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Few electric bicycles have regenerative braking. Most quality e-bikes seem to be settling on mid-drive, which even makes regenerative braking impossible because there is a freewheel between the wheel and the drive, and the rear derailleur as the chain tensioner!

Why is this the case? Why are regenerative brakes rare?

Criggie
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juhist
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    I will note that older battery-assisted bikes often had the ability to charge the battery when, eg, running downhill. I'm guessing this feature is not particularly common anymore, though. – Daniel R Hicks Aug 24 '20 at 22:04
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    The lead acid batteries in old bikes are much more tolerant of having arbitrary amounts of current driven back into them without a lot of fancy thermal management and safety circuitry! – Affe Aug 25 '20 at 16:40
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    Your impossible because there is a freewheel assertion assumes that the drive motor would be doing the regeneration; it's conceptually possible (and likely preferable) to have a generator elsewhere on the bike generating electricity (and thus providing speed retarding work), and all you need to do is carry the electricity somewhere; significantly easier to do than carrying mechanical effort that is being defeated by the freewheel – Caius Jard Aug 26 '20 at 19:33

6 Answers6

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There are several reasons for regenerative brakes not being common on bicycles unlike in electric cars:

  1. Electric bicycles have very poor acceleration, and the rider produces about half of the acceleration with the motor producing the rest. The power is limited by the low-speed motor, not the battery. Most quality electric motors like to spin at 5000 - 20000 RPM, but in e-bikes they do not spin at any such speed range. The best RPM situation in electric bikes is in mid-drive bikes that have enough space to use reduction gears, and the mid-drive arrangement does not allow regenerative braking due to the freewheel. The worst RPM situation in electric bikes (lowest RPM) is in hub motors, the only arrangement that would make regenerative braking possible. These hub motors cannot produce high forwards torque, and therefore, cannot produce high inverse torque for braking. In contrast, electric cars are well-known for their high acceleration, thus allowing rapid braking. An electric bicycle with regenerative braking could produce only half of its poor acceleration as braking force, and it is not at all significant.

  2. A common use case for regenerative braking in electric cars is going downhill. Most car drivers drive always very near the speed limit (unless traffic condition or safety requires otherwise). In contrast, most bicyclists ride at a very low speed that could be much higher. A car driver going downhill needs to brake to keep under the speed limit, whereas a bicyclist often uses a downhill as an opportunity for going fast. The most common braking at downhills for bicyclists is a 90-degree or hairpin turn after a downhill, and to maximize average speed, most bicyclists only brake very close to the turn, instead of braking all the time before the turn. Thus, a downhill is not a good location for an electric bicyclist to brake.

  3. Another common use case for regenerative braking in electric cars is stopping at a red light. In contrast to cars where braking close to red lights instead of anticipating the traffic lights far away is common even for internal combustion engine cars lacking regenerative braking, most bicyclists have learned an energy-saving way of going through traffic lights, avoiding unnecessary accelerations to high speeds only to need to brake right away. While a regenerative brake could be useful for an electric bicyclist adopting a different rapid start - rapid stop riding style, most bicyclists do not consider such a riding style necessary.

  4. The final killing blow making regenerative braking infeasible is the controlling of such brakes. In most cars, there is natural engine braking. Some electric or hybrid-electric cars (Toyota) simulate the typical engine braking amount with regenerative braking, while at the same time having complicated (expensive, heavy) machinery to adapt between regenerative braking and disc braking using the brake pedal. This approach has the advantage of familiarity for most automatic transmission car drivers. Such an expensive, complicated and heavy machine would not be acceptable on a bicycle. Other electric cars (Tesla) cheat somewhat and make the engine braking huge, to avoid having the complicated (expensive, heavy) machinery to adapt between regenerative braking and disc braking when the driver presses the brake pedal. In contrast, high-quality bicycles have extremely low rolling resistance and no engine braking -- there is a freewheel. The natural slight regenerative braking would be of no value in such a bicycle. The braking controls of a high-quality bicycle are independent levers for front and rear wheels. A regenerative brake would work on only one of those levers. Most of the time, the bicyclist uses the front brake only, and most quality hub motor electric bicycles are not front wheel drive for a good reason (when going uphill seated, the front wheel is practically unloaded and would slip). Even for a bicyclist that brakes with the rear brake, there would need to be complicated (expensive, heavy) machine to operate both the regenerative brake and the disc/rim brake with the same lever. Cyclists would not find such a piece of machinery acceptable. Thus, the only way to make regenerative braking possible would be to add a third regenerative brake only control working only for the rear brake. It would be a terrible brake due to the low torque of hub motor drive. Such a brake would be unused for most of the time.

