martymcfly wrote:If you are getting fumes, you are overcharging your battery. I am not even sure that you need to vent anywhere. The most produced model of car ever (vw bug) had a battery mounted under the back seat, with an ashtray about 2 feet away. Not vented. They made them that way for 50 years and never changed, so there must not have been any problems.
There are several sources for floor pan repair kits for the old beetles, made specifically because floor corrosion under the battery is commonplace and is due to the hydrogen being released as well as the mist that is carried along with the gasses.
Any time a battery is charged H and O2 are both given off by that normal chemical reaction. That can and does cause very small amounts of liquid to be expelled from the battery cells as a fine mist. It happens with any flooded battery. Every flooded battery I have installed in an off grid system eventually ends up with the tops of the batteries slightly moist with acid. Those are systems with high quality charge controllers... no overcharging going on. (There are special caps available for off grid use that have catalyst pellets in them and cause the H and O2 to recombine and drop back into the cells. Batteries with those caps usually have drier tops and less corrosion.)
I leave a copper penny on the battery top beside each terminal. The penny is sacrificial and corrodes before the terminal connections. But it does not stop the charge gassing or misting.
That is an advantage of an AGM battery. No gassing under normal use. But I like to check cells with a hydrometer and keep a log of the readings over the years. That helps see what the battery health is. But I baby my batteries more than most folks. I have used AGM batteries in my off road vehicles for decades though.