do I need a capasitor
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Registered User
Joined: Sep 2002
Posts: 1,429
From: plano texas
do I need a capasitor
System will include a 520w amp with a sub that is rated at 400rms and 1600max with an everlast battery, dunno what other info you'de need to judge whether its needed or not but I dont want to spend the money when its not needed.
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Joined: Nov 2000
Posts: 1,858
From: OHIO Go Buckeyes!
Whether or not you need one depends on how much current the stereo demands over the amount of current the car's charging system can supply. If your stereo needs only a little more than the car can provide a cap can help. However if your stereo is drawing considerably more current than the car can supply a cap is a waste of money. The cap would be doing more harm than good. Thats when you need to get a high output alternator.
If your lights dim on loud bass hits from the stereo I would forgo the cap all together and just get the HO alternator.
If your lights dim on loud bass hits from the stereo I would forgo the cap all together and just get the HO alternator.
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Joined: Dec 2003
Posts: 4
From: Racine,WI
If you have small dimming issues what I always do on Big systems or any system if you want....Run 4 ga wire from your alternator to your battery, Re-ground your engine to chassis with bigger or 4 ga wire...and Re ground your battery with 4 ga wire...always works GREAT!!
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Posts: n/a
you say it is going ot be 520 watts. now every manufacyurer rates there amps differently. what kind of amp is it?
the same goes with subs u say it i s400/1600( that i s quite a jump) just by them umbers i can tell u that is most likly not a high end sub. manufacturers put umbers like that to make u buy.
i would make sure the amp is really 500 watts b4 u consider upgrading ur charging system.
the same goes with subs u say it i s400/1600( that i s quite a jump) just by them umbers i can tell u that is most likly not a high end sub. manufacturers put umbers like that to make u buy.
i would make sure the amp is really 500 watts b4 u consider upgrading ur charging system.
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Joined: Jul 2001
Posts: 87
From: Birmingham, AL
You don't ever need a capacitor... unless it is just for looks.
Read this... http://www.welcometotheden.8k.com/caraudio/Captest.pdf
Read this... http://www.welcometotheden.8k.com/caraudio/Captest.pdf
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Joined: Jun 2003
Posts: 136
From: Flint, Michigan
Not sure I fully agree with the charts that GoghUA posted. Although it does show that with or without the cap the battery/alternator is able to stay charged. What the cap is actually used for is peaks, like when you get your big bass hit, not just the average music that is playing all through the song ---- ONLY for the BIG BASS HITS!!! This is when your headlights normally dim. This is what you are trying to prevent with a cap, not keeping the battery from discharging which is what the charts are suggesting. With the cap on while the music is playing normal and there are no big bass hits it will charge itself for the next hit. When this hit happens the cap will be discharged to supply the power and allow the battery/alternator to feed the rest of the car. This will keep your lights from dimming because the battery will now feed them instead of your subs. The cap is able to charge itself back up fully faster than a CD can have 2 bass hits in a row which means that every bass hit will now be covered by the cap and no longer by your battery/alternator. This is why sometimes only the amp powering the subs is hooked up to the cap and not the amps that power the mids and highs. You could hook the system up and try it without one and see how it goes. Then if your lights start dimming you could try what SSon22s suggested with rewiring the battery/alternator. If this does not help you could then price out a HO alternator like LSI RULZ suggested and make a choice between it and the cap. Of course this is only if you are not building custom enclosures and displaying you cap. If this is the case you would then end up redoing your enclosures to fit the cap at a later date. hope this all helps.
