Using Handcrank Dynamo to charge super cap + usb output

[Macgyver1]

Well I have one of those handcrank flashlights that uses the dynamo to charge a small battery that powers a few leds.

I want to design a circuit where I can use the handcrank dynamo to charge a super cap or 2. Then use the stored energy to supply a USB port of 5.0V @ 500ma MAX. I am making a crank powered battery pack to charge USB peripherals, but use a supercap instead of a battery to increase the life of the device.

I have intermediate experience with electronics and can construct any diagram or ideas you can suggest. I just don't have the indepth tech knowledge to construct this. Any suggestions would be greatly appreaciated.

NOTE:
If anyone could also include how I could vary the Voltage and Current output incase I wanted to upscale the project for larger powersource charged by crank.

Thanks for the advice in advance!!!

 

[Macgyver1]

Would a linear voltage regulator provide the needed stable output votage and current?

 

[crutschow]

It would take a very large supercap to provide 500mA for any significant period of time. For example, 1 farad of capacitance will decrease in voltage by one volt every two seconds when supplying 500mA. The formula is t = CV/I.

 

[Macgyver1]

Could you give me the units for t = CV/I please.

time = Columbs * Volts / Current??

thanks!


Note:
Found a PDF describing a similar project to mine using supercaps and voltage regulators. Fell free to check it out. https://www.electro-tech-online.com/custompdfs/2009/02/paper-ESS-013.pdf

 

[Macgyver1]

Im sorry, but what are the units for t = CV/I

time = Capacitance*Volts / Current?

Thanks!!

 

[mneary]

Seconds, Farads, Volts, Amperes.

 

[Macgyver1]

So no advice or ideas on how to impliment this?

 

[Nigel Goodwin]

So no advice or ideas on how to impliment this?

People seem obsessed by Supercaps these days? - yet their performance is a long way short of a much cheaper battery, and their discharge curve is far less useful as well.

So for a start dump the supercap idea, and simply connect the dynamo to the batteries via a diode to prevent it discharging them when not winding.

 

[Macgyver1]

People seem obsessed by Supercaps these days? - yet their performance is a long way short of a much cheaper battery, and their discharge curve is far less useful as well.

So for a start dump the supercap idea, and simply connect the dynamo to the batteries via a diode to prevent it discharging them when not winding.

I appreaciate your input, but this is a project I would like to learn with. If it doesn't work out, so be it. I learned something. Just as a counterpoint battery's take longer to charge and their charge cycle is limited, whereas supercaps seem to one up on these points. I agree with you they are limited in their capacity so far, but hey thats why we experiement and innovate.

Im having real trouble finding a simple diagram or project using supercaps as a powersource. I would also like to build a portable power supply using a bank of supercaps for powering things where no power is available. Seems solar cells can have a high current and low voltage thats perfect for charging these supercaps quickly. THAT would be nice, using a solar arrary and a few min of sun light can charge a bank of supercaps. A lot better than using batteries and hoping for clear skies all day to charge them.

Im still open to ideas or hints.. I will report what i find too. So stay tuned.

 

[Nigel Goodwin]

I appreaciate your input, but this is a project I would like to learn with. If it doesn't work out, so be it. I learned something. Just as a counterpoint battery's take longer to charge and their charge cycle is limited, whereas supercaps seem to one up on these points. I agree with you they are limited in their capacity so far, but hey thats why we experiement and innovate.

Supercaps charge faster, but simply because they don't hold very much - you will probably also be surprised to find how unreliable supercaps are as well - a batteries charge cycles will probably exceed it.

Think of a battery as a 5 gallon bucket, and a supercap as an egg cup - which fills faster?, why do you think that is?.

Im having real trouble finding a simple diagram or project using supercaps as a powersource.

That's because they are unsuited to the application.

I would also like to build a portable power supply using a bank of supercaps for powering things where no power is available. Seems solar cells can have a high current and low voltage thats perfect for charging these supercaps quickly. THAT would be nice, using a solar arrary and a few min of sun light can charge a bank of supercaps. A lot better than using batteries and hoping for clear skies all day to charge them.

Solar panels have low voltage AND low current, but as before supercaps charging for a few minutes are going to contain very little energy at all.

You've been given the formula, how much current are you hoping to be able to draw from them?, and for how long?.

 

[Macgyver1]

It would take a very large supercap to provide 500mA for any significant period of time. For example, 1 farad of capacitance will decrease in voltage by one volt every two seconds when supplying 500mA. The formula is t = CV/I.

How is my calculation flawed. I'm getting 10 secs instead of 2.

time = 1 farad * 5 volt / .5 A

It's 10 sec, isnt it? How did u get 2 secs?



Also I really appreciate all the help and advice. I'm no engineer by any means.

 

[Nigel Goodwin]

How is my calculation flawed. I'm getting 10 secs instead of 2.

time = 1 farad * 5 volt / .5 A

It's 10 sec, isnt it? How did u get 2 secs?

The time constant is 10 secs, NOT the time to drop 1V (which was what crutschow suggested was 2 seconds).

