some nice volts per rev with that. high resistance though. that resistance could be from many turns of tiny wire in a small package? (not so good) or many turns of larger wire in a larger package..have you looked at the wire? just a 2 wire brushed motor? with maybe some other wires for the encoder..was there a brake on the thing?
folks may be hesitant to comment with that "pulsed" amp rating? think that may be calling out a drive spec for running the thing and would guess that a brief (fraction of a second pulse) will not say much about the continious ratings of the motor.
could easily be all wet here, but using the 6 ohms we might expect a short circuit current of 3 amps @ 200 rpm. (18/6=3) and about 660mA for charging current into a 12v battery ...18voc-14v needed to charge=4volts/6ohms=.66amps and .66x.66x6=2watts (I*I*R) "heat" and .66x14=9watts charging
at 300 rpm it might look like this.. 300rpm x .087volts per rev =26voc and 26-14=12/6=2amps charge into a 12v battery. 24watts of "heat" in coils and 28watts charging.
at 500 rpm? 500x.087=43voc 43-14=29/6=4.8amps charge..with 138 watts of heat in the machine and a charge of 67 watts.
500 rpm at 24 volt charging? 43voc -28v=15/6=2.5amps and I*I*R=37 watts of generator heat and 70 watts of charging.
i'm sure there is more to it than that but a close guess?
perhaps a "more experienced eye" might jump in to help us both out in our understanding. i'm not afraid of being wrong as long as i can learn from it!