new to this forum - lots of nifty tips here.

Sort of a dumb question, but what sort of symptoms should someone look for if the 12V battery is dying/dead? Obviously, in a conventional car, the car wouldn't start. In a hybrid, the HV system has me confused. As I understand it, the HV battery powers the AC and even the stereo (through a HV-to-DC converter). So, what does the 12 battery do? Start the ICE? Help the power windows go up and down? Is that it?