I took this from a well known site:
"Excessive charging causes
electrolysis, emitting hydrogen and oxygen. This process is known as "gassing". Wet cells have open vents to release any gas produced, and
VRLA batteries rely on valves fitted to each cell.
Catalytic caps are available for flooded cells to recombine hydrogen and oxygen. A VRLA cell normally recombines any
hydrogen and
oxygen produced inside the cell, but malfunction or overheating may cause gas to build up. If this happens (for example, on overcharging) the valve vents the gas and normalizes the pressure, producing a characteristic acid smell. However, valves can fail, such as if dirt and debris accumulate, allowing pressure to build up.
Accumulated hydrogen and oxygen sometimes ignite in an internal
explosion. The force of the explosion can cause the battery's casing to burst, or cause its top to fly off, spraying acid and casing fragments. An explosion in one cell may ignite any combustible gas mixture in the remaining cells. Similarly, in a poorly ventilated area, connecting or disconnecting a closed circuit (such as a load or a charger) to the battery terminals can also cause sparks and an explosion, if any gas was vented from the cells."
The OP may not have asked, but best not have the charger connected and operating, just in case, while adding water.