I don't know why people are already concerned about battery life. Nintendo has a killer track record with battery life
The Wii U gamepad and original 3DS both have pretty notoriously poor battery life.
Plus the Gamepad and 3DS have low resolution screens. The screen on the Switch has to at least be 1080p, and if the screen part is what's housing the rest of the components of the console, which it would have to be to be able to take it out, away from the dock, then that means there's going to be even less room for a reasonably sized battery.