seaeagle2 wrote:
Lead acid batteries off gas Hydrogen, remember the Hindenberg ?
His AGM is a recombinant cell battery. As long as interior pressure doesn't reach the threshold of the safety release valves, any gases are recombined into the electrolyte paste. There are many AGM batteries in service in unvented areas. Typically, X-ray machines and telecom switching rooms are two of those.
Having said that and knowing the OP has tremendous DIY skills, and has air vent in hand, I'd suggest to install the vent. Depending on what style vent he has, the battery can be further enclosed to provide storage.
FWIW, when I vented my indoor battery box, I bought a really nice chrome plated vent with flapper from Best Converter. Don't think they sell that, anymore, but should be available elsewhere.
