If your battery keeps dying and it's a new or not very old battery and it holds a charge when you charge it back up then check to make sure nothing in your car is staying on such as the radio, lights etc.
If your car battery is still going dead then it could likely be the diode in the alternator being stuck shut causing your vehicles battery to go dead.
When the alternator stops turning and your car turns off the diode is supposed to open up.
If it doesn't open up and stays stuck then a circuit continues through the battery which causes the battery to die overnight or in a few hours.
Try disconnecting the battery cables overnight and if the battery stays up then it's likely either the diode in the alternator and in that case you'll need a new alternator.
Have your alternator and battery tested at Autozone and they can tell whether your battery has a short in it or if your alternator is bad.