Here is the answer to this question, after I did some research/searching.
The laptop power requirement is determined during design. In my case, my laptop's requirement is 19.2V/3.95A. But why does battery voltage is 10.8V or 11.1V? It is because every battery consist of some 18650 dry cell battery which its voltage is 3.6V or 3.7V. Three batteries are arranged in series to produce 10.8V or 11.1V (3.6*3=10.8V, 3.7V*3=11.1V). As the laptop power requirement is 19.2V*3.95A (=75.84 watt), then the charger must be made according to that spec. How about the power from the battery? Need boost converter to convert the 10.8V to 19.2V. And that is inside the laptop or battery.
What about the cell battery is not 3.6V?
Say if you can increase the voltage to be 10.4V like 9V dry cell 6LR641, and each two of them you arrange in series and then you parallel to another two series, then you will get voltage 20.8V. To meet the 19.2V you need to step the voltage down, also converter but to step down (rather then to step up).
So, when we plug that 19.2 charger to a laptop with a common battery we have (10.8V), then the voltage from the charger need to be stepped down inside the battery. Vice versa, when laptop is taking power from the battery, it need to be stepped up to 19.2V