You could also have a voltage leak that pulls the battery down. If it's been totally discharged a few times, chances are that it's damaged and won't hold a full charge any more.
To test for a voltage leak, have the battery charged and pull off the + lead. Make sure the key is off. Put a voltage meter between the lead and the post. If it registers anything, you have a leak.
You can use a test light, too, but if the leak is very small, you won't have enough stray juice to light it.
It's not common, but on occasion, a very dirty battery will leak voltage because it's going through conductive grime.
Dick