Also, to produce useful torque at low weight, an electric motor needs to spin fast. The only arrangement making the drive motor spin fast on an e-bike is mid-drive, where the motor drives the bottom bracket through reduction gears. Such an arrangement does not work with regenerative brakes due to rear derailleur being the chain tensioner, and due to the freewheel in the rear wheel.

Because regenerative brakes have terribly low power (reason #1), no valid use case (reasons #2 and #3) and no feasible means of control (reason #4), and because mid-drive is the most logical drive variant in e-bikes that makes regenerative braking impossible, regenerative brakes are not feasible on e-bikes unlike they are in electric cars.

juhist
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    Practically any hub-motor eBike you can buy pre-built in a store in 2020 is a geared motor. Direct Drive is almost entirely the domain of hobbyists trying to DIY regenerative bikes at this point :) (answer seems to imply only mid-drive bikes have internal reduction gears.) – Affe Aug 24 '20 at 18:45
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    Other lesser consideration: almost every electric failure mode of a retail 2020 hub motor eBike leaves you pedalling home on a rather heavy bike. Regenerative systems tends to have plenty of failure modes that leave you calling a friend with a pickup. – Affe Aug 24 '20 at 18:47
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    imo #3 should be #1++. Even if you solve every engineering problem, cyclists prefer to coast and are much more flexible about utilizing coasting that cars. Until every component of the electric system reaches some insane ideal of perfect efficiency coasting will always be a more efficient thing to do with your energy than trying to recover some portion to then use re-accelerating. – Affe Aug 24 '20 at 18:54
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    What is the evidence for #1? My ebike can accelerate with my producing almost no power. – kmm Aug 24 '20 at 20:26
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    I’ve got regenerative braking on my ebike, and the equipment is neither heavy nor cumbersome: The levers have sensors that detect when the brakes are squeezed, so that the regenerative brake kicks in when I first squeeze the lever, before the pads make contact with wheel rim. – RLH Aug 24 '20 at 21:11
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    @RLH Are they special brake levers, or generic ones you just attach a sensor to? – MaplePanda Aug 24 '20 at 23:33
  • @MaplePanda The ones I've got are special levers, https://www.ebikes.ca/shop/electric-bicycle-parts/ebrakes/ebrakewuxd.html . There are also retrofit kits, e.g., https://www.ebikes.ca/shop/electric-bicycle-parts/ebrakes/tripwire-pull.html . – RLH Aug 24 '20 at 23:48
  • @RLH Oh, that’s cool. I was just thinking from the perspective of high-end E-MTBs. It’s nice to see that the retrofit kit can work with “some” hydraulic levers. – MaplePanda Aug 25 '20 at 01:34
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    Can you explain or support the claim that "Most quality electric motors like to spin at 5000 - 20000 RPM" ? – Rsf Aug 25 '20 at 07:16
  • @Affe “more efficient” in what sense? Coasting fast is actually really inefficient – it wastes almost all the energy in air drag. Just, normally we don't care because there's nothing else that could be done with that energy. Electric motors meanwhile are very efficient already; it's mostly the gearing, regulators (on a small scale) and batteries that introduce inefficiencies, but I'm confident that these will continue to get better. – leftaroundabout Aug 25 '20 at 09:30
  • @Rsf it doesn't have as much to do with quality as with size/mass. The problem is this: a solenoid is efficient if the voltage induced by magnetic movement is much larger than the resistive voltage. To make the induced voltage higher you need to either increase RPM or add more windings; to make resistive losses lower you need to make the wire thicker. I.e., it's basically a tradeoff between either faster RPM or higher mass, and usually the gearings that allow high RPM are lighter than a motor that could work efficiently at low RPM. – leftaroundabout Aug 25 '20 at 09:36
