Only charge the battery from the terminals under the hood.
Here's why.
The charge circuit in the car limits the amperage to the battery to prevent damage to it based on age and condition. Bypassing this and charging directly to the battery will make the circuit think it's weaker than it is and then limits the alternator charge amperage to the battery while driving. Over time the battery will create a memory at 70, 80 or 90%. Who knows what the value will be, but it will be less than full capacity.
Here's how I know:
Before changing my battery, I tested charging the old one from the under hood terminals. It took .10 amps. At the battery it took 4amps.
After swapping the battery: it charged at 1 amp at the battery (before install) and
1 amp under the hood(after installing). After registration it took 2 amps from under the hood.
So the car circuit upped the voltage to the battery after registering it. I would expect after some time the charge circuit will determine the actual values and bring the voltage down. This will be the charge voltage from the terminals as well as the alternator.
This car is not your average bear....
It has a dedicated charge circuit that is separate from the rest of the car. In most cars the battery goes to everything all the same. Meaning all circuits get the same supply voltage. If a circuit needs say +5v, then that circuit has a voltage divider as a part of it, old school is a resistor, new school if a transistor or MOSFET.
The 6 series, and others I suspect, sends power directly to the battery with it's charge circuit. AND I suspect there is a car buss that is supplied by the alternator and the battery. And maybe even smart with switching to maintain constant voltage to other circuits.
I could dig into this further but why. I'm done and know what I need to. Besides I have another project to work on.
All software BWM pertaining to the 6. Stay tuned.