Have you tried inserting the battery while booted the BIOS? It might tell you if the battery needs replacement. At least that's what the some Dells and Lenovos do. They won't start with a dead or missing battery.
Anyway, I wouldn't go out buying a new battery, an Atom convertible has just as much (or more, on the GPU side) grunt as a Core2Duo T and costs as much as a replacement battery.