  • @leftaroundabout in the sense that with no other behavioural constraints about travel time and stop sign etiquette etc you'd cover the most distance per unit fuel by always gently rolling to a stop rather than trying to recover energy. Also one assumes when talking about wind resistance we're in the range of street legal eBike speeds – Affe Aug 25 '20 at 16:22
  • @Affe you seem to be assuming a mostly flat topography – in which case my opinion would be, why use an e-bike at all? But yeah, in that case regen braking is definitely no use. On reasonably steep downslopes however, you obviously don't “gently roll to a stop”, nor stay within “legal e-bike speeds” (assuming you mean the speed up to which e-bikes are allowed to power-assist, which really has nothing to do with legal speed limit). You still wouldn't get much out of regen when first coasting 50 km/h and then only braking hard at the end, but you would from braking all the time at 30 km/h. – leftaroundabout Aug 25 '20 at 16:59
  • IMHO the points 1 - 4 could be solved comparatively easily for certain usage scenarios. But they have to compete against "bigger battery", and the latter is easy to sell. 1 + 4: Poor acceleration, poor braking power for recuperation: I grew up with chromium-plated steel rims and back hub brake. Very low braking power compared to what we nowadays consider acceptable. But I could easily see a system where "back-pedaling" steers recuperative braking, and the usual levers do what they usually do. I think I've even seen bikes with 3 brakes (2 x rim + back hub) before (nothing e, though). – cbeleites unhappy with SX Aug 25 '20 at 18:43
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    For 2 + 3, see e.g. the article (in German, though) I linked in my comment under cmaster's answer. That article basically suggests that people who cannot bring the necessary power onto the pedals to reach safe speed sufficiently fast in urban stop-and-go stop cycling. So most bikers not thinking they'd need more acceleration is to some extent a self-fulfilling prophecy. I'd tentatively assume that that clientele also tends to do the downhill more slowly. Mid-motor recuperation would be possible if the freewheel mechanism moves from the back hub to the crank region. – cbeleites unhappy with SX Aug 25 '20 at 18:48
  • (and IGH doesn't need a tensioner) – cbeleites unhappy with SX Aug 25 '20 at 18:57
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    I'm not sure I agree with the idea that an electric motor MUST run at high speed to work well. High electrical speed, maybe, but not necessarily high mechanical speed. A brushless DC motor with oodles of poles, for example, as a sort of "electrical gear ratio". – AaronD Aug 25 '20 at 19:06
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    In fact, I used to work for a company that made BLDC controllers, and one of our "oddball" projects was a "scuba scooter" or whatever you like to call it. Basically, a propeller with handles. I've forgotten how many poles that motor had, but it was a LOT! Not very big wires to feed it, meaning not much power input, and not much speed, but good luck stalling it! It was very well suited for direct-drive. Now expand that concept to even more poles around the perimeter of a wheel... – AaronD Aug 25 '20 at 19:06
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    I agree with your conclusion, but controlling speed on a downhill is often needed, and the back brake can be used very well for that. A rear hub motor could do the same on a bike used for anything other than a race-like riding style with its late braking for bends, and would be beneficial in saving wear/heating of the main brakes – Chris H Aug 25 '20 at 20:36
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Short answer: it's not worthwhile.

Most bicycling energy goes toward overcoming wind resistance, especially for casual riders. That energy is lost, with no chance for regeneration.

Liberty Trike claims that the most you can expect to gain from regeneration is 5-10% of energy expended. Panda eBikes claims 10%, with some math.

