# Capacitors ??



## oddpowers (Dec 9, 2008)

Can some on tell me if my calculations are correct? I'm looking at capacitors to help fight voltage sag on take off. (racing)
72v system

Capacitor is mallory 10000MFD 200v
10000MFD = 1 Farad * nominal voltage of 76v = 76 amp/seconds
15 of these in parrallel = 1140 amp/seconds

So for one second, I could shore up my voltage with 1140 amps. Is this correct or am I missing something?


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## lazzer408 (May 18, 2008)

They'll be dead in .00000000000001 sec. 

Cap's are good for reducing losses when the controller is _under_ 100% PWM. They provide peak currents that the battery can not due to the battery's internal resistance and resistance in the cabling.


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## subcooledheatpump (Mar 5, 2012)

Let's see....
Energy in a capacitor = .5 * Capacitance (* voltage squared) 

so .5 * 15 Farad = 7.5 

76 V squared = 5776 

7.5 * 5776 = 43,320 joules 

Joules to watt hours, 1 watt hour = 3600 joules 

43,320 / 3600 = 12.03 watt hours 

convert to watt seconds, multiply 12.03 by 3600 

12.03* 3600 = 43308
So you will get about 43308 watts for one second

Original calculation called for 1140 amps at 76 volts which would be 86,640 watts 

divide 43308 by 86,640

Capacitors will stay charged for .499 seconds


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## oddpowers (Dec 9, 2008)

well crap....


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## major (Apr 4, 2008)

oddpowers said:


> Can some on tell me if my calculations are correct? I'm looking at capacitors to help fight voltage sag on take off. (racing)
> 72v system
> 
> Capacitor is mallory 10000MFD 200v
> ...


Hi odd,

First off, it is a shame they use the units representation of MFD. What is meant is uF or microFarads. So 10,000uF = 0.01F.

I am quite sure there is no 10F 200V capacitor made by Mallory. Here is a 42V 11F supercap: http://www.tavrima.com/pdf/10_42.pdf It weighs 25 pounds. A 10F 200V cap would be like 20 times bigger.

Also, capacitance = charge / Voltage. Farads = Coulombs / Volts. A Coulomb is an Ampere second. So IMMIC, Current = Farads*Volts/seconds.

IMMIC = If My Math Is Correct 

You may find this interesting. An EV drag racer using ultracapacitors. http://www.altenergymag.com/emagazine.php?issue_number=04.04.01&article=dragster


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## lazzer408 (May 18, 2008)

subcooledheatpump said:


> Capacitors will stay charged for .00831 seconds


So I was a little off. It's 831,000,000,000 times more power then I estimated, but equally insignificant.


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## subcooledheatpump (Mar 5, 2012)

oops, I made a mistake. 

I said to convert to watt seconds multiply by 60...that is incorrect 

I forgot minutes. 

multiply by 3600. above post will be corrected

so assuming a full 15F that would stay charged for half a second... not bad

Edit: Major is right though. it takes one million micro farads to make one farad. 

Atleast now you know what 15 (actual) farads at 76 volts will get you


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## lazzer408 (May 18, 2008)

Supercaps?


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## Duncan (Dec 8, 2008)

oddpowers said:


> Can some on tell me if my calculations are correct? I'm looking at capacitors to help fight voltage sag on take off. (racing)
> 72v system
> 
> Capacitor is mallory 10000MFD 200v
> ...


OK
10,000MFD = 0.01Farad - 1000,000 microFarad = 1 Farad

Anyway lets assume you got the 1 farad capacitor

76 volt battery - will sag say 20v (a lot) from 76v to 56v

So the capacitor will go from 72v to 56v 
The charge = voltage x capacitance

So the capacitor will release 20 coulombs - that is 20 amp seconds

Or 20 amps for one second

Not going to do you a whole lot of good


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## oddpowers (Dec 9, 2008)

Thanks for all the replys. As you can see I don't know much about electronics.

Thanks again!


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