Firstly, to clarify something you wrote (much) earlier...
The isolator and "battery guard" are two separate functions.
The isolator is to:
- isolate "aux" batteries form the main battery when NOT being charged (so as to
ensure normal cranking power);
- isolate paralleled batteries when not being charged or not needed for a load. (Batteries should not be left paralleled when not being used. A faulty battery can kill the others, and in some cases can be hazardous - eg, thermal runaway in AGM (and long-term, wet!) batteries.)
The "battery protector" is to cut off loads when the battery has discharged to whatever desired level - eg, 80%, 50%, 20% etc of remaining capacity - to keep within warranty specs or to prevent premature failure.
Both above are subject to widespread misinterpretation and subject to tremendous ripoffs by industry.
EG - battery sellers will often claim paralleling is no problem. And it isn't,
until one fails. And wrt to battery reliability, you will be replacing batteries 4 times as often for 2 permanently paralleled batteries, or 9 times for 3 batteries; 16 for 4, etc. Indeed, paralleling is "no problem" what$oever for battery $uppliers et al!!!. Others may claim the same based on a lack of knowledge or certain lucky else other maintained or monitored installations.
And few seem to incorporate ways to stop a battery from discharging to an excessive depth. (Why don't battery manufacturers highlight low-voltage cut-outs? Hmmmm...)
As to wiring, parallels are fine, though usually it is cheaper to go the equivalent gauge. But redundancy and path diversion may factor into that decision.
Whilst the GND may be easy to take from the engine (or alternator though that should not make any significant electrical difference), the +12V is usually a different issue.
The +12V is usually easier (and safer) from the battery else a common power point - eg, alternator/battery/fusebox common point.
Of course it is implicit that ALL paths be capable of the required current - eg, don't connect 300A to a "common" connection with less from the alternator or battery (unless each is fused for
their rated cable capacity).
The above can also depend on what source is the most important.
EG - if you want the best power available from the battery, then +12V & GND from the battery. (Same for the "cleanest" or least noisy source for amplifiers etc/)
If it's when charging, then +12V from the alternator with GND from the alternator body - though usually that's from the engine.
In theory, the GND at the chassis/body, alternator, and engine should all be the same. As too the alternator +12V output and battery +12V.
But in practice (and
practical or detailed theory), cables and fuses insert a resistance between them and hence a voltage drop of V=IR where I is the current and R is the path resistance (cable, fuse, joins).
And though some are pedantic and wire GND to the alternator case, I have yet to see where an alternator case to engine has any measurable else significant voltage drop (resistance) - except for some with remote alternators (including where alternator mounting bolts or brackets are not
solid enough). However, for those that weld cables to the alternator body - or better still, internally to the stator winding(s) - in order to negate terminal resistance (including later contamination, corrosion etc), though sometime overkill, that's cool.
But all are subject to consideration of what happens if the connections break down. (NEVER lose your alternator GND (engine) to body/chassis and battery GND! Hence multiple
redundant paths are often used. [Redundancy - where each is capable of full load (or) so failure of one (or more) is still within specs or capability.]
As an old example, in pre-HID & pre-LED days where lighting was important (I'm thinking of rally cars), the GND and +12V was often taken from the alternator. Where the wire gauge was too big to connect directly to the alternator, a thinner short interconnect was used from the alternator B+ (+12V) to the light's +12V supply. This cable might even be underrated wrt to the lights' current, but done in such a way (ie, SHORT) that the alternator or thicker gauge would adequately heat-sink excess heat. (As to the added resistance, it was only a short length.)
Keep in mind though that these installations would typically use gauges much thicker than normal wiring tables since they wanted minimal voltage drops - not the commonly accepted 3V or 1V drop etc that such tables were based on. Actually, even then, the GND was usually from the engine as connecting to the alternator body or mounting bolts was too risky, or inconvenient for fast alternator changeovers. (Keep in mind, engine movements and heat that fatigue electrical connections!)
FYI - Wise people (and sites that shall remain nameless) recommend "the BIG 3" as the first step for anything - even improving standard electrics. (Like, if up to a 3V or 1V is an
acceptable voltage drop for the industry, and hence in standard vehicles...).
That's trying to make the various grounds, and the various +12V
source points, the same. IE, battery GND, chassis/body GND, alternator GND (and hence starter motor GND?) the same (negligible resistance); and battery +12V, fusebox +12V supply, and alternator output the same. (Pity that the alternator-battery fuse "blows" (pun) the same alt +12V and battery +12V voltage.)
Or that's as I see it. But the effect of that is how many see it - ie, reducing voltage drops OR increasing current capability - maybe so that wires don't melt!
{That reminds me - should a cable be thickened or paralleled? Feel it after operating. The warmer it is, the higher its resistance. Starter motor cables may get warm, but that might not matter, and it should be a short-term load only. Warmth/heat is often good way to "feel" where any high-resistance portion exists or failure is about to occur, eg - switch contacts, connectors, wires, fuse contacts (but don't burn yourself on fuses - they can normally be quite hot!) etc.}
Ideally power connections should not be to the moving/hot engine for reliability reasons. (PED: And maybe increased resistance due to higher temperature.)
But for GND, it is usually easy to mount BIG and solid/secure cables/bolts to the engine. {Don't assume continued good connectivity. I once had a
2V drop([b/!!) between 2 BIG crimped & soldered eyelet connectors with a highly torqued 12mm (~1/2") thread bolt to the engine block! A typical remove & refit (with a clean) fixed it - but only after blowing my Alpine HU's display.}
I now have redundant grounds - a few separate engine (& gearbox) points to different chassis points. (BTW - most vehicles have only 2 GND cables - battery to chassis (body), engine to chassis, or engine to battery. It is usually easy to add the 3rd.)
{Alas, a problem with redundancy - how do you know if one or more has failed? The non-monitored non-maintenance answer - when the last fails and your equipment fails or blows! But that should be very unlikely - at least for a few years.)
Some vehicles now seem to have a main body-mounted connection point, eg, heavy battery +12V and alternator +12V cables to a heavy connection block. That then connects to the main fusebox if it isn't the fusebox itself.
That's great for us
add-on'ers if it's heavy enough, else we can install our own.
It is also a good compromise between having both the battery and the alternator as the prime power source. Then take all loads from there - unless you can directly battery mount etc. But I'd argue to make the single alternator cable as heavy as possible - unless the cable voltage drop is still a issue so you still want to run 2 cables to the alternator - one to the normal stuff & one for you. But one BIG is usually better and less risk - unless it's redundant.
Crikey - there is so much involved when getting real fussy and pedantic. So much - heat versus resistance, metal corrosion, vibration, fatigue, is it a "critical" load or merely "highly desirable" when available?
But it all starts with consideration of
what you want (after some careful/learned consideration). Hence a "line diagram" of your theoretical desire (showing inter-cabling and denoting where connections exist). It's then a case of "how", and maybe using some of the above methods to achieve it.
As many will say, "
the more, the merrier". IE - extra cables will reduce the resistance (increase voltage/power available). And IMO certainly no harm
adding multiple grounds. [Grounds are often overlooked when adding new loads - there was a guy that lost $thousands when his 7 DVD players and screens etc all blew. (lol).]
But especially for the +12 side, practicality & reliability & safety play a important part. EG - maybe ditch the alternator to battery fuse in favor of physical security as done with starter motors and battery to fusebox cabling? (Was it this thread where I mentioned the dude that (IMO) created a hazard by incorporating an alternator to battery (or similar; 200A?) fuse?)
Geez, I must be in one of my "pre-awake" verbose moods again....
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