question about putting an alternator on an E-bike

[DSJ]

Hello I am wondering if anyone can help me understand something. I read a similar thread on here where someone wanted to setup an alternator on the rear wheel of their e-bike. Everyone was very helpful in explaining the energy losses that would result from that setup. I just wanted to piggy back off of that person's question. What if I rigged the alternator to be connected to the pedal drive chain so that I would be charging the battery via the alternator when I manually pedaled. Im sure It wouldn't give a great deal of energy to the battery to the point of a complete recharge but enough to extend the battery range right?  

[Frank]

Regenerative braking utilizes the fact that (most) motors can be used as generators (alternators).  So every (most) e-bike has a built-in alternator.  Adding an alternator to an e-bike just adds extra weight and cost.

 

[Neil Obstat]

I suspect regenerative braking would be a performance problem too, since generating electricity via braking at the rear wheel would promote tire slippage on the road, since most of your braking power is through the front wheel not the rear wheel. Therefore, trying to slow down by applying resistance to the rear wheel which turns an alternator to charge a battery could mean that when you're approaching a red light at the bottom of a steep hill you'd find your rear wheel intermittently skidding down the hill and your bicycle not slowing down quickly enough, so then you'd be forced to apply the front brake which is not driving the alternator. Therefore, what good is having the alternator in the first place? You would say, "This bike weighs twice as much with the alternator, it makes pedaling harder, other cyclists pass me up, it never has enough power to move the bike forward, and my rear tire gets flat spots on it from skidding." 

But the OP asks about putting the alternator on the drive chain, which would be separated from braking at the rear wheel since the coasting ratchet frees the chain drive from the wheel. Unless your bike is a direct drive type which has no ratchet coaster and the pedals are ALWAYS connected to the rear wheel. I'm not aware of any e-bikes that have a full-time connection with the pedals. 

[Bender]

- regenerative braking is not worth the bother on a bike. The potential energy gain is negligible and you need additional electronics to convert the power back so it can actually load the battery.

- There would be no issue with regenerative braking on the rear wheel. The difference between rear and front only matters during an emergency break, something regenerative braking is not suited for.

- the pedals are (nearly) in all bikes permanently connected to the rear wheel. I don't think it would qualify as a bike otherwise. Usually there is a freewheeler, but that only decouples in one direction.

- putting an alternator on pedals which are connected to the wheel is completely pointless. Adding a clutch to be able to disconnect is even sillier. It would be possible to forgo having a chain altogether by loading a battery with the pedals and discharging on the wheel, but that would be less efficient than a simple chain. Again, I don't think this would still qualify as a bike.

Edited June 6, 2018 by Bender

[Frank]

On 6/6/2018 at 1:15 PM, Bender said:

- regenerative braking is not worth the bother on a bike. The potential energy gain is negligible and you need additional electronics to convert the power back so it can actually load the battery.

Huh.  Seems regenerative braking isn't used on most e-bikes despite having the capability because of increased battery and motor wear vs limited range extension (5% to 10%), also a freewheeling issue.  Might be worth doing on those e-scooters though because motors are more powerful and propel the full load.

Why Don't More E Bikes Use Regenerative Braking?:  https://www.evelo.com/blog/why-dont-more-bikes-use-regenerative-braking/

Electric Bike (eBike): Freewheel or Regenerative braking? - Bicycles Stack Exchange:  https://bicycles.stackexchange.com/questions/31124/electric-bike-ebike-freewheel-or-regenerative-braking

 

[Ken Fabian]

On 06/06/2018 at 8:56 AM, DSJ said:

What if I rigged the alternator to be connected to the pedal drive chain so that I would be charging the battery via the alternator when I manually pedaled.

You could do that - and you will have to pedal harder. A lot harder. Whoever told you about energy losses was right - you'd be better saving that effort for making the e-bike go when it needs pedalling. The energy losses in pedalling to make motion are small, but energy losses converting crank power to electricity via an altenator, to charge a battery, back to an electric motor to turn the crank to make motion are much higher. Running the alternator on down hill runs in place of braking will result in a gain, but if you have to pedal where otherwise you didn't, you'll be working harder for very little (no) gain. More exercise and better health outcomes maybe, but you'd get that by using an ordinary bicycle, not an e-bike.

Edited June 14, 2018 by Ken Fabian

[StefanLazic]

I think it depends on how hilly your terrain is. If there are many ups and downs, then renegerative braking would make A LOT of sense.

But if your terrain is mostly flat, I believe that the added weight, complexity and cost will far outweigh any gains. Bikes do not have much kinetic energy due to their low speed. So there is almost nothing to be gained by converting the kinetic energy to chemical energy (battery) during braking. But there would be A LOT to gain in a hilly terrain as the potential energy will be significant.

Here is a comparison:

Kinetic energy in a  80 kg bike+rider travelling at 10 m/s:  80 kg * (10 m/s)² / 2 = 4000 J

Potential energy for a height of 100m of a 80 kg bike+rider: 80 kg * 100 m * 9.81 m/s² = 78480 J

You would get 20 times more energy from a 100m drop in height than a 10 m/s difference in velocity.

At lower velocities the difference is even more significant. At 5 m/s you have about 1000 J of kinetic energy.

Imagine accelerating from a stop to 5 m/s and then stopping immediately. You'd have to do that 78 times to equal the work needed to gain 100m of height (or 328 feet).

 

Calculate how much acceleration and deceleration you do in a day, and to what speed. Then calculate the kinetic energy loss from braking.

Then check how hilly your terrain is and calculate the potential energy loss.

Then check out how much energy is stored in a battery. Then decide if it's best to carry a second battery or add regenerative braking.

[Bender]

On hilly terrain, don't forget the need to switch to a breaking resistance when your battery is full.

Can the batteries even handle the current on a steep hill?  It can be considerably larger than during normal operation.