The common Bose Soundlink Mini II battery charging issue. And a solution.

Bose Soundlinks tend to lose the ability to be charged after about two and a half years - combined with a repair cost of about 80$, this is really not nice, especially as a new machine would cost much more.

What however works for me is a downgrade of the formware version. Or, better to say, re-grade, as only the current formware (1.1.4) can be installed on the update page https://btu.bose.com/. So you go to that page, plug in your Soundlink Mini, and it tells you you actually have the formware you need. Then we need a little hack. Press the following secret combination while being on the update screen in order to enable options for downgrading: 'a' 'd' 'v' '<up arrow>' '<down arrow>'. Do the downgrade/upgrade/regrade (which takes about 2 min), and the Soundlink will charge completely afterwards. 

Well, not the best method since you need to do it always but.. better than 80$ :)