# How many watts can a 120amp alt. handle?



## NP (Apr 8, 2003)

I have a 98 VR6 with a 120 amp alt (if I remember correctly). How far can I go on amp size/power before I outgrow my alternator. To me, that means how much amp witout the lights dimming while driving at night?


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## Cyrus #1 (Sep 29, 2005)

*Re: How many watts can a 120amp alt. handle? (NP)*

I have a 90 amp (I believe) in my MK2. My 2 10" subs and my rear speakers are powered by amps, probably about 675 watts RMS. I have never noticed any dimming and the idle has never been affected. If you are worried about dimming you can also invest in a power cap.


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## afinley (Dec 29, 2004)

*Re: How many watts can a 120amp alt. handle? (NP)*

well, if you have a 120 amp alternator, it is because your car needs a 120 amp alternator. before you put the stereo in. you will always outgrow it evertually. i think if you keep it under 600 watts you should be fine.


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## pookz (Jul 5, 2005)

*Re: How many watts can a 120amp alt. handle? (afinley)*


_Quote, originally posted by *afinley* »_well, if you have a 120 amp alternator, it is because your car needs a 120 amp alternator. before you put the stereo in. you will always outgrow it evertually. i think if you keep it under 600 watts you should be fine.

That's not necessarily true all the time... TDI's have 120amp alts, and you KNOW they don't need that much considering 4cyl gas models usually come with a 90a. The vr6 however, comes closer to that necessity though. I have a 120a alt in mine, but I run my laptop, stereo with approx 800wrms, and all accessories on without a hitch.


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## afinley (Dec 29, 2004)

*Re: How many watts can a 120amp alt. handle? (pookz)*

yeah it has more to do with your electronics acc.'s. if you have power windows, foglights, and your rear defroster on, you might have some dimming.


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## JDriver1.8t (May 20, 2005)

*Re: How many watts can a 120amp alt. handle? (afinley)*

I have 1600W RMS, and it sucks even a 120 Amp dry.


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## jaybong (Jun 11, 2005)

it depends more on accessories and options. most alternators have a decent overhead- an amount that the alt can put out that isnt required even with all your toys running. my mk2 with ac has a 90 amp alt. my fuses for my amps total 40 amps- so thats the peak draw for my system. as long as my accessories and engine dont draw more than 50 amps then, Im fine. 
realistically, im pretty close- a barebones 1.8 8v comes with the 60 amp alternator, so the 30 i have overhead with the ac off isadequate, but as you see, Im pushing the limit when i crank the air.
you can get your alt rebuilt up to a 200 amp plus if you want. 1600wrms is a lotta juice IMHO, but in my car with a 120 amp alt it would be ok. a vr has a much higher draw than a 1.8 tho- so maybe with these #s and calculations, you can apply em to your own setup and figure out if you need an alt or not.
with massive power there will always be dimming lights to some degree- a cap isnt the bee all end all bandaid- it make the sub amp sound cleaner, and run more adequately regulated but it wont stop the dim any appreciable amount... and thats if you have a good cap- most caps dont have a good enough esr to be effective at all.


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## afinley (Dec 29, 2004)

*Re: (jaybong)*

yeah i just want to add that when you look at the fuses on your amps, that is the maximum draw defore they burst into flames. most amp will only draw half of thier amperage rating at max. fuses are designed to protect things from fire or melting.


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## GTIsilverstone (Feb 10, 2004)

