Background: The Technical Stuff
Inside Your Laptop: Power Distribution
Remember your desktop system? The power supply has 5 V, 3.3 V, and 12 V power rails. Those exist on your mobile system as well. But on a notebook, the power brick doesn't deliver those three voltages to the motherboard, as a PSU does. Instead, the brick delivers a steady voltage to your notebook's input connector. After that, voltage regulators on the laptop motherboard do the rest of the work to output voltages that the CPU, graphics processor, storage and add-in cards can use.
Displays are a different beast altogether. Older CCFL LCD panels need a voltage higher than what your notebook’s power supply can deliver. In order to achieve that higher voltage, a component called a power inverter is used to convert back to a low-frequency AC current. The existing generation of notebooks use LED-based LCD panels, which don't have this high voltage requirement, as they are solid-state devices. Instead, LED backlights operate off of +5 or +12 VDC. LEDs use less power than CCFL backlights; additionally, the lack of a power inverter translates into better efficiency as well.
How does the battery all tie in? The battery supplies power to the input side of the regulators, just like power coming from the wall socket would if you connected that way. However, there is a separate charging circuit. This diverts power from the AC adapter before it gets to the voltage regulators so that your battery charges. This charging circuit is the reason why the voltage on the AC adapter is always rated higher than the one stamped on the battery. You need a higher voltage to charge your battery.