i'm working a project using ultrasonic sensors, leds, glcd display and buzzer on arduino mega 2560. how much battery voltage should i use and how to connect it? 
The mega2560 must have 5V. Most other AVRs can run at lower voltages.
Simple voltage regulators require a minimum operating voltage. LDO versions offer a better solution.
Alternatively, use a boost-buck chip that can cope with a wide range of input voltages.
I am guessing that you actually have an "infinite" supply e.g. a car battery.
If you were really worried about battery life, you would not be using a Mega2560 in the first place.
David.
ok. thank you so much 