user25580: I have a 9V Lion 990mAh battery. I am not sure if I can use it to power an Arduino externally?
Can you please guide me how to understand current specifications of any device?
Thanks and Regards,
Manish

user-1: It says 7-12v on the site, so it should work https://www.arduino.cc/en/Main/ArduinoBoardUno
You can look at the datasheet for the Atmega328, but the current largely depends on what you connect to the Arduino. So it all depends what you do with it. It can certainly power and idle Arduino, driving motors and dozens of LED's maybe less so.

user6959: While this answer addresses the specific question it should also prove useful as a link to resources relating to current reduction in other microcontroller projects.
Short:  
The "C" rate for a cell is a current in mA numerically equal to the capacity in mAh
eg 1000 mA for a 1000 mAh cell.
In the absence of a specific data sheet a LiIon or LiPo cell is probably safe for maximum charge and discharge rates of C/2 and C.
ie 500 mA max charge rate for a 1000 mAh cell and
1000 mAh max discharge rate for a 1000 mAh cell.  
In many cases double those rates will be safe.
Charge rates are seldom higher than C but discharge rates of substantially higher than 2C MAY be safe - see data sheet!.
In your example case, the operating time will be approximately mAh/average_current. Consult the data sheet for your microcontroller and/or measure current drain.  
A typical "bare" Arduino with processor, USB IC, LEDs and "glue" components probably draws typically 50 mA and maybe up to 100 mA. Adding external loads can increase this - depending entirely on the load.
At say 100 mA average drain you'd get approximately 900/100= 9 hours operation. Running just the processor and on board components / peripherals you'd probabaly get twice times as long. By targeting low current methods (see refs ate end) you could get MUCH longer.

Most LiIon batteries are rated to charge at up to the "C" rate - ie 990 mA for a 990 mAh battery and to safely discharge at several times as much. Some manufacturers rate their batteries for lower maximum charge rates and this also can vary with the intended cell application. A "high energy" cell is liable to have lower charge and discharge rates than a "high power" cell. This relates to means of construction and to be safe the manufacturer's data sheet should be consulted.
The battery "capacity" rises somewhat as rates decrease below 'C' and fall significantly for rates above C. 
Here is an excellent example data sheet for selected Sanyo LiIon cells . These are only examples but give a reasonable idea of what can be expected from a reasonable quality product. 

Arduino per pin current limitations
Typical current draw](http://gadgetmakersblog.com/arduino-power-consumption/)
Excellent - Methods of reducing Arduino power consumption  & Related & more 
Stack exchange discussion - How much current do you require in Arduino and embedded systems projects?

25580: <p>I have a 9V Lion 990mAh battery. I am not sure if I can use it to power an Arduino externally?</p>

<p>Can you please guide me how to understand current specifications of any device?</p>

<p>Thanks and Regards,
Manish</p>
 -1: <p>It says 7-12v on the site, so it should work <a href="https://www.arduino.cc/en/Main/ArduinoBoardUno" rel="nofollow">https://www.arduino.cc/en/Main/ArduinoBoardUno</a></p>

<p>You can look at the datasheet for the Atmega328, but the current largely depends on what you connect to the Arduino. So it all depends what you do with it. It can certainly power and idle Arduino, driving motors and dozens of LED's maybe less so.</p>
 6959: <p>While this answer addresses the specific question it should also prove useful as a link to resources relating to current reduction in other microcontroller projects.</p>

<p>Short:  </p>

<p>The "C" rate for a cell is a current in mA numerically equal to the capacity in mAh<br>
eg 1000 mA for a 1000 mAh cell.</p>

<p>In the absence of a specific data sheet a LiIon or LiPo cell is <strong>probably</strong> safe for maximum charge and discharge rates of C/2 and C.<br>
ie 500 mA max charge rate for a 1000 mAh cell and<br>
1000 mAh max discharge rate for a 1000 mAh cell.  </p>

<p>In many cases double those rates will be safe.<br>
Charge rates are seldom higher than C but discharge rates of substantially higher than 2C MAY be safe - see data sheet!.</p>

<p>In your example case, the operating time will be approximately mAh/average_current. Consult the data sheet for your microcontroller and/or measure current drain.  </p>

<p>A typical "bare" Arduino with processor, USB IC, LEDs and "glue" components probably draws typically 50 mA and maybe up to 100 mA. Adding external loads can increase this - depending entirely on the load.</p>

<p>At say 100 mA average drain you'd get approximately 900/100= 9 hours operation. Running just the processor and on board components / peripherals you'd probabaly get twice times as long. By targeting low current methods (see refs ate end) you could get MUCH longer.</p>

<hr>

<p>Most LiIon batteries are rated to charge at up to the "C" rate - ie 990 mA for a 990 mAh battery and to safely discharge at several times as much. Some manufacturers rate their batteries for lower maximum charge rates and this also can vary with the intended cell application. A "high energy" cell is liable to have lower charge and discharge rates than a "high power" cell. This relates to means of construction and to be safe the manufacturer's data sheet should be consulted.</p>

<p>The battery "capacity" rises somewhat as rates decrease below 'C' and fall significantly for rates above C. </p>

<p>Here is <a href="http://www.awilco-multiplex.dk/files/pdf/Batteries%20Li-Ion/Sanyo_Lithium_Ion_catalogue_10_2007.pdf" rel="nofollow noreferrer"><strong>an excellent example data sheet for selected Sanyo LiIon cells</strong></a> . These are only examples but give a reasonable idea of what can be expected from a reasonable quality product. </p>

<hr>

<p><a href="http://playground.arduino.cc/Main/ArduinoPinCurrentLimitations" rel="nofollow noreferrer">Arduino per pin current limitations</a></p>

<p>Typical current draw](<a href="http://gadgetmakersblog.com/arduino-power-consumption/" rel="nofollow noreferrer">http://gadgetmakersblog.com/arduino-power-consumption/</a>)</p>

<p>Excellent - <a href="http://www.home-automation-community.com/arduino-low-power-how-to-run-atmega328p-for-a-year-on-coin-cell-battery/" rel="nofollow noreferrer">Methods of reducing Arduino power consumption</a>  &amp; <a href="https://alanbmitchell.wordpress.com/2011/10/02/operate-arduino-for-year-from-batteries/" rel="nofollow noreferrer">Related</a> &amp; <a href="https://alanbmitchell.wordpress.com/2011/10/09/design-of-the-extended-battery-life-power-controller-for-the-arduino/" rel="nofollow noreferrer">more</a> </p>

<p>Stack exchange discussion - <a href="https://electronics.stackexchange.com/q/1707/3288">How much current do you require in Arduino and embedded systems projects?</a></p>
