Gelled Batteries or Gel Cells are sealed, and some are valve regulated. They contain gelled acid that was gelled by adding silica gel, making like a battery acid jelly. If you have the space and especially EZ Battery Reconditioning the ventilation, the valves actually do help out on battery life. But ventilation is extremely important. However they have to be charged at a slower rate to prevent excess gas from destroying the cells. Usually, solar systems charge relatively slow, so this isn't really an issue, but if you have a aux.
generator on your system, you have to limit the current. Most of the better made inverters handle this without a problem. If they are overcharges they will die quick.AGM (Absorbed Glass Mat) batteries are similar to the gelled batteries but they also have fiberglass mat between the plates of the batter, which is then filled with gel. These batteries are the premier choice if you have any concerns about spilling of battery acid. While spills can still happen with AGM batteries, because of the viscosity of the gell, it takes a little more than just a h*** in the battery to come in contact with you or the floor.