By comparison, an electric car or truck has a lot more mass and momentum. This makes regenerative braking more worthwhile. These sorts of e-vehicles also often have more sophisticated battery thermal management, which makes it safer and more efficient to quickly dump energy back into the battery.

jeffB
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    Given the complication and cost of regen and the potential wear it could put on the battery, all to get 5-10% more range at best, I'd imagine that just outright buying and carrying a second battery, even with their high cost and weight, and getting 100% more range looks increasingly more attractive. – Zach Lipton Aug 25 '20 at 06:13
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    “an electric car or truck has a lot more mass” – sure, but that's kind of irrelevant because it also means you need much more energy for accelerating. The actual point here is that the mass is a vast factor higher whilst the ærodynamic drag is only a moderate factor higher, so inertial and hill-potential forces have a relatively speaking higher importance. “often moving considerably faster” actually has the opposite effect, because air drag increases with the square of velocity. The main problem with regen cycle braking is that cyclists are going too fast, not that they're going too slow. – leftaroundabout Aug 25 '20 at 09:43
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    @leftaroundabout You're right about speed; I've edited to eliminate that claim. – jeffB Aug 25 '20 at 14:47
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    I would think that "most energy goes toward overcoming wind resistance" especially for those who ride faster, i.e. athlettes. – Zeus Aug 26 '20 at 01:12
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    @Zeus Sure, but I assume athletes will have better gear and posture to reduce drag coefficient. Casual riders are more likely to be upright in street clothes (in my experience), but they can still crank up the e-boost to go fast (again in my experience). :) – jeffB Aug 26 '20 at 13:45
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Energy grows with the square of speed. If you are a normal weight cyclist (70 kg) with a heavy e-bike (30 kg), riding at 25.2 km/h (7 m/s), you will have an energy of

E = m * v^2 / 2 = 100 kg * (7 m/s)^2 / 2 = 2450 J = 2450 Ws

For an e-bike with a 250 W motor, that energy suffices for

t = E/P = 2450 Ws / 250 W = 9.8 s

If you have to stop for a red light every 1 km, you are riding for 1 km/7 m * s = 143 s. I.e., you only get about 7% range extension out of recuperation. I guess, manufacturers will prefer to give you 7% more battery (simple and good for marketing) rather than going through the extra effort of developing a recuperating drive. Especially since the larger battery will serve you well on long distance tours, where the effect of recuperation would be totally negligible.

The above covers the use in fairly flat terrain, which appears to be the major market for e-bikes. However, there are use cases that would indeed favor adding a regenerative brake:

  1. Recuperation would be nice for commuting through a city. In such a setting it might be worth riding with an empty battery, and only using the recovered energy to accelerate when the lights change. It would be possible to build some really light e-bike on that principle (only a tiny battery for roughly 5 kWs), but that's not the main market for e-bikes: Normal use is to ride with a full battery and stop before the battery becomes empty.

  2. Recuperation could be a game-changer in hilly terrain: 100 m elevation translate to

    E = g*m*h = 9.81 m/s^2 * 100 kg * 100 m = 98.1 kWs

    which is energy that needs to be expended on the ascent, and which needs to be removed on the descent. Recovering it on the descent for the next ascent would indeed be a very significant advantage.

    However, there's a catch: People ride downhill pretty fast. If our standard biker rides down a slope of 5% at 54 km/h (15 m/s), their weight has a power output of

    P = 5% * g*v*m = 0.05 * 9.81 m/s^2 * 15 m/s * 100 kg = 736 W

    That's almost three times the power rating of a typical e-bike motor. And I used a rather benign example, I have done descents that produced about 2 kW. To make this sort of recuperation feasible, the electric motor would need to be built much stronger than it is allowed to be. It would need to be roughly five times as large and be limited electronically to output only 250 W while accelerating. I think, it's obvious why e-bike manufacturers do not do this.

