Justin, could you improve the regen capture by using an ultracapacitor as intermediary, with an ultracapacitor such as JCG uses here:
I think that my first reply perhaps didn't quite address what I think you were getting at so I'll try again here. One of the main reasons that ultracapacitors are touted is because of their ability to absorb very large currents that a battery couldn't safely handle. For instance, an ebike and a rider (~100kg) together moving at say 40 kph have a kinetic energy of about 6kJ, and if you wanted to come to a stop in say 2 seconds, then that means absorbing some 3 kW of power on average. No ebike battery could take 3kW of charge current, so better absorb that with an ultracap right?!
But what I was showing with parabolic regen battery current curves is that for high rates of regen braking, the amount of regen current that actually flows back into the battery starts to decrease with higher braking forces. If you want to come to a fast stop, then just short all the windings, and you don't have to worry about any excessive recharge currents damaging the battery. This also illustrates how if you do want to come to a fast stop, the efficiency of the regen is quite poor, simply because high regen torque means massive amounts of I^2R losses in the motor winding, so increasing the rate at which you can recapture energy (as supposedly an ultracap would allow) actually decreases the total energy you end up getting. On the curve that showed the % of kinetic energy recaptured, you can see that beyond 12 amps of regen current, sure I could come to a stop faster and waste less energy to wind and rolling drag, but I would actually get fewer watt-hours back in the source because the generating efficiency is pretty low at these kinds of currents.
So even if an ultracap would allow me to do a maximum regen current of say 20 amps. Yes, I would be able to do more regen and come to a faster stop, but no, I would not get any more energy back. In fact, I would recover quite a bit less energy than had I come to a more gradual stop at 6 amps regen using just a battery to absorb the energy. So the idea that an ultracap could help 'improve' the percentage of recaptured energy is quite false in this case.
It is interesting to note that for similar reasons, if you have both mechanical brakes and regen brakes and need to come to a quick stop, you can actually get more energy back to the battery by using both the mechanical and a more modest regen together, than you would coming to that stop purely on regen alone.