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Self charging an electric bike? Answered

Hi all, I am always amazed at the ingenuity here and after many years of being fascinated I would like to ask my first question.  I'm thinking of getting or converting/building an electric cycle.  I've been told by local dealers that none except the very expensive have a recharge facility for sat when you go downhill or are freewheeling,  That seems a waste to me so I am thinking ... why not buy a used wheel with a hub motor and fit it at the front wheel and that raises the following questions in my mind ...

Can hub motors be used in reverse as generators?  

Would the output be speed (rpm) dependent and if so then if said cycle has say a 36 volt system would it only charge the battery if the 'generator' is producing 36v or close to?

If so would it be better to find perhaps a 24v hub motor to use as a generator and charge the battery via a variable input, 36v output step-up?

I appreciate it will be less than 100% efficient when both using electric at the motor and charging at the 'generator' but would anyone have any idea how inefficient?  I'm fit enough to cycle on the flat but would appreciate help on the hills!!

I assume I will need a diode or similar ... does anyone know where I can find a suitable circuit diagram

Lots of questions I know ... sorry.



Interesting stuff. Never even heard of electric bikes until today, but one question: Since it seems a lot or most have the motor in the rear hub, and the battery is elsewhere, would it be possible to charge a secondary battery pack from the front hub area, and be able to switch batteries on long commutes? I don't have any electronic background, but I'm looking at what would be the negatives: additional weight, resistance from the charging mechanism, etc. You gurus with the know-how can probably tell me why it's not feasible in a couple sentences. Thanks.

Beginner electric bike users who try to charge another battery (which adds weight) while motoring, find they get LESS range by about -45% then a solo system....

You know it takes power to charge the battery and that power comes as an extra load on the running battery.

This may surprise you but to charge a battery takes twice as much energy as the battery will return. So keep pedaling on :)


2 years ago

A lot depends on how the motor is driven, often the regeneration can use the same power electronics.

The RPM determines how much current is available to be pushed into the battery.

Yes, the hub must be producing more then the battery voltage to reverse current and charge the battery.

BTW batteries are good at storing electrical energy but they not efficient. As an example, a lead acid battery only returns half of the current used to charge it.


Thanks Iceng but I'm not understanding your "the hub must be producing more then the battery voltage to reverse current and charge the battery". I was seeing it the same as my solar panels and the Grid. My solar panels can't fight against the Grid but when I'm not using the juice it goes up the Grid for someone else to use.I was rather thinking that if the generated electricity gets past the diode and the motor is not calling for it then it would go to the batteries ... are you saying that's not the case and it effectively just disappears?

Your solar energy is "boosted" by passing through an inverter - the output of which is AC at mains voltage.

As your not trying to charge a storage device your AC can go directly onto the grid to join all the other electrons bouncing around.

Gotcha, thanks Rick, I see what you are saying. :)

It should be understood that the inverter is another drop in efficiency and the current finally going into the grid is much less then the solar panel "juice"...

Unless your machine with diode produces more voltage then your battery there can not be any current flow into the battery... Even if the sun is shining.

When the voltage is equal or less then the battery there is no juice as you say... Nothing happens !

Generating systems vary in efficiency from 50% to 80%.

The system you are talking about uses dynamic breaking for stopping or maintaining speed while going down hill to put some of the power back in the batteries to extend running life of your bike while on a trip.

Some DC motors can be used as a generator as is. When you let off the throttle it makes some switching to the charging system to charge the battery. It is not an easy after market rewire because you will need to make a custom control box but it can be done if you know what you are doing.

Hi Josehf, thanks for your help but there is no dynamic braking with the cycles that's the reason why I was looking to add a new generator. There is no 'back charging' from the motor to the batteries when free-wheeling as in an electric car. The suppliers are telling me that the technology is too expensive and the market won't bear it! So the system I'm talking about is to have a separate stand alone generator (from wheel hub) to simply put extra juice into the existing circuitry. I am not looking to muck about other than to have some form of diode and charging manager introduced.

You can do that but it will cost you power and it will not extend the charge life of your battery.

With a normal alternator or generator most only give you no more than 50% to 60% advanced systems 80%.

A motor used as a generator would be less efficient, if you ran it all the time it would increase the load on the drive motor costing you power and drive time. And if you wanted to use it as a dynamic braking system it must be completely separated from the battery and the rest of your system until you are breaking or going down hill.

Think of a generator this way you put two horses in and get one horse out, so if you ran it all the time like you would be with a diode it would just shorten your drive time.

