Possible Battery Failure

My battery icon in my taskbar says that it is running on AC Power. Usually it powers my battery while it's plugged in but it says unknown power remaining. Do I need a new battery?

My computer is a sony vaio pcg-k23 notebook. I had to keep my main post brief because i had my battery in my comp and the computer was running so damn slow. I took it out and it's running perfectly fine with the power adaptor plugged into the wall. Is it most likely that my battery is fudged if it's running fine without the battery?