By using some simple hardware and the Arduino analogue input pin, Darius has completed a simple device that can monitor the discharge rate of a lithium-ion battery pack. The data is sent back to a host PC via serial to allow constant monitoring. If you're making your own version, note that the maximum raw input voltage from the battery pack is 5V!
A simpler and more conveinent way of monitoring data generated by your Arduino projects is using an LCD. Here at Freetronics we have the LCD & Keypad shield which contains a bright 16x2 character LCD and five buttons that can be read from only one analog input pin, perfect the battery monitoring project: