[RyanN]

For the resistors, I used a potentiometer and set it to 7.5 volts and skipped the Zener Diode and the battery will just slowly charge using the resistors that go around the diode connected to the battery. I did not include the resistor from the 12V to the backup battery as I saw no real reason to include it. What I have right now supplies a steady 7.5V and I think it will work reliably. I also added a diode so it would not try to power the whole robot once the breaker was switched off.