# Supercapacitors instead of Battery pack



## frodus (Apr 12, 2008)

They're VERY expensive and not very energy dense.... batteries are a much better solution right now. You would need a lot of them and they're not volumetrically dense either.


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## mizlplix (May 1, 2011)

As I understand it:

*They have a higher self discharge rate* than lithium based batteries. 

*They are heavier * than lithium based batteries.

*They are expensive too*. {true electrolytic Ultra or Super caps.} 3,000 Farad used cells go for $25.00 or so. they are 2.7 volts each needing 45 of them for a 120 volt system, cost of $1125.00 [OK I did find some 2600F caps for $810.00}

Their only advantage over a good lithium battery would be a higher charge/discharge rate.

I plan to {someday}parallel a Cap bank with my traction pack as a small buffer for maybe a sudden fast acceleration or sudden regen. Other than that, I am not too sure they are worth the extra problems they introduce in regards to service and maintenance of the vehicle.

They need balanced just like lithium batteries. Only they can just be drained to ZERO. Then a "dumb" battery charger can be placed on them to recharge them before re hooking to your pack with a balance resistor, then when equalized can be hooked directly up.

Miz


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## Ziggythewiz (May 16, 2010)

joamanya89 said:


> Have someone tried it already?


Yes, Brigham Young University has an electric dragster that runs on supercaps. If your commute is less than 1/4 mile, go for it.


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## joamanya89 (Feb 13, 2012)

But it has several advantage comparing with lithium battery, first they have arround 1000000 discharge cicle, and second you can charge it back in a few minutes...

But what I would like to know is if some one can explain me about the a/h that this things handle


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## Ziggythewiz (May 16, 2010)

joamanya89 said:


> But what I would like to know is if some one can explain me about the a/h that this things handle


Just like batteries, each one will be different, but in any case the Wh/$ will be 2-3 orders of magnitude less.



To get some perspective on how big a farad is, think about this:

A standard alkaline AA battery holds about 2.8 amp-hours.
That means that a AA battery can produce 2.8 amps for an hour at 1.5 volts (about 4.2 watt-hours -- a AA battery can light a 4-watt bulb for a little more than an hour).
Let's call it 1 volt to make the math easier. To store one AA battery's energy in a capacitor, you would need 3,600 * 2.8 = 10,080 farads to hold it, because an amp-hour is 3,600 amp-seconds.
So according to that math, the above mentioned pack for $1125 would have the energy equivalent of 15 AA batteries.


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## electric85 (Apr 10, 2008)

expensive and not far enough along yet....

from what i know (could be wrong) there have been a few claims of supercaps having amazing specs and could put lithium to shame...but they either get bought/squashed by big companies that are interested in keeping it off the streets, or the claims were just fairy tales...

...batteries threaten the oil companies...but not like a fast recharge successful supercap would...


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## blackpanther-st (Apr 4, 2009)

another problem with supercapacitors that I don't see mentioned here, is that there voltage discharge graph is nearly linear, with a voltage strictly proportional to charge level. whereas batteries have a voltage plateau across there usable range of charge level; in other words they attempt to some degree to maintain a consistent voltage until depleted. this means that with batteries you typically have a usable voltage across the entire discharge range of the battery, whereas with a capacitor or super capacitor the voltage quickly falls below a usable level far before the full charge capacity of the capacitor is depleted.


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## frodus (Apr 12, 2008)

I totally forgot to mention that, nice catch!

because of the linear discharge, the controller would have to function throughout the entire voltage swing of the capacitor pack.


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## MN Driver (Sep 29, 2009)

The fun thing about linear discharge is when you want peak horsepower. At your highest motor RPM down a drag strip, you've burned off most of your energy and back EMF together with your reduced cap voltage becomes your worst enemy. Better get a mega voltage controller or a massively oversized cap pack so you still can push full motor voltage towards the end of the track.


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## major (Apr 4, 2008)

blackpanther-st said:


> another problem with supercapacitors that I don't see mentioned here, is that there voltage discharge graph is nearly linear, with a voltage strictly proportional to charge level.





MN Driver said:


> The fun thing about linear discharge is when you want peak horsepower.


You guys are right, but everything is loaded with compromise. Let's say the drag race is 10 seconds. Is the battery rated for 360C discharge? If not, you will have energy left in the battery after the race is over. In other words you have too large of a battery for the mission.

With an ultracapacitor you can use 75% of the total energy stored in the cap in the top 50% of the working voltage range. That would be equal to 270C in battery terms.

As far as the working voltage range, all you need to do is design that peak power/peak speed point for half voltage. The motor controller is a buck converter so it won't matter if it is starting out the run at V or 2V.

For your weekend warrior electric drag racer ultracapacitors won't be the cat's ass anytime soon. But they do have some interesting attributes which might fit the bill somewhere. And I don't think ultracapacitors have been pushed (or should I say abused) enough to find out what the true potential is. Drag racers don't pay any attention to the motor nameplate. Why do so with the cap nameplate


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