@Andy: Thanks for your suggestion. It will, however, probably not be possible to increase the amount of solarcells and SLA batteries are quite heavy (I am to use this for a bike trip). But I see the point in your idea.
@Crutschow: Hmmm...Thank you also for your ideas. I will consider your suggestions, although I had hoped that there might be a simpler solution like adding a few components and maybe making a few more connections ... Ideally, I would like to find a simple solution that measures the available power on the input and adjusts the output accordingly but I don't know if that can be done in a simple way?
'Best for your weekend,
Jesper