AGM batteries cost more and are generally not standard equipment on run-of-the-mill cars. Unlike traditional lead acid batteries, AGM batteries are sealed, meaning you cannot top off the water levels. If you can pop the cell covers off then it's not an AGM battery.Well first of all, AGM stands for "Absorbent Glass Mat", which is a type of a separator used in the construction of the battery. AGM batteries have a relatively small amount of acid, which is absorbed by the AGM separator. This allows the battery to be spill-proof. AGM batteries are also sealed, allowing them to be maintenance-free.