Bronze Member
Username: Taylor17

Kopperl, Texas

Post Number: 60
Registered: Jan-05
How does a capacitor help the output and how much, what brand, and where can I get one.

Silver Member
Username: Malby7

Post Number: 120
Registered: May-04
The capacitor will help when there's a sudden draw for a lot of power. There's been debates about whether or not you can actually tell if there's a gain in output. From what everyone tells me there's all the same except for the amount of farads. From what people also tell me you need 1 farad for every 1000 watts peak your system puts out.

Bronze Member
Username: Taylor17

Kopperl, Texas

Post Number: 62
Registered: Jan-05
When you say peak, does that mean like on on an amp when it says 800 rms and 2400 peak or is it different.
I will just make this easier, I have the Rockford p8002 that put's out 800 watt's rms, and the rockford p4004 and it put's out somthing like 400 watts rms. What would be good for my amp or amp's? Thank's for the help!

Bronze Member
Username: Taylor17

Kopperl, Texas

Post Number: 63
Registered: Jan-05
Also what is a farad, and what does it do? Sorry about all the dumb question's.

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 7251
Registered: Dec-03
Capacitors. Do I need one?

Well let's see. First, I'll define what a capacitor is, how it works, and what it does. This is best done by the following webpage:

Now with that out of the way, I'll give my take on what a capacitor will or will not do for you.

A capacitor WILL:
-Stiffen voltage rails. If you experience very brief, momentary periods of high current demand that cause the electrical system to falter only at these rare, peak draw times, then a capacitor will supply the additional current needed (when bass hits) to keep your voltage rails stiff, and prevent damage to the car or audio equipment.
-Increase response times for musical accuracy by reducing delay caused by transient response times between current demands from the amplifier, and response to this by the electrical system. In other words, your subs will respond more quickly, because they don't have to wait for the alternator to supply additional current at the moment of demand. Amplifiers have to provide a very dynamic and quick response many times. A capacitor can assist in this if the rest of the charging system is up to par.

A capacitor will NOT:
-replace the need for a larger, high-output alternator and/or a deep-cycle battery or batteries.
If your electrical system is inadequate, the ONLY way to fix this, and again I repeat, the ONLY WAY to fix this, is to replace the alternator. This is the SOLE source of electrical current for your car when the motor is running.
When the motor is turned off, the battery then becomes your source of electricity.
When the battery is run down, and when the capacitor(s) is/are depleted, the alternator has to work even harder in order to supply current to the car, the audio system, and also to recharge the capacitor(s) (which deplete very quickly) as well as recharge the car's battery(ies).
So by adding a capacitor to try taking the place of a high-output alternator, you are actually causing more work for your alternator, and causing even more damage to that stock alternator.
-make your system magically sound 10 times better.

Many people believe that a capacitor adds NO real benefit to an audio system, and this is why you never see before and after demonstrations, or factory capacitor company vehicles at competition events.
A capacitor does have it's uses, but it is not a magical fix for a lacking electrical system.

To calculate the capacitance needed for your system, you will need to find the peak or max power ratings of your amplifiers, and add those together. This is the only time peak amplifier power ratings are even remotely useful, since a capacitor is only used to cover very brief peak demands, and not cover for the continuous amplifier demands.
Take the peak power total and figure 1 farad of capacitance for every 1000 watts of power.

A capacitor is an electronic device which consists of two plates (electrically conductive material) separated by an insulator. The capacitor's value (its 'capacitance') is largely determined by the total surface area of the plates and the distance between the plates (determined by the insulator's thickness). A capacitor's value is commonly referred to in microfarads, one millionth of a farad. It is expressed in micro farads because the farad is such a large amount of capacitance that it would be impractical to use in most situations.
Capacitor and DC voltage:
When a DC voltage source is applied to a capacitor there is an initial surge of current, when the voltage across the terminals of the capacitor is equal to the applied voltage, the current flow stops. When the current stops flowing from the power supply to the capacitor, the capacitor is 'charged'. If the DC source is removed from the capacitor, the capacitor will retain a voltage across its terminals (it will remain charged). The capacitor can be discharged by touching the capacitor's external leads together. When using very large capacitors (1/2 farad or more) in your car, the capacitor partially discharges into the amplifier's power supply when the voltage from the alternator or battery starts to fall. Keep in mind that the discharge is only for a fraction of a second. The capacitor can not act like a battery. It only serves to fill in what would otherwise be very small dips in the supply voltage.
ESR is the equivalent series resistance of a capacitor. An ideal capacitor would have only capacitance. As you remember, all conductors have resistance. In a capacitor, there are multiple conductors like the wire leads, the foil and the electrolyte. The resistance of all of the conductors contribute to the capacitor's series resistance. It's essentially the same as having a resistor in series with an ideal capacitor. Capacitors with relatively high ESR will have less ability to pass current from its plates to the external circuit (to the amplifiers in the case of stiffening capacitors in car audio). Low ESR is desirable when using a capacitor as a filter.

