This is less common than the drive interface, but it happens. This allows you to bypass buying a separate video card and saves a slot. But, again, if you want to upgrade your video later on, you'll have to disable the video on the board and buy that fancy new card anyway. The battery is pretty noticeable. It is a little cylinder just like your run of the mill Energizer, but shorter.
Most batteries are small, round, and flat.
These types fit into a small socket and are held in my a small metal tab. Its job is mainly to keep the system time and a few other settings when the computer's power is off. If you are asked what time it is or what kind of hard drive you have when you turn on your computer, you probably need a new battery. You plug your keyboard into it. This complicated setup is usually located next to the battery.