 

[Hero999]

People seem obsessed by Supercaps these days? - yet their performance is a long way short of a much cheaper battery, and their discharge curve is far less useful as well.

I can think of three advantages of a super capacitor:

1. Fast charge time, a capacitor can fully charge un under a second.
2. Fast discharge time, capacitors can dump their charge quicker than batteries.
3. Longer life, batteries have a limited number of charge/discharge cycles, capacitors don't.

If you don't need any of the above then use a battery!

 

[Nigel Goodwin]

I can think of three advantages of a super capacitor:

1. Fast charge time, a capacitor can fully charge un under a second.

They only really charge faster because they only hold a tiny amount of charge.

2. Fast discharge time, capacitors can dump their charge quicker than batteries.

Again, they discharge fast because they don't hold much to begin with - and supercaps generally have very limited current capabilities. They also discharge exponentially, so become 'flat' very quickly.

3. Longer life, batteries have a limited number of charge/discharge cycles, capacitors don't.

Life of supercaps seems very limited, it's just been hopeful expecting them to last longer than batteries.

 

[Hero999]

They only really charge faster because they only hold a tiny amount of charge.

Again, they discharge fast because they don't hold much to begin with - and supercaps generally have very limited current capabilities.

Sorry but that's nonsense.

Capacitors can charge and discharge more quickly than batteries because they have a very low impedance. A super capacitor can supply a much higher instantaneous power than a battery of the same physical size, even though it may store one tenth of the energy. One of the reasons for this is because a battery relies on a chemical reaction which is much slower than simply piling/stripping off electrons on electrodes in a capacitor.

They also discharge exponentially, so become 'flat' very quickly.

That depends on the load, a resistive load will exhibit an exponential decay, a constant current load (e.g. a linear regulator) will discharge linearly and a constant power load (e.g. a switching regulator) will discharge exponentially, i.e. the current will increase as the voltage is reduced causing the voltage to drop more quickly as the capacitor discharges.

The useful energy in a capacitor is determined by the minimum operating voltage of the device connected to it. Some boost regulators can work down to 1V meaning most of the energy in a capacitor charged to 12V can be used.

I agree 99% of the time batteries are better but super capacitors are great for things such as: improving the transient response of a battery or re-claiming energy from a regenerative breaking system.

Life of supercaps seems very limited, it's just been hopeful expecting them to last longer than batteries.

It depends on the application. The life time of a capacitor is normally given hours at a specific temperature and voltage, the life time of a battery is given in cycles at a specific temperature and discharge depth. A battery is likely to last longer than a super capacitor in a typical standby application and a super capacitor is likely to last longer in something that requires many cycles.

 

[Nigel Goodwin]

Capacitors can charge and discharge more quickly than batteries because they have a very low impedance. A super capacitor can supply a much higher instantaneous power than a battery of the same physical size, even though it may store one tenth of the energy. One of the reasons for this is because a battery relies on a chemical reaction which is much slower than simply piling/stripping off electrons on electrodes in a capacitor.

I doubt they store anywhere near 1/10 of the energy for the same size - what sort's of currents are you imagining from them? - a simple NiCd AA battery has an EXTREMELY (in fact dangerously) low impedance and will happily provide enough current to destroy itself.

 

[Hero999]

Capacitors typically have ten times the power density of conventional batteries.
**broken link removed**
Electric double-layer capacitor - Wikipedia, the free encyclopedia

 

[Nigel Goodwin]

And from that same graph, between a 1/10 to 1/50 of the energy density of conventional batteries - all the power density really shows is that it's lighter than a battery.

Also, I presume 'conventional battery' means non-rechargeable?, NiCd and NiMh have massively higher power densities than conventional batteries. This is without mentioning Lithium Ion which are much higher still, but have more difficult charging routines.

Perhaps you would care to list a selection of items that use supercaps in place of rechargeable batteries?, laptops perhaps?, mobile phones? - or perhaps you can't find any examples?, why might that be?.

 

[Hero999]

And from that same graph, between a 1/10 to 1/50 of the energy density of conventional batteries - all the power density really shows is that it's lighter than a battery.

Also, I presume 'conventional battery' means non-rechargeable?, NiCd and NiMh have massively higher power densities than conventional batteries. This is without mentioning Lithium Ion which are much higher still, but have more difficult charging routines.

Going by the article accompanying the graph 'conventional batteries' does cover Li-ion as it mentions them.

NiCd and Lithium Ion have a similar power density to lead acid which is much lower than a super capacitor.

Perhaps you would care to list a selection of items that use supercaps in place of rechargeable batteries?, laptops perhaps?, mobile phones? - or perhaps you can't find any examples?, why might that be?.

Obviously you won't find a super capacitor in any small consumer device, except maybe for memory backup. They just don't store enough energy and portable devices don't need to supply huge amouns of energy in short bursts.

Super capacitors are used mainly for pulsed power e.g. coil/rail guns, electric armour starting nuclear reactions and will be used in buses and trains of the future - China is already using them in a bus see the Wikipedia article.