Does the 5v and 12v need to be fixed and regulated voltages? Batteries are normally referred to by their nominal voltage but all batteries have a voltage discharge curve decreasing from fully charged voltage to near empty of charge voltage, so it would be good to know what variation you can have for your 5v and 12v requirement. That is what is the minimum and maximum voltage you can handle for your 5v circuits and your 12v circuits?
Lefty