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Old 05-31-14, 09:04 PM   #1
Aznman
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Appropriate Super-Capacitors

Dynohubs have been the number one choice for the majority of the professional cyclists that I have talked to online and in real life. It is awesome that - with today's technology - super-capacitors can continue to power your headlamps even when you are not pedaling.

I have 2 questions:
1. Are there any super-capacitors that can power a 1,000 lumens light for 5 minutes without pedaling?
2. Would it be significantly safer if I add voltage regulators, or is it basically unnecessary?
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Old 06-01-14, 11:18 AM   #2
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String enough of them in parallel and they'll be able to. The issue, of course, is that as soon as you start drawing current their voltage starts to decrease, linearly at constant current and exponential decay with a resistive load. What is needed is a dc-dc converter that will convert the decreasing voltage of the capacitors as they discharge to the the constant current needed to drive the LEDs.
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Old 06-01-14, 11:30 AM   #3
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String enough of them in parallel and they'll be able to. The issue, of course, is that as soon as you start drawing current their voltage starts to decrease, linearly at constant current and exponential decay with a resistive load. What is needed is a dc-dc converter that will convert the decreasing voltage of the capacitors as they discharge to the the constant current needed to drive the LEDs.
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Old 06-01-14, 02:27 PM   #4
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The issue, of course, is that as soon as you start drawing current their voltage starts to decrease, linearly at constant current and exponential decay with a resistive load. .
To be fair, batteries experience a similar discharge curve, though it may not have exactly the same shape and since batteries hold a lot more energy than a similarly sized ultacapacitor, the effect isn't as noticeable as it's more spread out over time -- but it's certainly there, and anything that uses batteries as a power source should have been designed with that in mind.

Lights already have to deal with batteries that don't give a constant voltage, so that same mechanism would work with an ultracapacitator powered power source. However, in order to have a ultracapacitator that can provide 10 watts for 5 minutes -- 3000 joules -- with a limited range of acceptable voltages (though you could possibly increase that to some degree with the dc-dc converter that Loigi mentioned, something beyond whatever is in your light) ... it would be quite large.

Last edited by dougmc; 06-02-14 at 04:43 PM.
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Old 06-02-14, 07:48 AM   #5
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Exponential decay:

http://www.schoolphysics.co.uk/age16...ics/index.html

http://en.wikipedia.org/wiki/Exponential_decay

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Old 06-02-14, 03:42 PM   #6
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parts-express.com has a bunch of capacitors , they stock them for speaker building but have some really big ones .

Capacitors in the Electronic Parts Department at Parts Express | 68

really its the capacitance that you are asking about , the Lumenns is an effect of the light , loading it ..



amps is the draw it demands And how much for how long is amp hours . batteries are also measuring amp/hours.

Perhaps a 6v gel electro-lite lead-acid battery would do as well .. You choose.


Stand lights are coming in quite a few headlights these days .. just buy one ..

Last edited by fietsbob; 06-02-14 at 03:45 PM.
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Old 06-02-14, 08:01 PM   #7
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Yeah, the Edelux II headlight has a 4 minute standby included and it sells for $200. Great light also!
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Old 06-03-14, 01:12 AM   #8
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Originally Posted by Aznman View Post
Dynohubs have been the number one choice for the majority of the professional cyclists that I have talked to online and in real life. It is awesome that - with today's technology - super-capacitors can continue to power your headlamps even when you are not pedaling.

I have 2 questions:
1. Are there any super-capacitors that can power a 1,000 lumens light for 5 minutes without pedaling?
2. Would it be significantly safer if I add voltage regulators, or is it basically unnecessary?
To get the size of your supercap you could use this equation: C=I*Δt/ΔV, where C is in F, I in amps, Δt in sec, ΔV in volts. Assume you have a 5.5V supercap connected to a constant current LED driver able to work down to 3.5V to drive a 3A LED for 300sec to get 1000lm. This will give you 3*300/(5.5-3.5)=450F. Here's a 875F/5.5V bank:

So it's not going to be small. Nor cheap!
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Old 06-03-14, 02:17 AM   #9
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Originally Posted by choa View Post
To get the size of your supercap you could use this equation: C=I*Δt/ΔV, where C is in F, I in amps, Δt in sec, ΔV in volts. Assume you have a 5.5V supercap connected to a constant current LED driver able to work down to 3.5V to drive a 3A LED for 300sec to get 1000lm. This will give you 3*300/(5.5-3.5)=450F. Here's a 875F/5.5V bank:

So it's not going to be small. Nor cheap!
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Old 06-03-14, 02:55 AM   #10
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Regarding your question 1, where will you find a dynohub driven 1000 lumen headlight? DIY?

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Old 06-11-14, 09:45 AM   #11
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Originally Posted by fietsbob View Post
parts-express.com has a bunch of capacitors , they stock them for speaker building but have some really big ones .

Capacitors in the Electronic Parts Department at Parts Express | 68

really its the capacitance that you are asking about , the Lumenns is an effect of the light , loading it ..



amps is the draw it demands And how much for how long is amp hours . batteries are also measuring amp/hours.

Perhaps a 6v gel electro-lite lead-acid battery would do as well .. You choose.


Stand lights are coming in quite a few headlights these days .. just buy one ..
Parts-Express also carries a car audio cap that's 1 farad for about $100. It would seem more practical to me to use a modest rechargeable battery as part of such a dyno system.
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Old 06-11-14, 09:47 AM   #12
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you are probably right there ..
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