Our 2008 Chrysler 300c has 2 batteries instead of the usual one. This is common on boats and RVs but not on cars. It has a large battery in the back far away from the starter motor and a smaller battery in the front close to the starter motor. When Barbara left the lights on we went out and jump started the car on the front battery and it started fine. Why don't they separate the batteries for Engine and cabin like in marine applications? Why is the big battery not in the front? What is the purpose of two batteries when it was easy to flatten the system anyway?
The battery is located in the trunk next to the spare. There is a positive battery terminal in the engine compartment located next to the fuse box. Maybe this is what you are referring to as a smaller second battery? It's just a convenient place to access the positive battery connection from the engine compartment.