What types of batteries are considered sealed?

Sealed batteries are designed to prevent the escape of gases and are generally maintenance-free, which means they do not require regular checking of fluid levels or topping off with water, as is necessary with conventional lead-acid batteries.

Maintenance-free sealed batteries typically refer to types like absorbed glass mat (AGM) and valve-regulated lead-acid (VRLA) batteries, which contain a gel or fiberglass mat that immobilizes the electrolyte, minimizing the risk of spillage and allowing for safe operation in various orientations. This characteristic makes them particularly convenient for applications where regular maintenance is impractical.

While some types of batteries like nickel-cadmium can be sealed, they do not necessarily fall into the maintenance-free category since they might require other forms of maintenance. Conventional lead-acid batteries can also be sealed, but they typically require maintenance due to fluid level checks. SLA, or sealed lead-acid batteries, are indeed a type of maintenance-free battery, but the term "sealed" as used in the question is best encapsulated by the broader maintenance-free category, which emphasizes the lack of upkeep required for proper functioning.