I'm with the collective, this sounds like a failing, probably sulphated battery, that has been left too long in a mid/low state of charge at some point. Those of us who have ever dabbled in used cars/bikes, machinery that has possibly been laid up for several months, will be all too familiar. These days I fit a new battery as a matter of course & factor the outlay into the cost of change calculation.
This is a useful guide from Varta, admittedly car oriented, with statistical data from ......... you guessed, Germany: -
Discover your car battery. How to test with guidance from VARTA®. ► Get instructions for checking battery health and performance now.
www.varta-automotive.com