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.