You have a drain of voltage from somewhere. You may have a light left on or your lights like the tail lights are being left on. When you park it at night, check to see if the tail lights are still on. That could be your problem. Also, take that battery back to where you bought it from and have them check the cold cranking amps. While you are at it, take the alternator off and have them check it. Somewhere in the system, you have a FUBAR circuit that is draining your system or is not charging properly.