Capacitor Discharge Rate

Discussion in 'General Discussion' started by TZ250, Aug 3, 2008.

  1. TZ250

    TZ250

    267
    Jul 16, 2008
    Ohio, USA
    Recently a friend and I were chatting about a capacitor that I installed in front of a car audio amp.

    My question is, what is the maximum discharge current of a 1Farad capacitor at 12V? By maximum current, I mean a short circuit, not into a different load or amp.

    I know there are a few things that will have to be generallized because I don't have all of the specs of the capacitor.

    I know that capacitors can be dangerous. There was a guy nearby that died while installing his capacitor in the trunk of his car. That was about 15 years ago, but I still remember the lesson.

    Thanks in advance.
     
  2. Lurker

    Lurker

    Jul 21, 2007
    NJ
    1 Full Farad? :eek: 

    Are you sure it's not a milli or micro Farad?
     
  3. TZ250

    TZ250

    267
    Jul 16, 2008
    Ohio, USA
    It's supposed to be 1 Farad. I believe it. It is approx. 3.5" diameter and about 10" long.
     
  4. gadgetguy11

    gadgetguy11

    Nov 16, 2005
    Kentucky
    That is 1 full Farad, for sure. In the "old days" 1 Farad was not heard of. Today, I have a 5 Farad capacitor bank in the trunk of my Lexus to keep voltage rock solid to a 1000 watt JL Audio bass amp. The size you described is typical of 1 Farad at 12 volts DC.

    I do not know the maximum discharge current in the case of a short circuit.
     
  5. TZ250

    TZ250

    267
    Jul 16, 2008
    Ohio, USA
    :eek: 
    5 Farads! Wow!

    I'm using my 1F capacitor for a PPI 2150. (150 RMS, 2 channels) I'm running two 4-Ohms 10's wired in parallel. The amp is bridged. I don't need any more bass than that.

    Gadgetguy, which subs are you running?
     
  6. Lurker

    Lurker

    Jul 21, 2007
    NJ
    There's a very simple simulator for a "standard" capacitor/resistor/inductor circuit. If you plug the numbers in there, you'll get around 25 Amps for a resistance of .5 Ohm (which is, really, pretty low) and around 1000 Amps for a resistance of 0 Ohm (unless you have some supercooled helium laying around, not likely).

    As resistance in your circuit goes up, current will go down. The bad news: since the capacitor is fully charged it will simply take longer to completely discharge. That 1000 Amps might not be so bad since it will last for maybe 50ms, where that 25 Amps could last for the better part of a second - enough to do some serious damage.
     
  7. gadgetguy11

    gadgetguy11

    Nov 16, 2005
    Kentucky
    I run a JL Audio 12W7 in a 1.25Cu ft custom enclosure (it was reviewed as equivalent to 4x 15"'s and requires 500 Watts RMS to drive it). Amp is JL Audio 1000/1 1000 watt 1 channel with regulated power supply and EQ internal card, with remote gain knob hidden in the door pocket. Materials are not as costly as labor, so I bought the best cap, amp, and woofer I could find. The supply can drive 2) 12W7's in parallel easily, so I ran the conductor (I believe it was 4 AWG), the 5 Farad cap, and the 1000 watt amp, and am ready to add a second woofer / enclosure or a 13W7. - but hate to give up any more trunk space.

    We added 125 sq feet of Dynamat Xtreme so unlike the loud stereo's driving by, my Lexus keeps the tunes INSIDE - I focused on quality audio, not SPL. The Dynamat made a huge improvement, as did the FOCAL mid's and twets driven by Nakamichi amps.

    BTW, the 5 Farad is unusually shaped. It looks about the size of Apple TV only it is black with aluminum heatsink mounting plate. It contains calcium (not sure why).

