Imagine that we want to design a power supply circuit for a laptop,
we can provide the required number of 12V-5A batteries, the specifications of the laptop power supply adapter is inserted:
Some laptops can work direct on 12V Lead-Acid battery, connected instead original Li-Ion battery. Voltage of Li-Ion laptop batteries are around 10-14V, and voltage of 17-19V is used to ensure charging of Li-Ion Laptop battery.
There is many ways to make this, for example to make DC-DC converter which will boost voltage from 12V to 19V.