Meanwhile we are in Carinthia at my mums and stepfathers house and have a nice time. We really needed it after all the stress in Innsbruck in the last days.
The electrical system was finally connected to the dc-dc charger which gets 12 volts from the car and gives 14 volts at the output. The charger itself is connected to the battery of the car, giving the car the information that the main battery is not fully loaded, so that the alternator keeps charging while the car is running and charges the batteries in the back as well. Should take eight hours of driving from very empty to full. The charger is rated for 10 hours of heavy load usage, so this should work out. Within 20-30 mins of driving I could gain approximatly 6 % off load, so the calculation should work out.
I installed a fuse and a main switch so that the charger does not empty the cars main battery. It can work with key+ as well, but i don’t thrust this. If we want to use the car stereo or sth else, where the car needs to be on (without beeing started) it empties the main battery quickly. A good old switch should be the better solution (if we don’t forget it after driving :), but it gives an overload warning beep as soon as the electrical system in the back is pulled on, so it should be save). So there are three switches now – two which connect the two batteries to the power consuming stuff as well as the solar charger/battery computer and one which connects the dc-dc charger.