A VRLA battery (valve-regulated lead-acid battery), more commonly known as a Sealed Lead Acid battery (SLA battery) or Maintenance Free battery (MF battery), is a type of lead-acid rechargeable battery. Due to their construction, they can be mounted in any orientation, and do not require constant maintenance. During the normal operation, there are very less gas released from the battery due to the high efficiency of inner gas recommendation, so there is very less water loss during normal operation. So the VRLA battery are not required for filling water or acid during normal operation. The term "maintenance free" is a misnomer as VRLA batteries still require cleaning and regular functional testing. They are widely used in electric vehicles, 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.
There are two primary types of VRLA batteries, Gel type and AGM Type. Gel battery add silica dust to the electrolyte, forming a thick putty-like gel. These are sometimes referred to as "silicone batteries". AGM, short for "absorbed 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, as well as each other.