Just make sure there isn't a control module still "talking" when the car is off. That's called Parasitic Draw" When the car is off and the control modules are no longer talking via the CAN network the draw should be in the area of 30-40mA. Unhook your negative battery terminal and complete the circuit with an ammeter to read your parasitic draw. If there is a draw, it will still happen with a new battery. I think it was said before too, batteries and alternators usually fail hand in hand, the alternator is stressed to charge a poor battery.