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  Why Capacitors don't work
Why Capacitors don't work. Period.

BATTERY - this device has the ability to provide a very large amount of current. But due to its nature the current is provided at a voltage that is less than optimum -at least for a high powered stereo. Since its float point is 12.8 volts if fully charged, it can provide current only at voltages that are proportionally lower than 12.8 Volts.

ALTERNATOR - this device is electronically regulated at a point that allows it to recharge the battery. The alternator is usually designed to output voltage in the 13.8 to 14.5 volt range. Because its output is actively regulated it attempts to maintain this voltage with varying load conditions up to the point where it's output cannot keep up with the load at which time it's output drops off very rapidly. While relatively tight regulation is the strong point of the alternator, it's weak point is that it simply is not practical to obtain one that can provide large amounts of current like a battery is capable of.

CAPACITOR - The advantages of a cap are that it can charge up to whatever the highest voltage source in the system is, (in a car this would be the alternator) and provide current at this elevated voltage. The down side of a cap is that it cannot store very much total energy and only a portion of this energy is available at a usable voltage potential. The fourth type of device is an electronic voltage regulator. These devices have not been part of this discussion so I will pass over them for now.

Now modern car audio amplifiers are capable of consuming enormous amounts of power. Even with efficiencies in the range of 60% to 90% an audio system is capable of drawing hundreds or thousands of amps from the cars electrical system. Typically, the audio system is larger than any other electrical device in the car including the engine starter. Fortunately for the car, the demands of an audio system are rarely continuous in nature. The very nature of music rarely demands more than a duty cycle of 10% to 20% from a power standpoint. This means that the audio system is demanding short term, but repetitive peaks of current from the electrical system.

The primary source of this power is the alternator. It should be considered primary for two reasons. The alternator is the only first generation source of power. It ultimately provides all the power for the system either directly or indirectly by restoring power to the battery or cap. It is also primary as it is the power source with the highest voltage potential. In an electrical system current always flows from the source of highest voltage to circuits of lower of lower potential.

All three devices can be used in a system to great advantage. But the dynamic conditions present in a music system or car operations determine the role each device plays and to what degree. To understand this lets consider a low current drain condition. In this scenario the alternator will be at or near its set point.

This voltage is designed to be high enough to charge the battery meaning it will be one or two volts above 12.8 volts. This means that the battery will actually be a continuous load on the alternator and provides no power to the system. The size of load it presents is determined by the state of charge of the battery. The higher its state of charge the smaller the load will be. A cap if present in a system in this state will present a load for a finite amount of time until its charge voltage reaches equilibrium with the alternator.

Unlike the battery, the cap will cease to be a load after it is charged except for a factor known as dissipation, which for all practical purposes can be ignored in this application unless it is excessive. Under these circumstances, as long as the alternator can maintain its set point, it will provide all the power for the music system and the rest of the car's accessories. The battery and cap may as well not even be in the car.

Now if we increase the current demands of the music system to an amount that taxes the alternator its output voltage will begin to drop. Even so the alternator will continue to be a source of current to the system -i.e., the car, music system, and battery. It is at this time that the cap will begin to discharge and begin to augment the alternator as a source of current. The degree to which it provides current to the system is dependent on the actual voltage at the alternators terminals. Only when the alternator begins to drop below the caps charge potential does current flow out of the cap.

This is a continuous process and the current provided by the cap tries to maintain the voltage at its charge potential. The degree to which it can do this is dependent on two things. The current provided by the cap is limited to the total capacity of the cap and any series reactance's (resistive or inductive components) that are part of the cap. The instant the cap starts to output current its charge potential begins to drop.

Now just what can we expect the cap to provide? Suppose we happened to have a cap charged to 14 volts, with a total reactance (made up of either resistive or inductive components) of about .017 ohm. We could figure that at the first instant of discharge it could provide ten amps at 13.83 volts. Of course if we were playing the system at a level enough to load our alternator, ten amps is not likely to provide much relief. But perhaps 30 amps might help-at this modest level our cap could begin to provide current at a potential of 13.5 volts.

Of course this voltage level would drop at an exponential rate commensurate with the discharge curve that is standard with caps. No doubt the cap could help out a hundred amp alternator with the addition of an extra 30 amps even though it might be for only a brief instant. But it is sort of interesting that at even this modest power level of 130 amps (100 amps alternator + 30 amps cap) the cap is unable to maintain the voltage at 14 volts.

Of course in this scenario we are sitting at 13.5 volts for a brief instant and our poor battery is unable to help at all as its potential is at a lowly 12.8 volts. In fact the battery is still a load on the system!

