Hello Arduino World!
I am using an arduino to make a battery management system as my project, i just wanted help with the powering of the arduino.
I am going to use 12V (3 batteries to make 12v) to power a system and i was just wondering would i need to power the arduino with the 12v aswell or do i power it with the standard 5v?
Thanks
You can power a standard Arduino UNO via the barrel connector with 12V, that's fine. The on-board regulator will turn that into the 5V required by the UNO.
When you say you will be running on battery, ensure you have calculated the power requirements of the Arduino system otherwise your batteries may run flat very quickly. Ideally, measure the current going into the Arduino to get an accurate figure.
And just to nail this question, using 12V to power the UNO via a battery is wasteful as the onboard voltage regulator is just a very low efficiency linear converter - all the excess power is converted to heat. A better solution you may want to consider is using an efficient Buck converter to reduce the 12V to 5V and then power the UNO directly via the 5V pin on-board. Just don't get the voltage wrong when you do this as otherwise there will be no UNO left to power up 
- do a search for Buck converters, they are cheap and very efficient, > 80% and will save lots of battery power!
What kind of batteries are you using?