More research repeats the manufacturing literature that I received with the batteries, stating that the sealed agm does not vent, and is sealed and pressurized.
How does a gel cell work? A gel cell is a "recombinant" battery. This means that the oxygen that is normally produced on the positive plate in all lead-acid battery recombines with the hydrogen given off by the negative plate. The "recombination" of hydrogen and oxygen produces water (H2O), which replaces the moisture in the battery. Therefore, the battery is maintenance-free, as it never needs watering.
The oxygen is trapped in the cell by special pressurized sealing vents. It travels to the negative plate through tiny fissures or cracks in the gelled electrolyte.
The sealing vent is critical to the performance of the gel cell. The cell must maintain a positive internal pressure. Otherwise the recombination of the gasses will not take place, and the cell will dry out and not perform.
In addition, the valve must safely release any excess pressure that may be produced during overcharging. Otherwise, the cell would be irreparably damaged.
It's important to note that gel cell must never be opened. If opened, the cell loses its pressure, and the outside air will "poison" the plates and cause an imbalance that destroys the recombination chemistry.
And this appears to address the question of can it vent, and how to store:
Can gel cells be installed in sealed battery boxes?
NO! Never install any type of battery in completely sealed container. Although the normal gasses (oxygen and hydrogen) produced in a gel cell battery will be recombined and not escape. Oxygen and Hydrogen will escape from the battery in an Overcharge condition.
For safety, these potentially explosive gasses must be allowed to vent to the atmosphere and must never be trapped in a tightly enclosed space!
So, it appears the manufacturer says roughly:
1. Put the batteries in a battery enclosure that includes venting. ( for overcharging instances )
2. The batteries do not vent under normal use.
