Both maintenance and maintenance-free batteries still need to have corrosion removed periodically.
Maintenance-Free Battery. Many maintenance-free lead acid batteries are permanently sealed with a hydrometer built into the battery casing. An indicator on the top of the battery identifies a battery’s charge condition. Instructions for understanding the indicator are usually next to the indicator.
Maintenance Battery. If a lead acid battery is not maintenance-free, use a hydrometer to check the battery electrolyte’s specific gravity. Warning: Never recharge a battery that has a low electrolyte level or is frozen.