Hopefully you are talking about when the car is parked and not running. Otherwise your alternator isn't charging. But if this hapens while parked, hood light and trunk lights are big culprits. The best way to check this is to put a test light between tthe positive post and cable so that it lights. Then start pulling fuses out of the box to locate what circuit is causing the drain. Start with the courtesy circuit first. When the light goes out you've found where the drain is. If it stays on when all circuits have been checked then it may be your alternator as the diode bridge has blown out and letting power slip back through where it's not supposed to.