Valve-regulated lead-acid batteries
often have immobilized rather than free-flowing electrolyte. This
immobilization is done by a gel or by absorption of electrolyte into special
(AGM) glass mats which replace or supplement separators.
Gelling is done with small-particle
amorphous silica. The silica is mixed with the electrolyte and kept agitated
until the gel is inserted (usually by vacuum) into the cell. The gel then
gradually stiffens during subsequent charging.
Absorption into glass mat sometimes
requires a metered amount of electrolyte per cell. Cell filling is usually
done by a vacuum method.