Oh that's right 
I tried to connect the BEC to the 5V input but it did the same thing... I'll try to put in serie 2 BEC so I can get 10V and put it in the Vin to see if it works.
I also discovered something else. Actually, when connected on USB, the analogPin read the value 330, but when switching to Vin, it reads 400, like if voltage was raising up. But it didn't raise, I've checked with my multimeter : 1.6V so a 330 value is ok, but not 400...