Self-charging e-bike?

I’m very much not an engineer, so don’t shout at me, but I’m wondering why/if there isn’t an e-bike, that uses a dynamo-type arrangement to add charge to its battery during descents? Superficially it sounds like quite a good idea, you’d have extended range at the cost of slightly (?) slower descending and you could have the option of turning the top-up facility off for shorter rides.

I’m sure there’s a good reason why nobody does this – would it simply be a negligible charge relative to the amount needed by the motor or massively complicated? Feel free to enlighten me, please.

I’m not thinking of making one btw, just having an idle Monday morning…

I suspect it would result in negligible charge rates with a load of additional complexity,weight and cost.

some hub motor bikes have regenerative charging.  ie the motor acts as a dynamo on descents.  its very inefficient and while giving a bit of power back its not much.

Much harder with BB motors because of the freewheels involved

They break down enough as it is.

The motor is in the bottom bracket, right? When you brake, that’s not turning. So you’d have to fit dynamos to the wheels which would be big and heavy, and there’d be wires going up your fork leg. But to provide meaningful deceleration they’d have to be huge, as a lot of your braking is done from low speed. You wouldn’t want this much weight, but even so you’d get way more range for less weight by simply fitting a bigger battery.

would it simply be a negligible charge relative to the amount needed by the motor

This, basically. Think how much charge a dynamo hub produces – enough to run a light or charge a phone. You’d not get nearly enough to make any sort of charging dent in an eBike battery.

Regenerative breaking works for cars as, because of the weight of the car and the speed it’s generally travelling, you’ve got a s–t load of kinetic energy that needs to be got rid of when breaking, so that can be nicely turned into quite a lot of electrical energy. With bikes there’s not really that much as they don’t really weigh that much and don’t really go that fast, so there’s not much enerdy there to be ‘reused’

Oh well. I sort of thought as much.

With bikes there’s not really that much as they don’t really weigh that much and don’t really go that fast, so there’s not much enerdy there to be ‘reused’

Have you seen the average e-mtber? There’s a lot of mass there… 😉

The motor is in the bottom bracket, right?

Arguably the bottom bracket is in the motor, in the sense that the motor crank and bearings are effectively the bottom bracket spindle and bearings, but basically, yep.

I suspect you’re probably limited by the battery’s maximum charge rate and what people would be prepared to accept.

So under braking, 500W is probably about what your rear brake can achieve, while the front is doing 10x that, so you achieve very little as you’re probably only braking maybe 1% of the time?

Just coasting along, on a touring bike in big mountains it might work as a drag on long descents where you’re comfort braking anyway. But no one is going to accept a rear wheel on the verge of lock-up all the way down an off-road descent, it’d be dull and uncomfortable as the suspension get’s nerfed by the braking forces. If nothing else just consider the erosion!

Might work better as a switch in the rear brake so it kicked during the free stroke. But unlike cars I suspect it would only add a few percent at best. Car’s have far lower drag coefficients (0.25 Vs 1.0), and the regenerative system relies on the driver using the accelerator like they would with a normal engine (with engine braking) so it’s effectively 3 modes, power, coasting, or regen braking. So the electric car can very effectively recover it’s energy as:
a) it wastes relatively* little moving through the air.
b) the driver rarely has to use the friction brakes.

*relative to what it uses accelerating 2tonnes+ of mass.

I have ridden a hub motor bike with regen braking.  It had 4 levels you could switch thru and you could feel it working.  alpine road descents I think you would get a worthwhile amount back.  UK or off road – not really

yeah with the motor at the BB, you would have to get power to it via the drivetrain.

and not just rotating the chainring (ie moving the freewhell from hub to bottom brack) because the basic principle of the deraileur drivetrain precludes transfering force this way. You would need to either tension the bottom section of the chain or use the top part of the chain in compression.

So only possible on single speed/gearbox arrangements.

and then there is the amount of power. as noted above, a standard dynamo hub type thig gives you peanuts. the only way to get something useful, it would (quite obviously) need to have a significant drag. In some circumstnaces, thats plausible. You could flick a switch at the top of a steep descent, and utilise it as a drag brake all the way down. It would be like having your rear brake partly on all the way down; but still leaving you both actual brakes to negotiate technical features. Potentially useful on steep tech descents. Possibly some benefit on tarmac for e-touring bikes in hilly areas or commuters in incredibly hilly cities.

2 problems to all of this. It adds weight, complexity and cost. I imagine that it is significantly easier and cheaper to simply add a slightly larger battery.

And the biggest problem to the system I described? Human stupidty. If there was a “flick this on to recover energy” switch, the average idiot would turn it on and leave it on, pedalling against the resistance (its an ebike so they wont notice) and then moan that they are getting terrible range.

you’d need to use regenerative braking as your rear brake, or it would drag you down a lot, while not being very efficient. And as you know, the back brake tends to not do a massive amount at times..might be useful on something like a touring bike..but on a road or mountain bike it’s be pretty pants, for the complexity and inefficiency to eek out a little more battery range.

I have a radwagon e-Cargo bike which has regenerative braking. There’s a little switch on the brake lever so when you put the brakes on slightly, regen activates and generates about 300w according to the display. You can feel the drag from it when it kicks in. Pull the lever a bit more and the disc brakes come on.

The main advantage for that bike is that on long gradual descents you don’t cook the brakes.

From an MTB point of view, I’d imagine that it would be quite difficult to generate significant power in a controllable way. On my cargo bike, it’s just an on/off switch, not great when you are finding the limits of traction, hence the relatively low power generation.