cmaster - reinstate monica
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  • Calculation assumes braking is only done with brakes, and no "rolling" to a stop or to a slower speed which i guess is more common if you predict the red lights to some degree. So I would cut that 7% even lower. – Viktor Mellgren Aug 25 '20 at 14:30
  • @ViktorMellgren Any "rolling" to a stop can only recover a part of the energy I calculated, because that's the entire energy that's available for recovery. This is a strict upper limit to the effectiveness of recuperation. If you don't believe me, please revisit your school's physics textbook. – cmaster - reinstate monica Aug 25 '20 at 14:46
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    You are both saying the same thing. – Michael come lately Aug 25 '20 at 15:46
  • In other words, a comparably small battery would allow a useful e-bike construction for daily urban use (as opposed to day tour use). In case you read German, here's an article that discussed such thoughts already > 10 years ago: https://fahrradzukunft.de/9/elektrorad-reichweite/ – cbeleites unhappy with SX Aug 25 '20 at 18:04
  • The author argues that practical utility of an e-bike that assists only during this short acceleration period would already help a lot, e.g. to help elderly people to keep using the bike for their everyday errands. They insist to not use the electric help to hide low efficiency of the overall drivetrain!). Those people would also not go downhill at 50+ km/h on a bike, they'd go at maybe 20 - 30 km/h (and probably slower on steeper roads). Which would make a recuperation of 250 W most if not all that is needed. – cbeleites unhappy with SX Aug 25 '20 at 18:18
  • @cbeleitesunhappywithSX Riding down a 10% descent at 18 km/h still yields 500 W. Twice as much as the motor is rated for. In the areas where you would stand to gain most from recuperation, you would loose more than half the available energy to the normal brake. – cmaster - reinstate monica Aug 25 '20 at 18:24
  • @cbeleitesunhappywithSX Or ride down the descent at 9 km/h to keep within the limits of recuperation. But, who would do that? Descents are the most fun part of cycling! :-) – cmaster - reinstate monica Aug 25 '20 at 18:28
  • @cmaster: the braking power depends largely on the speed, so there is some leeway (and yes, I'd probably be faster. But then I'm anyways hearing a lot "oh, and leg-drive only!?" ;-)). No, recuperation is not feasible as a replacement for good brakes that allow an emergency stop, and yes, you can harvest only a fraction of the potential energy. However, when going up the hill later on (or the other side), even only 1/3 of the potential energy harvested may mean going up at 9 km/h instead of 5 km/h. – cbeleites unhappy with SX Aug 25 '20 at 18:32
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    Or, iif I manage to harvest 1/3 of the 100 m elevation, the other hill is nevertheless 30 m "flatter". But I admit I had been thinking of the urban stop-and-go target group of the article I linked: elderly people that profit a lot from a "kick-start" and from being able to keep a speed uphill where the bike behaves stably. Quite different use case from what I tend to do (for another several decades, hopefully). Which still doesn't say that the alternative, i.e. a battery that can store the required energy isn't the better solution - even though for this target group, ... – cbeleites unhappy with SX Aug 25 '20 at 18:35
  • ... a much smaller battery that they manage to safely take out of the bike may be very much of a point. – cbeleites unhappy with SX Aug 25 '20 at 18:37
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    Excellent answer now. I'd just remark that your power consideration may be overly pessimistic: up to a (high) limiting RPM, electric motors/generators are rather constant-torque than constant-power, so a motor that outputs 250W on an 8 km/h ascend is probably quite capable of munching 500W on a 18 km/h descent – if the battery and regulator allow it. At least in cars, the battery actually seems to be the limiting factor for regen performance. – leftaroundabout Aug 25 '20 at 21:59
  • This calculation assumed you always cruise at max power output (250W) but on flat terrain, when electronically speed limited to 25kph, the motor is probably only expending 50-100W to balance air resistance. – benjimin Aug 26 '20 at 07:48
  • @benjimin That's only half true. For one, most of the 9.8 seconds would be expended accelerating to cruising speed. And for another, typical e-bike usage is by people who do not output significant amounts of power. You need somewhere between 200 and 250 W power to keep riding at 25 km/h, more for a heavy e-bike with less than optimal tire pressure. I'd guess that the typical e-bike user does not output more than about 100 W, leaving roughly 150 W for the electric motor to supply. – cmaster - reinstate monica Aug 26 '20 at 08:09
  • @benjimin After all, the electronic regulation creates a harsh wall at 25 km/h where pedaling becomes much less effective. This relatively sudden decrease in effectiveness will train most e-bikers to stay clear of this area, because any additional effort that they would expend would simply seem to evaporate without effect. So, most e-bikers will be riding at an operating point where the motor does the bulk of the work. – cmaster - reinstate monica Aug 26 '20 at 08:12
  • Your calculation shows that coming to a complete stop from the assisted cruise speed (25kph) might recover about enough energy (2.5kJ) for 10s at full power. By offsetting the battery energy that would otherwise immediately be spent accelerating back to the cruise speed, the battery could last for an extra 25-50 seconds of cruising. This affects the efficiency gain in your particular example (15-25% not 7%), but your overall point (that the benefit of regenerative braking will depend on how frequently you use the brake, and may only be marginal in some use cases) still stands. – benjimin Aug 26 '20 at 08:51
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    Another factor that may support your argument (that the gains may be too marginal to justify drive complexity) is that in hilly terrain riders typically descend at faster speeds, and the non-linear increase in wind resistance losses will reduce what is recoverable by braking. – benjimin Aug 26 '20 at 08:58
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    Sucks to have a stop sign at the bottom of a hill. – Michael come lately Aug 26 '20 at 16:31
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It's uncommon because cyclists don't care for it enough.

