Thanks for the quick replies.
I'll definitely put in a diode. Just to clarify, in this situtaion the diode is there to protect the relay, not the Arduino right?
What is the correct value diode to use?
Run separate wires from the battery to the relay and to the Arduino supply, ie use a star-
ground arrangement for minimal interaction / interference.
Is this necessary, or just the optimal way to do it? The relay is 5v, so can't go direct to the battery.
If I'm
not going to run the relay directly off the battery, should I run the relay ground to the battery, or to the Arduino?