‘Maintenance Free’ is an unfortunate label for an automotive lead acid battery as all batteries require some maintenance (keeping the battery top clean, keeping attached cables tight and clean, keeping the battery charged or re-charged as soon as possible following a significant discharge etc. The maintenance free label is usually used to convey that the battery will not require water additions. This can be facilitated by the selection of grid hardeners (e.g. calcium). The Maintenance free classification also often attracts the always incorrect description of ‘sealed battery’ (not possible in a lead acid automotive battery as the build-up of charging gasses in a permanently ‘sealed’ battery would eventually cause it to burst).
Water additions are also eliminated for the Valve Regulated Lead Acid (VRLA) class of batteries in both their AGM and GEL sub categories. In the case of VRLA, most of the charging gasses (which are derived from the water content of the battery fluid) are combined back to water inside the battery. To accommodate the un-recombined gasses, VRLA batteries are fitted with pressure valves which allow these gasses to be periodically released at pre-set pressures.