Some things to consider -
The ground current path resistance is low, but not zero (cable / body / frame, regardless of which you choose).
So - whenever current flows, there
will be voltage drop from one end of that ground return path to the other.
V = I * R
Ground resistance is usually (better be!) so low that it's very difficult to actually measure, but the effects of voltage drop from transient current events can be troublesome.
Concept to grasp:
Ground loops - if there is more than one possible ground return path, then the current will split up and take all possible paths.
How will it split? Depends on both DC resistance, and AC inductance - gets very difficult to predict.
Chassis grounding (or ground plane) is like an infinite number of parallel loops, which is much better than a few.
The issue is that there are other bits of circuitry scattered around the vehicle that rely on the chassis for 2 things.
1) Ground current return of their own smaller load currents.
2) Zero volt reference - this is the important one.
Vulnerable to I*R drop & noise trouble:
- Audio equipment - you'll hear it.
- Vehicle computers, like the ECU - measurement & control circuitry acting unpredictably.
- Added in computers - acting glitchy.
These are probably the troublemakers:
- Starter motor current -
very high DC for a few seconds.
- Ignition / spark - high short transients, constantly varying with engine speed.
- Fuel injectors - medium short transients, constantly varying with engine speed.
- Inverters - 60Hz or DC, chopped & modulated at high frequency.
Dedicated isolated cable ground return:
Pro :
- Most "pure" and idealized.
- Copper cable - lowest resistance, size selectable.
- Most predictable.
- Resistance here can be thought of as internal battery resistance, with no cable.
- Least likely to cause interference with other circuitry.
- Any interference can be addressed by filtering components, if needed.
Con :
- Expense of heavy cable.
- Routing of cable.
- Body passthrough of cable.
Chassis distributed ground return:
Pro :
- Free for the taking.
- Massively parallel.
- Lowest possible AC inductance / impedance to AC loads.
- Resistance
may be low enough.
Con :
- Other
sensitive loads attached mid-way, sensing ground current I*R drop as noise.
- Steel - medium high resistance.
- Resistance
may not be low enough.
- Difficult to predict.
Blended / hybrid approach - chassis / cable parallel ground return:
It's definitely got ground looping going on, but impossible to really control it.
Plus, what did it actually gain over the other 2 approaches? You still invested in long cables.
I would suggest staying away from this, as a basic strategy.
But, this could be added in later, as a band-aid "fix" to remedy trouble with the chassis ground approach.
Notice that modern vehicles have ground straps sprinkled all over - frame, sub-frames, several around engine.
p.s. I realized that I sort of unconsciously use the word "loops" in connection with ground current paths, by long habit, and the meaning of this might not be clear to the casual observer.
- If you can draw a schematic circuit diagram, somewhat representing the physical arrangement of connections, both to and from source and load, and returning current has a single physical pathway back to the source, then no ground loop. Situation is stable and repeatable. You always know how the current is getting back home.
- But, if there are multiple possible current return paths, then you have "loops" in the ground path - ground loops. Then, when various loads located on the various pathways turn on and off, you have return current voltage drops adding together in strange ways, causing changes in behavior. Not stable. And possibly "shocking".
