I thought that I would add to this thread the only technical reason (as in electrical theory), in my understanding, that there is for using a separate battery negative bus wired directly to the battery as opposed to using the vehicle's engine/frame as a path back to the battery negative terminal. This, as stated previously, is excluding such reasons as install convenience, trouble shooting convenience, reducing the number of connection points, inadequate wire size, insufficient negative current carrying capacity of the vehicle's existing wiring system, factoring in poor wiring practice, etc..
My interest was in knowing if there is a theoretical benefit in the conveyance of the electricity simply by doing this if all other variables are equal.
From what I have discerned the most compelling reason for doing this is to isolate any sensitive added on accessories from potential voltage spikes. Voltage spikes of varying intensities are occurring all the time in a vehicle's electrical system as components cycle on and off. The biggest spikes would be caused by items that draw a lot of power. Items such as the cooling fan motors, especially at start-up, as they cycle on and off. The battery acts as a buffer to these voltage spikes and tends to help reduce their intensity. The theory is that wiring the negative as well as the positive sides of the circuit directly to the battery terminals helps to reduce how much of the the voltage spike is transmitted through your accessories.
Based on this it would seem that items whose operation is sensitive to voltage spikes, such as audio/video/radio equipment as has been mentioned by a few of you, benefit from this wiring approach. Any electronic item that could suffer erroneous operation or be damaged by a voltage spike would also benefit.
With regards to non electronic things such as horns, lights, heated gear, there doesn't seem to be a theoretical advantage to this wiring approach as I understand it. Theoretical advantage in the sense that the actual conveyance of the electricity to these items is not superior simply because a negative bus bar wired directly to the negative battery terminal was used. If these items are being controlled by an electronic controller that is sensitive to voltage spikes than the advantage is in the protection afforded to the controller, not in superior delivery of the electricity to that item per se.
I was fully aware when I asked this question that this does not have a whole lot of relevance to most applications in the real world. I was curious and wanted to know so I asked.
On a motorcycle, single source star is my preferred method, but if just using raw power it probably isn't needed.