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Joined: Mar 2001
Posts: 2,667
From: SLC, UT
Originally posted by browneye97ss
Not sure I fully agree with the charts that GoghUA posted. Although it does show that with or without the cap the battery/alternator is able to stay charged. What the cap is actually used for is peaks, like when you get your big bass hit, not just the average music that is playing all through the song ---- ONLY for the BIG BASS HITS!!! This is when your headlights normally dim. This is what you are trying to prevent with a cap, not keeping the battery from discharging which is what the charts are suggesting. With the cap on while the music is playing normal and there are no big bass hits it will charge itself for the next hit. When this hit happens the cap will be discharged to supply the power and allow the battery/alternator to feed the rest of the car. This will keep your lights from dimming because the battery will now feed them instead of your subs. The cap is able to charge itself back up fully faster than a CD can have 2 bass hits in a row which means that every bass hit will now be covered by the cap and no longer by your battery/alternator. This is why sometimes only the amp powering the subs is hooked up to the cap and not the amps that power the mids and highs. You could hook the system up and try it without one and see how it goes. Then if your lights start dimming you could try what SSon22s suggested with rewiring the battery/alternator. If this does not help you could then price out a HO alternator like LSI RULZ suggested and make a choice between it and the cap. Of course this is only if you are not building custom enclosures and displaying you cap. If this is the case you would then end up redoing your enclosures to fit the cap at a later date. hope this all helps.
Not sure I fully agree with the charts that GoghUA posted. Although it does show that with or without the cap the battery/alternator is able to stay charged. What the cap is actually used for is peaks, like when you get your big bass hit, not just the average music that is playing all through the song ---- ONLY for the BIG BASS HITS!!! This is when your headlights normally dim. This is what you are trying to prevent with a cap, not keeping the battery from discharging which is what the charts are suggesting. With the cap on while the music is playing normal and there are no big bass hits it will charge itself for the next hit. When this hit happens the cap will be discharged to supply the power and allow the battery/alternator to feed the rest of the car. This will keep your lights from dimming because the battery will now feed them instead of your subs. The cap is able to charge itself back up fully faster than a CD can have 2 bass hits in a row which means that every bass hit will now be covered by the cap and no longer by your battery/alternator. This is why sometimes only the amp powering the subs is hooked up to the cap and not the amps that power the mids and highs. You could hook the system up and try it without one and see how it goes. Then if your lights start dimming you could try what SSon22s suggested with rewiring the battery/alternator. If this does not help you could then price out a HO alternator like LSI RULZ suggested and make a choice between it and the cap. Of course this is only if you are not building custom enclosures and displaying you cap. If this is the case you would then end up redoing your enclosures to fit the cap at a later date. hope this all helps.
I'm running an HO alternator, and 2 1-Farad Caps... without the caps the lights were horrible, now with the Caps, it's lessened it about 90%.
But then again I'm drawing about 120 amps on that amp when it "hits" the subs.
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Joined: Oct 2001
Posts: 117
From: Atlanta, GA (in a burb)
Originally posted by browneye97ss
Not sure I fully agree with the charts that GoghUA posted. Although it does show that with or without the cap the battery/alternator is able to stay charged. What the cap is actually used for is peaks, like when you get your big bass hit, not just the average music that is playing all through the song ---- ONLY for the BIG BASS HITS!!! This is when your headlights normally dim. This is what you are trying to prevent with a cap, not keeping the battery from discharging which is what the charts are suggesting. With the cap on while the music is playing normal and there are no big bass hits it will charge itself for the next hit. When this hit happens the cap will be discharged to supply the power and allow the battery/alternator to feed the rest of the car. This will keep your lights from dimming because the battery will now feed them instead of your subs. The cap is able to charge itself back up fully faster than a CD can have 2 bass hits in a row which means that every bass hit will now be covered by the cap and no longer by your battery/alternator. This is why sometimes only the amp powering the subs is hooked up to the cap and not the amps that power the mids and highs. You could hook the system up and try it without one and see how it goes. Then if your lights start dimming you could try what SSon22s suggested with rewiring the battery/alternator. If this does not help you could then price out a HO alternator like LSI RULZ suggested and make a choice between it and the cap. Of course this is only if you are not building custom enclosures and displaying you cap. If this is the case you would then end up redoing your enclosures to fit the cap at a later date. hope this all helps.
