There is a formula for figuring out how much hydrogen batteries generate. Lemme see if I can find it...
They say, "motive power battery" - that's pretty much what a golf cart battery is. Their example shows a 24 cell 450ah bank generates about 8 cu.' of hydrogen per hour of charging. Your two 6v golf cart batteries will have a total of 6 cells (so that's 25% of what they used in their example), and also a pair of 6v 225ah batteries rigged in series to get 12v will be a 225ah bank (which is half of the number they used).
So a decent WAG would be about 1 cu.' of hydrogen per hour (of charging) for a pair of 6v 225ah batteries.
A 2" hose would likely be enough to allow that much to escape - even without a fan or intake duct. It'll just rise through the column of air in the hose...sort of like bubbles in a glass of beer rising to the top.
A drain in the bottom is also a good idea, even if you don't need it as an air intake. You'll have to add water to those batteries once in a while, and you might spill some. There also might be some normal battery surface corrosion which you might want to wash off now and again.
Personally, I would use an intake vent anyway, and the drain can serve double duty by also being the intake.
There is a drawback if the intake draws air from outside; When it's cold out, the vent system will do a nice job of cooling the batteries, which isn't great since batteries don't work as well when they are cold. YMMV.
Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
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