Sony Vaio VGN-S1XP problem!

[Greekdude!:D]

Hello guys, I am new to this forum and I have a problem with my Vaio VGN-S1XP battery! Please help me!

I've got couple of games installed on it. I play on Steam. So everytime I try to open the laptop even if it is plugged-in, the battery light 'lights' on. And the orange light keeps lighting without stopping. That means the battery loses power. Then I check to the power and says 85% for example. But it stops and then re charges. When I try to play games suddenly when I have for example 50 fps. it turns to 10 and then 40 and this happens all the time. So I check and the battery light keeps lighting like it isn't plugged in. So I have to quit the game. Then the battery energy is at 70% for example. I've got the laptop plugged in with an Adaptor 16V.

So I scanned the laptop, defragmented the disks and nothing changes. I put the power scheme to the Minimal Power Management. Same problem all the time which doesn't let me play games .

I thought that the battery dying because it is an old laptop and as I know batteries have some kind of 'life'. So one day I put out the plug and let the laptop till the power goes to 10%. Almost 40 minutes passed and it was at 13%. Anyway. I think it is the adaptor's fault. Should I put it out? I mean just plug it in without adaptor? Should I buy new battery or AC Adaptor? Please let me know what should I do! I am so confused! Thanks for reading my post .