A valve-regulated lead-acid battery (VRLA battery) sometimes called sealed lead-acid (SLA) or maintenance free battery.
There are three primary types of VRLA batteries, sealed VR wet cell, absorbent glass mat (AGM) and gel cell. Gel cells add silica dust to the electrolyte, forming a thick putty-like gel. These are sometimes referred to as “silicone batteries”. AGM (absorbent glass mat) batteries feature fiberglass mesh between the battery plates which serves to contain the electrolyte. Both designs offer advantages and disadvantages compared to conventional batteries and sealed VR wet cells, as well as each other.
Due to their construction, the gel cell and AGM types of VRLA can be mounted in any orientation, and do not require constant maintenance. The term “maintenance free” is a misnomer as VRLA batteries still require cleaning and regular functional testing. They are widely used in large portable electrical devices, off-grid power systems and similar roles, where large amounts of storage are needed at a lower cost than other low-maintenance technologies like lithium-ion.