There are five main types of car batteries: nonsealed wet, sealed wet, wet marine starting, standard absorbed glass mat and sealed spiral-wound absorbed glass mat. Wet batteries contain a liquid electrolyte and are also called flooded-cell batteries. AGM batteries suspend the electrolyte in a mesh of fine glass fibers so there is no free liquid in the battery cells.
Wet batteries are the most common type of car battery and contain a sulfuric acid-water solution where lead and lead dioxide plates are suspended. The solution creates a chemical reaction between the plates that produces voltage to start the car's engine. Nonsealed wet batteries have removable caps for maintaining the electrolyte solution. This is an advantage in dry climates where the solution is likely to lose water. Sealed wet batteries reduce corrosion on the battery terminals and are less likely to spill than nonsealed batteries. Wet marine-starting batteries were originally designed for boats and have special construction to reduce vibration. They work well in off-road vehicles and recreational vehicles.
AGM batteries work similarly to wet batteries, but contain only a small amount of sulfuric acid solution inside a mesh of glass fibers. The chemical reaction that produces voltage takes place inside this mesh. Standard AGM batteries have the mesh in a flat mat while spiral-wound AGM batteries roll up the mesh and the plates into six smaller tubular configurations. Spiral-wound AGM batteries are sturdy and work well in vehicles with a lot of vibration and work regardless of mounting position.