Juhist made some good points, but none of them are really show-stoppers.

First off, clearly regenerative brakes are only useful when you actually use them sufficiently much. Well, it turns out cyclists don't like to brake – which makes sense because normally it's just wasted energy/time, and unnecessary pad-wear. In relatively flat settings, there's nothing to be gained from braking at one place to get a boost in another, whereas on mountain roads or prepared MTB tracks, we're happy to just get a really high speed out of the decents, and with increasing speed air drag becomes much stronger so then there's not much left for regen. However, if one were disciplined enough to use the regen brakes all the way on the downhill passages (and maybe even continue pedalling lightly), then you would get back nearly all the energy for the next ascent, because electric motors and batteries are pretty efficient.

Of course that would mean going down you wouldn't be much faster than going up, which I guess most cyclists would feel as taking the satisfaction away from the downhill passages. (I personally find it much more satisfying to arrive at a mountaintop knowing all the energy came from my legs, but I seem to be in a minority there.)

Where the situation is a bit different is in MTB on natural singletrails. Anyone but downhill racers will tackle these with a lot of braking anyway because it's just too dangerous to go down very fast. But unfortunately, in MTBs hub motors are particularly problematic because they don't offer as much torque and add unsprung mass, whereas mid-motors aren't capable of regen braking.

IMO regenerative braking has one place where it really should be popular, and definitely would work well: long distance touring on mountainous roads. Time doesn't matter much there – taking longer for the descents is actually a welcome break and opportunity to see more of the nature; also you need to be extra careful when hours away from any help in event of a crash. The extra mass of the baggage would also add to the energy that can be reclaimed, whereas the unsprung mass of a hub motor becomes insignificant.

Common wisdom seems to be that e-bikes don't make sense at all for long-distance as there's nowhere to charge, but properly implemented regen braking is exactly what would make it sensible – if the cyclist is willing to actually use it in braking mode as much as in power mode.

leftaroundabout
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  • The extra mass of baggage will let you reclaim more energy, but it will also require that you spend more energy in the first place - I don't think adding mass to the bike makes regenerative braking any more efficient (although it could affect drag). – Nuclear Hoagie Aug 25 '20 at 13:44
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    @NuclearWang correct. My point is that the potential & kinetic energy is substantially bigger but the drag only a bit bigger (or even lower because you'll be riding slower), so the ratio of lost energy to total energy is smaller and therefore the ratio of regen-energy to uphill-energy bigger. – leftaroundabout Aug 25 '20 at 13:57
  • Braking in one place and getting a burst in another: that's what happens a lot in urban settings, though. Moreover while I live in a quite flat region in the middle of Germany, on everyday biking I frequently encounter downhill routes where I cannot coast at full speed (visibility, crossing roads, ...). Recuperative braking could easily be used. – cbeleites unhappy with SX Aug 25 '20 at 18:06
  • I'm not convinced by the cited common wisdom about long-distance. Except in the most remote areas of the world, charging an e-bike every 200 km should be feasible. – gerrit Aug 26 '20 at 06:59
  • @gerrit Sure, you could recharge regularly, but do you want to? A major part of the appeal of long trips is the total independence – no need to plan any stops ahead, just set up the tent when there's a nice spot... And even if you do charge enough in between, the ability to brake regeneratively would still be reassuring, and you wouldn't need to use the battery so sparingly to actually make it through the 200 km of mountains. – leftaroundabout Aug 26 '20 at 11:14
  • @leftaroundabout Yes, there are parts of the world where one can do that. For reasons mentioned in the various answers, I doubt regenerative braking would bring nearly enough to not need to recharge at all. Or just bring some solar panels :) – gerrit Aug 26 '20 at 11:55
  • Since regenerative braking is only going to add on the order of 5-10% more range, even if you manage to do a bit better than that, I'm not sure how its that material for long-distance e-bike touring. It would extend the distance between recharging stops a little bit, but it certainly wouldn't eliminate the need for recharging. Electric cars with regenerative braking still need to be recharged regularly. – Zach Lipton Aug 27 '20 at 17:58
  • @ZachLipton that's because electric cars don't have pedals. On a bike, you can manage to do not just “a bit better” than 10%, but orders of magnitude better – it just requires conscious use of the regen brake, i.e. braking to prevent speeding up on downhills rather than just to slow down, and saving the electric drive as just a slight assist rather than as the primary propulsion. – leftaroundabout Aug 27 '20 at 21:17
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We can identify scenarios where recuperation could help a lot:

