ORIGINAL: budgetdude
Any problem running a 6v regulated 2800 lithium ion in conjunction with a regulated 6v 700 nicad pack as a redundant battery setup?
If I were going to do what you describe, I think I would regulate the nicad pack to a lower voltage, so it will be a backup in case the lithium battery goes south.
The nicad would need to be able to supply enough power for a whole flight, and you would need to check the batteries independently (separate switches) before and after each flight.
JMHO.