You can have a dead cell in the battery which pulls the other cells down when it is sitting. If the battery is over 3 years old, I would suspect this is your problem. You may also have something on that is draining the battery. A glove box, dome, interior, trunk, or under hood light, for example. If you remove the negative battery cable when it is fairly dark and see a spark there is something drawing power. The radio presets will draw very little power and you will see a tiny spark which is normal. If you see more than a tiny spark something else is on. I suspect you just need a new battery.