A standard "flooded" lead acid battery has the electrodes immersed in liquid sulfuric acid. Several modifications to the electrolyte are used to improve battery performance in one of several areas. The key parameters of the electrolte which control the performance of the battery are the volume and concetration of the electrolte and forming a 'captive' electrolyte.
Changes in the volume of the electrolyte can be used to improve the robustness of a battery. Increasing the volume of an electrolyte makes the battery less sensitive to water losses, and hence makes regular maintenance less critical. Adding to the volume of the battery will also increase its weigth and reduce the energy density of the battery.
In 'captive' electrolyte batteries, the sulfuric acid is immobilises by either 'gelling' the sulfuric acid or by using and 'absorptive glass mat'. Both have lower gassing compared to a flooded lead acid battery and are consequently often found in "maintenance-free" sealed lead acid batteries.
Gelling. In a "gelled" lead acid battery, the electrolyte may be immobilized by gelling the sulfuric acid using silica gel. The gelled electrolyte has an advantage in that gassing is reduced, and consequently the batteries are low-maintenance. In addition, stratification of the electrolyte does not occur with gelled batteries and therefore boost charging is not required, and because the electrolyte is gelled, the chances of spilling sulfuric acid are also reduced. However, in order to further reduce gassing, these "gell-cell" batteries also typically use lead calcium plates, making them unsuited to deep discharge applications. A further drawback is that the charging conditions of a gelled lead acid battery must be more carefully controlled to prevent overcharging and damage to the battery.
Absorbtive Glass Matting. A second technology which can be used to immobilize the sulfuric acid is "absorptive glass mat" or AGM batteries. In an AGM battery, the sulfuric acid is absorbed in a fiberglass mat which is placed between the electrodes plates. AGM batteries have numerous advantages including the ability to be deeply discharged without affecting lifetime, allowing high rates of charge/discharges and an extended temperature range for operation. The key disadvantage with these batteries is their need for more carefully controlled charging regimes and their higher initial cost.