[Al Skierkiewicz]

I feel the need to chime in here on a few points.
1. The charging circuit that IFI is likely to publish will include charge current limiting and diode protection so that the backup battery is not able to supply current to the 12 volt circuitry on the robot. There are several usable designs that can accomplish this, just be patient.
2. Mike, I have to disagree on the NiCad memory issue. It does exist, particularly in inexpensive cell technologies and older batteries (we make extensive use of rechargeable packs in camera, VCR and light applications for the field). New designs have managed to overcome the chemical issues that led to this problem. (much of the problem occurred with internal construction and material choice.) It is unlikely that any of the newer, high charge current or extended use NiCad cells will exhibit the problem. As a team, you should not consider it to be a factor in your battery choice.
3. Many of you seem to be fully on board with a 12 volt charging circuit operated from the main battery. I am not, as it adds weight to your robot and steals power from the main battery. Simple buy two backup batteries and charge one while you use the other on the robot. If you find that you require higher current (due to multiple servos in your design) Digikey or other vendors can assemble battery packs in any current or package size you might desire.