# How to discharge 19000 watt capacitor bank



## ChargeCar (Jun 17, 2008)

I was curious how one would go about building a dummy load to discharge a bank of (total) 19000 watt ultracapacitors? I don't own these, I was just reading about them and thinking about how to use them in an EV and was curious how you would build a device to discharge them in the event you want to move them around (after charging). 

I recall in EE power classes (which I paid too little attention to thinking all this high power stuff was boring) we had these massive ceramic coated resistors and fans blowing over them but in looking around on the net I don't see where you might get those or how you would go about building something to do this...

Thanks


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## major (Apr 4, 2008)

ChargeCar said:


> I was curious how one would go about building a dummy load to discharge a bank of (total) 19000 watt ultracapacitors? I don't own these, I was just reading about them and thinking about how to use them in an EV and was curious how you would build a device to discharge them in the event you want to move them around (after charging).
> 
> Thanks


Hi ChargeCar,

You don't give the energy for the cap. Or how fast you need it discharged. I've discharged large banks of ultracaps often. Resistors work well. At higher voltages, you draw an arc on contact closure. So you have to take some measures. Maybe discharge in lower voltage sections, or with more Ohms which would take longer. For a fast discharge of a high voltage pack, with repeatability and reliability, you'll probably need a buck converter and power resistor.

Then again, why do you need to discharge it? Do you discharge batteries? Guess I do for testing, not for in service use. Battery and capacitor discharge test stands are pretty much the same. I use a bunch of one kilowatt resistors. They're common as locomotive braking resistors. Actually can be found on the surplus market.

Hope that helps,

major


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## ChargeCar (Jun 17, 2008)

major said:


> Hi ChargeCar,
> 
> You don't give the energy for the cap. Or how fast you need it discharged.
> 
> I've discharged large banks of ultracaps often. Resistors work well. At higher voltages, you draw an arc on contact closure. So you have to take some measures. Maybe discharge in lower voltage sections, or with more Ohms which would take longer. For a fast discharge of a high voltage pack, with repeatability and reliability, you'll probably need a buck converter and power resistor.


This is the thing I was looking at:
http://www.maxwell.com/ultracapacitors/products/modules/bmod0063-125v.asp

Since this isn't something you would do regularly, time wouldn't matter. So discharging over say an hour or 2 would be fine. I don't think it would be possible to do it piecemeal so you would need to do it via the main terminals (high voltage/current).



major said:


> Then again, why do you need to discharge it? Do you discharge batteries? Guess I do for testing, not for in service use. Battery and capacitor discharge test stands are pretty much the same. I use a bunch of one kilowatt resistors. They're common as locomotive braking resistors. Actually can be found on the surplus market.


You wouldn't need to discharge it necessarily, unless you wanted to move it around and were paranoid of high voltage, as I am  

Do you have a link to the resistors and/or discharge test stands?



major said:


> Hope that helps,
> 
> major


It does, thanks for the reply!!

CC


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## major (Apr 4, 2008)

ChargeCar said:


> This is the thing I was looking at:
> http://www.maxwell.com/ultracapacitors/products/modules/bmod0063-125v.asp
> 
> Since this isn't something you would do regularly, time wouldn't matter. So discharging over say an hour or 2 would be fine.
> CC


Hi CC,

That's 125 volt pack, so chances are a 100 watt light bulb or two would do the trick because you can wait for it to dim out. Would take a while to discharge to a safe voltage, 10 volts or so. You might want to switch in parallel bulbs as voltage decays. That'd be an inexpensive way to go.

Next question: How you going to charge it back up? Well, you can use the bulbs (or resistors) in series with a battery pack if you don't have a power supply with a current limit. I think that is how BYU charged their ultracap drag car from batteries at the track.

I'll see if I can scan in an old picture of my test stand. Be a few days.

Regards,

major


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## major (Apr 4, 2008)

ChargeCar said:


> Do you have a link to the resistors and/or discharge test stands?
> CC


Hey CC,

Here is a test stand I was able to use for ultracapacitor testing. Man, is it sweet. I think retail is a bit pricey $$$$$$. And installation, more $$$$.

http://www.avinc.com/PowerProcessing_product_details.asp?Prodid=19 

Lucky I was able to "borrow" it. When on my own resources, I made my own. I'll try to post some pics later.

Regards,

major


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## ChargeCar (Jun 17, 2008)

major said:


> Hey CC,
> 
> Here is a test stand I was able to use for ultracapacitor testing. Man, is it sweet. I think retail is a bit pricey $$$$$$. And installation, more $$$$.
> 
> ...


Very nice. I am sure that is indeed pricey! Thanks, look forward to the pics.

CC


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## kixGas (May 2, 2008)

major said:


> Hi CC,
> 
> That's 125 volt pack, so chances are a 100 watt light bulb or two would do the trick because you can wait for it to dim out. Would take a while to discharge to a safe voltage, 10 volts or so. You might want to switch in parallel bulbs as voltage decays. That'd be an inexpensive way to go.
> 
> ...


Just throw them in your pool!! LOL
Off topic- but I think it is cool that BYU is doing a drag car. I got my degree there and did a little work on the formula 1 car- electric blue with Perry Carter. It was a cool project but I never got to drive it


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## lazzer408 (May 18, 2008)

100w household lightbulb.


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## major (Apr 4, 2008)

major said:


> I'll see if I can scan in an old picture of my test stand. Be a few days.
> 
> Regards,
> 
> major


Hey,

I finally got the picture scanned in. So here is my resistor load bank. Twelve 1 kilowatt locomotive braking resistors. Since that picture, I have doubled the number. I can discharge battery banks up to 400 volts at up to about 400 amps. Usually do about 200 volts at 300 amps for up to 10 minutes. I have used it for ultracapacitors also.

Hope it isn't too late for this thread.

major


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## ChargeCar (Jun 17, 2008)

major said:


> Hey,
> 
> I finally got the picture scanned in. So here is my resistor load bank. Twelve 1 kilowatt locomotive braking resistors. Since that picture, I have doubled the number. I can discharge battery banks up to 400 volts at up to about 400 amps. Usually do about 200 volts at 300 amps for up to 10 minutes. I have used it for ultracapacitors also.
> 
> ...


Thanks, very nice. Where did you say you get those things?
-CC


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