I believe the "Inteligent Key" system is constantly in the "receive" mode waiting for a RF signal from the key FOB itself, like a cell phone in the on mode but not making or receiving a phone call. Both my 09 FX50S and my sons 09 G37 with inteligent key systems have had our car batterys go bad. The dealer replaced both batteries under warranty and after asking the dealer why the batteries failed so soon they said the inteligent key systems is constantly using current from the main car battery 24/7 and therefore the larger CCA and reserve amp-hrs size batteries are required. It would be interesting to see what the current draw is by connecting a amp meter between one of the battery terminals and the cable (disconnected from the battery) with the ignition off and all accessories off (doors closed, no radio on, no parking lights, etc.) Has anyone out there tried this test?