Alternator or battery? How to tell the difference

Easy test: with the engine running, check voltage at the battery terminals with a multimeter. Should read 13.5-14.5V. If it's below 13V, your alternator isn't charging. If voltage is good at the alternator but the battery still dies overnight, you have a parasitic draw — something is staying on when the car is off.

On Nissan Altimas of that generation, the intelligent key system is notorious for parasitic battery drain. There's a known issue where the BCM doesn't go to sleep properly. Check your current draw with the car off — should be under 50mA after everything shuts down (give it 30 minutes). If it's 300mA+ you've got a draw. Start pulling fuses one at a time to isolate which circuit.