Yesterday I received a generic replacement battery that I had ordered for my Sony VGN-FS840/W laptop. When placed in the laptop the battery indicator flashes rapidly. The laptop will not turn on with the battery inserted, even if the AC cord is also used. The battery shows no charge and does not charge. Reading online I learned that Sony apparently uses a hardware lock in certain of its laptop models to prevent the use of non-Sony replacement batteries. See, for example, these blog posts.
As I said before, avoid all products with the Sony label – they’re out to screw you.