Laptop Computer Batteries By Brand

Why we don't have 20,000 mAh or 40,000 mAh battery in laptop whereas we are getting up to 5,000 mAh battery in a small mobile? Compared to the size of mobiles, can't we have a high capacity battery for laptops?

There are several factors here.

1. mAh is a measure of charge storage, not energy storage.
A mobile phone with 5000 mAh at 3.7 volts stores 18.5 watt hours
A laptop battery with 5000 mAH at 11.1 volts stores 56 watt hours
The latter will be 3 times as big and 3 times as heavy as the former.

2. Airlines have a 100 Wh battery limit for electronic devices - hence even if laptop manufacturers managed to add more capacity, you could not carry it in a plane - no one would buy laptops if you could not take them on an airplane in your cabin baggage.