Low-maintenance, maintenance-free and absolutely maintenance-free

Low-maintenance, flooded lead-acid batteries lose fluid during normal use. They must be maintained, i.e. the fluid level should be checked regularly and the fluid loss compensated with distilled water if necessary. These batteries usually have vent caps on the top and vent holes as well as a vent hose. It is also advisable to check regularly whether liquid has overflown during heavy charging, as acid can lead to corrosion and damage the cables or the vehicle.

In contrast, maintenance-free batteries leak almost no fluid. They don't have vent caps on the top, but a lid that you can't open. In some cases, there is a labyrinth in the lid where rising vapor is condensed and returned to the system. These closed batteries lose liquid if they are tilted heavily or used or charged improperly.

Absolutely maintenance free batteries are designed to produce extremely little gas. The gas produced remains in the circuit of the battery by gas recombination, excess pressure is prevented by a safety valve. Because of this valve, these batteries are also called valve-regulated lead-acid batteries or VRLA batteries. These batteries do not lose liquid during normal operation. Refilling is also not possible, because the battery cannot be opened!