To view the approximate battery life on the console:
From the HOME Menu, select "System Settings."
Scroll down the options on the left and select "System."
On the right-hand side select "Console Battery (%)." Press A to display the approximate battery life percentage on the HOME Menu.
Press A again to turn off this option.
To view the battery life for each paired controller:
From the HOME Menu select "Controllers."
The approximate battery life will be displayed for each controller that is paired to the Nintendo Switch console.