[Al Skierkiewicz]

I agree that an additional diode would help this circuit and prevent back feeding the rest of the robot.
The diode in the schematic is a Schottky diode with a 0.4 volt drop instead of the 0.6 volt drop of normal diodes.
What we all need to realize is the 317 regulator will provide all the power needed by the 7.2 volt buss (at 8.5 volts) until the main battery falls to about 9.5 voltsor less. When this occurs, the output of the regulator will fall and when the output is less than about 7 volts, the backup battery will begin to supply current to the backup buss at 0.4 volts less the terminal voltage of the 7.2 volt battery. The resistor in parallel with the diode will also pass some current in each direction. The effect is that servos will not be running on 6.8 volts or less so teams should not be surprised if there servo operated devices run slower at lower main battery voltage.
There is still no substitute for a fully chargd backup battery working alone.