Battery compartment ventilation?

[giterdone72]

I am in the middle of doing a deck repair/replacement on my 85 bayliner. I thought it would be cool and useful to get a hatch cover for the battery compartment, the center seat used to cover it. I am eliminating that seat to make it easier to move around in the boat. I ordered a tempress hatch cover from great lakes skipper and its supposed to be here today. BUT I was now wondering how much ventilation the battery needs if any to eliminate any explosive gasses? maybe I will have to put a few 'Breather' holes in the side of the batt compartment?attached is a pic of the original seat configuration
Any advice is welcomed thanks guys-