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First, there is no 12 V battery under the hood. The 12 V battery is located under the floor in the hatch area. The big High Voltage Battery is also in the back. Which battery did you have replaced under warranty? To start your car, the 12 V battery needs to be able to power up all the control modules in the car. The control modules then run checks to insure your car is “ready to drive” which primarily means that the HVB is properly connected to the High voltage system and the control modules are functioning. Your car will not start if the HVB is not properly connected. When your HVB is plugged in and charging from wall power, the 12 V battery is also being charged by a separate 12 V charger only used when car is plugged in and charging. When the car in in “ready to drive” mode (HVB is connected), the 12 V battery is charged from the HVB by a DCDC converter module (there is no alternator like a conventional car). When car is off, the HVB is not connected to the HV system. It sounds that you have a significant 12 V battery drain causing the 12V battery to completely discharge in a few days. The APIM behind the radio / display has been know to cause battery drain. You can try pulling fuse 79 in the fuse panel located under the glove box and leaving it out over night and see If battery drain stops. Put it back in when you plan on driving car. Here’s a document you can read and learn a lot about the hybrid / Energi. OBDSM1700_HEV.pdf