A maintenance-free vehicle battery is one that is not designed to be opened or refilled. The liquid solution in conventional lead-acid batteries can evaporate, lowering the level in the battery and rendering it inoperable. Sealed batteries use materials to fix the electrolyte in place, removing this necessity.
Typically, a maintenance-free battery uses a specialized system, such as a glass mat or gel, to absorb and immobilize the electrolyte, preventing it from converting to a gas and escaping the battery. When a conventional battery begins to lose its charge, adding distilled water may revitalize the battery by recovering the plates with electrolyte solution. When a maintenance-free battery loses its ability to hold a charge, however, it must be replaced.