Hello! Welcome to the site! Thanks for coming! I'm Ron Z. I'm here to provide as much info and insight as I can, to best answer your question.
From what you are describing here, it sounds like you may have a problem in the Charging System. Possibly, a failing Alternator
. There is a quick easy test for this, with the use of a voltage meter. With the engine
running, take a voltage reading at the battery terminals. If the voltage is not between 13.2-14.4v, or fluctuates in and out of this area, the Alternator will need to be replaced. If the Alternator passes this test, turn the engine off and wait 15min. Take another reading at the battery terminals, this time with the engine off. If the voltage is anything less than 12v, the battery may have a "bad cell" and is not accepting the alternator's charge. If you do not have access to a voltage meter, most local "big chain" parts stores, like AutoZone, will run these tests for Free right in their parking lot.
If the tests prove the charging system is in good working order, than there is going to be an electrical draw in a system somewhere. This takes a bit of time and testing to find a draw. The fuses will need to be pulled one at a time while a voltage meter is connected. When the volt meter shows no draw, whatever fuse was pulled is the system that has the draw. From there, the effected system will need to be inspected looking for any electrical problems.