How can I find the discharge rate of the battery?

You can try powerstat, which gives you the watts used over a period of 5 minutes. To install it, try the following commands:

sudo add-apt-repository ppa:colin-king/powermanagement
sudo apt-get update
sudo apt-get install powerstat

Or you can try the power-usage-report, which I guess, depends on fatrace and powertop.

Linked Question:

Maybe you're looking for "Rate" in Power Statistics > Laptop battery > Details?

Power Statistics screenshot

"Linux Power Top" may help (currently available from https://01.org/powertop )

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