Not sure I fully agree with the charts that GoghUA posted. Although it does show that with or without the cap the battery/alternator is able to stay charged. What the cap is actually used for is peaks, like when you get your big bass hit, not just the average music that is playing all through the song ---- ONLY for the BIG BASS HITS!!! This is when your headlights normally dim. This is what you are trying to prevent with a cap, not keeping the battery from discharging which is what the charts are suggesting. With the cap on while the music is playing normal and there are no big bass hits it will charge itself for the next hit. When this hit happens the cap will be discharged to supply the power and allow the battery/alternator to feed the rest of the car. This will keep your lights from dimming because the battery will now feed them instead of your subs. The cap is able to charge itself back up fully faster than a CD can have 2 bass hits in a row which means that every bass hit will now be covered by the cap and no longer by your battery/alternator. This is why sometimes only the amp powering the subs is hooked up to the cap and not the amps that power the mids and highs. You could hook the system up and try it without one and see how it goes. Then if your lights start dimming you could try what SSon22s suggested with rewiring the battery/alternator. If this does not help you could then price out a HO alternator like LSI RULZ suggested and make a choice between it and the cap. Of course this is only if you are not building custom enclosures and displaying you cap. If this is the case you would then end up redoing your enclosures to fit the cap at a later date. hope this all helps.
"One note I might add is that this was a two thousand watt system driven riht to clipping...with a stock 80 amp alternator."
I edited out some points of the statement but left the important points. In that test they were driving the system to clipping on a 2K watt system and if you notice during some points of the test (when the 15F cap was hooked up) the votlage of the electrical system was 'lower' because it was charging up the cap.
The arguement of the cap discharging all this butt load of current is all marketing. Sure they will discharge some but not as much as you'd think. If you'd like the math behind all of this I will post it later when I get home but all in all a cap is not worth the money so don't waste your money on it. You see above in the test what a 15F cap does on a system with 2K watts and a 80 amp alternator so what do yout think a 1F is going to do?
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Joined: Mar 2001
Posts: 2,667
From: SLC, UT
Oh, I can do a TON of math on it too and can make it show things as well...
Plus in that test you're trying to compensate for a ****-POOR charging system.. that's not what a Cap is for.. of course you're going to see a lower voltage...
A Cap in a Stereo system is used in conjuction with a good charging/voltage supply system to smooth out the voltage flucsuations (sp?), not to compensate for an amp that draws almost 2x what is being supplied to it... It compensates for those Mili-second spikes in draw by an amplifier. Ask anyone who's an electrical engineer who works on Powersupplies, If you've ever looked in one you'll see some HUGE caps, What are those for?
Basically that test you posted just shows what happens when you put a band-aid on a wound that needs massive amounts of stiches...
Plus in that test you're trying to compensate for a ****-POOR charging system.. that's not what a Cap is for.. of course you're going to see a lower voltage...
A Cap in a Stereo system is used in conjuction with a good charging/voltage supply system to smooth out the voltage flucsuations (sp?), not to compensate for an amp that draws almost 2x what is being supplied to it... It compensates for those Mili-second spikes in draw by an amplifier. Ask anyone who's an electrical engineer who works on Powersupplies, If you've ever looked in one you'll see some HUGE caps, What are those for?
Basically that test you posted just shows what happens when you put a band-aid on a wound that needs massive amounts of stiches...
Registered User
Joined: Oct 2001
Posts: 117
From: Atlanta, GA (in a burb)
You're correct about caps being used in power supplies - except you need to understand the SIZE of what we're talking about here.
First, how big is that cap?
J = 1/2 * C * V^2
Note the voltage is squared. So if I have a amp and I want to add enough capacitance to the power supply, do I add it before the DC/DC converter where its 12-14V, or do I add it later at 60-70V? Well, after the voltage is 5 times higher, so the capacitance has 25 times more energy per farad.
Second, how much current are we gonna get before the voltage drops? Rather than worry about our amps at 50A or 100A of current, how about we do the math for a pair of headlights. This is, by the way, calculated by a EE.
Capacitor voltage as a function of time can be expressed by:
V(t) = V0 * e^(-t/RC)
Where V0 is the initial voltage, R is the load resistance, C is the capacitance, and t is time.
Assuming our 55W headlights are at 12V nominal, they've got a resistance of 2.6 ohms each, or 1.3 ohms in parallel. Assuming we want only a 0.1V drop over 0.1 second, we'll solve for the necessary C; we start at 13.8V and end up at 13.7V.
C = - t / R * (ln V(t) / V0)
C = - 0.1 / ( 1.3 * ln (13.7 / 13.8) ) = 10F.