Now what if we get serious with our stereo and we really crank it up? Let's say we have something like a manufacturers demo van with lots of amplifiers that can draw hundreds of amps on musical peaks. Lets pick a nice round number like 500 ("Cade" said 490) amps. Let's say we have a 200 ("Cade" said 190) amp alternator. Typically such an alternator can maintain a voltage near its set point up to perhaps 80% of its rating-after which its voltage begins to drop as it provides large amounts of current. As I am not familiar with all the different alternators let's just assume these assumptions are close and our alternator is putting out 200 amps. Well our amplifiers in an instant are asking for 500 amps so what happens?

In any constant voltage system when the current capability is exceeded the voltage drops. So let's say our alternator voltage starts dropping. What does our cap do? Since its charge potential is at 14 volts it starts to discharge and provide a source of current. Since the cap is now sharing the load with the alternator it is called on to provide what the alternator can't-that would be 300 amps.

What happens to the terminal voltage of our cap when 300amps is flowing? Well for starters, the voltage tries to drop nearly 5 volts inside the cap before it can even get out. Not in a short time but instantly. There is no time constant in the formulas for ohms law. They are instantaneous calculations! But wait. The voltage doesn't really drop to 9 volts because we have our battery sitting in reserve waiting at 12.8 volts.

Our cap lets our poor alternator down as the voltage plummets and when things hit 12.8 volts our battery jumps in and starts to take over. The battery with its enormous storehouse begins to provide vast amounts of current until things lighten up for our poor cap and alternator. Of course we could add another cap to halve our ESR loss to only 2.5 volts but that would still cause the cap terminal voltage to drop to 11.5 volts.

Let's see how many caps of this spec we would have to add to keep the voltage at 13.5 for even a few milliseconds. We would need a cap bank with a total ESL of about .001 ohm. Gee it looks like it would take over thirty caps paralleled to maintain 13.5 volts at 300 amps for even a brief instant. And let's hope we don't need to do this for long, as the total power contained in thirty units is only about what is in a dozen 9v alkaline batteries!

It should be clear that if the voltage doesn't drop the caps don't do anything. The voltage MUST drop for them to start discharging.

Now, is it possible to have a steady 14 V because we added caps? I don't think so.

After studying a little history on large 1 Farad capacitors in car audio, you'd be amazed that they even sell at all. How useful are they? What do they really do? Will a Cap 'improve' my sound quality? Will it prevent my lights from dimming? Will it audibly affect my audio system in any way?

Before you get the truth to any of the above questions, chances are, you've probably spent $100 or more on one of these devices. However, let's study a little history regarding this issue.

A long time ago, in a land far away, 2 elves...Ok, Richard Clark & Wayne Harris (Carsound magazine and the inventor of DB Drag, respectively) separately came up with a solution to preventing their lights from dimming.


As you may know, Richard Clark is one of the founders of AutoSound2000 Tech Briefs, Carsound magazine, and a published author of the industry of mobile electronics. In SQ competitions, he posted a record of 1234 1st place finishes, and only ended up NOT 1st in his first event. I've heard that he had minor system problems, but judging by his record, he must have corrected it.

Wayne Harris was previously a leader at Rockford Fosgate in their development. Later, in his free time, he created the organization we call DB DRAG. Wayne was the first SQ World Champion from the organization we know as IASCA (International Auto Sound Challenge Association).

Both of these gurus are both legends, and considered the leading experts in the field. During their competition days, both guys came up with a way to assist in the prevention of voltage drops. In SQ competitions, the look of your system is actually more important than the sound, and having your lights NOT dim under high playing levels is a competitive advantage.

As you may know, amplifiers are made up a bank of little capacitors, resistors, etc. What has been common engineering knowledge is that capacitors store energy, and more or bigger ones assist in balancing the power supply.

Wayne came up with the idea of putting several dozen 'little' (approx 100uF) capacitors on a circuit board to 'extend' the power supplies storage. At about the same time, or shortly afterward, Richard came up with the idea of one huge mondo capacitor (I believe it was 800,000uF or 0.8F) to do the job.

Eventually, Richard won. The large cylindrical tubes won over the complicated 48 caps strapped to a circuit board. However, what did this really accomplish? Let's start here:


Basically, capacitors are an energy storage device. Large, 1 Farad or more capacitors store energy (electrons) between their plates. Capacitors differ from batteries because batteries store energy in the form of chemical energy--and rely on acid and lead plates, as the place of storage.


