Elektrik: zwei Batterien im 750i - Fragen

[Sinclair]

I have a slight different view, please correct me, if anyone knows better:

0. car is not started, but only measurements are done
1. if I measure voltage across the motors' jump start poles, I get 12.8 V
2. if I measure at the cigarette plug inside, I get 12.1 V
3. if I start the car, after 3 sec the system voltage ramps up to 14 V
and the smaller board battery is charged, too.

I conclude:
a) at point 1), see above, I measure the larger starter battery
b) if the car is switched off, the two batteries are not connected via the switch, they are electrically separated.
c) the smaller board battery on-top (see point 2), is charged during normal running, together with the larger starter battery, using the automatic interconnection switch, then closed.
d) if one wants only, to recharge the smaller board battery (70Ah), then it is usefull, to connect the external charger directly to the bord battery. I use an additionally build-in 12V connector to that battery (where I otherwise connect e.g. a cooling box), the additional connector is 16A-fused for safety.
But be careful, to stop "constant current charging process", when 14.2 V are reached. I recognized a buzzer, when voltage is higher than charge end voltage ?
e) I'm not sure, if by charging the large starter battery (110Ah) using the start poles at the engine compartment, that the smaller board battery will be charged two, because the separation switch seems normally opened, when the car is not running.