The most important feature of a notebook is its mobility.
Rechargeable batteries supply the power of the notebook.
Like everything in the IT business there are several different options from cheap and heavy to expensive and light. But wait before you start running to the next computer shack to get a Li-Ion battery pack. The charger monitors the battery voltage and temperature. It is looking for a rise of the battery voltage that usually occurs at the end of the charging cycle. Ni-Cad and NiMH have similar behavior except for the point close to the end of the charging process.