Go into settings and move the second slider for mobile data to the left.
To my knowledge there is no such functionality. You can try to solve this with the energy saving functionality in your os to only allow f-droid to do things in the background when its charging. Maybe this is somewhere possible. The always working solution would be to change the free F-Droid code the way you like and then that one.
FYI, the update scheduler is set to prefer running only when being charged. It will run on battery if it hasn’t been updated in a while, as controlled by the time slider in settings.