How to make a buck work over some voltage limit?

When I power my project with a battery I don’t need a buck. With such a small voltage difference it has a lower current limit. But when I use an ac-dc 12v adaptor I want the current to go through it. Is it possible?

Look at the power source switching in the Uno schematic between USB and the dc barrel jack, it does exactly what describe.

Cool! Thanks! Trying to find details and calculations.

Tofer:
With such a small voltage difference it has a lower current limit.

Have you taken into account that max-charged charged batteries have a considerable higher voltage? Fully charged LiPos at 4.2V may often kill electronics that would be fine at the rated 3.7V.