I have Arduino Mega 2560 which I want to connect to DC battery as power source. My query is that how to ensure that the software terminal along with serial port communication is established when connected externally
assuming you'll connect the software terminal using USB, the Arduino will have two power sources: USB and the one coming from your battery (jack ?)
The Arduino will draw current from the highest power source automatically (likely the Jack if you power with more than 7V) and the GND of the USB will be joined on the GND of the MEGA and so all will be fine.
@sagnik_95, your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with (nor for advise on) your project 
See About the Installation & Troubleshooting category.
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