Maintenance charger:

If there is one thing that a lead-acid battery does not like it is persistent neglect. When such a battery is being discharged, its energy is produced by the acid in the electrolyte gradually combining with the active material of the plates. This combination produces lead sulphate in both negative and positive plates. A cell is completely discharged when both plates are entirely sulphated. Since the plates are then (irreversibly) composed of identical material the terminal voltage collapses. In normal practice, discharging would be stopped long before this condition is reached.