Interesting idea in theory, but probably less so in practice. Regenerative braking rarely makes sense on bikes. Humans and bikes just don't weigh enough to hold a lot of kinetic energy, and few bikeable hills are tall enough and steep enough to require continuous braking. Unless the grade is really intense, I'd rather just coast at a fun 30mph.
this post was submitted on 21 Nov 2024
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Keep in mind, they are using super capacitors, not batteries. They can charge VERY QUICKLY by comparison, especially when pedalling. And they have much greater longevity (think decades rather than a few years) vs lithium-ion batteries.
If this project works, it would be a very exciting thing for e-bikes. Even if they aren't giving you the same range as a traditional e-bike with a battery, they could offer e-assist or “hill assist”, and that alone could attract a lot of new riders.
Interesting point. Did some napkin math on that:
If a 100kg person + bike was going 50kph and came to a stop in 5 seconds, the average power generated would be 2kW.
I consider that to be a fairly extreme circumstance and requires 100% efficiency. A typical e-bike battery is around 500Wh, so that's a 4C charging rate which isn't really that extreme especially if it's only maintained for a few seconds.
Now in a more sustained situation, it might be helpful. If you're trying to maintain a comfortable 30kph while descending a 45% grade, that's a continuous 4kW that needs to be shed. I suspect a lot of it would go to wind resistance, but if 100% of it went into your battery, a standard lithium pack may struggle to keep up.
So maybe it'd help in extremely hilly situations, but I'd still like to know what it weighs. Supercap specific energy per wikipedia is around 90Wh/kg vs 270Wh/kg for lithium ion. So a cap weighing the same as a 500Wh e-bike battery would have a capacity of around 160Wh.
In the above descending example, it would be fully charged in 2.4 minutes.
So I guess it depends on the scenario.
The other thing to consider is comfort. It's generally more comfortable to coast to a stop on a bike (burn off energy to wind resistance), and assuming the bike has conventional brakes, I wonder what it would feel like switching from regen to friction braking.
Can you explain what is meant by a "4C charging rate"?
The current a battery can accept/provide tends to scale with the size of the battery. So rather than measuring it in Amperes, it's usually measured in C. 1C means that the battery will be full charged or discharged in 1 hour. 4C means it'll be charged in 1/4 hour. The charge rating tends to be lower than the discharge rating though it depends on the cell.
Basically, you take the battery capacity and divide it by the charge rate. If you exceed the C rating of the battery, it could cause it to overheat or even catch fire.
When it comes to higher performance requirements (like in drones), you can get insanely high C rates like this though those batteries aren't typically designed to survive too many charge/discharge cycles.
Thank you!
I think regen braking is fine on a bike, but it's mostly just about descent control and saving wear/tear on your disk brakes, the nominal charging done is just icing on the cake (at least with traditional batteries, maybe it's more efficient with caps?)
I think efficiency would be about the same between caps and batteries. The issue is that both are heavy. Capacitors moreso for the same capacity though this application isn't looking for a long-haul energy storage solution.
when I was originally pricing out parts for a diy build I was looking to implement regen as a way to maintain a slower speed downhill without riding the brakes. I feel like that on its own is worthwhile if it doesn't add weight or make maintenance harder.
Definitely adds some weight IIRC, but not a crazy amount.
Bicycle riding is very efficient and effortless at 10-15 mph, if it wasn’t for those pesky stops. This is a great solution - even if it only gives back 75% of the braking energy. For urban utility, a bike really only needs to boost up to 10 mph and then I can take over.
I think the real killer for biking is anytime I hit a steep hill. Doesn't matter if I'm slow or fast, I lose so much energy climbing hills. And the wattage for climbing even at slow speeds can be high enough that smaller motors struggle.
The only hills around me are bridges over rivers or highways. But for those kinds of hills a super capacitor bike may still perform well.