VRLA battery - AbsoluteAstronomy.com

A VRLA battery is a type of low-maintenance lead–acid rechargeable battery. Because of their construction, VRLA batteries do not require regular addition of water to the cells.

VRLA batteries are commonly further classified as:

Absorbed glass mat (AGM) battery
Gel battery ("gel cell")

An absorbed glass mat battery has the electrolyte

Electrolyte

In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

absorbed in a fiber-glass mat separator.