Controlling alternator load for peddle bike
Alternators are given an excitation current, which controls the torque versus speed.   There's no benefit to be had in changing the batteries' load (just
trickle a little more or a little less  excitation current to control
the speed-versus-torque curve).

A normal bicycle cadence is something like 1 revolution of the pedals per
second.   So, consider biasing the alternator so that low speeds (say,
one third revolution per second) offer low power (30W, for instance) and
highest speeds (which are associated with best blood circulation
in the leg muscles) give 300W output, at about 1.3 rps.

A racer might top 2 rps, so an adjustment range for different riders is
appropriate.

Output current of an alternator at the various speeds can be measured
at a variety of excitation currents, and an excitation current versus
average cadence can be implemented to make any desired curve.   But,
you cannot just DO this theoretically, someone has to get on the bike
and decide what's comfortable and productive.