recharge your battery via a dynamo

[simonineaston]

Given that modern dynamos are so good why don't e-bikes have them fitted, so that there's a constant albeit small charge going into the battery?

[Nearholmer]

If used during “motoring”, such a system would simply take energy from the battery, waste some in various system losses, and put back less energy.

If implemented only under human power, it would work, but add probably unwelcome resistance to pedalling, and the low rate at which a battery could take-up energy would make it pretty ineffective as a regen brake, although it might be a useful thing as a “drag brake” on very long descents.

[DevonDamo]

It's an interesting question. Nearholmer's pointed out the problem with always-on charging via a dynamo, and that the only time it might make sense is during braking. I've had this discussion with friends that use ebikes and it's always been a bit of a murky topic. A quick search on Google found the following website summarising the challenges of regenerative braking on eBikes, but no doubt technology will eventually evolve to overcome these:

https://evelo.com/blogs/learn/why-don-t ... ve-braking

[rareposter]

Given that modern dynamos are so good why don't e-bikes have them fitted, so that there's a constant albeit small charge going into the battery?

Because it adds loads of extra weight, complexity and rolling resistance which more than negates any tiny charge that the dynamo can provide to the battery.

Basically it'd cost more energy to run the dynamo than you'd get back from it, added to which batteries don't like being charged piecemeal, they need a steady current which a dynamo can't give.

Same reason that e-bikes don't have dynamo lights - you're already carrying a battery to power the bike, it's a lot easier to run the (very low drain) bike lights off that than it is to have an extra power source.

[Nearholmer]

One of the big challenges of regen braking in any application is the rate of energy transfer that is desired, which is often greater during braking than during acceleration.

Where the energy source/sink is a battery that becomes a particular problem, which is why things like super-capacitors, flywheels, hydro-pneumatic stores etc get used in some applications. Even in railways and tramways, where the power comes and goes via a collector and a fixed conductor, things aren’t always simple, because there need to be receptive loads (other vehicles in motoring phase) in the right places at the right moments, to act as sinks, and there have certainly been experiments (I’m less sure about long term installations) of line side energy stores at stations. The best solution is a “humpy” route profile, with the stopping places at the tops of humps, but other considerations often quite literally get in the way of that, and I’m not sure a cycle ride could easily be organised to have all stops at the tops of just the right sized hills.

[simonineaston]

Because it adds loads of extra weight, complexity and rolling resistance

I think that's what started me wondering. The last two hub dynamos I’ve had were light, straightforwardly reliable and added no discernable rr.

[stodd]

Because it adds loads of extra weight, complexity and rolling resistance

I think that's what started me wondering. The last two hub dynamos I’ve had were light, straightforwardly reliable and added no discernable rr.

If they added no discernible rr then they would add even less discernible charging capacity.

A typical dynamo generates 3w, so an hours pedalling would give 3wh.
That would be enough to give you 250w for a hill for around 40 seconds.
That is with no losses in the conversion, and even a typical 250w continuous motor is actually consuming much more than 250w when under load on a hill.

For comparison, typical charging from a slow charger is around 70w.

[Nearholmer]

I’ve tried up to now not to say this, because it sounds a bit blunt, but: what is being proposed is perilously close to being a perpetual motion machine, and they don’t work, however many people post videos on TikTok claiming to show ones that do.

[Carlton green]

Because it adds loads of extra weight, complexity and rolling resistance

I think that's what started me wondering. The last two hub dynamos I’ve had were light, straightforwardly reliable and added no discernable rr.

If they added no discernible rr then they would add even less discernible charging capacity.

A typical dynamo generates 3w, so an hours pedalling would give 3wh.
That would be enough to give you 250w for a hill for around 40 seconds.
That is with no losses in the conversion, and even a typical 250w continuous motor is actually consuming much more than 250w when under load on a hill.

For comparison, typical charging from a slow charger is around 70w.

In principle it sounds easy but in practise less so. I note that electric cars have regenerative braking so something should be possible but whether it’s worthwhile I’m not so sure. If something like this ever got established then it would form a load leveller over a journey and potentially reduce the size of battery required. A 70 watt dynamo (if the motor can’t be reverse used to generate power) would be pretty big and certainly would have a noticeable drag, inevitably more time would have to be spent in lower gears.

[Biospace]

I’ve tried up to now not to say this, because it sounds a bit blunt, but: what is being proposed is perilously close to being a perpetual motion machine, and they don’t work, however many people post videos on TikTok claiming to show ones that do.

It is, better with 30w of solar PV trickling a charge in whether riding or not, than making use of single figure watts from a dynamo.

Many e-bike owners wonder about regenerative braking, but the long and short of it is that there isn't enough energy involved, even in electrically assisted cycling, to make this work.

