In an open, flooded battery, any gas which is generated can escape to the atmosphere, causing both safety and maintenance problems. A sealed lead acid (SLA), valve-regulated lead acid (VRLA) or recombining lead acid battery prevent the loss of water from the electrolyte by preventing or minimizing the escape of hydrogen gas from the battery. In a sealed lead acid (SLA) battery, the hydrogen does not escape into the atmosphere but rather moves or migrates to the other electrode where it recombines (possibly assisted by a catalytic conversion process) to form water. Rather than being completely sealed, these batteries include a pressure vent to prevent the build-up of excess pressure in the battery. Sealed batteries require stringent charging controls to prevent the build-up of hydrogen faster than it can recombine, but they require less maintenance than open batteries.

Valve regulated lead acid (VRLA) batteries are similar in concept to sealed lead acid (SLA) batteries except that the valves are expected to release some hydrogen near full charge. SLA or VRLA batteries typically have additional design features such as the use of gelled electrolytes and the use of lead calcium plates to keep the evolution of hydrogen gas to a minimum.