This is from the Arduino Nano specifications:
As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high.
I understand that if I don't power from the USB, only from de Vin the 3.3V output is useless.