    Here is a link to 5 Farad Caps. They dropped in price 5 fold since I bought one :redface::

    http://www.nextag.com/5-farad-capacitor/search-html
     
  8. genera

    genera

    Oct 6, 2005
    California
    I think that if the terminals were shorted the capacitor's internal resistance would be the only thing limiting the discharge current. Check with the manufacturer, that info should be available.



    0 ohms may not be practical but a 1' length of 12 awg copper wire has a resitance of about 0.0015 ohms.
     
  9. John, can you install a car stereo for me?! You've got it made, man.
     
  10. gadgetguy11

    gadgetguy11

    Nov 16, 2005
    Kentucky
    LOL! Shane, this is a shot of the 12W7 in my trunk. The 1:18 model with the trunk open is to show the car and give perspective of where we are looking. JL does not offer a grill (the sub cone is extremely tough so there is no need), so I fabricated a fan cover, & stuck the Lexus trunk ornament on it. It gives it a "factory" appearance in the trunk.

    The 1000/1 amp manual with photo of the amp is on the lower left; W7 manual on the lower right. The shiny silver at the top is the back side of 125 square feet of Dynamat Xtreme sound deadening. The 1000/1 can barely be seen at the top of photo. 5 Farad capacitor, Nakamichi amps and active crossovers are behind a panel.

    :wink:

    344898649_bWbLo-X3.
     
  11. WOWZA! I have a puny little Kenwood in my trunk (250W) and a 250W Amp in the back. How much does that setup cost?!
     
  12. gadgetguy11

    gadgetguy11

    Nov 16, 2005
    Kentucky
    Amp $1K
    Woofer $1K
    Cap $250
    Focal tweets: 8 x $500
    Focal mids: 4 x $500 (approx)
    Xtreme Insulation $500 approx
    Power block, 4 gauge power wire, battery terminal, Esoteric Audio speaker wire (I don't remember $)
    Active crossovers (?)
    Nakamichi amps (?)

    With helper labor and the items I forgot exact $, total was between $10K and $15K.
     
  13. Power capacitors do more harm than good in car audio set ups.

    When your bass peaks and drains your capacitor, not only does your battery and alternator have to keep up the demand for the amplifiers, but it also has to recharge the capacitor.

    The best thing to do is rather than a capacitor, run a 2nd battery on dual charge, or a larger battery. Also upgrade your earth cable from the battery, and your alternator cables. These will do a much better job than a capacitor in an audio set up.
     
  14. I envy you, John. YOu truly have the best of everything :) 
     
  15. gadgetguy11

    gadgetguy11

    Nov 16, 2005
    Kentucky
    Shane, it may seem that way because we share common passions and hobbies in life. I am (a lot) older than you, so I've accumulated more stuff (junk).

    I value friendships & relationships most of all. The more time passes, the truer this becomes!
     
  16. Either way, John, you're a great role model! Thanks!
     
  17. tintingkc

    tintingkc Guest

    A second battery can help but never cures the problem. A capacitor takes the load off the battery and supplies needed current to the amp. Much more than a battery and can handle the spikes that a battery can't. I have tried all different combos in my 14 years of mobile installations and a second battery never works as well as a cap. Caps charge very fast and store more power for the amp when you system is layed out right. 1 farad for every thousand watts. When you get nuts then more batteries are needed but they will never replace a cap in a properly laid out system.

    For the OPs questions though. There are many factors. All caps are not the same. Wire guage will also play a part. Most caps will release all energy in less then 20 seconds when at a dead short. Some quicker and some longer. It doesn't take long. They can be very dangerous if not installed correctly.
    Sounds like you have heard a story. I have yet to hear of a death due to a cap for automotive use. Seen plenty of burns and welded parts but a death is a bit far fetched.
     
  18. tintingkc

    tintingkc Guest

    The GS is what got my attention. GS guy here too.
     
  19. gadgetguy11

    gadgetguy11

    Nov 16, 2005
    Kentucky
    GS= "Good Stuff!" :wink:
     
  20. tintingkc

    tintingkc Guest

    CRW_0001.
    Thread jack over...
     
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