Sounds like your alternator isn't charging your battery. If you have a voltmeter you can test it. The battery should be around 12.7V while the car is off. When the car is running it should be around 13.5-14.5V.
If your alternator is charging, its like ragamuffin said and you have something drawing power while your car is off. You need to find it and stop it.
I also merged your two threads because this is the same problem.