Ok,
compromise points, where to GND, etc. Geez, where to start...??
Maybe first a little anecdote.
Ok, you got me - maybe first an
little anecdote.
I recall discussions of "
ABSOLUTE ground". I often find them amusing because they fail to state the conditions - ie, when the alternator supplies the load or when the battery supplies - eg, at idle, under high total loading, load transients, or when not charging? (So MANY web opposing arguments I see are a waste of existence - both or many are correct (or rather, not incorrect) - they simply miss defining the conditions!)
I recall one
audio system example where the big deal seemed to be how the alternator supplied the most -ve voltage and hence was '
the absolute GND' and therefore the alternator case/bolt was used as the GND. In my experience that is problematic and unnecessary except for certain rare situations...
But there I was thinking (apart from the various electrical considerations!) how do you securely mount BIG audio GNDs to an alternator case or mounting bolt... which in turn is on a hot and vibrating engine? A 0/2G or 00G cable
flying across to the engine to an eyelet to a (smallish?) mounting bolt, or bolted into an alloy alternator case...?
Then it turned out the audio system involved was a mere 100W or similar! Now if their added wiring couldn't handle 10 or a few tens of Amps without significant voltage drop...
Now if being that pedantic, IMO the alternator case might not be the 'absolute' GND anyhow. But forgetting that and the other issues, they obviously didn't care about DC supply noise nor the GND that is used during amplifier burps (surges) etc (like I said, it was a small system...). Nor if chassis or batt- grounded accessories connected to such an HU or amp or PC there could be GND path issues and hence noise and maybe even damage.
BTW - for my discussions, chassis & body (and dash grounds etc) are equivalent. It is assumed that they are one & the same electrically - ie they have adequate & good condition interconnection if they are separate
mechanical items.
Anyhow, the above was an example of arguments I have oft heard & LOL'd. Without sufficient details or conditions a
correct answer cannot be assigned. But I'll tackle if from a ground up POV. (That
ground up pun is simply natural!)
First and foremost is physical (or economic) practicality. A heavy gauge audio power cable mounted the engine or alternator bracket?
It's bad enough having a fat cable attached to the alternator output - it gets hot and vibrates hence might fatigue, anneal, suffer insulation breakdown, etc - but to
double up on that by attaching a GND to the (probably hotter) alternator mounting bolts or engine block...
Likewise multiple heavy cables attached to battery terminals? Even with suitable terminals, will it strain the battery housing or mean longer paths or be too messy? (Batteries are usually mounted to the front of the engine bay. Main fuseboxes or may be closer to loads than the bat or alternator, etc.)
Then there's the electrical aspect. If the alternator can't supply the load, the battery
supplies the GND & +12V - ie, the batt- is the most -ve. That occurs when the alternator falls short - eg, transients, overloads, engine off.
Sure, normally the alternator supplies the voltage (hence its GND is the most -ve), but then we get into the issue of running cables to the engineblock or alternator.
What is usually done in practice is chassis-based 'power' points. Relay- or fuse-boxes else special insulated studs are used to be a
junction of the alt output and batt+ to supply +12V to everything else. (Startermotors, winches, big audio/loads excluded.)
Similarly batt- and alt/engine GND join at some chassis point.
Those common or '
central' points are often closer to the loads than batt+ or batt- or alternator/engine and they have the advantage of fixed solid chassis mounting which is probably cooler and vibrates less than engine mounting. (Hence no need for multiple very multi-stranded and heat tolerant cables to the engine.)
IMO if any the above suffer voltage drops, upgrade the applicable cable or connections.
Ideally all points wired together are the same electrical point but that ignores cable & joint resistance. Hence why we minimise cable resistance and temperature - eg, use copper instead of Al, steel, or gold; appropriate purity, stranding and annealing etc; and use appropriate interconnection techniques.
From Ohm's Law, every conducting wire has a voltage drop - V=IR. (Let's ignore superconductors.)
The higher the current, the bigger the voltage drop. If it's too high, we need to reduce the resistance (eg, bigger cable, shorter cable, better conductors, better joints, lower temperature). Usually cable size is increased (since we assume we already have the shortest paths and good connections), and often merely by doubling up - eg, another parallel cable of the same assuming that's practical.
I think there's enough above to enable you to decide, but maybe a few clarifications or examples...
Audio and PCs etc often use the battery as their power source because it's the cleanest DC power (a battery acts like a cap and smooths out much electrical variation/noise). That may include the batt- for GND if the main or central GND is inadequate.
Big audio often uses the battery because it's the most likely to provide for burps (a battery is like a huge capacitor).
Even if an alternator can handle a burp or surge, the battery might be faster reacting. That's certainly true for older mechanical voltage regulators but with modern electronic
all in one alternators with regulators it depends on so many factors.
Suffice to say, when the load or transient magnitude exceeds alternator capability, it will be the battery that (eventually?) supplies the load.
Another old example which is irrelevant to modern LED & HID headlights etc...
I used to power my halogen lights from the alternator output for max light output when driving. That could make considerable difference - and extra 0.5V makes a big difference with halogens, far more than with tungstens).
But now they are direct off the battery because (1) I use heavy wiring too inconvenient to mount at the alternator output; (2) alternator vibration etc & inconvenience; (3) my alt to batt+ wiring voltage drop should be negligible anyway; (4) my battery is usually 14.2V else 14.4V anyhow (it used to be 13.8V in ye olde days).
There are issues I haven't covered - especially transient and sub-transient response of alternators... and batteries; source (battery) resistance or impedances; the
non-capacitive power desirability of high peak SPL competitive setups etc - but it's merely a generic discussion and not any specific solutions.
And I'll avoid alternator fusing. Just note that if used, the fuse should be at the batt+ end (the fuse is not to protect the alternator).