I agree with Larry when he said his answer wouldn't be very accurate. "Sulfating" is lead sulfate that forms on the surface of the plates and decreases the efficiency of electron transfer from the plates to the electrolyte.
Its the oxygen and hydrogen that explodes. These 2 elements are created by the electrolisis of the water inside the battery when it is being overcharged.
Newer batteries have a substance added to the lead plates to prevent this electrolisis and that why it not necessary to add water as often or never to modern batteries.
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