This is a simple voltmeter meant to tell if a battery is charged or not and its approximate % via an Arduino Uno. The main.ino file contains a class named BatteryTester that has a abstract function, getPercent(voltage), where you fill in the formula for any battery's discharge curve such that you can get an approximate %. An example for a 3000 mAh 7.2 Vex Batery is shown.
As for any type of battery, make sure to apply a load to the battery momentarily to get rid of surface charge. Without this step, the battery has an elevated voltage (from having time to fully soak and spread out electrons throughout itself) that may give a false reading. By applying a load for a short period of time, you drain the excess electrons.
The wiring for this is as follows:
... where the battery to the far left can be replaced with any battery less than 10 V.