Idling issue; From what RPM should the genny do i'ts job?

Dear folks,
To tackle a strange idling problem;
From what RPM should the genny give enough to keep the battery sufficiëntly charged to keep the engine with lights on?

XJ900F 1988.
Starts very quickly, when riding with lights 13,8V onboard.

When idling for a traffic light some time, drops to 11,0V or lower and starts to misfire.
When i turn off the light, idling is okay.

Battery as suspect or charging not enough?

 

2000 rpm.

 

that would depend on the bike and model and year, your avatar says 1988 xj900f, although i wouldnt know for sure, but does a 1988 have a brushless 3 phase alternator, these have a much lower charging speed than the earlier brush type alternator, my 1989 xj600 will still charge at idle revs around 1000rpm at 12.9v