Ac Power Cord Figure 8 Laptop Question: Do I Need A New Battery Or A New DC Jack?

Laptop question: Do I need a new battery or a new DC jack? - ac power cord figure 8

Now I have a Gateway MX3228, which is 3 years. Over the last 6 months, the performance of my laptop running on battery power only at the point that if I is reduced the power cord, the laptop turned off immediately. I left him in a long time when burning DVDs.

I try to know if my problem is the DC socket, my battery, or even something else. Has anyone any suggestions on how I can my problem before diagnosing computer to a repair shop? (A set of batteries I get a lot cheaper than I would if I was in a shop computer repair buy)

Thanks for any help.