I have a circuit board powered by 12v battery. Some shielded cables are are connected to this board. Should I connect shield on the cable to the battery negative (system ground) or to a metal box/sheet I'll have to add-on (frame ground)? And why? Thanks!

 

Are you talking about RF shielding or insulation?

Better safe than sorry with insulation. An accidental bump of an exposed wire or live terminal can destroy components, Zap you (12v hurts, and can cause small burns, I know from experience), or even start a fire.

The only time to skimp on insulation (or any safety) is when you know exactly what is going on with the circuit and exactly what the dangers of a short could be..

Even under those circumstances, extreme care should be taken until you have thoroughly tested everything...

Even the best engineers make mistakes.

I know little about other types of shielding, so I'll leave that for other members.

 

connect to battery -, and only battery -. Do not connect at both ends, especially if your frame ground is also connected to battery -, because this will create a ground loop.