  • urban stop and go as discussed in this (German language) article: they estimate that starting from a stop happens with a total power input of 200 - 300 W (depending on whether there's e.g. the groceries to transport etc.) for the first few seconds.
  • tours with many not-too-long ascends and not-too-steep descends. Steep descends mean that only a small fraction of the potential energy can be harvested.

The article argues that someone who is not a fit biker (but e.g. an elderly person doing their shopping) may easily have the muscle power to go at an acceptable speed in the flat once at speed, but they may have difficulties producing the power output to accelerate the bike/hold the speed uphill that allows a safe and stable operation of the bike.

In fact, they basically argue for an electrically assisted system that helps only at low speed - while not hindering the normal muscle output above that speed. The idea is to get rid of peak power, so the biker can get along with their own continuous lower power output.

Hills need some more power stored than acceleration after a stop, but such a system could get away with very small batteries (see below).

The idea here is very different from sportive e-biking: the assistance is meant solely to help people avoid people becoming so slow that the bike becomes unstable.

The competitor of recuperation is a bigger battery, and IMHO this is where it is not considerd worth while.

A glance through the internet tells me that one can get 1300 kWs (360 Wh) in maybe 5 kg of battery.

Going with @cmaster's back-of-the-envelope calculations, that translates either to 500x accelerating from stand to cruising speed or 1300 m of elevation gain.

With a recuperative system as outlined above, we could get away with a small battery of < 1 kg. Without recuperation, 1 kg of battery using the electical assistance only as outlined above would still give us the equivalent of 250 m elevation gain or 100 starts. Plenty for every-day use (and the scenarion is not to stop pedaling, so this would cut 500 m elevation gain in half or so). But 1 kg battery is still small compared to the weight of the drive. (Yes, with the target clientele of the scenario above, a 1 kg battery instead of a 5 kg may be an argument...)

And 5 kg battery can be marketed as assisting not only for acceleration and getting us to go uphill at maybe 8 or 9 km/h but at getting us to an acceptable overall speed for so many km. And it can be marketed in line with sportive e-bike - which the outlined system would not be.

And incidentally, going assisted all the time will hide the resistance due to big low-pressure (or even knobby) tires and a not-so-very-efficient drive train.

cbeleites unhappy with SX
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The electricity costs for charging an e-bike for a 18 km ride are in the order of US$0.01–$0.03. Even if you ride 15000 km/year, that's less than $20/year in electricity costs. Apart from all the reasons already mentioned, regenerative breaking on e-bikes is uncommon because the electricity costs for charging and e-bike regularly are negligible compared to the other costs of e-bikes, most notably purchase, depreciation, and maintenance. Most users can recharge every day and would notice no advantage whatsoever from regenerative braking.

gerrit
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    The travel distance also matters. When after four hours of riding the battery is running out and there is no where (and no time) to charge, the remaining energy is worth more than these cents. – nightrider Aug 26 '20 at 21:04
  • @h22 Yes, but as pointed out elsewhere: (1) that's not a typical behaviour of ebike users, and (2) you won't lengthen those 4 hours very much. – gerrit Aug 27 '20 at 05:49