Sealed, maintenance free, lead-acid ACUMAX batteries with a 6 - 9 years designed lifetime are made in AGM technology - electrolyte is absorbed in highly porous fiber glass separators placed between the plates. They use a gas recombination cycle which prevents electrolyte loss and allows the batteries to be used in rooms without forced ventilation, that are occupied by people. One-way, self-regulating pressure relief valves prevent the case from blowing up, by preventing excessive buildup of pressure in the cells.