Regular lead acid batteries give off hydrogen gas, so venting is a good idea, but I think all trunks have some sort of ventilation, they aren't air-tight. Many cars have trunk-mounted batteries without extra venting. That being said, the reason a lot of folk go with Optima's is because they're AGM (absorbed glass mat) batteries and they don't leak or vent like regular batteries, beside being a better battery technology and usually having more available power. Optima, while the most popular and ubiquitous, is not the only AGM battery brand, but generally AGM batteries are more expensive than regular lead acid batteries. Personally if I were trunk-mounting a battery, I'd pony up for an AGM battery, I keep hearing good things about XS Power from car audio enthusiasts, as well as Shuriken and Kinetik batteries, and if you search you can find Kinetics and Shurikens for around the same prices as regular batteries (XS Power is not cheap). Hope this helps bro!
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