Triumph Terrier 150cc; a smaller Tiger Cub.
It didn't charge on high beam (a whopping 30 watts). It had a Lucas RM12 alternator (a whopping 60 watts).
After enormous trial and error I found it had the wrong wiring system, via the dreaded PRS8 switch. I rewired it and it charged perfectly, according to the ammeter I'd installed by then.
I don't blame Lucas; I blame Triumph.