On mine I have a Supercheap Autos solar panel connected to a Bosch plug I have fitted to the side of fairing & connected to the battery directly.I have the panel hanging in the window of my garage & this seems to keep battery up to scratch well. If left alone the battery seems to discharge in 2 or3 weeks.

I've never had a problem with BMW bikes and there batteries and I never charge the battery when lift for 3-4 weeks.