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What to do with a Boostcap?

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Inducktion

Thu Dec 08 2011, 09:00PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

I just got this massive 2600 farad (yes, farad) ultracapacitor from electronic goldmine... Rated at 2.5 volts, this thing can hold around 8 kJ of stored energy, and has an amazingly low ESR for an ultracapacitor (something along the lines of .7 milliohms, so it can discharge really nicely)

As for what to do with it, I'm not quite sure! Being 2.5 volts, It's not useful for too much, as a boost converter circuit usually required a minimum of around 3 volts. (Though, I have seem one that can do 2 volts!) Plus, charging it is an absolute pain in the butt. Me and gren actually tried charging it via a car battery, with some series resistors to limit current somewhat.... With .5 ohms of resistance the cap was charging at 24 amps, and wasn't even charging that fast, not to mention the resistor actually smoked, because...according to what I calculated, it was dissipating 288 watts through a 15 watt resistor...) Is there another...faster way of charging it?

I would've gotten another one, but I could only afford one of them. (they were $16 or so dollars on sale) If I can't find something to do with it, I can just resell it on ebay for $40, and make a decent profit off of it.

Daedronus

Thu Dec 08 2011, 09:19PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
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Posts: 370

I don't know what to do with it, but I want a set of them.
I wanted them since I so them on goldmine....long time ago
Too bad goldmine run out of them, now my $ is safe....for now...

ScotchTapeLord

Thu Dec 08 2011, 10:44PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

The only application for a capacitor of those ratings would be an energy harvesting unit. Even then, 2600F is a lot.

The idea is that you have some tiny source of power to trickle charge the capacitor (solar cell, piezo transducer, a combination, etc) and then have a specialized low voltage boost converter convert it to a reasonable voltage for light or intermittent duty. The thing is, everything in the system has to be specialized for low voltage and high efficiency. They make chips that do all of this stuff in one.

Doesn't sound like a fun project, but it has its uses. One example is a device that periodically transmits a signal maybe once every few minutes that doesn't need a formal or reliable power source. i.e. You can put it outside to charge with the sun, and the boostcap will hold the charge overnight (if it's that good).
Though 2600F is a lot for this, and the size is pretty large.

You could also rewire something that needs one double A battery to accept the boostcap and make a better charger so you can recharge it quickly...

It wasn't dissipating 288 watts. If it had, it wouldn't have just been smoking. Resistance of a resistor rises with temperature, which resulted in fewer amps flowing than what you calculated, and a much lower wattage dissipation. That would explain why it wasn't charging quickly.

To charge it reliably, you should have a constant current source, either a buck step-down converter or linear. You'll get more power out with the buck. And to make sure you don't overcharge it, a feedback system.

Killa-X

Thu Dec 08 2011, 11:01PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

i bought 20 of these for $10 each and sold on ebay for $30 each. huge profit, i thought id have a use but nope, couldnt find any..

klugesmith

Fri Dec 09 2011, 06:56AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Dr. Duck wrote ...
Is there another...faster way of charging it?

Well, uh, ..., how about increasing the current?
Max charge = 2.6 kF x 2.5 V = 6500 coulombs. At 25 A, you would reach the rated voltage in only 4 minutes and 20 seconds!

Re. the smoking resistor.
If you charge the capacitor from a 12 V source with resistive current limiting
(perhaps some 12-volt lamps in parallel),
the first volt will put 1300 J into the cap and 30 kJ into the resistor.
(2600 coulombs x average voltages of 0.5 V and 11.5 V.)

Think of the Boostcap as a special kind of rechargeable battery.
It can deliver 1800 mAh at an average of 1.25 volts,
comparable to a very ordinary AA-size NiMH cell.
As a battery, it's outstanding in terms of:
* Fast charging and discharging;
* Many thousands of cycles without reduction in capacity
* Low energy dissipation from losses in the cycle.

They use ultracaps stacked to hundreds of volts in some hybrid city buses. Regenerative braking at every stop can absorb almost enough energy to get the bus back up to speed.

Steve Conner

Fri Dec 09 2011, 10:14AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Re "discharge really nicely":

Time constant = RC = 0.007 * 2600 = 18 seconds
Peak current = V/R = 2.5/0.007 = 357A
Peak power to load = V^2/(2R) = 446W

Not exactly a can crushing powerhouse. It'll be useless for the kinds of pulsed power experiments usually done around here.

Inducktion

Fri Dec 09 2011, 12:04PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

.7 milliohms. Not 7 milliohms.

But, yes, I know very well it won't work as a can crusher, that's not what they're made for!

Marko

Fri Dec 09 2011, 01:00PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

You could make a rapid fire handheld coilgun. You'd probably want's to use at least 2 boostcaps in series to get reasonable 5V from which you can drive a mazzili inverter made with motherboard mosfets, or whatever.

Or even use them to power a small handheld railgun directly (hint: neo magnets for augmentation + copper brushed projectile)

Marko

klugesmith

Fri Dec 09 2011, 05:43PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Dr. Duck wrote ...
.7 milliohms. Not 7 milliohms. But, yes, I know very well it won't work as a can crusher, that's not what they're made for!

Based on that worst-case ESR, which is probably conservative:

If you discharge the boostcap into a 6.3-mOhm load,
(ESR + R) x C will be 18 seconds, and 10% of the stored energy will heat the capacitor.

If you discharge it at a constant 260 amps (not a likely or useful load behavior),
it will run for 25 seconds, and 15% of stored energy will heat the capacitor.

So you should look for an application with a longer cycle time. How about a dynamo-powered bicycle headlight that stays lit while you're standing at a red light?

Daedronus

Fri Dec 09 2011, 09:40PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

12 in series = portable DC TIG welder.

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