I know they have low energy density, but what could be done in a 18650 or 2650 sized package, i assume they have little voltage sag, but it would be very quickly chargeable and last for many, many charges
How much would it cost to build and how many watt-hours would it give?
How long could it hold a charge (are we talking minutes, hours or days)?
No criticisms please, i know they would not be very practical, but as a thought experiment please humour me
There are/were a few comercially available flashlights that used capacitors instead of batteries. I would like to build one sometime myself. There are a few DIY projects on the web, but the electronics part will get me
Your best bet is ultracapacitors, but they still fall short in terms of energy density:
Source: http://gigaom.com/2011/07/12/how-ultracapacitors-work-and-why-they-fall-short/
Their voltage drain graph is also linear, unlike the flat ones for li-ions.
I don’t think ultracaps are expensive, probably as expensive at good li-ions themselves at the same size.
Perhaps a good use would be lights that need to be always ready. You can float charge the caps and when you need them, you can pull them out and turn on the light. A car light that powers off the cigarette jack?
They're available in D cell size: http://www.ebay.com/itm/300945126145
The catch is, they can provide full amperage all the way down (or nearly) to zero volts, so to use a good portion of their capacity in a constant drain circuit like a flashlight you'd need a boost driver of the sort that more or less violates the laws of physics. They're best suited for short duration high current situations (like a cranking battery), which is about the exact opposite from what you need to power a flashlight.
Now if you had a flashlight that needed a couple hundred amps for only 20-30 seconds...
My Nightstar is one of those.
Oh man. |(
Carbon arc 3D Maglite? :evil:
Yeah, Comfy hit the problem. The voltage drops are they are used. So you may start at 4.2 volts, turn on and one second later be at 2.5 volts (depending on cap sized and current to emitter).
And even worse, what happens when you're down to .6 volts but they're still capable of giving several hundred amps?
Anybody else read the Maxwell datasheets on some of these things? Completely crazy stuff.
My “forever flashlight” used a capacitor instead of a battery. The capacitor was a bit smaller than a 16340/RCR123 battery, and was able to power a fairly dim LED for a few minutes at a time. However, I don’t think it ever got fully charged, since its only power input was a magnet and copper coil plus shaking it back and forth. IIRC it got only a couple lumens, for a maximum of about 5 minutes. And it was easily the biggest flashlight I’ve ever had, and hard to carry anywhere because its magnetic field extended several feet outward.
I still have it, but the capacitor doesn’t seem to hold much of a charge any more. I guess it wasn’t as “forever” as it claimed, considering I’ve had non-LSD NiMH batteries last longer (years of reusability) than it did.
I also had some sort of dynamo crank light, but it died relatively quickly. I think it had a tiny battery inside, which wasn’t well-suited to that type of use and stopped holding a charge after about a year.
In both cases I find I’m much better off with a tiny 1xAAA or 1xAA light and a few extra batteries — smaller, brighter, easier, quieter, and no significant difference in total runtime unless I’m planning on attending a zombie apocalypse. If I wanted to survive the end of the world, I’d probably rely on my ZL SC52. It’s the toughest light I’ve tried and it gets the longest run times of pretty much any common-battery light on the market.
I have a couple dozen Maxwell 58 farad/15 volt modules… came out of windmill prop pitch controller. Each module can do around 6500 watt-seconds, 12 millohm ESR, 1000 amp short circuit current.
Btw, an interesting observation on the winder type torches.
The CHEAP ones are a bit of a trick. They have a switch with 2 positions, On and Off (generator).
In the “off” position, you wind the generator to generate power, and the LED lights up for a bit. very quickly though the LED stops working. In the ON position, the LED lights brightly, and appears to last forever. Winding the generator crank doesnt seem to give you much more boost.
When I took one apart, what I found was that in the OFF position, the LED was connected to the motor. your hard winding would only barely light up the LED. In the ON position, the generator is completely disconnected, and power to the LED comes from a button cell lithium (non rechargeable). Battery power keeps the led shining significantly brighter, but still nothing to write home about. So all your winding is not actually charging anything, except supplying the LED itself, and totally not worth the effort for the miserly output.
These were anywhere from $1 to $10 each, small and relatively light. Not to mention that the winder breaks apart relatively quickly. Really all it is is a button cell keychain light, with a little motor setup to power the led to sell it as a “generator” torch. You would be much better off with a couple keychain torches for the same expense, and end up with a lot less waste.
:O
Also, note that when you connect the capacitors in series to raise the working voltage of the module, you have to put a balancing circuit across each cap. For low end modules this is just a resistor which considerably increases the apparent self-discharge leakage current.
More complex modules that are not kept continuously charged use an active balancing circuit. Either way, the caps self discharge faster than you can say “leaky NiCad.” Caps are no substitute for batteries…