Ok, batteries...
If the +ve current sensing loop is around the one & only battery +12V cable EXCEPT for the startermotor +12V, then (I'd assume) it is for battery monitoring.
Based on your wiring description where the alternator output splits to the battery and to everything else (ie, fuse box and onwards), the battery supplies all loads aka everything-else (except startermotor) via the alternator.
If so, if you take a feed straight off battery +12V you will stuff up whatever
they are trying to do with their battery current monitoring.
IMO they would therefore have warnings or notes NOT to tap from the battery direct. But if you did it probably means the EMS or BCM etc does not reduce the alternator to float voltage and hence wastes a MASSIVE 1/10th horsepower or whatever.
And I guess battery life will be compromised... they won't last more than the normal 6-10 years that ordinary wet cells get with non-current limited fulltime charging at 14.4V or 14.2V or alternators that drop to lower voltages (eg, 13.8V) depending on temperature else after a delay.
With dual batteries, the usual rule is that they do not have to be matched unless permanently paralleled.
Certainly parallel charging 2 or more mismatched batteries will not matter (
**) and keeping them paralleled whilst supplying a load depends on preferences - eg, max capacity or minimal battery strain for winching or big audio versus the desire for an independent cranking reserve/battery.
My (
**) above is to note an assumed caveat - namely that neither battery suffers any cell collapse. (And that alternators have reasonable output capability.)
But collapsed batteries should NEVER be charged and that is irrespective of parallel battery(s) - though a collapsed AGM paralleled with any battery can be a recipe for disaster.
There are those that insist that paralleled batteries must be matched. That is true for permanently paralleled batteries but my (UIBI or) battery isolator and other multi-battery discussions are not about prime movers etc with such permanent series &/or parallel battery configurations.
One avid such claimant is unable to explain how one battery can discharge into another while both are being charged (ie, >13V) off the same alternator despite that being the basis of her or his "matched" argument. Furthermore they support
single battery isolators (including diode types!) yet have never explained that they must be in EACH pole and that the batteries must be co-located and symmetrically connected (ie, +12V or +24V taken from one and GND from the other (string). IOW it merely prevents parasitic loading and collapsed cell issues but can NOT be used for those wanti8ng independent batteries as most of us want - eg, to run fridges, PCs, audio, etc.
Alternatively they have not explained at what point batteries become mismatched, nor how to determine that. It's as if you buy 2 matched batteries you'll be fine - it doesn't matter if one is in the engine bay and one in the trunk; how they are connected or isolated, or if one is used for loads whilst isolated. I presume they magically continue to be matched (LOL!!). [Ah yes, these young(?) KIAs have a right to arrogance and flaming.]
If BCMs & EMSs do vary charging with battery type & age, then you have problems.
If you substitute an AGM for the wet cell cranker I wonder if the alternator will increase its output to 14.8V or whatever is recommended for the AGM (generally higher than for wet cells).
Or if it was an AGM, was it charged at 14.6V or 14.8V and will it reduce to 14.4V for a wet cell or just corrode its plates to an early grave?
And if you substitute a 7AH or 15AH cranker will it lower its max charging current?
I suspect no to all the above.
Smart chargers can (allegedly) do all the above. So are vehicle manufacturers now adding the cost of smart charging so we get extra life out of our batteries?
So I get back to what I hinted at earlier - wth are
they doing with their current sensing? (Or ytf are they doing it?)
The only stories I have read or heard IMO do not stack up. Some are (IMO) obviously self contradictory. Others MAY have merit, but either battery understanding and technology has recently leaped else they have tightly defined parameters for battery replacement do not allow added loads direct off battery terminals.
And whether there is any
practical or profitable point in doing the above for lead acid batteries... Lithium batteries yes, but then you wouldn't be bothering with lead acids; their behaviour is simply too different.