Laptop BATTERIES!! HP T4200.<Life Span>?

Ok so my laptop is a HP T4200. The battery is 6-Cell and Lithium-Ion. The battery life is like 2.5 hours when it is unplugged. I was just wondering if there is a possibility that I can buy another battery for my laptop that will have more battery life of more than 3-4 hours?
Update: Yes it is a Pavillion. And I do have it on power saver... its sucks. I really want my laptop to stay on longer when its unplugged.
There should be something I can do. I have many friends who's laptops can be on for 4 hours. (different laptop model).
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