Hello I'm building a small weather monitoring. It is really a arduino pro mini with a Nokia 5110 LCD and a DHT22 temperature sensor. I would like to monitor the Double AA battery. But I'm using a 0.8V to 3.3v Up boost converter i got from ebay Here. Everything is working great but i would like to monitor the battery somehow. Sense it is using the Up boost converter not sure how i would be able to monitor it. The output on the Up boost converter is 3.3v i did put my multi meter on it and checked 1.54v on the battery side and 3.28v on the output side going to the arduino and other parts. So not sure how i would be able to do it can someone please help me out? Thank you.
what exactly would you like to monitor? The battery voltage? or temperature? or level? or discharge curve? or current out? or anything else?
josephchrzempiec:
Double AA battery.
you mean 2x AA battery?
Please have a look at this forum topic if that's what you're looking for 
http://forum.arduino.cc/index.php?topic=92074.0
Hello I'm sorry if I'm not clear. It is a Single AA battery I'm just trying to monitor the voltage and give a Percentage like 100% 99% 98% and so forth? Everything else i do have.
So far from the same pack of batteries they range from 1.56v up to 1.62v I'm guessing that is okay.
In order to calculate percentage based on battery voltage, first you need to know both the upper limit and lower limit. In your case you know the upper limit can be set at average 1.5v, so now you need to know the lower limit. Measure the battery voltage when the boost converter can no longer power your project (that is when the voltage is too low for the module to convert, the converter will usually stop before battery voltage goes below 0.3 or 0.2 volts ) and that voltage will be the lower limit. Now you can convert the voltage range in between those limits into % value.
However there is a drawback to this method, this works only with the same type of battery, same manufacturer, same model. So make sure if you use this method you use only the exact type of battery or be ready to change the code whenever you use a new type of AA battery or from a different brand.
eg:( just a rough suggestion )
a = 1.5; // define upper voltage limit
b = 0.6; // define lower voltage limit
c = (a-b); // define voltage range
d = ((b*100)/c); // convert lower voltage limit into lowest percentage
e = analogIN; // voltage input read from arduino analog pin OR from voltage sensorx = (((e*100)/c)-d); // convert analog read voltage value into percentage
print (x); // display battery percentage on screen