Just starting this research myself. However I would reason that the coil and its core will be a variable that needs to be tuned to the type, size, strength and distribution of the magnets used. For that mater, even the size and speed of the rotor.
I am going to be working up my battery knowledge for a bit yet, but do look forward to hearing what you find out.
****Its like building a puzzle but you have never seen the picture, and you only get 3 pieces a week.****