Aternator Noise

I think I finally figured it out.  I was getting an alternator noise in my sound system only after I played my music for long periods with the car turned off.  I was wondering why it only happended after this and today I was reading the MECP basic installer handbook and found an explination of ground loops and battery functions. 

A ground loop is a difference in current between two grounding points.  You have to have close to no resistance in your ground wires and locations between  the amp ground and the battery ground or less than a .2v voltage difference.  If you have higher than this then you might get a ground loop if you don't have any other devices to suppress the voltage ripple.

A battery stores energy but also helps suppress a small ripple ( voltage variation) that by-passes the voltage rectifier of the alternator.  So the DC voltage still has a miniscule sine wave like ac but curving around 12v or whatever the charge is.  A battery helps maintan the voltage constant thus helping smooth out the sine wave to look flat.

In my car I have a second battery in the wheel well.  After paying my music with my car off it discharded the battery slightly.  When I turned on my car the electrical components were relying on the alternator without the battery's help in smootheing out the ripple thus feeding the electrical components a varing voltage. 

Since the alt is charging the battery will the battery be able to smooth out the ripple as before?

Will the varying voltage cause a current diference of more than .01amps? According to ohms law it would right.

Do the internal capacitors of amps cause the electrical components to be out of phase of is this only in AC?

I'm really not sure if this justifiable but that's my explination so far.  What do you think?

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Poly Dollies

Who told you that batteries smooth out ripples?  That's the function of a capacitor, and is the reason why there are capacitors in most good power supplies INSIDE the amp.  The sound you are likely hearing is not "voltage ripple."  The most likely cause for your noticable alternator whine after partially discharging your battery is simply the extra current being drawn from the alternator to recharge the battery, addded to the load of your car and the system, is overloading the alternator.  Many alternators - especially those with internal voltage regulators - operate more noisily when they get hot, and placing extra load on one by charging up your battery makes them get hot.  The solution is to add a ba=ttery for your extended off-engine listening, upgrade your alternator, or both.

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The MECP book states that the battery helps maintain the voltage.  I'm not sure if it helps smooth out the ripples significantly or not.  Does any MECP certified person here recall this or can reference it to the book?

Most good amplifiers have internal capacitor so is there really a need for and external capacitor?

You stated that some alternator operate more noisily when hot but the noise I'm hearing does not relate since instantly hear it after turning on my car.  The alternator noise is a gound loop only noticable when my alternator is supplying more amprage.

Does the battery help stabalize voltage?

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Poly Dollies

You stated you can hear the alternator whine only after you operate for a while with the car off, thus my assumption that your battery was placing too much load on your alternator, overloading it.  Do you hear it all the time?  This is a different thing entirely.  If you hear it all the time it is definately either a ground loop or a defet in your electrical system or ignition system.

The purpose of a battery in an automobile electrical system is to start the car and to provide power while the engine is off.  When the engine is running, the alternator supplies 100% of the power.  If the load on the altrnator is too great, the battery can help supply poweras long as its voltage is greater than what the alternator is putting out.  So in effect, yes, it will "maintain voltage" in a severly overloaded system for a finite time period.  But as long as the alternator output voltage is greater than the battery voltage, the battery "sits there" and does nothing.

In a properly designed system, there is no reason for an external "stiffening" capacitor at all.  I have run 5000+ watt systems without them.  That being said, caps can help minimize "voltage ripple" when operating near your alternator's load limit in some systems.

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That does make sense now.  How much do high current amps cost?  I'll be running about 1000w RMS once I get my P3's.  Do you know of a place to get them or can I just order them at autozone?

I just wanted to quote what I found on the MECP book : " The alternator produces pulsating DC voltage, and the battery filters out most - but not all - of the residual ripple.  Ripple is the residual AC left on the line after it has been rectified into DC.  The ripple is generally variable in frequency and determined by engine speed (alternator spin)...  Most good audio products have sufficient filtering on their +12 Volt lines, but not on the ground side."(p 55-56)

"The vehicle's battery is sometimes referred to as the electrical system's largest capacitor.  This is due to its characteristic of resisting any change in the voltage across it, and to its ability to be discharged and recharged.  These characteristics enable it to smooth out transient ripple currents in a system." (p 67)  MECP basic Installer Study Guide, CR2000 First Edition.

It also states that the bigger the alternator the louder the ground loop sounds.  That's why I didn't want to upgrade but if the noise is being caused by an overloaded alternator then I should.  I still want to buy a DMM with a clamp on ammeter so I can measure how much my system and accesories are drawing.

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Poly Dollies

No wonder there is so much poor actual understanding of basic electrical theory out there.  That study guide is using gross over-simplifications and should be re-written.  It is not "wrong" per se, but it is not "correct" either. A battery and a capacitor are not the same thing, they are in fact nothing alike, and the idea that a battery will function as a filter for the ripple found in automotive rectified DC systems is just plain wrong.  A battery will do nothing to suppress any positive ripple above it's full-charged state voltage.  That statement from your page 67 is a gross, gross oversimplification.  And that other statement is just plain incorrect.  An alternator produces AC voltage.  There is an internal rectifier that converts it to DC.  In many cases, it is a pulsed DC because auto manufacturers use cheap rectifier bridges.

A high-output alternator will not cause "louder ground loops."  That also is a mis-statement.  Now, if someone were to install a high-output alt and NOT upgrade the vehicle's ground straps at the same time, you can create ground loops from frying the ground strap, but that has nothing to do with the alternator and more to do with ignorant installers.

Get a good 180 to 200 amp alternator.  They run anywhere from about $200 up.  Yes, go to AutoZone and tell them what you want, they should be able to help you.  If they say there is no HO alternator for your car, go to a car stereo shop or look online at a place like Zena or Stinger or Ohio Generator.

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