I have been trying to get a stable base for a Arduino track bot. I had trouble with coming up with a good battery for powering the 2 motors. I tried using a 9V battery but it consumed the power in a few minutes. I found a good alternative by using a blackberry phone battery which is about 3.7V DC (BAT-11004-001). The power lasted fairly long time and was sufficient enough for the track bots motor. I was able to connect it as an external power to the motor shield with an on/off switch allowing me to cut the power to the motor quickly. The code is similar to what I had before. Just a simple Ping sensor looking for the distance, backs up and turns when it senses something 8 inches away.