I think the general equation for an electric motor is :- Vapplied = Zloss X I + Vback emf. So if the rotor is spinning off load Vback emf is very close to V applied( 95%), because the running current is low. As the rotor is loaded it either slows down or its phase slips so Vback emf is not in phase with Vapplied, and the difference is Zloss times the increased running current. So I would think that something like 90%+ Vrunning should be available. As the braking in vehicle is normally about 5 times faster then its acceleration, you have to boost the generated voltage to charge the battery at about 5 times the acceleration current. This is also required as the vehicles speed falls.
Frank