On Sun, Oct 28, 2012 at 06:18:51PM +0000, Wayne Stallwood wrote:
If he measures upstream from the batteries on their feed wires then his local ground will have a DC offset to the battery negative post, particularly under load. The reason is that the voltage drop in the battery cabling affects both sides of the circuit. So it would be fairly easy to create an unintentional ground loop.
Chris now tells us that the batteries are grounded to the hull things have become a bit simpler (as long as his ground straps are all good).
As in nearly all well engineered electrical systems like this (i.e. like our boat) it has as far as it's possible to ensure a single ground point.
I.e. there is only one electrical connection to the hull of the boat and that is close to the batteries and has a short and very fat cable from the battery negatives to the ground point. All connections throughout the boat comprise two wires whose negative wire (eventually) gets back to the single ground point.
The 'grounding' problem (if there is one) arises from the distance between the computer/electronics measuring the voltage and the voltage to be measured (which is essentially that across the battery terminals in the nether regions of the boat).
If the voltage measuring device has a floating/balanced/differential input there is not so much of a problem, a pair of wires to the battery terminals (or as close as necessary/needed) should work OK and if it's a twisted pair most electrical interference will be fairly well rejected.
However if the input is 'unbalanced', i.e. the A2D input on the Arduino or whatever has one side at ground potential (as seen by the Arduino) then one has to make very sure that ground potential is *actually* at the same potential as the battery -ve terminal. If there is significant current being drawn by other devices fed by the same piece of wire then this may well not be true.