Originally Posted by
Darth Lefty
You are nibbling at the edge of the problem, but it's not just the battery. It's power. The amount of power it takes to go slightly slower downhill is the same it takes to go that amount slightly faster uphill... a hundred watts, for a bicycle? How long is that downhill, a minute? If you were in a vacuum. But you are still making drag, too, you don't get that back. Single digit watt-hours at best. If you wanted to use the motor as a generator for The Brakes, to recover that, just think how much faster you can, must, slow down than you can speed up. It would have to be a lot more powerful, several times more powerful than the forward drive, to make as much power as the friction brakes. And then it would be on the rear, in most bikes, which is not where you want brake power. It then recovers some % of your ~1wh kinetic energy.
As the vehicle gets heavier, more aero, faster, it will make more sense.
I’ll have to repeat the experiment, but my recollection was that I added around 5% of charge to the 370wh battery on my scooter during a very controlled 5 minute descent. that suggests a higher charge rate than the thing averages on the wall charger (which takes around 180 min to fully charge, not 100 min) so perhaps there are some battery level reporting shenanigans.
one minor (maybe) point for a bike is that the potential energy from the descent is not symmetric to what the motor contributed going uphill - it’s symmetric to what the motor PLUS the rider contributed, minus drag and friction of course. if I ride up a 10 minute climb at 10mph by putting out 240w with my legs and 60w from the motor, and then descended at 10mph for 10 minutes the potential energy available is far more than the 10wH that the battery contribute to the climb, it’s some significant fraction of the full 50wH expended, again, minus drag and friction and waste heat and noise and all that. this is actually the way I ride my turbo creo, with the assist in the 50-75w (1/4 of my 200-300w) range, and completely turned off otherwise… but of course I get nothing back riding the brakes on a long fast descent.