The number 1 reason would have to be because their lights dim when their system is playing HARD. In car audio, we are told that a capacitor is designed to prevent the voltage drop associated with your lights dimming. The number2 reason is that it is rumored to 'improve' sound quality or 'stiffen' the power supply/source. The number3 reason is that it improves performance and battery life when used.


Headlights brightness is in direct proportion to the source voltage. For instance, if your car is running, system voltage is ~12.5 -14.4 VOLTS. Your lights will be much brighter than when your car is turned off--where battery voltage is ~12V. Most car alternators put out between 75 to 120 amps of current. When this current draw threshold of the charging system is exceeded, system voltage will drop as power demands are now shared by the alternator and the storage devices (battery & cap). We are using battery reserves beyond this point until the demand lessens

When playing your system really hard, your lights dim because your alternator can't keep up it's charging voltage (around 13.5V) and therefore, demand exceeds output. When this happens, your electronic devices are dipping into the power storage of the battery. Since the battery stores power at ~ 12-12.5V, there is a 1.3 to 1.8V drop in voltage available. This in turn is why your lights dim down.


1 Farad = 100 joules or 100W/second
800 CCA battery = ~2,200,000 farads

For storage purposes, you'd need ~2,200 units of 1 Farad capacitors to equal the energy of your battery. But this is physically impossible to place in a car!!

Due to its impedance (ESR & ESL), a cap's energy is only 50% available. What's worse is that in order for a 1 Farad cap to discharge, first the alternator output must have maxed out, and the voltage must have dropped around 1.5 volts. But I thought a cap was supposed to prevent that (voltage drop)!!!!!????? Yep, you got the point.


My sentiments exactly. Marketing is the reason why people buy caps. In many cases, upgrading wiring will help your system get the maximum transfer of current. Once that has been reached, adding a capacitor may have a minor effect on your system. 50W over the course of a second is not a lot of power considering an amplifier may draw 2000W to put out 1400 watts. Let's look at the situation from a resources standpoint.

Alternator - 80 amps
Car accessories (minus stereo) - 40 amps
A large Car Audio system (DRAWS ) - ~200 amps AT FULL OUTPUT

In this case, you have 240 amps of draw, but only 80 amps of current from the alternator. In your case, you need 160 amps x 12 volts or or let's say 1920 watts of energy. Since a cap stores 50W, how much of a difference do you think it's going to make? A cap is basically a peashooter. We need a Howitzer cannon here, to do the job well.

Also, once a cap is discharged, where does it get its power from? Answer is the alternator, which is already overloaded. Once a cap is discharged, it's worthless. Like SWEZ says, " The cap already shot its wad, and goes limp till its recharged" (I never said it quite like that...!)

  1. Audio Jewellery- impress chicks with large cylindrical shiny thingy
  2. Extra weight during winter time
  3. A very POOR... BUT expensive distribution block
  4. A projectile in the event of a crash LOL.
  5. Rolling pin--for cooking purposes
  6. A neat thing to tell your friend, "..Hey man, lick the top of this.."
Please do not try # 6. New hairstyles are always refreshing, but if you are wearing railroad tracks across your teeth, you might have one big filling after its over.


They can't. Sound quality is not dependent upon the presence of large bulky 1 Farad capacitors. How many 1 Farad Capacitors do you think the Boston Pops, Aerosmith, or Snoop dog use in the recording studio?


When Richard, our fearless inventor, became World renown for winning every competition under the sun, people began copying what he did. Soon, every 'serious' competitor had a 'stiffening' capacitor--not to be confused with the 'loosening' capacitor.


In the late 80s, people began sticking out their tongue when dunking the basketball because Michael Jordan did. Did sticking out your tongue improve your dunking ability? Same here with adding a capacitor or battcap to your electrical system or battery cell.

Phoenix Gold's marketing guru had just posted information on how their Powercore (basically the Alumapro CAP15 in a Phoenix shell) had both stabilized their voltage and improved the sound quality. Richard called him on it (all in another post) and the marketing geek was unable to quantify any of the conditions that resulted in the voltage being HELD at 14.2V and the 'improved' sound quality.

Please do not read every stinking post as valid. There are a lot of people that have had the efficacy of capacitors inbred to their minds, and were not (and still not) convinced in the futility of a 1 Farad storage device.

In a final note, Richard relayed a quote regarding battcaps : "The audio industry is the only place i know of where you can publish specs that show your product is useless and still be able to sell them------and what's worse is that technically ignorant people will argue against the math!!!!!!!.........". This also relates to most digital readout capacitors, and I wish my Archie Bunker skills could have said it better myself.

Cliff Notes: Caps are pointless. Get a better alternator, do the Big 3 and get a stronger battery.