power arduino using cell phone batteries

Hi.
Im new to arduino and need some help as to which direction to take on this project im working on.
Im trying to find a way to power up a arduino- machine system using a cellphone battery. What are rating requirements for the arduino and battery that will work and prevent the system from blowing off or be short on voltage? Any tip and help will be very much appreciated. Thank you

You will need at least 6V to get it to turn on, and about 7V for perfect function.

Put two 3.6V li-po cell phone batteries in series should work. Most cell phone batteries can supply enough current, so voltage is your concern. Not sure if there is a 7.2V cell battery, but you can get cheap camera battery with 7.2V (max 8.4V).

Just a thought.

Thank you for the great answers. I have a few more questions: first: i'm a little bit confused as to how much power the arduino itself needs. Let's say i'm using a 5v arduino, wouldn't the voltage needed to be fed to it be less than 5v?

second: If i need to keep my arduino-machine system working on this (these) batteries for a certain number of hours before switing batteries, would 7.4v keep the system on long enough?

Sorry if these questions are kind of stupid =(, I'm just trying to learn. Thank you for your patience

If you want a battery to power your arduino, go with one of these. http://www.batteryspace.com/96vrccarbatterypackseries.aspx

zigo:
Thank you for the great answers. I have a few more questions:
first: i’m a little bit confused as to how much power the arduino itself needs. Let’s say i’m using a 5v arduino, wouldn’t the voltage needed to be fed to it be less than 5v?

second: If i need to keep my arduino-machine system working on this (these) batteries for a certain number of hours before switing batteries, would 7.4v keep the system on long enough?

Sorry if these questions are kind of stupid =(, I’m just trying to learn.
Thank you for your patience

Most Li-Po battery, be it for cell phone or not, has a capacity rating in (milli) ampage-hour, like 1000maH means 1000 hours if you draw it at 1mA rate or 1 hour if you draw it at 1A rate.

I think I have read somewhere that Arduino draws 50mA at idle (a bit too much, but I can be wrong), so with 1000mah 7.4V battery (or two 3.7V 1000mah in series), you get about 20 hours.

Energy will be wasted because of the voltage regulator, in this case close to 1/3 are wasted (7.4V to 5V assuming in current and out current are close to be the same)