Q: Can you update an iOS device without plugging it in? A: Yes, but . . . .
So you're on a business trip or vacation and get a notice that their's an update for your iOS device. You'd like to update, but the charging cable is back in your hotel room. Is there a way to update the device without having to plug it in?  Yes, but it depends.
Their is a little known fact that all iOS devices can be updated while on battery that is if the device has more than 50% of battery left.
Anything lower than 50% would allow one to download the update, but the device will not update until it is either above 50% battery power, or it is plugged in.
Now you know. :-)