I am working from my experience, with car type alternators fitted to motorcycles (Moto-Guzzi, BMW R series (2 valve) and Triumph (Hinckley era).
On all of them I have experienced regulator failure, where a broken D+ feed results in minimal charge. Residual magnetism in the rotor means that charge happens, but because the field isn't being boosted by the regulator feed, it tends to sit at around (battery +0.2v). Which works with no real load and enough revs, but the system dies quickly with any load.
The Bosch generator on the Moto-Guzzi and BMW uses separate external regulator and rectifier assembles, and police spec regulator units with a higher set point are available and common retro fitments on heavily electrically loaded bikes.
The Triumph alternator is a ND unit with an integral regulator / rectifier assembly. Because of the compact size it costs over £100 and is only potentially interchangeable with a Suzuki / Kawasaki part from a single model, costing not a lot less. When mine blew the regulator, I looked at the schematics and mechanics and ventured £15 on an external regulator as fitted to the Guzzi. I insulated the alternator internal feed to the D+ terminal and substituted with a feed to it from under the seat, linked to the battery and switched live circuits. The result is a consistent 14.5v charge rate.