On cars, I think they do quite a bit of work to make the brake pedal feel “normal” whilst behind the scenes there’s quite a bit of work going on via an ECU to work out whether to deliver that braking via the disc brakes or the regenerative braking, depending on speed, battery state etc.. Adding all that to an MTB would be a lot of work for a tiny gain.

My Decathlon R500 e-cargo hub motor apparently generates power above 45km/h. But I don’t think the system is really set up to deal with it and recharge the battery, so they simply advise you to never go above 45km/h. I think if you did it when the battery was already full it would kill the electrics.

If one built the regeneration system into the freehub / hub interface it would have zero drag while pedalling, but instantly become regenerative as you freewheel.

That gives the intuitive single pedal EV response, and could be arranged with a suitable (ie high) derailleur tension to both a) work and b) give a soft pickup.

Making it small enough and getting gear changes to work would be the interesting bits thereafter.

And whether it gave any meaning output.

Note – tarmac based bikes only even with a working version of this setup

The way to do regen is basically as a speed-limiter for road bikes, so you don’t have to hang onto the drops at 40-50mph absolutely terrified as your arms go to mush and you wonder if you can slow down for the next corner. Imagine cruising down the Colombiere at a sensible speed and feeling fresh as a daisy when you get to the bottom and you’ve added 70% of another go up to the top to the battery too.

No idea how you would sort the mechanics, some sort of direct drive to the motor then put the freewheel only on the cranks somehow I guess.

become regenerative as you freewheel

Except you would not be freewheeling – you’d be actively braking as power is transferred back to the battery. Fine when braking, less so on the flat when you’d have to pedal harder in order to charger the battery !

I did wonder if this thread was prompted by this being in the news:

https://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-64964294

A bob trailer with solar and a small wind turbine seems to be the answer 🙂

Regenerative breaking works for cars as, because of the weight of the car and the speed it’s generally travelling, you’ve got a s–t load of kinetic energy that needs to be got rid of when breaking, so that can be nicely turned into quite a lot of electrical energy.

Even then it peters out the slower you go. The friction brakes come in as you brake gently to a stop at a junction or whatever. If you’ve had rain and developed a sheen of rust on the discs you can hear it.

Ebikes and Ecars are not the same thing. A car climbs a hill under battery power, and then descends, recovering some energy via regenerative braking. As the system is inefficient, the battery will never be fully recharged.

On an ebike, the power to climb the hill is provided by the battery AND the rider. If this input is 50/50, then the energy recovery only needs to be 50% efficient to have the battery fully charged again at the bottom.

I fully agree with all the concerns regarding feel and the mechanics and weight of energy recovery, but one day it will be feasible to significantly increase the range from a battery by recovering energy. It won’t be on MTBs to start with, but it will come…

You can get ebikes with regen braking!  I have ridden one and there are plenty on line.  You don’t get a lot or regen but it can be done – hub motor only

@whatgoesup

Fine when braking, less so on the flat when you’d have to pedal harder in order to charger the battery !

No you wouldn’t – you’d have to pedal fast enough that you weren’t freewheeling, i.e. just turning the pedals over until the freewheel went silent no noticeable effort required really. When you were actually pedalling to provide forward motion there would be no movement in the (re)generator and therefore no drag.

Works ok in concept but not much cop in reality I suspect.

Perfectly do-able (as per Troutwreslters comment).

Currently cost prohibits the wide adoption of regen’ braking on bikes.

However electric cars have very high levels of very efficient regen’ braking and only use the friction brakes IF the battery is fully charged or the car is below approx’ 20kmh.

So we’re just waiting for cost of regen’ braking to come down or for the purchase price if ebikes to go up even further :o)

I fully agree with all the concerns regarding feel and the mechanics and weight of energy recovery, but one day it will be feasible to significantly increase the range from a battery by recovering energy. It won’t be on MTBs to start with, but it will come…

Except on a bike, it’s all about maintaining speed, so regenerating on anything other than actual positive braking will be a no no. People will want to coast on descents, it’s actually very efficient to coast as much as possible, and no laws will be broken, in a car, you need to control speed, so there’s a bit of regen a lot of the time.
And braking to a halt from speed (any speed) gives a massive spike of current, which most e-bike batteries won’t be able to take in. It’s actually a problem with 48V/light hybrid cars, regen is very limited because the battery RMS limit is reached very quickly. It’s one factor of many that controls how much regen we can do, even in a hybrid/EV.

However electric cars have very high levels of very efficient regen’ braking and only use the friction brakes IF the battery is fully charged or the car is below approx’ 20kmh.

Or if the RMS limit of the battery has been reached. Decently calibrated and properly designed regen braking can go down to <2kph.

If you’ve had rain and developed a sheen of rust on the discs you can hear it.

Also not a problem in properly designed systems, disc cleaning is a Thing.

https://newatlas.com/nua-electrica-stealth-ebike/59481/

Decently calibrated and properly designed regen braking can go down to <2kph.

My Vauxhall van will slow down to walking pace on the regen mode, brakes only needed to come to a full stop. I reckon it does about 75% of its regen mode by releasing the accelerator, the first bit of brake travel is regen, and you can feel when the brakes come on as it tends to be a little snatchy.

Back to bikes, regen would never make pedaling harder as it would be assisting, and it can’t assist and regen at the same time. However, to coast you would have to turn the pedals lightly.