A GEL battery is a type of sealed lead-acid battery that uses a gel electrolyte instead of a liquid one. The gel electrolyte is made by adding silica to sulfuric acid, which results in a thick, viscous substance that is immobilized within the battery's casing. This design makes GEL batteries maintenance-free and spill-proof, as the electrolyte cannot evaporate or leak out of the battery. 

GEL batteries are similar to AGM (Absorbent Glass Mat) batteries in that they are both valve-regulated lead-acid (VRLA) batteries. However, GEL batteries use a gelled electrolyte instead of an absorbent glass mat to immobilize the electrolyte. 

GEL batteries have several advantages over other types of batteries. They are maintenance-free, have a long lifespan, and can be used in virtually any position without leaking. They also have better heat tolerance than other types of batteries and are less prone to sulfation.