A gel battery design is typically a modification of the standard lead acid automotive or marine battery. A gelling agent is added to the electrolyte to reduce movement inside the battery case. Many gel batteries also use one way valves in place of open vents, this helps the normal internal gasses to recombine back into water in the battery, reducing gassing. "Gel Cell" batteries are non-spillable even if they are broken. Gel cells must be charged at a lower voltage (C/20) than flooded or AGM to prevent excess gas from damaging the cells. Fast charging them on a conventional automotive charger may be permanently damage a Gel Battery.