If you are placing the battery on the inside firewall (I am for the second battery) consider using this: ETX900 Vent I have been a long time user of Lithium based batteries and have seen one very small battery emit a lot of noxious fumes.
RV-14 QB building, Lycoming 390 Thunderbolt arriving Feb 2019, EFII-32 Ignition and Fuel, hope to be flying fall of 2019 !! Dues paid+ of course !!