The battery is usually on the system board (although depends on the model).
If you are lucky then removing the keyboard should show you the battery, if not you will have to probably take the case off, best bet is to check the userguide as it should tell you in there how to replace it.
This is what you'd think. It's a Sony PCG-FX804, and the user manual I download says everything else possible to know about the laptop, except for this one thing.