We have an AGM in our airplane. It doesn't emit very much vapors. Although the technology is different it is easier to think of it as an over grown flashlight battery. It won't leak unless it it way over charged. It will get hot and turn into a big ball. Ignore it and it will explode and emit acid all over the place. The advantage for an airplane is that it doesn't leak and can be stored in an open container. The other is it is very light at about half the weight and size compared to other batteries of it's same capacity. There are other benefits to it except that it needs a special battery charger if it gets low on charge and also to keep it from over charging.

HudginJ3 wrote:

We have an AGM in our airplane. It doesn't emit very much vapors. Although the technology is different it is easier to think of it as an over grown flashlight battery. It won't leak unless it it way over charged. It will get hot and turn into a big ball. Ignore it and it will explode and emit acid all over the place. The advantage for an airplane is that it doesn't leak and can be stored in an open container. The other is it is very light at about half the weight and size compared to other batteries of it's same capacity. There are other benefits to it except that it needs a special battery charger if it gets low on charge and also to keep it from over charging.

I have to agree with HudginJ3. I also run a AGM battery in my plane. I have never had a problem with outgassing. With that said I had and older battery out of my airplane in my shop and hooked it up to a "Mondern" battery charger. I don't know what went wrong but the batter swelled in size and stunk of acid. The moral of the story is that even AGM and sealed batteries can over pressure the valves. 

All batteries IMHO, should be encased in a vented case if inside the passenger compartment. Our trunks are passenger compartment. 
 

I'm running a Lithium battery. I was going to relocate my lead acid battery to the trunk similar to what Shelby did with the 65 GT-350 and like I did on my 68 Cougar. After considering all of the details, the weight of the welding wire, the overall weight of the battery is still there albeit it's in a different spot, and the always existing potential for a catastrophic electrical short. I decided on the lithium battery. It's not for everyone. It's very expensive, but it spins the engine over like a top with a slightly higher voltage to the starter and it allows me to keep the battery in the stock location. The battery is only a little over 6 lbs and it is the size of a garden tractor battery. I had to make some adjustments to the mounting tray to mount the smaller battery box and after having the trunk mount battery explode in my Sunbeam Tiger, I decided to make a smaller battery box and use it there as well. It only takes 5 minutes or so to switch it between the two vehicles and I always bring it inside during the winter. Something to think about, it works for me. But then my cars are toys and not daily drivers.-Brad

Lithium batteries are again smaller and lighter than AGM batteries and again another safety concern. 
Here is a site for use of the battery in experimental airplanes. They are pretty straight forward about what could happen to it and how to maintain it. They are certified to be used in airliners and helicopters.

https://earthxbatteries.com/product-category/automobile

It appears that the Earthx Batteries are similar and potentially cheaper. Well worth a look.-Brad

There are rules to be followed with the lithiums as well. You can't let them ever go below a certain voltage, they become very dangerous at that point. I believe they told me to never put the charger on it if it ever read less than 8 volts, and that I was to send it back to them for possible rebuilding. I also had to be sure to never allow more than 14.4 volts or thereabouts from the charging circuit so I needed to pick a proper voltage regulator. All worth it in my opinion, I could not believe how fast it spun the engine over the first time I hit the key.-Brad