For anyone who would like to run their electric bike off of capacitors let me save you some time.
Simple Answer:
With current technology capacitors are not a viable option for powering an electric bike.
Medium Answer:
Currently, the energy density for ultra capacitors is at best about 1/10 that of lead acid batteries so it would take about 10 times the weight in ultra capacitors as it would for lead acid batteries. Unless you have room for 300lb of capacitors this isn’t really an option.
A little wiki action:
(3–5 W·h/kg for a standard ultracapacitor, although 85 W.h/kg has been achieved in the lab as of 2010 compared to 30-40 W·h/kg for a lead acid battery)
| Parameters | Electrostatic Capacitor | Ultracapacitor | Battery |
|---|---|---|---|
| Discharge Time | 10-6~10-3sec | 1~30 sec | 0.3~3 hrs |
| Charge Time | 10-6~10-3sec | 1~30 sec | 1~5 hrs |
| Energy Density (Wh/kg) | < 0.1 | 1~10 | 20~100 |
| Power Density (W/kg) | < 10,000 | 10,000 | 50~200 |
| Charge / Discharge Efficiency | ~ 1.0 | ~10 | 0.7~0.85 |
| Cycle Life | Infinite | > 500,000 | 500~2,000 |
http://www.nesscap.com/product/overview.jsp
http://en.wikipedia.org/wiki/Energy_density
http://en.wikipedia.org/wiki/Ultracapacitor
Long Answer:
Even short trips require 4 or 5 amp hours of power at the rated voltage to be effective.
Others have asked the same question on multiple web forums, and the best answer I could find was this.
1 Farad is 1 Ampere second per Volt. 1 F = 1 As/V.
1 Ah = 3600 As
3600 As / 12V = 300 As/V = 300F
But you need more then that because the voltage will drop as the capacitor discharges. Half the charge means half the voltage.
With a lead acid battery that’s different. A battery that is half empty still has nearly the same voltage as it had at the start.
Maybe 600F and some electronics to automatically step up the voltage so it stays at 12V.
So you’d need to buy 160 capacitors with 3V and 60F each.
http://www.physicsforums.com/showthread.php?t=256751
Even after reading the above statement I decided to do more research and see if there is any new technology that could be useful. Ultra capacitors are starting to come down in price and are readily available, but they still are not cheap.
It is possible buy capacitors that are rated for 3000F and 2.7 volts, and their data sheets state that they have 3.30 Wh (~1.2Ah = 3.3Wh / 2.7V) of usable stored energy. The problem is that the calculation does not factor in the voltage drop that takes place as the capacitor discharges.
Ideally, with a bike you would want to run somewhere between 60 and 36 volts for a hub motor.
There are two options:
1. A voltage multiplier circuit.
2. A string of ultra capacitors in series.
The first idea is probably the better one, but it is fairly technical and goes beyond the scope of a blog post.
The basic math behind a series string of capacitors would be this. (Capacitance drops as capacitors are added in series.) 1/((1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)+(1/3000)) = 166. 7F
Theoretical storage would be
166. 7F*48V = 8000As
8000As/ 3600 As = 2.2 Ah
However, the usable potential is likely 1.2Ah which is stated on the data sheet, and the hub motor will operate most effectively between 48 and 36 volts. That is a 12V change and happens to provide access to about 25% of the usable 1.2 Ah in a simple series circuit.
……………………………. This is getting too boring.
If you still want to know more about sizing ultra capacitors for electric vehicles this article provides the required calculations. http://people.clarkson.edu/~pillayp/c1.pdf
Conclusion:
Start considering ultra capacitors for application in electric bikes when ratings of 12000F + capacitors become available. There were a lot of claims in 2007 and 2008 that we would see the end of batteries, but it looks like someone is getting close.
Can I ask you for a copy of the article you linked to (http://people.clarkson.edu/~pillayp/c1.pdf)?
The link seems to be broken, but the article titles sounds interesting.
Regards,
Salman
The link is dead…
I can not find a copy, and I am currently away on business for about a month. I may have a copy on my desktop at home. If I remember I will try to send you a copy, but it will probably be too late to do you any good.
Sorry I can not be more help.
Hi, please can you advise what kind of capacitor i would need to fit to my electric moped and wire the power through and boost style button. Currently does 18kph and wish to 35kph.
Many Thanks and Kind Regards,
Tony Ward
It would take me quite awhile to figure out exactly what you need(probably not cheap), but you could probably make a voltage multiplier circuit out of capacitors to make it work. It might be a little dangerous as it could put a serious load on your battery. I’m afraid I just do not have time to figure it all out right now. If you do find an answer please post it back here. I would also be interested to see what others have done.
I found this because I wanted to outfit my bike not with batteries to be totally lazy. but to use the braking or petal power to charge up the capacitors so at a green light or a stop sign I can use the capacitors to bring my bike up to 80% of my previous cruising speed before I stopped at the red light or stop sign. That acceeleration from a red light is the toughest part, just like with hybrid cars, the cells charge up when you brake and the motor turns into a generator. the same thing could be used on a bike but with a way less weight and bulk of equipment.
when you say not viable it depends upon exactly what you mean. if your idea of an electric bike is a bicycle shaped electric moped then capacitors aren’t up to the job yet. if on the other hand you want an electric road bike with about 100W of assistance (enough to go from 16 to 20mph on the flat with pedal assistance) then capacitors can work. you will need a regenerative brake and a current limited dc/dc boost circuit to make it work. the boost circuits are redly available on the internet and will take a variable input to put out an adjustable fixed output, you’ll want to limit the current to keep efficiency high. all you need do is calculate the amount of energy necessary to climb the steepest/longest continuous hill you are likely to encounter and size the caps appropriately. the system can be used to average the gradient of your entire route which even in a hilly area might be surprisingly flat, my commute crosses two steep valleys but the average gradient is only -0.75%. tools like Google Maps Find Altitude and the simulator available at ebikes.ca can be helpful to work everything out. my personal set-up uses the current limiter on a small battery putting out 100W max which charges some capacitors which go through a second booster to power the controller, the capacitors alone store enough charge for 30 seconds of riding. this arrangement is efficient because batteries don’t like supplying peaks of current but the the capacitors are fine with it. I have used the second boost circuit so I can drain most of the energy from the capacitors when I need it without pulling the battery voltage down.
Lenny, could you tell us more about your setup? I’ve just begun looking into using capacitors to assist the battery pack in order to reduce demand on it. Could you point me in the direction of where to learn more?
Thanks!
I am interested in designing the same thing. A Capacitor bank that runs the motor controller, with constant charging from the battery, and maybe charge the battery w/ the regenerative breaks, to reduce a lot of stress on the battery. *shrugs* I have yet to see any working setups, it should be possible now in 2017.
My idea is to press a high torque motor/generator against the rear wheel instead of the standard brake calipers on bikes with them. I have this setup already but without the capacitors or the electronics set up needed.
Capacitors in parallel with an invtduice load will reduce the running current, this will be useful if you generator your own power, or if your cable is overloaded, or if you are charged for your power in kVAh rather than kWh. If you are a domestic consumer your electricity charge will not be reduced as you will be charged on a kWh basis for electricity used.
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Now I’m already aware
This is quite an old article (although accurate at the time) and things have marched on since 2011
https://phys.org/news/2018-02-alternative-traditional-batteries-closer-reality.html
What Lenny is doing is perfect and I know, from talking to a NASA engineer, this is exactly the capability of ultra/super capacitors for e-bikes. This is the 1st explanation I have been able to find after searching, off and on, for several years.