which battery starts the car

[alx5]

I have an 04 CTT and it was running OK until this morning. It decided to not crank. Radio, lights, and display panel were on.  I know I have to check the battery.  I would like to know which battery starts the car. I know I have two batteries in my car.  Most people seems to pay more attention to the one under the driver seat.  However, there is a link said the one in the trunk starts the car.  Which one is this?  Thanks for any suggestion.  

ALX5  

[Loren]

"Depending on specification and air-conditioning zones, the Cayenne is fitted with a battery having a capacity of 340 A/70 Ah to 450 A/95 Ah. As on the Cayenne S and Cayenne Turbo, the alternator is water-cooled and has a nominal output of 2100/2660 watts, depending on battery capacity. All Cayenne models can optionally be equipped with a backup battery (starter battery). This backup battery ensures that the vehicle can still be started even if the vehicle battery (main battery) is discharged. The capacity of the backup battery is 480 A/85 Ah.

If two batteries are fitted to the vehicle (main battery and backup battery), these are connected or disconnected to the bus bar in the power distributor through a relay. The switching of the two batteries is controlled independent of the load management (function is integrated in the vehicle electrical system control unit). The starter relay of the 2nd battery is actuated parallel to the starter relay. That means for every start the starter is supplied from both batteries. The backup battery is disconnected from the vehicle electrical system when the engine is switched off and then supplies just one consumer, the vehicle electrical system control unit."