If the 17 vdc was the output without the charger connected to the battery, I think that is about the voltage to be expected.
A 12 volt battery needs the charging voltage to be at least 14.7 volts to become fully charged and without a battery connected to absorb the RR's output, the voltage will read higher.
Even with the battery connected, a charging voltage up to around 15.8 volts isn't uncommon for a motorcycle charging system.