Peter, first let me say that I am talking about a water flooded "regular" battery, not a AGM battery. All regular 12 volt batteries have 6 holes in the top where the manufacturer put in the electrolyte. These holes are covered by a vent which may be a long one piece cover that is flush with the top of the battery. Using a small flat screw driver, start to pry up one end of the cover. As soon as you have a little space, switch to a larger screw driver which will give you more leverage. Gentlly pry up until the vent pops off. Add your filtered water, distilled is best, and don't over fill. Replace the cover(s) and press down hard to snap the cover completely Back on. My 3 year old Walmart battery in the ML, my 1 year old Advance Auto battery in the SDL, and my daughter's 1 week old Die Hard all have this type of vent. All are Johnson Controls batteries. Walmart has now switched to Exide.

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Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback