A category of lead-acid batteries that can be operated with diluted sulphuric acid, which is in liquid form, i.e. not immobilized as GEL or held glass fibre mats.
They are available in two versions: Low-maintenance and maintenance-free. In the case of the low-maintenance battery, it is necessary to regularly (every 2 to 3 months) check the state of charge and the fluid level and, if necessary, refill with distilled water. It can be identified by the removable plugs and the venting hose. It can only be installed upright, otherwise it will leak. Maintenance-free flooded batteries, on the other hand, cannot be opened or refilled. Compared to the low-maintenance flooded battery, they are more leak-proof.
Until about 1995, vehicles were equipped with alternators designed for flooded batteries. These vehicles cannot be equipped with a different battery class. If in doubt, replace an existing flooded battery with a flooded battery again.