| Sounds like it could be a alarm prob, as mine was (see post above).
It pays to check that the voltmeter still reads 14-ish volts with engine running when the engine is hot.
The voltmeters as recommended by Shazaam are a useful idea, but, be warned, they are not at all waterproof, and if left out the rain, or even during enthusiastic bike-washing, they will clap out. Best not to fix one permanently to the bike, I think.
Don't forget to examine the famous yellow wires, and their connections, between the alternator and reg/rect (behind the battery tray). | 
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