Thanks for the info, CrossRoads. It is indeed the standard smoke detector-type square 9V battery. I looked up the info on Wikipedia; a standard AA battery supplies something around 400 to 900 mAh, whereas the one 9V only has 565 mAh (according to Wikipedia). So as you said, putting three together gives me (~400*3) mAh which is plenty more power than the 9V. Does my logic sound right? If so, I guess it's back to Radio Shack to see if they have something like that! One question though, should I be using Vin or 5V when running on external power on the Uno? The docs seem to suggest Vin is the way to go.