Hi biggame63, if you are looking to mount your battery in your trunk (or any enclosed space), your battery should be properly-vented to the outside. Even though our batteries have a “sealed” design, all lead-acid batteries can vent gas. Under normal operating conditions, an AGM battery will not vent gas. Since alternators or chargers can fail, the safest and correct mounting method for trunks and passenger compartments is to make sure that any possible gas venting will escape to the outside of the vehicle. All vehicles with original equipment battery locations in the trunks or passenger compartments will have a vent provision that should be used.
Our group 27, 51, 34C, and 31 batteries all have ports for connecting a vent hose. The Group 34/78 YellowTop you mentioned does not. If you are custom-mounting and isolating a battery in the trunk, a Group 27 YellowTop can be vented and comes with a three-year, free replacement warranty. Although people do it anyway, we would never recommend installing an unvented battery in any enclosed space, because there's a legitimate, albeit unlikely, safety risk involved.
For example, IF there is a voltage regulator failure, and IF the battery is severely overcharged, and IF this goes unnoticed, and IF the battery vents because the internal pressure exceeds the release pressure of the vents, the gasses are both flammable and toxic. This may sound like a lot of “ifs,” but attorneys and engineers get paid to plan for every worst-case scenario. If anyone has any questions about our batteries, I'll do my best to answer them.
Jim McIlvaine
eCare Manager, OPTIMA Batteries, Inc.
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