I never got into the arduino since it did not have the processing power I was looking for at the time of another project. I use the Parallax Propeller for all my projects now since it can handle multiple tasks at once. I did think about building my own charge controller, but I don't know enough about charging batteries to know where to start.
I did get a charge controller from a local solar installer who said it just died on him and he did not know how to fix it. I wound up replacing a regulator on the charge circuit and a diode on the battery voltage display and got it working again...I think.... I have a 24V solar panel and am charging a 12V sealed battery. Once everything was connected though the charge controller, I tested it with a meter and saw that the output voltage to the battery was pulsing from 12V to 18V+ I don't know if this is how to properly charge a battery, nor if putting higher than 14.5 V directly to a 12V battery was a good idea or not. The solar installer guy said that once the battery gets to 14.5V, the controller will slow the pulsing and keep the battery regulated at 14.5V