So you're on your ebike, going 15mph. Using 140w or so. You're spending 15-20w on the drivetrain, and the remainder is entirely aerodynamic drag. You're putting 120-125w into making wind move. You're also losing 5-10 watts to the drive electronics and dashboard. So your total power use is 150w.
If you're going to recharge a battery. In the same time it takes to ride, you need to get at least 150w of power into another battery. Sadly, batteries don't actually "just cleanly charge" there are some losses, but since we're going to take an hour to charge it, lets call it 5%. So to fill up that battery we need 157watts of input power.
Your bike, moving at a steady speed, is absorbing 150w of power. If we want another 155w of power, where is that going to come from? If we take it from.. say.. the front wheel, we are now absorbing another 155w of power. So to maintain the same speed, we now need to push the bike along with 305w of power. And.. now we need a bike that makes 305w of power, to go the same speed we were going with 150w. .... And we're only generating 155w.
There is no free lunch. If you're doing work, you need to get that power from somewhere.
needs more upvotes