I have a 2009 Chevy Aveo which has had an ongoing problem for months. Every night the battery completely dies. The first thing I did was replace the battery with a brand new one (and, later, replaced that one too - hey, you can never be too careful) but it kept happening. Two mechanics have told me that the alternator seems to be working fine and is definitely charging the car while it runs, and that there's got to be a draw on the battery when the car is off.
But here's the weird part. When they test the battery for a load with a voltimeter/ampmeter/whatever, there's no unusual draw at all. Obviously there has to be a draw sometime, because the thing is at 12.6V when I go to sleep and 4.0V in the morning. I know this because I had to buy a big stand-up battery charger that I use every day.
The conclusion the mechanics have come to is that something is "turning on" after the car has been off for some time, and draining a lot of power. How can I find this? What could it be? Help me, Car Talk Community, you're my only hope!