Should be vented, but it doesn't have to be forced air. In truth, any holes/vents/cracks in the upper spaces of the chamber would be the most beneficial, as the hazard is hydrogen which goes upwards.
As to the original question, a car battery will work, but you shouldn't run a starting battery below about 70% of capacity. Deep cycle batteries can be run much lower, around 40% (as I recall.)
jky
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