I am looking for an automated way to switch between two 12V power banks for an Arduino project. It is intended that the system switches whenever the power source that is being used reaches 15% of total battery, to use the battery of the other power source.

 

The most reliable switching over will be with diodes as an "OR" function. If the diode drop under load can be tolerated. That provides the lowest parts count and the simplest circuit. But power is wasted in the diode drop.

 

Per MisterBill2, after wrestling with the question for a day, that's the solution I settled on.

 

The 7805 regulator also needs the 0.1 MFD capacitors to common on both input and output. I know I am being picky, but at least some of the time they will oscillate very well.