My understanding is that even AGM batteries can off gas so I'm looking for a sealed battery box that I can vent to the outside:
6.1 Release of Ignitable Gasses
All lead acid batteries, including VRLA batteries, produce hydrogen and oxygen gases during
normal charging. Even though VRLA batteries are designed to recombine these gases
internally, the recombination efficiency is less than 100%. Small amounts of hydrogen and
oxygen are released from the pressure relief valve during charging. Normally, the hydrogen gas
dissipates very rapidly and never reaches a concentration level that is hazardous. However, if
the battery is installed in an enclosure with minimal airflow, the concentration of hydrogen could
build up to a high enough concentration to be of concern. Hydrogen can ignite at
concentrations as low as 4% in air. For this reason, never install a Lifeline® AGM battery in
a sealed or an airtight container.