We spent the Saturday morning at the car mechanical. Rather useful use of time. During our travel from Darwin to Broome we collected an error list of 6. It turned out that one of them was a 'false alarm', caused by our lack of knowledge. But the others seemed to be valid and resulted to change both(!) of the batteries.
In case you are not familiar with camper vans: There are a battery in the front that starts the engine. And an other one at the back that operates other facilities: fridge, water pump, inside lights in the 'living' area, etc. The second one is charged both by the engine and the power cable in camping places.
So we were a bit surprised that the hire company let us take the car with bad batteries though they were fully aware that we travel to Perth. We even reported that the car hardly start and we suspected that there could be problem with the battery but they convinced us that this is normal.:(
But fortunately we reached Broome even with this conditions. And the staff in the service was really kind and helpful and started to examine the campervan promptly and I believe that they did whatever they can for us. Thanks for them in this way as well.