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Old 17th October 2010, 04:36 AM   #1
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Default Charging a 1 Farad Cap - Concerns?

I recently picked up a couple of 1 farad car type capacitors. I was curious and a found a pair cheap. I know this is more of a car stereo related item but i am more interested in the technical side of it.

I see these car stereo guys connecting these caps directly to there battery. Now at first connection there is one helluva lot of charging current. Is there any concerns about this? I mean i gotta think that connecting a fully discharged 1F cap across a 12V battery has some concerns?

Once it is connected it should be ok. But what about constant current draw? how much current does it take to keep a 1F cap charged?

Are these caps really 1F??

I used my regulated power supply and set the current limit at .25a and let the caps charge up slowly (very) for the first couple of cycles and then used a .5amp 12V light bulb to discharge the cap. each cap would keep that bulb lit a good amount of time. maybe 40-50 seconds or more. I then opened the current limit to the max which is a little over 1.5amps. connecting a discharged cap would peg the supply for several seconds until the cap could equalize. I would imagine across a car batt the current draw would be huge for a few milliseconds.

Wouldn't a bank of smaller caps totaling the same 1F be better? I have to imagine that the same rules apply as in power supply design??

Zc
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Old 17th October 2010, 05:50 AM   #2
wwenze is offline wwenze  Singapore
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Car battery can provide up to 100A to start the car.

And wouldn't a bank of smaller caps result in lower ESR enough for enough current for a railgun? :P
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Old 17th October 2010, 07:30 AM   #3
djQUAN is offline djQUAN  Philippines
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yup. those are indeed 1F. as far as branded ones are concerned. I have measured only 0.85F for the cheapie no name ones from asia.

car audio installers usually use a light bulb in place of the main fuse to charge the cap then replace it with the proper rated fuse when fully charged. first time I played around a 2.5V 50F super cap and it kept my 2.5A CC/CV PS in CC mode quite a while before going to CV mode.
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Old 17th October 2010, 03:02 PM   #4
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Quote:
Originally Posted by wwenze View Post
Car battery can provide up to 100A to start the car.
A car battery can provide 500-1000amps into a dead short! and a fully discharged cap looks a lot like a dead short until it is charged. any chance these things can explode if charged to fast? I mean with the way i am seeing these connected. directly across the battery. if an unknowing auto tech disconnected the battery. that cap would discharge. said auto tech now goes to connect the battery again and BANG that cap is suddenly drawing hundreds of amps of current(for a few ms anyway).
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Old 17th October 2010, 05:33 PM   #5
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Quote:
Originally Posted by Zero Cool View Post
A car battery can provide 500-1000amps into a dead short! and a fully discharged cap looks a lot like a dead short until it is charged. any chance these things can explode if charged to fast? I mean with the way i am seeing these connected. directly across the battery. if an unknowing auto tech disconnected the battery. that cap would discharge. said auto tech now goes to connect the battery again and BANG that cap is suddenly drawing hundreds of amps of current(for a few ms anyway).
You are right, it can be potentially dangerous to connect such a huge cap uncharged to a car battery.
As a safety provision any such caps I have seen have an excess pressure seal (the small round rubber thingy You can see mostly located between or nearby the screw terminals). In case the thing boils the seal pops out (together with the electrolytic liquid, I think You can figure what nice mess this would be). If this seal doesn`t work correctly (old cap with hardened rubber) I could imagine that in rare cases such a cap could explode as well.

To prevent such a scenario, before connecting the cap it should always be charged from the battery via a small light bulb or a low value resistor in series with the cap (anything between about let`s say 10 to 50 Ohms / 5W will work). Depending on the value of the resistor or wattage of the bulb this should take from about a 20 seconds to maybe a minute for a 1F cap.
There are also caps for car audio applications which do have some sort of automatic soft charging (transistor) circuit already integrated.
While working on the car electricity, the cap should be disconnected and discharged via said bulb or resistor in parallel.

If I`d have such a cap in my car, I´d put a warning sign (maybe even together with short instructions how to unconnect and connect it) nearby or on the car battery so that a car mechanic can immediately recognise it after lifting the hood.
Better yet, disconnect the cap Yourself before You bring the car to the garage.
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Old 17th October 2010, 07:52 PM   #6
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Speaking of large F capacitors; I just came across these caps the other day.......

Maxwell BCAP 3000E 3000 Farad 2.7VDC BOOSTCAP® (Used)-The Electronic Goldmine

Yes, that's 3000F.


BTW, I think the only real danger of connecting a discharged 1F cap to a car battery is the resulting spark and damage to the connector. If it's some costly gold plated brass piece then that can be pretty upsetting.

Last edited by theAnonymous1; 17th October 2010 at 07:56 PM.
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