Toshiba Satellite L355 | Battery Life

Hi,
I am a beginner when it comes to Toshiba laptops since I just recently bought one (Satellite L355) 6 months ago. And I have a problem with its battery life which started last week. When I am traveling and running it by battery, it only lasts for a couple of minutes, 17 minutes is the most. I tried lowering the brightness of the display but still, the same result. I want to buy an extra battery for it, but I am a bit hesitant since the battery has only been used for 6 months considering that I only travel twice a month.
And also, I am using Windows Vista as my Operating System. What should I do?
