I thought this alternator was a little higher output than previous ones, but on earlier Guzzi's, if you weren't running above 2K rpm, there wasn't enough output to charge. My initial guess would be that there is some oxidation built up on either the three connections at the alternator, a ground wire, or at the voltage regulator.
If the light goes out at higher rpm, it's not an immediate crisis. It's almost certainly not your battery.