A modern gel battery (also known as a gel cell) is a VRLA battery with a gelified electrolyte; the sulfuric acid is mixed with fumed silica, which makes the resulting mass gel like and immobile. Unlike a flooded wet cell lead acid battery, these batteries do not need to be kept upright. Gel batteries reduce the electrolyte evaporation, spillage (and subsequent corrosion problems) common to the wet cell battery, and boast greater resistance to shock and vibration. Chemically they are almost the same as wet (non sealed) batteries except that the antimony in the lead plates are replaced by calcium, and gas recombination can take place.
The modern gel formulation was invented by Otto Jache's and Heinz Schroeder's U.S. Patent 4,414,302 assigned to the German company Accumulatorenfabrik Sonnenschein. With gel electrolyte used as the separator, it was no longer such a critical and difficult to make component, as a result, the cycle life was increased, sometimes drastically, along with a reduction of the active material being shedded from the plates.
Gas recombination is used to make this type of battery without the requirement to occasionally add water to them in order to maintain the strength of the electrolyte, and are therefore referred to as maintenance free batteries. The one way valve on each cell is set at 2 psi, which allows for full recombination to take place within the sealed enclosure. When charging is complete and the battery is allowed to continue charging unregulated, oxygen is created by the overcharging condition on the positive plate. The oxygen then travels through the shrinkage cracks in the gel directly to the negative plate which is made from a high surface area pure sponge lead and causes a reaction that combines the oxygen with the hydrogen that is adsorbed on the surface of the sponge lead metal negative plate to create water that is retained in the cell. This chemical reaction removes the requirement to occasionally add water to the cells as there is no evaporation taking place from the sealed enclosure.
This sealed, non spill feature made it possible to make very small VRLA batteries (1–12 amp-hour range) that fit into the growing portable electronics market. A large market for inexpensive smaller sealed lead acid batteries was generated quickly. Portable TV, light for news cameras, children's toy riding cars, emergency lighting, and UPS systems for computer backup, to name a few, were powered with small sealed VRLA batteries.