If i put my amp in the trunk, how many feet should i get to hook up the power cable to the battery, anyone have this info ??

(for a SCION TC)

 

I'd say to get 20 feet, that way you can have plenty and be able to move it whereever you want in the trunk. From 16 to 20 feet, depending on how much leeway you want.

 

^^ Yea. Always get more than you need. You don't want to be splicing together 4 guage. I would buy an amp kit from circuit city or best buy (PLEASE NO WALMART) They have plenty of wire for power, ground, and amp turn on and all the connectors.

 

well i got a kit from best buy - but now i'm wondering how i should connect the capacitor to the system or should i use 8-guage to ground it ?? (or should i splice off some of the 4-guage)

or should i go buy an xtra couple feet of 4-guage to hook up the cap?

 

Walmart kits are fine depending on your requirements. Wire is wire.

 

Why did you buy a cap? You don't need one.

 

.. how do you know he doesnt need one.. i dont even know what he has!

PS. i used 20ft, and i have about 2 feet left over or so.

 

You never, ever, "need" a cap. Anyone that tells you that you "need" a cap is an idiot.

 

cuz its a new car i wanted everything to be better quality: ie. the sound system, i remember my past cars the headlights would dim all the time..

plus my friend got a new job so he'll be quitting best buy in 2 weeks, so i said wtf and i'm splurging on things like this before he loses his discount.

got my rockford fosgate 1-farad cap for about $50

 

and anyone who tells you that you NEED a car is an idiot then.

anyone who tells you that you need anything besides water and freakin' soy beans is an idiot.

dont be so techical, egads.

 

Ok, if you believe in the "You need a cap" phenomena/idealogy, please tell me why one ever "needs" a cap? What does it do for your system? Be careful with your answer.

 

i dont believe anyone NEEDS one.. that is the point of my post. no one NEEDS anything

your sub box doesnt NEED a sub in it.. looks cool w/o one!

lol.

if nothing else, caps keep embarassing headlight dim to a minimum. they also look cool on display.

and i do NOT have a cap.

and no need to get crabby...

 

20 feet sounds about right. I used a walmart/scoshe (sp?) kit and it barely reaches. I am going to have to change the wire out from the distribution block now as it doesn't allow me to rotate the amp the direction I want it to go.

I also have a cap for my system (like you, got a great deal). Haven't installed it quite yet (hopefully this weekend), but I figured what the hell. It should look cool and may help in certain situations, so why not.

Good luck,
B-

 

I'm not getting crabby. Your original post suggested that he could possibly need a cap. That ideaology is what the companies that manufacture products target. People buying into commonly circulated myths whether internet forum circulated, words around the shop, etc. Caps combat dimming, in some instances. Some people they do nothing for. BUT, their only use is to combat dimming. There are more proper methods to go about eliminating dimming, ones that are guranteed to have an effect on it regardless of application, vehicle, etc. It's unprofessional to suggest something, whether or not the suggestion is inadvertant or not. Just be careful with the way you word things is all I am saying. Sorry if it seemed like I was being eliteist, or if it seemed like I was being a stickler, but caps are my hot button. And being as I work in the industry I like to give advice based on the individual person's situation, regardless of profit margin.

 

Tripe, there is sound engineering reasons for caps. So, let me provide a little professional technical information regarding caps from an EE's point of view. Oh, and I'll be careful of my answer

This definition will be in its simplest form. You will see audio capacitors referred to as Stiffening Capacitors. A capacitor is a device that stores electrical energy. Well, you say that’s what a battery does. Well yes it does, but the difference is the ability and speed that this electrical energy is able to be discharged.

Batteries are designed to store electrical energy for long periods of time. A battery also discharges slowly. A capacitor charges and discharges very rapidly. The reason behind this is due to the internal resistance of both devices. A battery has a high resistance that allows for long-term energy storage and a slow discharge. The capacitor has virtually no internal resistance, which allows for rapid charges and discharges.

An amplifier requires current as music signals passes through it. This creates transient signals in the amplifier. The problem occurs when many of these signals come in rapid succession in a short period of time. The time we are talking about are very small fractions of a second (milliseconds). If your electrical system is unable to react to this it causes a sagging effect in the audio output.

As the current goes up, so must the voltage. Since you can’t increase the voltage as the current requirement increases we get a decrease in voltage and this decrease in voltage causes a sagging power supply (and I'm sure you understand what that means...right?). Amplifiers work best at 14.5 volts and can easily overcome a factory installed alternator current output capacity. When the engine is turned off, the system requirements are supplied by the battery increasing the problem of supplying the transient requirements. You could add 10 or 20 more batteries and it wouldn’t help. Remember, as we explained above batteries can’t supply energy quickly enough to supply and meet the transient needs of the amplifier. Please note that these transients may be well over 100 amps. They occur so fast that your fuse will not be effected, remember this takes place in milliseconds.