Note that if you increase the time, the cap size increases. Yes, that's 10 farads just to provide 9.2 amps of current for .1 second. How big a cap do you need to provide 50A?
The cap is just a drop in the bucket compared to the alternator and battery.
First, how big is that cap?
J = 1/2 * C * V^2
Note the voltage is squared. So if I have a amp and I want to add enough capacitance to the power supply, do I add it before the DC/DC converter where its 12-14V, or do I add it later at 60-70V? Well, after the voltage is 5 times higher, so the capacitance has 25 times more energy per farad.
Second, how much current are we gonna get before the voltage drops? Rather than worry about our amps at 50A or 100A of current, how about we do the math for a pair of headlights. This is, by the way, calculated by a EE.
Capacitor voltage as a function of time can be expressed by:
V(t) = V0 * e^(-t/RC)
Where V0 is the initial voltage, R is the load resistance, C is the capacitance, and t is time.
Assuming our 55W headlights are at 12V nominal, they've got a resistance of 2.6 ohms each, or 1.3 ohms in parallel. Assuming we want only a 0.1V drop over 0.1 second, we'll solve for the necessary C; we start at 13.8V and end up at 13.7V.
C = - t / R * (ln V(t) / V0)
C = - 0.1 / ( 1.3 * ln (13.7 / 13.8) ) = 10F.
Note that if you increase the time, the cap size increases. Yes, that's 10 farads just to provide 9.2 amps of current for .1 second. How big a cap do you need to provide 50A?
The cap is just a drop in the bucket compared to the alternator and battery.
Registered User
Joined: Mar 2001
Posts: 2,667
From: SLC, UT
I somewhat agree...but...
They are useful when used correctly... that is in conjuction with a good power supply system... And most Caps for car audio use are rated at 24V, although most never get to that potential. And you're still missing my point, what you calculated is correct, for powering the amplifier off the Cap, not using it for filtering or buffering the power supplied to the Amplifier, which is the CORRECT usage...
They are useful when used correctly... that is in conjuction with a good power supply system... And most Caps for car audio use are rated at 24V, although most never get to that potential. And you're still missing my point, what you calculated is correct, for powering the amplifier off the Cap, not using it for filtering or buffering the power supplied to the Amplifier, which is the CORRECT usage...
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Joined: Oct 2001
Posts: 117
From: Atlanta, GA (in a burb)
Originally posted by 97FormulaWS-6
And most Caps for car audio use are rated at 24V, although most never get to that potential.
And most Caps for car audio use are rated at 24V, although most never get to that potential.
Originally posted by 97FormulaWS-6
And you're still missing my point, what you calculated is correct, for powering the amplifier off the Cap, not using it for filtering or buffering the power supplied to the Amplifier, which is the CORRECT usage...
And you're still missing my point, what you calculated is correct, for powering the amplifier off the Cap, not using it for filtering or buffering the power supplied to the Amplifier, which is the CORRECT usage...
If you have a light dimming issue then cross that bridge when you get there.
Ted
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Joined: Jun 2003
Posts: 136
From: Flint, Michigan
So what you are now saying is that the reason he may get dimming lights and have a battery discharging issue is due to him buying a bad amp?? Sorry, but that doesn't float with me. I've worked on a lot of car and home audio equipment before. High end and low end products and ran into issues using all sorts of products. Lights dimming is probably the most common thing there is when people start building aftermarket systems especially when they add a lot of bass. Guess how the issue is almost always resolved ---- yup thats right by adding a cap into the system. the most obvious thing is to have a new healthy battery, if you know that you are building a big system then get a HO alternator, and if there are still issues add a cap or 2 or 3 or even add a second battery. I guess the best way to put it is by saying this. Charts and graphs and formulas are all great and usefull tools to help us all out, but it comes down to what works for all of us. And I can say that from experience caps work for us. Wether the chart/graph backs it up or not they work. Ask anyone who put a system in and had light dimming issues or a battery draining problem and then added a cap into the system to help it out what they think about caps. I bet they will say that they like them. Of course there are going to be the few that had more issues than just adding a system. People with bad batteries, alternators, wires, blah, blah, blah!!! for these people i would say NO a cap will not help you or at least it won't until the other issues are taken care of. JMO 
