A method for selecting a power source for a load is provided. The method includes monitoring the primary power source, when the primary power source is providing power to the load, determining if a condition of the primary power source crosses a first threshold, when the condition crosses the first threshold, turning on a first power field effect transistor to couple a back-up power source to the load through a second power field effect transistor, when the primary power source is not providing power to the load, determining if a condition of the primary power source crosses a second threshold, and when the condition crosses the second threshold, switching off the first power field effect transistor to couple the primary power source to the load through a third power field effect transistor.