Nintendo Switch Manual page 12

Attaching them to the console
What happens if I attach the Joy-Con controllers
to the console when it's not charging?
The Joy-Con controllers will only be charged if
their battery is low.
They will stop charging at around half battery life
to keep from draining the console battery too
much.
Attaching the controllers to a Joy-Con charging
grip (sold separately)
Connect the Joy-Con controllers to the charging
grip accessory, then connect it to the Nintendo
Switch dock.
A USB charging cable is provided for connecting
the accessory to the dock.
You'll need to connect the dock to the AC adapter.