Wet or flooded batteries do not have special pressurized sealing vents, as they do not work on the recombination principle
Wet batteries contain excess liquid electrolyte that can spill and cause corrosion if tipped or punctured
Wet batteries should not be used near sensitive electronic equipment
Wet batteries can only be installed “upright”.
A key feature of VRLA-batteries is the process of recombination of oxygen during the charge process. The cycle starts at the positive plate. Water is broken down and gaseous oxygen is formed. The hydrogen ions remain dissolved in the electrolyte and are not released as gas, which is the case with vented wet type batteries. The electrons move away via the positive plate.
What happens now to the oxygen as it makes its way to the negative plate is different in wet vented batteries than it is in VRLA batteries.
In wet or flooded lead-acid batteries of the vented design with excess electrolyte, it is practically impossible for the oxygen to move to the negative plate. Immediately after having left the positive plate, it bubbles up and escapes through the vent plug.
In VRLA batteries, a densely porous medium is offered to the oxygen to facilitate its movement. The porous medium in an AGM VRLA battery is the glass mat. The porous medium in a Gel VRLA battery is the cracks in the gelled electrolyte.
Figure 1 - Vented Battery Gassing and VRLA Battery Recombination