Hi Beat
Thank you for the feedback and your suggestion with the 5K-Inverters. I will look if there are available here in Turkey.
But concerning the cloudy sky problem, I will point you to the message of Nullmoden a few hours ago. Its only one example of many others I found on the web. No one is saying, that the Axpert has always a problem with that situation. But this problem seems to be happen from time to time.
He also claims, that a simple PWM charger was able to do the job.