I want something that will Switch from powering my fridge from my Lithium Battery which is only charged from a mttp 75/15. I have a 3rd battery in the engine bay that I can currently power the fridge off as well. Since this is charged from the alternator with a dc dc charger. I want the fridge to power off this when the car is running. When I turn the car off I want it to automatically switch back over the the lithium battery. Which device from Victron will do this ?