You want to vent the gases outside of the battrery area. YOu have to many important things in that area. Plus you have the cabin air filter in that area. No good for these gases to come into the car.

 

If it is a sealed battery it will not have a vent tube.

 

"sealed" batteries still off gas. Thay is why the terminals corrode.

 

http://www.k0bg.com/alternator.html


In an AGM (Absorbent Glass Mat), like an Optima Red Top® shown at left, the electrolyte is a gel-like. As a result, they may be mounted in any position, even up side down. Their plates are spirally wound, which makes them almost impervious to vibration. What's more, they don't out gas under normal conditions so they can be mounted inside a vehicle.
All lead acid batteries generate hydrogen gas during normal operation, but its generation becomes excessive during overcharge conditions. Hydrogen gas is very explosive, and even a minor spark can ignite it. Any resulting explosion isn't a pretty sight, as the electrolyte is sulfuric acid! Since the gas is vented to the outside, flooded batteries shouldn't be used in enclosed areas like the trunk of a vehicle.
AGM batteries outgas too, but the hydrogen gas is absorbed by the glass mat. Here too, overcharging can cause more gas to form than the mat can absorb. In any case, manufacturer's maximum charge rates shouldn't be exceeded.