Thanks for the replies.
So how can I give it a common ground? Would that not create a ground loop? I intend doing this on a larger array so want to understand it correctly.
PV powers PV controller
Controller charges battery
Battery powers Arduino
If I start mucking about with grounds I could create more trouble?
Noted on wiring up the DAC before the power. Will be more careful 
How does this guy's example differ what what I am doing?
No common ground (although in his case you could easily have done that)