everyone here gave good advice. just an added note about AGMs and Gell Cells. In the industry these are called Sealed Valve Regulated Batteries. Normally they do not vent. But each cell has a little valve in it. If the battery gets too hot or is over charged, pressure build up is released by the valve. This can occur if you are using the wrong type of charger. Chargers for these batteries need to be voltage regulated to the battery manufacturers specs, usually between 12 and 14 volts but it varies depending on brand. Also and very important there must be a temperature sensor fro the charger to sense the battery temp.
But that doesn't answer your question about where to put them. Putting them in a people space is not a good idea. There are simply a whole lot of sources of ignition. However, hydrogen dissipates at an extremely rapid rate so any ventilation above the battery is probably adequate. But battery outgas is very corrosive as is the electrolyte. That is why the fed regs and ABYC standards do not allow any fuel system components directly above or below batteries.
"Don't tell me I can't. Tell me how I can!"New Boat Builders Home PageMy Boating Safety BlogMy Boat Building News Blog