I see little reason in having having an ammeter permanently installed on any BMW motorcycle, unless you have special circumstances. I foresee several drawbacks, not the least of which is a potential fire hazard, which includes burning up your bike’s wiring; besides the usual situation of LESS reliability with many modifications; especially with added electrical items.  I will lay out the benefits, and the problems, for both ammeters and voltmeters. The GEN lamp provides quite a bit of information if you interpret it correctly….but it may not be enough FOR YOU.