The myth of free energy
Oh ye seekers after perpetual motion, how many vain chimeras have you pursued? Go and take your place with the alchemists.
— Leonardo da Vinci, 1494
I keep on seeing articles referencing a new (proposed) ebike on Kickstarter that can “charge itself while you ride” and it drives me insane. I also field lots of questions about ebikes from people who think that all bikes should have regeneration and they’d be stupid not to. My bike does indeed have regeneration but it is largely a gimmick. Not entirely useless but not exactly useful either.
Here is how I explain it, and generally most people get it and wander off vaguely disappointed:
- Most ebike chargers run at 2Amps and take around 6 hours to fully charge a battery. That’s using mains power.
- The system runs at 36V so it’s theoretically pushing around 72watts of power into the battery.
- If you were to generate 72W with your legs, you’d notice it like a decent uphill or the worst headwind ever.
- Going downhill 72W against you would be like putting the brakes half on.
- So the opportunities for recharging are going down steep hills and stopping. On a two hour ride (which is probably how long it takes to flatten the battery) that could add up to maybe 5 minutes.
- So if you could get 2A (72W) recharge, it would extend your battery life by 5/(6*60) = 1.4%
- Snag is, batteries and chargers are inefficient so you might not actually do as well as that.
Disappointing, eh?
The particular VELLO bike referenced claims: “The internal 160 Wh battery pack can also be recharged via an external power source, with a charge time of approximately 3 hours.”
What? A 160Wh battery is miniscule, and if it takes 3hrs to charge on mains…. heck, I’m not even going to bother explaining this.
No wonder it is the world’s first first self-charging folding bike. Because it is an impossible goal. Go and take you place with the alchemists.
Edit: the only motor type that is practical for regeneration is a direct drive. They are large, heavy, provide less torque and are less efficient than geared hub or mid drive motors. That seems at odds with the goals of the Vello.
Update 6 Apr 2017: Now there is the Volta, “the bike that doesn’t scream ebike”. It has “40 miles of power” from 2.9Ah. They have been very economical with the truth on their Kickstarter and FAQ. Here is a clip from an online forum where the answer is “science”. Yeah, whatever…
A fit cyclist can easily produce 200+W on flat ground at steady effort, so your 75W is out by a factor of 3x. And given that is likely half the ride or more, then the 5min you quote is out by a factor of 12x. Therefore I’d suggest the self charging could displace hours of charge time. The bike only needs to support uphill effort not flat or downhill to be really useful.
Right, two problems with your analysis 1) You need motive power AND charging power in your scenario, so if you want 200W to ride with and 200W to charge the battery, that’s 400W, pro cyclist territory. 2) Most people riding ebikes don’t want to produce much power, and certainly don’t want to be penalised further by having to charge a battery — that’s why most ebikers actually prefer a throttle.
Tell you what, you ride with 200W extra resistance for half your ride and see how much you will yearn for your regular bike…
My own bike has regeneration capability so I do speak from practical experience.