I am working on the same thing right now.
Have you tried just 4 diodes, 1 diode for each coil, commons to ground?
It lowers the voltage and raises the amps.
I bet you are very suprised what it will do into a 12V battery compared to a pair of Ni-Mh (3V).
Connected like yours...
One of mine at the same RPM, is 50V open also.
With 3V is 64ma.
With 6V is 64ma.
With 12.8V is 55ma.
No explaination for any of that.
High coil resistance maybe.
Another is 25.3V open.
With 6V is 100ma.
With 3V is 105ma.
With 12.8V is 63ma.
That makes a little more sense.
Definitely watch those bearings!
G-