Do you have anything else in the vehicle that draws off that battery? Anything aftermarket (radio, amplifier, ext.) I have had this happen to my previous vehicle. I had just installed new audio equipment in my truck and the battery presented the same thing you're describing. Does your car have an AC Delco battery? What brand battery did you purchase? How old was the original battery? Answering these questions might provide other or myself a way to better diagnosis the issue.