I’m doing a project which prints signatures read by the Pixy2 on an LCD. It’s currently working with an Arduino Uno powered by a 9V battery. Whenever I safely disconnect the Pixy2 from the laptop and disconnect the micro USB cable, it stops working. The LED on the Pixy shows a very, very bright red color, and the LCD turns off entirely.
It sounds like your power setup isn’t providing enough power. It could be that the battery can’t provide enough current or if you are powering Pixy through the Arduino, the Arduino isn’t providing enough current. In this case, you should consider powering the Pixy by battery and have Pixy power the Arduino.
Please check out this page, particularly the section on unregulated power, which is what you are doing: