I use the gy-521 to control some motors, but I've noticed that if I upload the program and I have the USB connected everything works fine, the problem is that when I connect the robot to the battery, the motors work but the gy-521 just send nan instead of the values. It's like that part of the program it's ereased.
Some ideas?
What is a GY 521?
What Arduino are you using?
What batteries are you using to power it?
Post your program. And please use the code button </>. See How to use the forum
...R