Moris_zen has created a simple yet versatile battery tester using an Arduino, LCD module and external circuitry. It can display various parameters of the battery under test, and also send the data to a PC for further analysis. It can also detect the type of battery and perform a controlled discharge. Not bad considering this was one of the author's first projects.
Once again this type of project shows that you can do more than just play games or blink an LED with an Arduino. To build your own version of the battery tester, consider using the Freetronics Eleven - the Uno-compatible with onboard prototyping space: