I have a cell phone battery (this one http://www.clove.co.uk/product-images/fullsize/note2battery-sam-n2bat-a31.jpg).
And I'm wondering how can I check the charge state (or mah in other words) of the battery and then send it over to the arduino.
This article will shed some light on the subject:
The relevant introduction is enlightning:
There is no direct way of measuring the State Of Charge (SOC) of a Li-Ion battery. There are indirect ways of estimating it, but each suffers from limitations. This paper describes how combining two techniques can result in a reasonable estimate of SOC.
This is why LiON packs for notebooks have their own uC inside the battery pack.
Ray