NeoThermic:
Thanks for the replies.As for the redundancy, the arduino is being used to measure (via a resistor voltage divider) the voltage of the battery pack (usually 11.2v) BEFORE the regulator, as measuring it after the regulator would give me no change as the voltage dropped (due to the arduino reference dropping with it) .
This monitoring didn't work until i linked the negative term on the battery to the arduino's ground - hence the reason for what may seem like a redundant connection.
If it makes more sense, I have 11.2v of battery power, running (via the regulator) a raspberry pi, which is running the arduino.
Sounds like i'll be safe, i imagine any currents heading to ground will take the direct path to the battery pack, rather than going through the arduino -> usb connection -> pi -> voltage regulator -> battery.
I would have to review a schematic drawing of at least the complete power circuitry from battery to the various modules. Not sure why you needed to add a 'redundant' ground wire from the voltage divider/battery ground to the arduino for it to measure. The fact that the arduino was running at all means it had both good +5vdc and ground return to the regulator's output via the USB connection and the regulator's ground has to come from the battery ground, so who knows? A schematic drawing is worth a thousand posted words, so most likely we will not know the true answer unless you draw it out as built.
Lefty