Looking for ideas (dual battery inside cab)

Cuz IF you're going lithium it'll be too hot for it under the hood.

 

You do know thousands of car audio people have batteries inside the car. I had 6 16v inside my bravada.

 

If im correct, some Porsche's have batteries that are located within the passenger compartment, but they aren't the standard battery.
Standard batteries are vented and are under the hood so that the toxic gasses are vented to the outside air and dissipate.

Batteries that are inside the car are sealed, but have a small opening that connects to a special vent tube that carries fumes outside the car.

 

I know I'm not who you're referring to, but properly installed vented AGM or even flooded cell batteries in the cabin are just fine and used all over. When I ran SCCA, the area behind the passenger seat wasn't technically inside the passenger compartment and I ran a battery there for years. I'm more concerned with Li-Ions in cabin as they periodically "rapidly vent with flames"

 

You're referring to Lithium-Ion batteries that have been known to burst into flames. Which you will NOT find in 12v car/RV/home style batteries because those are typically Lithium Iron Phosphate (LiFePo4). LiFePo4 batteries are much safer.

 

There's companies like Meritsun and CyclenPO that make ultra thin LiFePo4 batteries. not sure who they OEM for though.