I have had a similar issue recently. Battery went flat, cause traced to faulty rectifier.
This a known issue, especially with the original two point mount system.
S&$ people in NZ commented that standard unit was "delicate" and that a Harley unit would be more robust. This was put onto my M3W and adapted to suit the new three point mount.Cost same as stock unit better delivery on HD unit. Mind this is NZ at the end of the world !
I believe that another car in NZ has also had this issue and had the same repair
