If it's "new new", take it back. It may be the battery, but if it's worse "in" than "not in" the machine, summat's not right.
My laptop battery doesn't change noticeably if left for a month. That's an IBM ThinkPad - I've had a long series of those, and they've all been excellent. Friends with Sony and HP and Compaq have problems: I don't.
If the battery in yours is more than about two years old (since manufacture), it's approaching the end of its lifetime. LiIon ones can last longer, but in my experience two years is close to the knee of the curve. How long have you had the machine, and what's the "manufacture date" on it and the battery?