Power:
The Arduino Nano can be powered via the MiniB USB connection, 6-20V unregulated external power supply (pin 30), or 5V regulated external power supply (pin 27). The power source is automatically selected to the highest voltage source..
The FTDI FT232RL chip is only powered if the board is being powered over USB. As a result, when running on external (nonUSB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TXLEDs will flicker if digital pins 0 or 1 are high.