[apalrd]

It seems as though the backup battery is completely useless for the big robots. Here is why:

You already have a separate battery to run the controller off of. The main purpose of the backup battery is to run the controller when the main battery temporarily (not for extended periods of time) dips below normal, such as when many motors stall and the battery is beginning to die. It is designed for normal vex robots, which pull all of their power from that little 7.2v 2000mAh (or 3000mAh) battery. Running only the controller, it should never do this. In this case, you are now powering your robot with 3 batteries, the main battery, the controller battery (just so it isn't affected by main battery low-ness), and a backup battery for what is essentially a backup battery.

You could alternatively connect the cortex to the 12v main battery. In this case, your main battery would have to dip below around 6.5v for the cortex to become unhappy. If your main battery is that low, it is probably very dead and under very heavy load. However, we lived this way for a really long time (using the IFI control system without its backup battery) without issues, so I don't think this should cause worry.

For the vex robots, the backup battery becomes more necessary. But, the old PIC processor lived without it just fine. (Although it didn't have a link process to run, it just kept waiting for data and processing it, and could boot in under a second)


Side note:
If your main battery is a vex battery (as in the "main for the cortex" battery, not main robot battery, or if this is a vex robot), then it should never get to be 10.2 volts. It is 7.2v nominal, so 8v is probably around its maximum. You could be using the 9.6v battery for the old joysticks (its wider but more flat, and says 9.6v on it), and that is not good.
If you have connected your cortex to your main big robot battery, then 10.2v is very very very dead and should be charged.