# Electric bicycle that can re-charge itself?

1. Oct 14, 2008

### muddyboots

(Complete physics novice here - I know very little about physics, so please excuse any stupid questions!)

I was reading the 'Electromagnet' thread, about how movement (preferably rotation) is best for generating power using magnets...

What I'm wondering, could you use the rotation of the wheels/pedals on a bicycle, and magnets, to recharge the battery on an electric bicycle? I have been googling to see if such a product exists, but was unable to find it. If you generate power in this way, can you store it easily in a battery?

Thanks :) (This is not for a school project or anything - simply my own curiousity!)

2. Oct 14, 2008

### Staff: Mentor

I suppose technically it is possible, question is - what for?

3. Oct 14, 2008

### Staff: Mentor

Just to be clear, you have to be pedalling for that to work: you can't use a motor to drive the wheel and a generator to recharge the battery.

4. Oct 14, 2008

### Pumblechook

Free energy eh??

All the energy comes from the pedals. All you will do is add weight and make the pedalling harder.

5. Oct 14, 2008

### f95toli

Electric bikes have been available for a very long time. Try google

see e.g.
http://www.bionx.ca/en/main/default/31.shtml

Now, the point is of course not to "generate energy", the motor is essentially there to help you up steep hills etc. You can -at least in principle- then re-charge the battery on the flats or when going downhill.
However, most of the bikes that are available today seem to use batteries that can only be re-charged from the wall; but the BionX that I linked to above is one exception.

6. Oct 14, 2008

### mgb_phys

Cos I have to wear out brake pads going down a 300m hill form my house, then climb up a 100m hill on the other side. What makes it worse is that you can see the destination below you as you scream downwards, then above you as you pant up the hill.
Curse you potential energy!

Edit - of course a smooth straight road, no traffic / speed limits and a large amount of insane courage would also work!

7. Oct 14, 2008

### brewnog

Muddyboots, if you like the sound of mgb_phys's suggestion, perhaps you should research 'regenerative braking'. However, you must first be clear that this energy must come from somewhere, whether it's just from gravity (when going down hills, in which case you need to get to the top of them first!) or from pedalling (in which case you'll be pedalling a lot harder when you're charging your batteries!).

8. Oct 14, 2008

### mgb_phys

Unfortunately regenerative braking doesn't really work for bicycles.
The problem is that they don't weight very much and have a lot of aerodynamic drag so when you go downhill you lose a lot more energy to the air than you can recover from the potential energy, add that to the extra weigth and cost of the batteries and it doesn't really work.

What would work is a series of viaducts and flyovers to make the world (or at least this city) flatter for cyclists - or they could fit the amazing Norwegain bicycle lift http://www.trampe.no/english/

9. Oct 14, 2008

### Pumblechook

The problem with any electric vehicle is the very poor energy to weight ratio which is about 1/100 of that of petrol.

Strap a car battery to a bike and see how much harder it is to pedal.

10. Oct 14, 2008

### Staff: Mentor

11. Oct 14, 2008

### mgb_phys

12. Oct 14, 2008

### muddyboots

Thanks for the all the replies!

Love the Trampe - what a great idea.

I guess what I was imagining, is that it would be on the downhill parts that you would be charging your battery, rather than by the actual peddling itself (though if you could recharge to a tiny degree from that too, then great).

Sounds like the amount of charge needed would be much greater than you would generate from going down a few hills on your everyday sort of ride....rats!

But thanks anyway :) My curiousity is satisfied...