this prob oughta be a sticky, since it seems half the threads started here are "do i need a cap?" "why are my headlights dimming?", etc etc.
this was originally taken from http://www.betteraudio.com/geolemon/newmain/battcapalt/
i'll copy it over. it's easy reading, and explains ALL the in's and out's of what you need to be looking for if you think you need to upgrade you electrical system.
Alternators, Batteries, Capacitors
Making sense of it all through all the chatter
Car electrical systems come with a simple single battery, alternator charged electrical system that is more than sufficient for the stock car's needs. It is even sufficient for a car with a relatively large custom audio system.
There is a lot of talk among the audio forums about what people "need to do" to their car's electrical systems to support moderately large audio systems. Unfortunately, often much of this talk is unfounded.
At some point, you'll find yourself pondering what you really do need in your vehicle to support your system and listening habits, and it's often difficult to see the forest through the trees, with all the rumors and myths.
One big cause of confusion for many is the application and use of capacitors.
There are many rumors out there that "caps don't do anything", or that "it's been proven that they aren't effective". These rumors began largely because of some tests conducted by Richard Clark (Autosound 2000), tests that were ironically enough performed to dispel some myths that capacitors help amplifiers make more power (which they certainly don't).
The scope of Richard's testing was along a much larger span and grain of time than the fine-grain, short-duration time period that capacitors contribute when activated. As such, Richard's plots had a minimum resolution of full seconds, spanning a period of several minutes.
As capacitors typically discharge down to their lowest charge level in an installation in under a second, it is easy to understand the misinterpretations, and hence the rumors.
Anyone who has experienced poor sounding MP3's (ones that were encoded at a bitrate low enough to cause audible degradations) should understand that our ears and brains don't process sound with minimum time resolution of a full second.
Understand also, that a the intent of installing a capacitor in the car is not to provide any semblance of a continuous supply of current - that is the alternator and/or battery's job.
So what is the purpose of the capacitor?
Read along as we explore the purpose of all these components in your electrical system.
Understanding what each does
You can't make an educated decision on what should be upgraded, if you don't understand what each component fundamentally does, much less what the limitations of each are.
An alternator can supply higher current levels when the car is running, but provides nothing when the car is off.
In addition, it puts a higher drag on your engine, which reduces your horsepower at the wheels and accelleration, it is expensive, and it might not even be avaliable for your car, depending on what car you have obviously.
They are also tricky to shop for, because you don't want to end up with a "200 amp" alternator that in reality produces the same or lower output current at idle.
But obviously, the benefits are that a good one can produce more current any time the car is running, if you do fall into the category who needs this.
Realistically though, that's not the majority- there are a whole number of interesting factors to consider though, including not just the wattage of your amps, but your listening habits, the "duty cycle" of the bass that you are playing, recording levels, volume levels, and your batteries recharge/discharge rates and the "duty cycle" there, also...
A Battery is always connected, but not always used. When the car is off, obviously it is. Most of the time, the alternator can supply enough current at it's 14.4v level that everything gets it's juice from the alternator.
When more current is demanded though, for that instant, the extra current is drawn from the battery... when that happens, your electrical system temporarily drops to the 12v level of the battery... and since lights aren't as bright at 12v, you see them dim. This isn't a symptom of a problem, the electrical system is doing what it's supposed to. As long as the battery has enough time to recharge in between (it only needs to recharge the small amount of current that was drawn) - as long as the battery voltage is able to rise back to 12v each time, it's OK.
Adding a second battery, close coupled to the amp, will help things in several ways... the most important being that your main power wire is resistive, and the second battery will essentially eliminate that, speeding the response of the battery to current demands from the amp.
A capacitor is added because batteries are slow to give up charge, relatively speaking, and are slow to recharge, where capacitors are nearly instantanious.
Think of it like eliminating a speed bump in the road.
Your amp demands current, to really slam a huge instantanious explosion or bass drum... in that instant, it exceeds the current capability of your alternator, so it must draw the extra current from the battery... the battery is like Droopy Dog.. it's saying "Ooooohhh... kaaaayyyy... heeeeeerrreeeesss yooouuurrr currreeeennnntttt", and finally the amp gets the current. But there is a rise time involved. Granted it is tiny, but here's where the lightning fast capacitor can help out. It gets rid of that speed bump. While waiting for Droopy Dog, your capacitor discharged for the amp.
Capacitors are also good, in scenarios where you have fast kick-drum type bass. They don't help much at all on those long drone-tone extended bass tones like bass CD's, like sine waves, but that's fine... they don't hurt performance in those scenarios, either.
Picture what happens when your system suddenly demands 200a of current for some huge, high-volume transient burst:
You have a point in time where the alternator reaches it's limits.
Before this point, there's no need for a capacitor.
And you have a point in time after that where the battery was called upon due to the dropping voltage level, where the battery actually has risen to the demand, and is fully providing the current that's being demanded.
After this point, there is no need for capacitor.
There's a gap in between those two points in time, where the voltage is initially falling, and where the current flow is rising, where the supply of current isn't meeting the demand for current.
And that gap in time only is milliseconds.
Reference the illustration below, to compare how a capacitor can augment the electrical system, during this transition time, when the current capacity of the alternator has been exceeded, but before the battery has fully risen to the task at hand.
