# capacitor bank question



## vmrod (Jul 2, 2010)

anyone use a cap-bank? (with battery pack)

At first it sounds great that they can supply a short burst of power (say at take-off), also the life-span...etc.

However, once the cap-bank is empty, won't the battery pack now be trying to replentish the cap-bank while also powering the vehicle? Is it possible that this process could harm your battery? (If the cap-bank pulls greater amps than the battery bank can put out.)

I don't plan on using caps in my Opel GT, but MAYBE it could be a consideration for my trike.


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## Siwastaja (Aug 1, 2012)

vmrod said:


> At first it sounds great that they can supply a short burst of power (say at take-off), also the life-span...etc.


You would need a capacitor bank that would weigh in order of 100 kg / 200 lbs and cost thousands of $ to make any difference. Just use the same money and weight to buy more batteries, and you will get the same burst power effect, and for much longer bursts, including an increase in continuous efficiency and power (and of course, range as an added bonus).



> However, once the cap-bank is empty, won't the battery pack now be trying to replentish the cap-bank while also powering the vehicle? Is it possible that this process could harm your battery? (If the cap-bank pulls greater amps than the battery bank can put out.)


No, the current flows all the time from the battery to the capacitor bank because it is connected all the time. So the cap bank smooths out the current the battery sees and does not increase the peak current but lowers it just like you want. It's just that with current capacitor technology, not enough to be usable.

You want your peaks with as little voltage sag as possible; otherwise, it's the battery supplying, not the capacitor. Unfortunately, capacitors do not have flat discharge voltage curve like batteries do; they drop in voltage right from the start. This is why you can use just a small fraction of the capacitor's real capacitance, say 1/10th. Combine this with poor energy density of current ultracapacitors even if you counted every coulomb.

Summary: No, ultracaps are practically useless technology in EVs. Buy more/larger batteries instead.


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## Ziggythewiz (May 16, 2010)

You're not the first hundred to ask. You wouldn't be the first to try it. If it's possible, you would be the first to make something useful.

As Siwastasa said, whatever you can gain with caps you'd gain much more just by using the weight and $$$ for more lithium.


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## vmrod (Jul 2, 2010)

Thanks for the replies. They are logical. Just kinda curious.


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## dtbaker (Jan 5, 2008)

a couple years ago I poked around and found a huge cap bank.... built for capture/return of braking energy in trains and buses I think. I found one big enough to capture a braking event from 60mph, with the intention of being in parallel and dumping its voltage from 2x pack level to pack level on accelleration.

it was close to 200#, 30"x26"x14", and cost around $9k.

.....so, obviously, not a cost effective energy spring even for short braking events.


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## TexasCotton (Sep 18, 2008)

Siwastaja said:


> You would need a capacitor bank that would weigh in order of 100 kg / 200 lbs and cost thousands of $ to make any difference. Just use the same money and weight to buy more batteries, and you will get the same burst power effect, and for much longer bursts, including an increase in continuous efficiency and power (and of course, range as an added bonus).
> 
> No, the current flows all the time from the battery to the capacitor bank because it is connected all the time. So the cap bank smooths out the current the battery sees and does not increase the peak current but lowers it just like you want. It's just that with current capacitor technology, not enough to be usable.
> 
> ...


What about between two different battery banks/packs?


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## Ziggythewiz (May 16, 2010)

Why would you need capacitors between two battery banks?


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## Siwastaja (Aug 1, 2012)

And you should practically never need two or more battery banks. It just lowers efficiency and performance compared to one larger bank, and switching the banks is a complex process which needs active safety circuitry.


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## PStechPaul (May 1, 2012)

One reason that capacitors are not suitable for energy storage in EVs is that their energy is proportional to V^2, while battery packs are essentially a constant voltage with a variable series resistance and considerable inductance. The controller needs to supply high peak current during PWM transitions so it has its own low impedance capacitor bank built-in, although an external bank may be helpful. The capacitors also absorb transients from the inductive switching. But these are very fast events in the order of a few microseconds, so the capacitance does not need to be huge. 

As an example, if you have a 144V pack with 0.01 ohm series resistance and 0.1 ohm inductive impedance, a 1000 amp surge will cause a drop of 10 volts to 134V from the resistance, but 100V during the transition. To keep the voltage at least 134V for, say, 10uSec, you need to store 144*1000*10=1.44 J (W-sec). The formula to determine the capacitance in this case would be:

1.44 = 0.5 * C * (144^2 - 134^2) 

so

C = 2.88 / (20736-17956) = 1036 uF

That should be enough to clean up the PWM waveform and give maximum efficiency and power. This may also increase efficiency because energy will be transferred between the capacitors and the supply inductance which have lower losses than the series resistance. Under low values of PWM it will have the effect of a smoother current flow from the batteries at a much lower current so the power loss will be reduced. At 10% PWM and 14.4V at 100A out of the controller, the battery current will be 10A and the power loss will be 100*0.01 = 1W. But without the capacitors, there will be 1000A pulses about 10uSec wide every 100uSec (10 kHz), so there may be 1000A^2*0.01 = 10kW at an effective PWM of 1% or 100W. at 1.44 kW that's about 7% loss. 

I just made up the resistance and inductance values, and I don't guarantee that my calculations are 100% correct, but you can see the principles involved. If there is a lot more impedance in the battery circuit, a larger capacitor may be useful, up to the point where it would hold a solid bus voltage over an entire PWM cycle, or about 100uSec. So in that case maybe 10,000 uF would be good, and that is still a reasonable size and cost. But you also need to sure it has low ESR and high current capacity, which may be best achieved with multiple capacitors in parallel and film type construction.


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## TexasCotton (Sep 18, 2008)

Siwastaja said:


> And you should practically never need two or more battery banks. .


Well most hybrid are going to have a OEM traction pack .....Therefore to supplement(add too) the OEM pack a secondary pack is the next step.


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## Siwastaja (Aug 1, 2012)

TexasCotton said:


> Well most hybrid are going to have a OEM traction pack .....Therefore to supplement(add too) the OEM pack a secondary pack is the next step.


You should add it parallel with the original pack, not as a separate, switchable bank.

Or, if it really is an "old-style" non-plugin hybrid, the pack may be too small to be of much use, just sell it and replace with a large enough pack.


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## TexasCotton (Sep 18, 2008)

All the previous post are logical. Kind a of lost on OLD style. I do realize caps have limited/specific use.
http://en.wikipedia.org/wiki/Electric_double-layer_capacitor


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## mizlplix (May 1, 2011)

Caps also have a higher self discharge rate than a lithium traction pack.

Which complicates parallel connecting to a battery long term...like parked...

Miz


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## SHARKBITEATTACK (Nov 12, 2012)

I've though a lot about as well. I had thought of using some of those capacitors that are used on high end car stereo systems with the big subwoofers. 

This is pretty interesting too. EV Buses in china that use capacitors and rapidly recharge at every station. http://en.m.wikipedia.org/wiki/Capa_vehicle


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