CrossRoads:
Diode would be a good choice to keep the battery from back driving the converter.You should have a battery charge control circuit as well so the battery doesn't get overcharged.
Alright great I'll look into getting a diode. Thanks for the suggestion about the battery charger too. I wasn't sure if it would be necessary.
johnerrington:
What exactly are you trying to achieve?
Several things. Trying to monitor performance of a few motors and send to database. On top of that there is a real problem with theft of these motors or the 12V battery, so need to be able to power the circuit if the 12V battery is disconnected and get GPS info.