte
Pressure control valve
The microfibre fiberglass mats, placed between the lead plates, absorb and immobilize the electrolyte.

The glass mats also act as separators and insulators between the positive and negative plates.

This design is what makes AGM batteries spill-proof.

Water droplets with a crossed-out circle around them
AGM batteries - like gel batteries - are sealed and spill-proof
Like gel batteries, AGM batteries are recombinant.

What does this mean?

The safety pressure relief valve prevents the release of hydrogen and oxygen gases.

Instead, they’re able to recombine during charging.

And this helps prevent subsequent water evaporation from the cells.

Also, the glass mats wrapped around the battery's positive plate help prevent damage from vibration and impacts.

An illustration of a vibration
The glass mats in AGM batteries protect it from vibrations
They also help extend its cycling.

Plus, AGM batteries have a lower internal resistance.

This lower resistance reduces losses to heat as power flows through the system.

And that’s why AGM batteries have more output voltage and lower charging times.