What is a gel cell?

Silica gel is added to the acid in a gelled battery turning it into a gelatinous solid mass. This prevents both acid spillage and electrolyte evaporation. Gel cells cannot be charged while they're inverted but do not need to be kept upright during use making them convenient to use in limited spaces such as boats, automobiles and other motorized vehicles.

Charging a Sealed Lead Acid Battery with a constant voltage can cause a rapid initial charging which is bad for the battery. It is suggested that the battery be charged using a constant current, using constant voltage only for the final portion of the charging.

What is an Absorbed Class Mat (AGM) battery?

AGM batteries have a very fine fiber Boron silicate glass mat between their plates. The mats are called 'starved electrolyte' because the mats are only about 95% saturated with electrolytes rather than fully soaked. This means that the battery will not leak acid even if is broken. AGM batteries can be used in colder climates as there is no liquid in the battery to freeze and expand. Almost no water is lost from the battery because the hydrogen and oxygen recombine into water in the charging process. Most modern motorcycles use AGM batteries because of the reduced likelihood of acid-spilling during accidents, because they are small and because the batteries can be installed at odd angles.

Why do gel batteries and AGM batteries have lower natural discharge rates than standard batteries?

Many standard batteries use lead antimony plates which can discharge as much as 1% per day on an older battery. Gel and AGM batteries use lead calcium plates in their construction. A typical discharge rate can be as low as 1-2% per month with an old battery discharging as much as 2% per week, still significantly less than standard batteries.