user3219: In context of getting the MCU on Pro Mini and ATMega2560 (chip is ATmega328 and ATMega 2560) to sleep and power down modes, so as to run on one set of battery for months, appreciate idea and comment.
Initially, I looked at web postings and get the following general idea. Did I miss anything?
Is sleep mode done via ATMel factory library, Arduino-level user contributed library, or the Arduino IDE? Many web posting are along the line of getting it partially working but only at half expected current reduction, etc. That is, current do reduced, but, not enough. 
Are there several such library? 
How are they? 
Which one is better support and more functions? (no need for easy to use, can do registers programming, when needed)
Many thanks in advance 

user6888: I could reproduce these power requirements - main thing is to get rid of the Arduino board and start with a Bareduino or sth similar. (find a detailed description here)

user10794: 
I have a page about saving power with the Atmega328P chips (similar techniques apply to other chips): Power saving techniques for microprocessors
I made a Temperature and humidity sensor - battery powered - this runs from 3 x AA batteries, and writes to an SD card. From memory it ran for 2 years before I needed to replace the batteries.
Another project flashes an LED every second: Simple project - torch-locator. I made something similar a couple of years ago that is still running from the same button battery.

As a rule-of-thumb, you should be able to achieve 100 nA of current consumption, and still be "alive" and able to respond to external events, like button-presses (eg. a calculator or TV remote). To wake periodically (without an external event) takes more current, because you need the watch-dog timer to be active, but that can still be as low as 6 µA.

user1566: Have a read through the Jeelabs blog which documents the author's very successful attempt at making a very low power wireless sensor node that can run for months off coin cells.
http://jeelabs.org
There is also a library based on this project called Jeelib:
http://jeelabs.net/pub/docs/jeelib/
Jeelib contains a class called Sleepy that allows for easy sleeping.
Rocketscream did a thorough test of the current consumptions with the different modes here:
http://www.rocketscream.com/blog/2011/07/04/lightweight-low-power-arduino-library/
According to them, you can get the ATMega328P down to 1.7uA.
Unless you do something about the power supply on a normal Arduino board you won't be able to extend the battery life by much at all. See my answer here for some more details:
Arduino powering from 9V battery

3219: <p>In context of getting the MCU on Pro Mini and ATMega2560 (chip is ATmega328 and ATMega 2560) to <strong>sleep and power down modes,</strong> so as to run on <strong>one set of battery for months</strong>, appreciate idea and comment.</p>

<p>Initially, I looked at web postings and get the following general idea. Did I miss anything?</p>

<p>Is sleep mode done via ATMel factory library, Arduino-level user contributed library, or the Arduino IDE? Many web posting are along the line of getting it partially working but only at half expected current reduction, etc. That is, current do reduced, but, not enough. </p>

<p>Are there several such library? </p>

<p>How are they? </p>

<p>Which one is better support and more functions? (no need for easy to use, can do registers programming, when needed)</p>

<p>Many thanks in advance </p>
 6888: <p>I could reproduce these power requirements - main thing is to get rid of the Arduino board and start with a Bareduino or sth similar. (find a detailed description <a href="https://holadimake.wordpress.com/2015/01/02/building-a-cellular-tracker-part3-running-on-batteries-for-months/" rel="nofollow">here</a>)</p>
 10794: <ul>
<li><p>I have a page about saving power with the Atmega328P chips (similar techniques apply to other chips): <a href="http://www.gammon.com.au/power" rel="nofollow">Power saving techniques for microprocessors</a></p></li>
<li><p>I made a <a href="http://www.gammon.com.au/forum/?id=12106" rel="nofollow">Temperature and humidity sensor - battery powered</a> - this runs from 3 x AA batteries, and writes to an SD card. From memory it ran for 2 years before I needed to replace the batteries.</p></li>
<li><p>Another project flashes an LED every second: <a href="http://www.gammon.com.au/forum/?id=12769" rel="nofollow">Simple project - torch-locator</a>. I made something similar a couple of years ago that is still running from the same button battery.</p></li>
</ul>

<p>As a rule-of-thumb, you should be able to achieve 100 nA of current consumption, and still be "alive" and able to respond to external events, like button-presses (eg. a calculator or TV remote). To wake periodically (without an external event) takes more current, because you need the watch-dog timer to be active, but that can still be as low as 6 µA.</p>
 1566: <p>Have a read through the Jeelabs blog which documents the author's very successful attempt at making a very low power wireless sensor node that can run for months off coin cells.</p>

<p><a href="http://jeelabs.org" rel="nofollow">http://jeelabs.org</a></p>

<p>There is also a library based on this project called Jeelib:</p>

<p><a href="http://jeelabs.net/pub/docs/jeelib/" rel="nofollow">http://jeelabs.net/pub/docs/jeelib/</a></p>

<p>Jeelib contains a class called <code>Sleepy</code> that allows for easy sleeping.</p>

<p>Rocketscream did a thorough test of the current consumptions with the different modes here:</p>

<p><a href="http://www.rocketscream.com/blog/2011/07/04/lightweight-low-power-arduino-library/" rel="nofollow">http://www.rocketscream.com/blog/2011/07/04/lightweight-low-power-arduino-library/</a></p>

<p>According to them, you can get the ATMega328P down to 1.7uA.</p>

<p>Unless you do something about the power supply on a normal Arduino board you won't be able to extend the battery life by much at all. See my answer here for some more details:</p>

<p><a href="https://arduino.stackexchange.com/questions/3688/arduino-powering-from-9v-battery/3689#3689">Arduino powering from 9V battery</a></p>
