Gel batteries are non-spillable, a silica gel is added to the electrolyte to reduce movement inside the battery case. To reduce gassing many gel batteries also use one-way valves in place of open vents, this helps the normal internal gasses to recombine back into water. Gel cells must be charged at a lower voltage than flooded or AGM to prevent excess gas from damaging the cells. Fast charging them on a conventional charger without using the appropriate Gel setting may permanently damage the battery.