Originally Posted by svfinnishline
. . . This boat has an isolated ground system - there are separate ground wires from both the alternators negative posts to a large central ground bus.
The battery negative cable is connected to the amp hour shunt and from there to the central ground bus - this cable - perhaps 9 feet in length, has not been upgraded or increased in diameter. . . .
Usually the "AmpHour Meter" also includes a volt meter. It measures the DC voltage from the battery to the central ground bus, measured at the shunt. If the central ground bus has a potential above zero the volts on the meter would be less than actual across the battery.
- - I find the idea that the central ground bus is "floating" above battery zero strange and unusual. "Never say never" but highly unusual. A meter across the actual battery terminals compared to the meter across the battery positive and central ground bus should read very close to each other. The difference would be accounted for in a resistance to current flow caused by an improperly sized battery negative cable from the batteries to the central ground bus.
- - Forget the instrument fluctuations - if that battery negative cable system is under-sized then you have a potential fire hazard
as that battery negative cable cannot handle the ampere load of the whole system at maximum. That is a serious problem that needs correcting immediately if not sooner.
- - I also find it unusual that the DC system could be "isolated" from earth/water grounding. There are electrical
sensors on the engine that usually have only one wire to them. The return DC wire is the engine itself which through the propeller
shaft is grounded to sea water
. And the alternators would have to totally non-standard as the case of the alternator is part of its "grounding" and is mechanically bolted to the engine. Again, never say never, as I have seen some unusual electrical systems coming out of other countries that are not bound by the US ABYC system of conventions.