The electrolyte used in lead-acid batteries. In GEL batteries it is a sulphuric acid (H2SO4) thickened with silica.
In AGM batteries, it is a sulphuric acid that is absorbed in a glass mat. In flooded lead-acid batteries, the electrolyte is in liquid form and is a sulphuric acid diluted with battery water. Their concentration, and therefore their density, changes with the state of charge, so by way of an acid siphon it is possible to determine whether a flooded battery is charged.