A VRLA battery (valve-regulated lead–acid battery), more commonly known as a sealed battery or maintenance free battery, is a type of lead–acid rechargeable battery. Due to their construction, they do not require ventilation, can be mounted in any orientation, and do not require constant maintenance. The reduced venting is an advantage since they can be used in confined or poorly ventilated spaces. 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 li-ion.
There are two primary types of VRLA batteries, gel cells and AGM. Gel cells add silica dust to the electrolyte, forming a thick putty-like gel. These are sometimes referred to as "silicone batteries". AGM's, short for "Absorbed Glass Matt", forces fibreglass mesh between the battery plates with the same basic outcome. Both designs offer advantages and disadvantages compared to conventional batteries, as well as each other. (from Wikipedia)