By adding a large stiffening capacitor to your amplifier input power line you should be able to meet these transient requirements. Remember as we explained above, a capacitor can store energy very fast and discharge energy very fast. This should solve the transient current requirement to the amplifier.

The voltage of the capacitor can only be the same as the alternator output or if the engine is off, to the level of the battery. So, as you can see a capacitor doesn’t maintain battery levels. If you are still having energy shortages to operate your system you will need to look into a high output alternator.

Oh, and I may not make a living installing this trivial crap...but I sure understand how to design them and how they operate down to the electron flow. Seriously tho, on small systems, not really necessary, but high current requirements......pretty ignorant (I mean that in a literal way) not to.

 

wal mart stocks scosche which is one of the top brands and some of the thicker wires i've used. i love it. i hate the rest of the kit but love their 4ga and 8ga wire.

 

First and foremost, electrical engineer or not, batteries do not store energy. They have the potential to create energy by means of a chemical reaction.

The 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.

The 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.

Size of the capacitor effects the discharge rate minutely.

This from Richard Clark (I can provide the more emperical data, should you wish):

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 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 cars 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. (lesson 2).

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? Lets 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. Lets 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 lets 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 (see footnote) 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! (lesson 7)

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.

 

d00d, i sooo lost interest in this entire topic after about 45 seconds of reading lol.

I am horribly sorry that there is SOME chance that i COUld HAVE implied he MIGHT NEED a capacitor.

i dunno, i got mine for 50 bucks in my old car, my headlgihts didnt dim, and it hada c ool voltage read out. *shrug* i couldnt hear a difference.

now stop being a crabby pants!

PS. 3 more classes and i will have a degree in computer engineering (very close to EE....)

 

Apples and oranges from your cut and past job sparky (get it, cap...sparky). Is this your canned response to cap discussions, because the last line of your cut and paste was never a question....now was it? What your not getting and Dick ignored (probably because it wasn't the question he is ACTUALLY answering) is the transient needs of the amp and power supply. At what point did I state the cap was going to do anything more than provide the (Instantaneous), transient needs of the amp. IF, and that is kinda a GIVEN, the alt is not even close to being taxed, then the cap would have no use to the amp. However, if the alt is at 80-90% load (alot of gizmos runnin on electricity now a days in a modern automobile ) and your superduper audio system places transient needs on the system which the Alt is unable to meet, can you figure it out from here? The cap is ONLY of use for Instant Catch up. Got it! Plus, the chicks dig em


Batteries have the potential to create energy by means of a chemical reaction. Did you cut and paste that one too

You know, when people spout off tripe little comments just to bust someone elses chops is what causes this type of topic. You don't like caps, don't use them and don't sell them. But don't misguide and say they do nothing but stop dimming. That my Scion friend, is not true. It is just not doing what YOU think everyone is saying it will do (and there are alot of false claims, I certainly admit that). However, I told you what it WILL do and that is certainly more than nothing.

The proper way to STOP lights dimming is to increase alt output....cheapest way is to get it rewound. But you already knew that

 

Correct, but, the thing is, caps are only good for dimming. I cut and pasted because, ho0nestly, I'm a lazy ___, and didn't feel like typing out anything. But, again, I'll state, caps do nothing except combat dimming. That's a physically proven fact. If you can prove to him that it increases voltage, and help keep the constant at 14.4 volts((hell, more than 12.8v even) and he'll give you 5 grand. No, I didn't cut and paste the part about batteries not storing, but chemically creating power. It's basic science. I gained that knowledge in high school in a science class (actually, it was physical science, but I digress). I never said they stop dimming, I said they have the potential to help dimming. Caps aren't a guranteed cure to stop dimming, they can combat it, possilby. Some cases they work, some they don't.

I'm sure, being an electrical engineer, you're familiar with a joule. Look at the joules of energy the cap creates. Then take away all of the power that's stored and usable below 10 volts, as the charging system never goes below 10 volts. Those joules of energy are unusable. Also subtract the joules above 12.8 volts, as the cap doesn't kick in until that point. The number you're left with are the joules available and used on the cap. Divide that by 1000. That's how many watts it can supply for a minute. Kind of small number eh? Milliseconds even? That's how long the cap is effective for. That's a short anwswer version. Again, I'll provide you with Mr. Clark's emperical data (including experiments he's done on caps, as it's a tad more detailed and complete).