Average outputs would be 18v, 36v, 54v your stator seems to degrade but provide enough to charge @ higher RPMs
Sorry no one wants to hear bad stator but may be possible.

 

My volt meter was dropping to about 8 volts at idle but reading normal while riding. I was getting a low voltage error which would go away by increasing the rpm. Check and clean my battery cables but corrected nothing.
Took it to the Dealer at 2 pm they check it out and found a bad stator and regulator by 4:15 they replaced both and I was on my way. Over $500 in parts, Paid my $50 deducible and extended warranty cover the rest.

 

Thank God for warranties, best $50.00 spent enjoy!

 

Remove the derby and smell your primary oil. If it really stinks, it's likely your stator. A bad stator can also ruin a new regulator. Learn to do the work yourself and save money.