Originally Posted by Doc_Wui

Justin Le, who own Grin Ebikes, has numbers that show regen gets you 10% more battery in hill country, He is pretty credible. Last year, he competed in a solar bike race where they started in France, crossed the Alps and coasted downhill to China. Regen sounds good til you realize that's 5 miles if you have a relatively robust 50 mile battery. Then again two miles is worth it if you don't have to pedal a heavy unpowered direct drive motor with its magnets dragging.

Some outfit in Europe, might be academic, has a sleek looking fuel cell prototype on a bike. Needs tanks of O2 and H2.

The long range commuter, and the touring biker will have their concerns about battery life. The guy who rides for fun, like me, can either plan his ride for the battery size or carry a spare.

I think the original poster could discuss what kind of bike he wants to build, and what it will be used for. The type of motor is the first thing to sort out. Motors are not too expensive. Good batteries are the biggest expense. If you do a conversion, you can always use that battery on the next one, and then you can try out different motors w/o spending a lot more money.