ESL is the equivalent series inductance of a capacitor. Since most electrolytic capacitors are basically a large coil of flat wire, it will have even more inductance than it would have if it were flat. This inductance, along with the small amount of inductance from the wire leads, will make up the ESL of the capacitor. The ESL is essentially the same as having an inductor in series with an ideal capacitor. Low ESL is desirable when using capacitors for filtering purposes.

Even though a capacitor's plates are insulated from each other, there is a small amount of 'leakage' current between its plates. This current is generally insignificant but will cause a capacitor to slowly discharge with no external circuit path between the capacitor's leads.

Some large capacitors used in car audio systems have a digital voltmeter on them. Some of these displays will have a remote turn on lead to turn on the LED display. Others will have a timer that will turn the display off after a few minutes. If, in either case, the capacitor's positive lead was removed from the power source (and the display remained on), the capacitor would be quickly discharged by the display. This is not the same as the leakage current that we previously discussed.
cited from:

Bronze Member
Username: Taylor17

Kopperl, Texas

Post Number: 67
Registered: Jan-05
I just replaced the battery, so I am not gettint a capacitor for that perpose. Getting a capacitor may not help me that much, but it couldn't hert either. So if I understand this, my amp's total power is 3600 watt's peak power and 1200 rms. Wen I look for a capacitor do I try to match the rms power like with subs, and amp's, or find one to match the total peak power.
Also is there a brand to look at or stay away from, I noticed Boss mad som capacitors and they aren't knowen for there audio stuff does this go for capacitors too?
Thank's for the help Glass!

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 7277
Registered: Dec-03
yes, it could hurt.
the capacitor will mask the signs of the weak charging system and continue to cause wear on the overworked alternator.
the result will be a destroyed alternator, clipping amplifiers and blown speakers.

my article on capacitors explains matching sizes.

the brand shouldn't matter. just the specs of the capacitor. they're all pretty much the same thing. foil wrapped electrolytic capacitors. you want one in the farad range you need, 20VDC with the lowest ESR possible.


Unregistered guest
Whats up, I have a problem like this myself, I have 2100 watts total, out of 3 amps, one 1000 watt, one 600 watt, and one 500 watt amp. The 1000 is pushing two 12's and the 600 is pushing two 10's. The 500 is pushing my highs which are two 6 x 9's and two 4 x 10's. I have a 2 Farad Capacitor. Everything electrical wise works fine in the truck when I'm in park with the engine at idle speed. I just boutght a new Die Hard battery with 660 cranking amps. I bought this because the system drained out the old battery. Anyway, when I'm in park and engine on idle speed, when I turn up the volume the battery meter drops from a full 15 volts to the red line immedietly. Even when I'm going down the road at 60 mph the battery guage on the dashboard is still jumping up and down with the beat of the music, and its not the vibration making it do either. Now from what I know about cars/trucks, electrical stuff etc. Obviously I need a new alternator, I just need to know how many amps is suitable for my set up. My stock alternator is 100 amps, and thats in a 1994 Chevy K-1500 Blazer Full Size 4x4 2 door model.
What alternator size in amps do you suggest I get that can handle the truck electrical system itself like the A/C, Windows, Head Lights etc., and that can handle my 2100 watt stereo system? I would really appreciate anyones help. Thanks


Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 7401
Registered: Dec-03
please stop cross-posting.

Unregistered guest
I've been reading this thread. I have a comment. If you have voltage drops, a cap is not the answer.

Instead, do the big 3: Add a 4 guage or lower wire:
1. From your battery ground to the chassis
2. From your alternator case to the chassis
3. From your alternator positive to your battery positive

This gives a huge drop in overall resistance in your vehicles electrical system, the result is less voltage drops and an easier workload for your alternator. It's at most $25 in wire and terminals. It's worth a shot if you're looking into a good cap, which runs about $100 - $200. It is also QUITE necessary if upgrading to a high output alternator.
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