Spitballin' an idea...

Sep 14, 2013
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Bradley Illinoiz
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I know there's no such thing as a perpetual engine but...
Anybody make a powerful generator hub...
to help offset the energy needed for the other (motored) hub?
I would think this could cut down the amount of batteries needed.

Carl.
 

OCD

Nov 21, 2009
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Unfortunately you'd increase the current draw on the driving motor pushing the load of the generator hub if it puts out enough to add charge to the battery. You can use regenerative braking with the driven hub which does what you describe when braking, it's good for about 10% greater distance (assuming you have decent hills to regen down). There has been discussion on endless sphere about building a mini gas generator to mount on a bike which may work. That's how the Chevy Volt works, the motor is electric only but an onboard gas generator kicks in when your battery voltage gets too low.
 
Sep 14, 2013
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Unfortunately you'd increase the current draw on the driving motor pushing the load of the generator hub if it puts out enough to add charge to the battery. You can use regenerative braking with the driven hub which does what you describe when braking, it's good for about 10% greater distance (assuming you have decent hills to regen down). There has been discussion on endless sphere about building a mini gas generator to mount on a bike which may work. That's how the Chevy Volt works, the motor is electric only but an onboard gas generator kicks in when your battery voltage gets too low.
Yeah, I knew it wouldn't work without some help, the drag alone (pushing the load) is always the problem, but that's where pedaling comes in. In my daydream... the cyclist is the equalizer between the hubs. Also the possibility of charging a pair of smaller battery banks and cycling between the two, to extend the range. I don't see it working without pedaling, but maybe the eMotor would be more of an assist to the cyclist, than the actual drive. It's my understanding that the banks of batteries is the main drawback of eBikes/cost.

Carl.
 

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Nov 21, 2009
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Yes, (quality) batteries are usually the most expensive part of an e-bike build making it harder for someone unfamiliar to take that financial plunge. SLA's are cheap but don't last many charge cycles before they begin to deteriorate (charge cycle basically means if you use 33% of battery total capacity 3 times that equals 1 charge cycle, not each time the pack is charged). LiFePo4 are cheapest in the long run as you can recharge them 1500-2000 cycles and they still retain aprox 80% of their original capacity. If you're not looking for max speed then 36 volt is cheaper than 48 volt for a given amp/hr capacity. I bought a Chinese 48v 20 amp/hr battery for about $600cdn off ebay (shipped to Canada) that lasted 4 years for my daily commute to work before a couple of cells died, that costs out to about $0.60/day for the battery pack itself and about $0.10/day to charge it. As I've said before, the cost is mostly up front, the $$$ savings happen over time, which is the exact opposite of a gasoline motor. There is no difference in range whether you switch out two 10amp batteries or connect them together in parallel for 20amp/hr.
 
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Oct 15, 2008
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My personal opinion is regen, or enroute battery recharging is only practical to use during long downhill stretches. Trying to regen during a level ride doesn't seem to work well. JMHO.
 
Dec 18, 2015
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There's no regen that adds any significant juice back into the system unless you can find a route that is all downhill. But, then, you wouldn't need a motor, eh? :)

The "simple" answer to this is: (besides just saying "no, it won't work.")

1 - The physical laws that govern matter and energy determine that you can't input a certain amount of energy into a system and get more energy back out of it, at least not without also converting some form of matter into energy.

(Which IS what you'd be doing if you pedaled, converting food calories into kinetic or mechanical energy - or if you added a gas powered generator.)

You COULD theoretically have a generator that is run directly by your pedals and which produces enough electricity to run the motor.

But, generating electricity that it then turned into movement would require more energy than just using the kinetic energy you converted into electricity. You would have to pedal the generator much harder than you would pedal the bike with no motor or generator, because:

2 - By the time you convert one type of energy to another, there is always a certain amount of loss through friction and other processes that result in unintended conversion of some of that energy into other forms like heat or light, that you cannot practically harness for use.

The generator is always producing less electrical energy than the equivalent kinetic energy it consumes.

The motor is always producing less kinetic energy than the equivalent amount of electrical energy it consumes.

If you could put this system of generator and motor into an incredibly sophisticated laboratory setting where you could turn all the circuits into superconductors, and make all of the bearing totally friction free, you might come closer to "breaking even" on energy use and generation in the system. But, good luck hauling the truck load of liquid nitrogen and anti-gravity device around on the bike. :)

You might ask, so - WHY do electric cars that use a gas motor to generate electricity get better gas mileage than just using the same motor to push the car?

That motor runs at a more efficient constant speed, trickling juice into the battery gradually instead of speeding up and slowing down as the car does. It's also a smaller motor since it does not have to produce great bursts of power, just a steady trickle of power that gets saved up and released in bursts from the battery as needed. A smaller motor has less weight and less inertia working against it when it runs at a constant speed. There are other factors, but these kinds of factors all result in a more efficient transfer of energy from the motor into moving the car.

Bottom line - using a generator on an electric bike to make more electricity? Not gonna happen without pedaling a lot harder or using a gas motor.

The generator and battery system to make a "hybrid" bike in the same sense as a hybrid car? That's absolutely doable. However, the weight and expense of this using today's technology would not be practical.
 
Jan 21, 2009
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The locomotives that pull the nation's trains have been electric powered for years, with a big diesels generating the electricity. I always thought they were using the diesel directly, but it's huge electric motors. On bikes, a gas motor to help charge up the batteries wouldn't be much smaller than the gas motors that power the bike alone. College students at some of the big universities are always trying new ideas and have contests, as are the battery manufacturers looking to extend and improve batteries. That's where we'll see some breakthroughs I hope.
 
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Dec 18, 2015
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Good points, all. It will be someone who doesn't buy my argument - who doesn't know it can't be done - who will be able to do it! :)
 
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Jun 13, 2015
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That's why I like when people ask questions, even if they seem like the answer may be obvious—going through the exercises of thinking about it can lead to new ideas that do work. Maybe not this time, but it happens.

I spend way too much time thinking about electric bikes, but I keep running into the problem of escalation—I keep building myself into a cool, but terrible motorcycle when I could just buy a decent used real motorcycle for a lot cheaper (bah, where's the fun in that?!).
 
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Jan 8, 2013
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The most practical way to use regen is as an electronic brake. For that it works pretty well, but for charging your battery it's pretty useless.
 
Oct 15, 2008
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Formula 1 race cars, though petro powered, use regen braking. During braking, motor/generators charge a capacitor. Later, the capacitor can release the charge to the motor/generators for more acceleration. You can charge that capacitor pretty well while decelerating from 180mph for a tight turn.
 
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