You may have a bad diode and your alternator may be draining the battery,,,i've seen this before,,,it's a wierd one but the lights on the dash sometimes come on when the battery is draing off via the alternator.
You need to stick a voltage meter on the alternator and see what it's putting out when idleing,,,and then turn the car off,,disconnect the ground terminal,,,put the neg terminal of your test light or voltmeter on the neg terminal of the battery,,,and the positive on your test lamp/meter, connect up to the field wire of your alternator,,,if you get a reading when the key is off then your diode pack is shot.
I'm going from memory here boy's so help me out if i've got it wrong.