In this picture i dont understand why is the 2nd battery (if it was not a starter battery) not connected with a big red cable to the plus pol of the 1st battery. 
asked
Many users use batteries in parallel connection without connection to Starterbattery. Why is there no diagram in the manual for the BMV712smart that shows this circuit variant.
When there are two or more batteries connected in parallel (or in series), the batteries are considered as one battery. That's why it's enough to just draw one red thick cable.
is this the right connection?
No, that's wrong. I will update it once I know what the Schaudt EBL 252 is, a charger or a battery?
Seems the EBL 252 is a charger or something like that, so it goes to the load side of the shunt. All loads/chargers minus go to the load side of the shunt. The negative from the one battery connects to the negative terminal of the other battery and then to the battery side of the shunt. Nothing else connects to the minus of the battery. If you connect more chargers or loads, then you connect the negative wires of the new charger/loads to the load side of the shunt.
I updated the your image:
Hi Stefanie, many many thanks for your help. I understand this update an i will change my connection yet.
