The quality of the battery has a large part to do with this as well cheaper batteries are already going to be compromised. Luke i can see what your saying but there is run in time on batteries when i got a laptop last year on a 6cell i got 3:45 at 100% then after a few months it settled down to 3 hours.
Also the sticks advice is possibly the best i have read in this thread he knows his electronics i wouldn't contradict him on this subject.