This shows the effect on voltage of a heavy bass note beginning at the point in time of T1, and ending at T4:
Illustration showing voltage momentarily dipping when no capacitor installed
this was originally taken from http://www.betteraudio.com/geolemon/newmain/battcapalt/
i'll copy it over. it's easy reading, and explains ALL the in's and out's of what you need to be looking for if you think you need to upgrade you electrical system.
Alternators, Batteries, Capacitors
Making sense of it all through all the chatter
Car electrical systems come with a simple single battery, alternator charged electrical system that is more than sufficient for the stock car's needs. It is even sufficient for a car with a relatively large custom audio system.
There is a lot of talk among the audio forums about what people "need to do" to their car's electrical systems to support moderately large audio systems. Unfortunately, often much of this talk is unfounded.
At some point, you'll find yourself pondering what you really do need in your vehicle to support your system and listening habits, and it's often difficult to see the forest through the trees, with all the rumors and myths.
One big cause of confusion for many is the application and use of capacitors.
There are many rumors out there that "caps don't do anything", or that "it's been proven that they aren't effective". These rumors began largely because of some tests conducted by Richard Clark (Autosound 2000), tests that were ironically enough performed to dispel some myths that capacitors help amplifiers make more power (which they certainly don't).
The scope of Richard's testing was along a much larger span and grain of time than the fine-grain, short-duration time period that capacitors contribute when activated. As such, Richard's plots had a minimum resolution of full seconds, spanning a period of several minutes.
As capacitors typically discharge down to their lowest charge level in an installation in under a second, it is easy to understand the misinterpretations, and hence the rumors.
Anyone who has experienced poor sounding MP3's (ones that were encoded at a bitrate low enough to cause audible degradations) should understand that our ears and brains don't process sound with minimum time resolution of a full second.
Understand also, that a the intent of installing a capacitor in the car is not to provide any semblance of a continuous supply of current - that is the alternator and/or battery's job.
So what is the purpose of the capacitor?
Read along as we explore the purpose of all these components in your electrical system.
Understanding what each does
You can't make an educated decision on what should be upgraded, if you don't understand what each component fundamentally does, much less what the limitations of each are.
An alternator can supply higher current levels when the car is running, but provides nothing when the car is off.
In addition, it puts a higher drag on your engine, which reduces your horsepower at the wheels and accelleration, it is expensive, and it might not even be avaliable for your car, depending on what car you have obviously.
They are also tricky to shop for, because you don't want to end up with a "200 amp" alternator that in reality produces the same or lower output current at idle.
But obviously, the benefits are that a good one can produce more current any time the car is running, if you do fall into the category who needs this.
Realistically though, that's not the majority- there are a whole number of interesting factors to consider though, including not just the wattage of your amps, but your listening habits, the "duty cycle" of the bass that you are playing, recording levels, volume levels, and your batteries recharge/discharge rates and the "duty cycle" there, also...
A Battery is always connected, but not always used. When the car is off, obviously it is. Most of the time, the alternator can supply enough current at it's 14.4v level that everything gets it's juice from the alternator.
When more current is demanded though, for that instant, the extra current is drawn from the battery... when that happens, your electrical system temporarily drops to the 12v level of the battery... and since lights aren't as bright at 12v, you see them dim. This isn't a symptom of a problem, the electrical system is doing what it's supposed to. As long as the battery has enough time to recharge in between (it only needs to recharge the small amount of current that was drawn) - as long as the battery voltage is able to rise back to 12v each time, it's OK.
Adding a second battery, close coupled to the amp, will help things in several ways... the most important being that your main power wire is resistive, and the second battery will essentially eliminate that, speeding the response of the battery to current demands from the amp.
A capacitor is added because batteries are slow to give up charge, relatively speaking, and are slow to recharge, where capacitors are nearly instantanious.
Think of it like eliminating a speed bump in the road.
Your amp demands current, to really slam a huge instantanious explosion or bass drum... in that instant, it exceeds the current capability of your alternator, so it must draw the extra current from the battery... the battery is like Droopy Dog.. it's saying "Ooooohhh... kaaaayyyy... heeeeeerrreeeesss yooouuurrr currreeeennnntttt", and finally the amp gets the current. But there is a rise time involved. Granted it is tiny, but here's where the lightning fast capacitor can help out. It gets rid of that speed bump. While waiting for Droopy Dog, your capacitor discharged for the amp.
Capacitors are also good, in scenarios where you have fast kick-drum type bass. They don't help much at all on those long drone-tone extended bass tones like bass CD's, like sine waves, but that's fine... they don't hurt performance in those scenarios, either.
Picture what happens when your system suddenly demands 200a of current for some huge, high-volume transient burst:
You have a point in time where the alternator reaches it's limits.
Before this point, there's no need for a capacitor.
And you have a point in time after that where the battery was called upon due to the dropping voltage level, where the battery actually has risen to the demand, and is fully providing the current that's being demanded.
After this point, there is no need for capacitor.
There's a gap in between those two points in time, where the voltage is initially falling, and where the current flow is rising, where the supply of current isn't meeting the demand for current.
And that gap in time only is milliseconds.
Reference the illustration below, to compare how a capacitor can augment the electrical system, during this transition time, when the current capacity of the alternator has been exceeded, but before the battery has fully risen to the task at hand.
This shows the effect on voltage of a heavy bass note beginning at the point in time of T1, and ending at T4:
Illustration showing voltage momentarily dipping when no capacitor installed