Battery and alternator questions

[trollbait]

Over the winter I've had the battery in the HHR die on me twice. So I'm planning ahead on what to replace it with, but I have some questions that my google-fu isn't handling.

First of all, the HHR battery is mounted in the rear with the spare tire. It is vented to the outside. How much does a battery regularly vent, and hazardous are the fumes? Where do batteries without hook ups for venting tubes vent from? Are there holes that vent lines can jury-rigged too, or does the entire thing need to be vented?

Since I EOC, I would like to get a dual purpose or deep cycle battery. Finding one that has the vent hook ups as been problem, and space next to the spare might be tight. An AGM battery might be ideal, but they require a lower float charge, and I haven't had luck on finding out how to adjust it, if possible, or what exactly the HHR is even putting out.

If, for what ever reason, I'm stuck with an OEM battery, would using two to spread out the strain of FASing be worth the effort?