Capacitor Voltages

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I'm an EE major and can't understand what is going on with attaching a 1 farad capacitor inbetween (and in parallel) an amp/sub and a battey for a car audio system. You can buy two kinds, a 16V and a 20V (or 20V and 24V peak).

My questions is, since the relationship is Q = CV, how can the 16V or 20V capacitor every be fully charged if the battery is 12V (or 13.8 running, whatever)? Basically, I don't see any benefit in getting a 20V capacitor if it can never charge fully, not even on a 16V capacitor. Also, how does that work; how can you have 16V/20V in parallel with 12V? Does the capacitor simply charge up to 12V and stop (which would make sense, because at that point the capacitor would act as an open since no current would pass through it)? Is it pointless to spend the extra $50 and get a 20V capactior?

I'm thinking of getting a Rockford-Fosgate CP7436 if what I said is right (1 farad, 16V, digital display, $100). Theroy and the real world sometime don't match up.

If you have better recomendations in that price range, please tell me.

If you care, I'm running a 12" Infiniti Kappa 120.1dvc (wired at 2 ohms) in a Q-logic type 2 sealed box and bridged on an Autotek Street Machines 300xi to run at around 250W rms. Head unit is a Pioneer DEH-P7000R running two Pioneer component 4X6s and two Pioneer 6.5 2 way compression horn.

16 volts should be just fine,

However 20 volts buys you a bit more reliability in case of over voltage. Some of these big caps are marginal to begin with. You could probably find big caps much cheaper if you look around. Generic ones don't look as nice as the ones in car stereo shops, but cost much less. For example has 16volt 230,000uF boomer caps for $5.95 ea.
Overvoltage? Boomer Caps?

When you say over voltage, how could the rms go above 16V? Are you talking about spikes in the voltage going above 16V? That would make sense, since 14 * sqrt(2) is about 20 volts, which is the max peak that most 16V capactiors can take.

Yeah, as I was looking through capacitors, I noticed that appearance was the major factor in cost ... I could care less what it looks like; I just need a capacitor, but I want the digital read out.

Aren't boomer caps really small? It says computer grade, so I'm thinking about .25cm^2; that can't handle a 250W amp, and I want 1 farad, not 230,000 micro farads (is that how they represent micro, M, it can't be mega ....).
Volt Stiffening

Another question, some capacitors claim to be volt stiffening while others don't - aren't all capacitors volt stiffening since they all resist changes in voltage? I thought that was just a given.

Right now I'm leaning towards a 1 farad 20 V Lightning Audio LSD10TB (has digital display) for $100 shipped.
to say that the cap would never fully charge is really not correct ..... a cap is fully charged when it reads the same as the supply voltage ....

caps should alway be over-rated for the purpose and yes unless you are using an aftermarket alternator, a second battery and a protection circuit an auto electrical system can provide spikes measuring hundreds of volts.

Also 12V is 13.8V fully charged and 14.4V while charging ie. while the ignition is running.

[Edited by AudioFreak on 12-06-2001 at 09:31 AM]
Battery Charge

The voltage across a battery is 12.6 volts since it has 6 cells and each cell is 2.1 volts - that's when it is fully charged. As for as the voltage when the car is running, it is a function of tempterature. At about 80 degrees F, the minimum voltage is 13.8 and the max is 14.4. The regulator increases the voltage when it gets colder because the battery is more reluctant to charge and decreases when it gets hotter. Since the engine compartment gets hot, the voltage is about 13.8V or less while the engine is running. That was straight from a book.

When I said the capacitor will never be fully charged, I meant that it could hold more charge since Q = CV. If I buy a 20V capacitor and hook it up to a 20V battery, it will hold much more charge than when I hook it up to a 12V. Yes, it will be fully charged in any system as soon as the battery and the capacitor are at the same voltage.

As for as the boomer caps, you would put 4 in parallel, but again, I think those are very tiny .25cm^2 (area) capacitors intended for computers and not high power car audio.

You could put 400V caps in there and the charge will be the same.If the price is the same or close always get over rated caps. The cap will help with spikes but isn't the reason for the cap to supply that surge of power for something like a sudden big bass note so that it doesn't tap out the amp and cause it to clip? I never have done any reading up on this because I do not want to roll around inside of a speaker cabinet but this has gotten my curiosity. Wouldn't a cap that is for filter purposes be between the battery and the amp and not the amp and the speaker?

I was thinking of getting the capacitor direct from Mallory, who supposedly supplies the capacitors to most of the audio companies. I went to their website and found three models that would work:

1 Farad 20V working, 22V surge, 40 amps max rms
1.5 Farad 20V working, 22V, surge, 41 amps max rms
.74 Farad, 25V working, 30V surge, 48.3 amps max rms (would buy two of these and wire in parallel)

Does anyone know if they sell over the counter through their distributors, or know how much they cost?


[Edited by dih1118 on 12-12-2001 at 05:12 PM]
get the ESL/ESR rating and decide.

there's an archived forum thread from that was some 10 pages long that discussed how caps with poor ESL/ESR ratings did not help (but actually hurt) the voltage level in the car system they were trying to 'stiffen' all along.

the following image (which i did have the link for) is of a true A/B/C/D test:

An externally hosted image should be here but it was not working when we last tested it.

the test (and post) were conducted by Richard Clark, the resident sage. (don't hold it against him that he believes all amps sound the same).

the colors are as follows (from memory, so forgive me if i'm wrong)

YELLOW: car is on, cap is disconnected
RED: car is on, cap is connected
GREEN: car is off, cap is disconnected
LT.BLUE: car is off, cap is connected

the same audio system, same car, and same musical track was played each time (and the car's engine/alt were allowed to operate at the same temp for 'car on' portions)

his discourse, and the test, showed that caps with ESLs and ESRs of even mild levels, when called upon to deliver the very high current demands of some musical peaks, developed some instantaneously high internal resistance and inductance levels in the cap PREVENTING it from doling out it's juice even when the voltage OUTSIDE the cap was LOWER than the voltage inside the cap.

The latter parts of the graph show how the cap becomes an additional burden to the charging system.

so as a tool to STABILIZE the voltage levels and lessen spikes, they should work, but as a "voltage stiffener" -buyer beware. If you need more power, you need a bigger alternator.

PS- a digital readout will only increase your ESL/ESR levels.

PPS- try your system without a cap first. I had a Zapco Studio 500 (1000 /real/ watts into 4 ohms mono) in my cherokee and i needed no cap (119 amp alternator). i had no dimming and no electrical problems pushing my two A/D/S 12" subs to their max
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