I'm so stupid I'll answer this. It's not going to make any difference, but I'll answer it.
I don't want a second battery in my car. It's small and the trunk is REALLY small. The computer is in the trunk and already takes up a lot of space. Putting a battery in there complicates the set up and takes up too much room.
The passenger compartment, front and back seats are used daily and have no space for a battery. The only practical place I could put a reasonable sized battery for sleep is in the engine compartment and I'm not sure it will fit in there.