Also don't be afraid of running it down. When my laptops got to 20% they really meant it, and soon after were shutting down. But last night at 20% I kept on using it for a couple more hours, and when I shut it down I think it may have been at 18% so I know I still had at least a couple more hours if not more. They way it reports battery life is less than accurate.
If all else fails though you might just have a defective one, swap it out for a new one.
If all else fails though you might just have a defective one, swap it out for a new one.