Running from a USB battery pack through the barrel jack should work just fine, for the most part. Because the voltage is lower than optimal, however, you're going to find that the supply voltage to the processor is going to be lower than 5V (probably around 4.3-4.5V) and may be noisier than normal, which would be reflected in noisy analogRead() results.

You could get away with it, provided you aren't running a lot of stuff off the 5V or 3.3V supply lines.

If you're running, say, line sensors or a bunch of LEDs or something, you may have trouble. In general, though, the stuff hanging on the input is rated to well over 12V. We've derated it to 9V because the linear regulator that powers the rest of the circuitry will tend to get hot (and possibly either fail or go into overtemp protect shutdown) if you try to put too much voltage into the system.

Also, very few motors require their rated voltage to operate. You'll lose some torque but you can probably run them at lower than 12V.

https://www.sparkfun.com/products/11601

As long as you use A4 and A5 you should be fine. The Redbot accelerometer actually uses I2C and is designed for that header (which also provides power).

Thanks, we'll get that updated post-haste.

Yep! The XBee header on there can be used to route serial data either to the hardware serial port or to a software serial port on pins 2 and 3. You can, of course, make a software serial port on any of the header pins on the board, but if you want hardware serial you'll have to go through the XBee header.

When you flip the switch over to HW mode, the XBee and the FTDI chip are contending for the pins. If you've got the board plugged in over USB, that's likely to cause problems, and it will definitely make programming impossible.

The Atmega328P on this board only has one hardware serial port, and that gets used for programming, so you have to share, and sharing is not always nice in the Arduino environment.

That's actually why we have the switch, it changes the XBee connection from the hardware serial line to D2/D3 for software serial. The ATMega328 can only do one thing at a time on the hardware line, either talk to the XBee, or talk to the computer for programming so if you have an XBee installed you must flip the switch to disconnect the XBee from the hardware line to upload code. You can use the XBee in hardware mode, just flip the switch when programming and then switch back to run (and make sure your code is using the hardware serial lines and not software serial).

Try changing the following line:

pinMode(buttonPin, INPUT_PULLUP);

Not much; some of the labels, and the addition of a button on D12. Also, at some point since the first version, I fixed the power switching circuitry that disconnects power from the I/O headers labeled "Servo". I'm not saying that was changed in this version, just that it has changed.

If you look at your board from the top you'll see the silkscreen outlining the direction the XBee is supposed to go. :)

No, 0.08" diameter is implausibly small. I think the holes must be based on the Dimension layer (20) rather than the Holes layer (45), which makes them about 0.065" radius, or about the right size for a #5 machine screw.