Thanks again Josehf, yes, I do understand efficiencies and %ages but you are referring to immediate work and energy whereas I am looking at immediate work with stored energy for future use. I will be pedalling and the electric motor is only used to assist when needed (uphill) ... the amount of assistance is a function of the load being applied to the pedals. So ... I'm seeing this as being inefficient use of the motor in an energy/work sense only when I am using the electric power to assist with hills and therefore also assist in driving the 'generator' (but I will still be pedalling so it could be argued that I am using my own power inefficiently at those times). When on the flat and pedalling with no assistance or free-wheeling downhill with no assistance the motor will not be involved in driving the 'generator'.

To extend the range of the cycle I will need to have a means of creating and saving energy when the going is good and storing it until it is needed to assist. It just seemed to me that something like 99% of the effort would be directed at moving me and the cycle forward and 1% at making the 'generator' turn so the extra consumption would be minimal in the scale of things but the energy created from my pedalling and gravity pulling me and the cycle downhill or coasting up to lights and junctions would be beneficial in the long run (please don't quote me on those %ages, they are for illustration only ... I have no idea what the actual split would be!).

As I have said the cycle from the manufacturers does not have dynamic braking and I have no intention of rigging for dynamic braking. Perhaps the diagram will help to show my very simple thinking ... I'm not bright enough to consider bells & whistles! :-D I thought of using a wheel with a hub motor because they are easy to buy used on eBay!


I think you miss-understand me.

You can do it with just a ten amp diode on the red wire of the hub motor.

However it will be on all the time.

The hub motor will become or act like a dynamic brake.

So it will be like riding your bike with the brakes on all the time.

You need a switch or a relay so it is only on when you want it on.

Ah, OK, in that case yes I did misunderstand you! Sorry, I thought you were thinking I was having a dynamic braking circuit! So how does turning it on and off affect the drag is it that the electricity generated produces a resisting magnetic force inside the motor turned generator?

One question I've not asked yet is what is the best type of 'home made' generator system ... I was only looking at the wheel and hub motor arrangement because if it's availability and simplicity but if there is something more efficient maybe I could mount it above the wheel and drive it with an added chain and sprocket.

Taking your last point maybe I could rig some form of sensing relay on the supply to the drive motor to switch the 'generator' off when the load on the pedals reaches a certain level.

Anyway, thanks for all you help it's most interesting.

The drag is the electricity generated, it produces a resisting magnetic field inside the motor turned generator.

Some motors will act like a generator as is. I think this would be the best if you are not very good at electricity. Unfortunately I do not know which make or model number.

Other motors will need rewiring.

If you look at the diagram iceng put up you may need to rewire the motor with diodes if it is a delta or a Y like in iceng's diagram.

Then you just need a switch or a relay to disconnect it from your system. a toggle switch on the red wire will do.

so if it's disconnected from the consuming unit (battery) does that mean the resistive effect goes away because it's not producing electricity?

I can follow a wiring diagram OK and have a basic knowledge that's more than changing a light bulb ... about 40 years ago I build a computer with 8K of RAM!! :-D So I feel confident enough to open up the motor and have a look ... I was born with a spanner in my hand but have had to learn electricity. I can understand Iceng's diagram and can see why it works except I'm not sure about his earth notation ... is it just saying "negative earth" and use the frame?

Yes when it is disconnected the resistance goes away because you no longer have a circuit.

A motor with brushes and magnets for the stator converts to a generator just by spinning the armature.

If the motor you convert to a generator is a brush-less DC motor its windings are in a delta or a why configuration just like this CD motor.


So you will need to add full bridge rectifiers to the windings.

Many electric motors can also be used as a generator - you may be able to get away with a single motor with control circuitry that switches between modes when you pull or release the brake lever - as you pedal uphill, it aids your efforts, but as you brake on the roll downhill, it slows you down by regeneratively charging the battery.

You will not get anything approaching 100% efficiency, but it would (potentially) greatly increase your range.

Kiteman, I now realise I was misunderstanding you (and Josehf) over this daynamic braking point. Sorry, I thought you were getting all sophisicated thinking I was installing a dynamic braking system! :-D

Thanks Kiteman, I wasn't expecting anything like 100%, I just wanted to know if it was possible and worth doing. :) As I have just explained to Josehf there is no form of switching to back charge nor dynamically brake at the moment so I'm not looking to interfere other than to introduce an additional source of electricity to charge the batteries when there is an excess in the circuits.