The only situation in which I could see it could work is if you lived at the top of a hill and commuted for 50 to 60 minutes downhill where several hundreds watts or more of braking effort was required to maintain a reasonable speed. Then, a direct drive motor or geared motor with bi-directional clutch could be used to charge the battery pack which could assist for the return uphill slog.

Adding a few tens of extra kilos in the form of stones for the downhill journey, to be discarded for the return, could partially make up for the inefficiencies of the energy recovery. Specific software would be required for such an installation.

[Nearholmer]

I note that electric cars have regenerative braking so something should be possible but whether it’s worthwhile I’m not so sure

I think the two cases, car and bike, have enough differences to make regen on the one very viable, and on the other probably not.

A car has a large battery, which can deliver and receive energy at a high rate, and it has the ‘tech’ to be able to blend regen and friction brakes to achieve a desired rate of deceleration. Also, regen will be most useful where most energy normally gets wasted in friction braking, in making frequent decelerations from relatively low speeds, the sort of thing that happens in urban and suburban driving. In such decelerations, the rate at which energy is dumped is low enough that the battery can soak it up, which possibly isn’t the case with, say, an emergency brake application from motorway speed, which I’d wager even on an EV is handled by the friction brake.

A bike battery is quite modest in size, and presumably can’t soak-up energy at a particularly high rate (any EAPC users present who can say what the maximum allowable charge rate for their battery is?). Even quite modest decelerations may be beyond the ability of the battery to handle. A quick bit of mental arithmetic suggests (someone please check!) that a 100kg bike+rider decelerating from 4 m/s (about 9mph) to rest in 4s will dump energy at 200W, which I have a feeling would be beyond allowable charge rate. An emergency application from the same speed might quadruple that to 800W (the achievable safe maximum deceleration rate for a bike seems to be something between 5 and 8m/s^2).

So, to be functional and safe, bike regen would probably have to be blended with the friction brakes as on a car, which implies a load of complexity (electronic sensors in the brake lever; a braking computer; accelerometers; automatic operation of the friction brakes), and a load of expensive design safety verification of the entire system, and bike techs needing a whole load of new skills and competence testing. Or, the blending might be left to the human control system, but that would involve an extra brake lever (I’m imagining it as two thin-section levers together) and some form of electronic sensor in the electrical brake lever.

As per my initial reaction, I can just about imagine it being viable as a supplementary drag brake for long descents, where one would switch it on, leave it on for the duration, and use the friction brakes in parallel to ‘fine tune’ the control of speed (always being ready to go harder on them in the event of a failure, or fade due to battery reaching peak voltage, of the electrical system).

Do BE mopeds and motorbikes use regen? [apparently some do, and the sophistication of it is rapidly advancing ….. maybe that’s how it will get into EAPCs]

[pjclinch]

There have been regen braking setups for e-bikes, e.g. BionX, but e.g. BionX going bust a few years ago suggests its game-changing level was on the order of putting rent on the Old Kent Road up to £3 from £2...

Pete.

[simonineaston]

I think that answers the Q nicely, nearholmer.

[stodd]

Yes, Nearholmer has it right.
Also for more comments see https://www.pedelecs.co.uk/forum/thread ... ike.34892/

Regenerative braking is more common on direct drive (no internal gears) hub motors where the mechanical complexity is much reduced and only the electric complications to consider. Unfortunately these motors are inherently heavier and much less efficient for normal ebike use, and they still only get limited return on the regen.

[PH]

Putting aside the science for a moment, I'd ask who wants it? I understand why range anxiety is a thing for EV drivers, why the costs involved in charging are significant, why commercial operators look for maximum usable drive time, why a flat battery could be a serious issue, but for the average E-bike user where's the advantage?
If I were to gain 10% in a days riding, which seems pretty optimistic, that would increase my range from 50 to 55 miles. How many riders would that make a difference to? The cost saving would be less than 1p a day, so that doesn't tempt me. For the additional cost of a dynamo and charger I could have chosen a bike with a bigger capacity battery. If I really wanted to push what was possible, I'd have bought a model with two batteries. A couple of times in three years I would have liked more capacity, but it would have needed to be 30 - 50% more.
Look at the most popular hybrid style of E-bike, the weight, size, riding position, tyres, luggage.... don't you think there would be easier ways to increase range by 10%? That most riders don't consider this demonstrates to me they see no need, I have tyres on my E-bike that I probably wouldn't consider if it weren't for the motor. The other style of E-bike, those intended to be more like non assisted bikes, tend to have a lower capacity battery, wouldn't that be the first consideration if more range was required?
If regen braking was free to implement and use, why wouldn't you, sign me up. If there's any cost, in ££ or effort, we'd have to match the cost to benefit and IMO the benefit for most riders is zero.