If I left a simple 16x2 LCD display hooked up via my arduino turned on with a 9v battery, would the 9v battery die quickly? I'm making a small sign with a LCD to hang up on my door, and I'm wondering if it takes up a ton of power, and if I should just power it with an outlet.
EDIT: Basically I'm just wondering how long a 16x2 LCD screen can run on a 9v battery
We have much merriment here when people refer to a "9v battery" if they mean the common "PP3" or "smoke alarm battery". It will not run an Arduino for very long at all.
You do not want to power the Arduino by "Vin" or the "barrel jack". The on-board regulator is not very capable and you are just wasting four volts out of nine. Four Ni-Mh "AA" cells in a holder would be far more appropriate, connected to the "5V" pin.
Now the backlight on the LCD draws about 20 mA (in most cases) while the LCD itself draws less than 1 mA. It will in fact draw only half of that if you correct a common mistake and remove the connection from the contrast potentiometer to 5 V, leaving it only connected to pin 3 and ground.
The Arduino itself however consumes as much or more power than the backlight, particularly a UNO or Nano etc. with a USB to serial chip. The Pro Mini does not have this USB to serial chip (you have to connect an adapter to program it) so is much better in this respect.
Exactly how long will the battery last? Can't say really. Try it out. 
Just to clarify, the "long and short of it" is that if you want a display sign on the door, presumably to operate every day for much or all of the day, and it uses any form of LED illumination or animation requiring the microcontroller to be active, then it must be powered from a power outlet, generally using a USB "phone charger" to provide the correct regulated 5 V.
is it better to use the barrel connector or 5v pin for the same project.
akshat_raj:
is it better to use the barrel connector or 5v pin for the same project.
Read the second paragraph above! 
But the 5v pin is being used by the lcd.
https://www.sunfounder.com/media/wysiwyg/swatches/starter_basic_kit_v2-0_for_arduino/4_i2c_lcd1602/i2c_lcd1602_fri.png
In electronics, terminals are not "occupied".
The battery I suggested - four Ni-Mh "AA" cells in a holder or more appropriately, a 5 V regulated power supply such as a USB "phone charger" - connects to the "5V" pin and ground as well as the LCD module.
Actually, a USB "phone charger" could simply plug into the USB jack with a common USB cable if the display is the only other module connected to the Arduino, but I suspect you may wish to connect other things as well so it is time to learn electronics - how to connect things together.
Exactly. "Dupont" wires have a plug or socket pin at each end - in the case of that display, you need a socket pin on the display end and a plug end for the UNO. You just have to figure out how to connect more than one thing. In the case of just using "Dupont" wires, you might use a mini-breadboard as a "bus" connector to connect many wires to the one line.
One blunder in that diagram is that it shows 5 V and ground being picked up on opposite sides of the UNO, but the ground next to the "5V" terminal should be used instead for the LCD as well as the battery you show, in order to keep all the connecting wires grouped together.
And a Nano is a much more practical version for experimentation as it can be plugged into a (larger) solderless breadboard or a "sensor shield".
akshat_raj:
Something like this?
Definitely NOT with that 9 volt battery you have been talking about. You must use Vin so that the current goes through the regulator.
I was trusting that we had completely forgotten any suggestion of using a 9 V battery or the on-board regulator.
