You won't be sealing flood lead acid batteries in a
capsize and venting them properly. If you use
AGM batteries than all you need to do is just secure them as that is the only thing they do require other than proper
charging. AGM's are the best for
boating. I switched to them ii the prior boat and loved them but have not switched yet in the
new boat as the whole
charging system will require replacement. The charging system has to be set properly for one or the other else it's
money wasted.
Flood batteries will out gas and that will include Sulfur Dioxide as well as Hydrogen Sulfide. Pure hydrogen also is possible. The out gas is a corrosive mixture and is what needs venting else any non Nobel metals will show signs of
corrosion. On the last boat the PO put the A/C unit in the same compartment. The
aluminum heat exchanger was corroding when I got the boat. Not fatal but could have headed in that direction.
The
battery box is more for the overflow that can happen when the batteries boil from poor charging. Not that it is normal to happen but because it could happen even on a proper boat should you not
monitor the water levels in the cells. You want to contain the spill no matter what.
In the case of a
capsize your
electrical system has but a short lifespan. The
salt water and the electricity will quickly destroy anything with
current in it. Putting batteries inside something won't help all of that much. The bigger
danger is that they come loose and a 70 lb battery flying loose could put some nice sized dents in your skull. In a capsize you would expect all heck to break loose and violent motions can make deadly weapons of even small objects. A battery could smash through a bulkhead with enough force launching it.