How is it that dual battery setups decrease power? If they were wired in parallel (they'd have to be, or on seperate circuits) you'd have more available amps / amp hours. You'd of course want to use the same battery if you were doing them on the same circuit in parallel.

I'm not sure about typical car batteries or the ones you used, but someone designed a battery box where I work that used "sealed" AGM style batteries (glass mat). They are very sealed, as they can be left in any position, these were installed on end. Someone used the wrong charger setting, charging them for typical style batteries which took them about a volt too high, causing them to gas (hydrogen). They do have little pressure relief valves in them. For electromagnetic insulation, the box it was in was sealed with EMI gaskets and blew up...

I forget the % of hydrogen in the air that is combustable, its like a 5 or 10% range, and its a low number. Below that won't blow up, and above it won't either. Either way, I'd be weary of putting a battery in my passenger cabin. There is probably enough air volume for it to not matter, but being a clumsy smoker I think I'll pass

My Perko acts as the isolator,don't need one of those.
The dual batt was actually my first mod to my jeep,and the extra batt in cab has never been a problem.
The setup I run is TRIED AND TRUE,and would advise anyone to mirror my dual setup,for it has worked fine
for MANY years.And yes it's mainly for use when engine is off,but has saved my can many of times when I run down the main batt(radio),and switch to all for jump start(all by myself in the woods).Your batt runs down and you're making a phone call.

Question was about fuses.

well

fuse it and make it even more flawless