*Re:*

The other thing you have to realize is this. Lets say you've got an amp thats rated at 1000 watts RMS.... and you've got the optimum power set up for that amp to acheive its full potential. RMS power is only ever really acheived (at least as far as sub amps go) during a competition when you'd be playing a 1 note drone. RMS Music power is usually close to half of the RMS rating... so your 1000 watt amp is probably only drawing somewhere in the neighborhood of 500 watts continuous... thats an average between output surges. Thats just sort of a relative rule of thumb. The other sorta relative rule of thumb is that 10 amps can translate to somewhere in the neighborhood of 100 watts... so a 120 amp alternator could do 1200 watts. My stock 90 amp alternator from my mk4 gti couldnt handle my ORION 1200 watt amplifier... not even with 0 Gage power wire, a system regulator and an auxilary battery. I ended up getting an upgrade alternator that fixed all my problems. My searches online for alternators produced at best 160-190 amp alternators that required major modifications, new brackets, etc. etc. big pain in the ass.... and costing in the neighborhood of 500-1000 dollars depending on the place selling. I was able to find a guy that builds custom alternators especially for car audio competitors. His name is Dominick Iraggi, his company is Iraggi alternator. He sells on ebay among other things. I was able to get a 240 amp custom alternator that just bolts up to where the stock one used to be.... with a new pulley i'm able to run 14.5 volts constantly, even during bass competition. End cost to me for the alternator was 450 Shipped. Also check out Kinetik high output power cells (fancy car audio batteries).... for around 250-300 bucks you can pick up they're smallest battery dubbed the CAP KILLER.... stores the charge of about 100 caps... and isnt rediculously priced when it comes down to it.


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## Non_Affiliated (Feb 12, 2002)

*Re: Re: (GTIsilverstone)*

120A Alt. x 14V = 1680 watts.
On older Diesel engines the battery is used for the Glow plugs and starting. After that minimal power is used from the charging system to run a diesel.


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## ssssly (May 4, 2005)

*Re: Re: (Non_Affiliated)*

how many watts your amps are has little to do with the size alternator you will need. Has more to do with the topology which governs how many Amps the Amplifier will draw. If the Amp has a regulated power supply then it will cause little to no problem with your charging system. High powered amplifiers with unregulated power supplies will suck juice out of your car right quick. 
If you are worried or having problems with dimming just install another battery.


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## Non_Affiliated (Feb 12, 2002)

*Re: Re: (ssssly)*


_Quote, originally posted by *ssssly* »_how many watts your amps are has little to do with the size alternator you will need. Has more to do with the topology which governs how many Amps the Amplifier will draw. If the Amp has a regulated power supply then it will cause little to no problem with your charging system. High powered amplifiers with unregulated power supplies will suck juice out of your car right quick. 
If you are worried or having problems with dimming just install another battery.

This I believe couldn't be further from the truth, it does matter how many watts your amp can put ou, nad the efficency of the amp. I am not ragging on just you here, but when are people going to figure out that its not just the output power of the amp that you have to consider.
You have to take into account of the *POWER INPUT* for the amp.i.e. the DC power needed to create the AC power out. If your amp is 500 Watts and is 50% efficent in making that power, it will need 750 Watts in power from the electrical system to create 500 Watts of AC power. Granted that this is also typacally averaged due to the nature of a complex audio signal.
People have to also realize that a 1000 Watt Class AB amp may consome more input power to make 1000 Watts, while a Class D may requires the only advantage is thet the Class AB amp will generate more heat which is wasted power. So yes a Class be will require less input power but 1000 Watts of ouput power is still the same amount of energy weither it is a Class AB, or Class B topology.


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## ssssly (May 4, 2005)

*Re: Re: (Non_Affiliated)*


_Quote, originally posted by *Non_Affiliated* »_This I believe couldn't be further from the truth, it does matter how many watts your amp can put ou, nad the efficency of the amp. I am not ragging on just you here, but when are people going to figure out that its not just the output power of the amp that you have to consider.
You have to take into account of the *POWER INPUT* for the amp.i.e. the DC power needed to create the AC power out. If your amp is 500 Watts and is 50% efficent in making that power, it will need 750 Watts in power from the electrical system to create 500 Watts of AC power. Granted that this is also typacally averaged due to the nature of a complex audio signal.
People have to also realize that a 1000 Watt Class AB amp may consome more input power to make 1000 Watts, while a Class D may requires the only advantage is thet the Class AB amp will generate more heat which is wasted power. So yes a Class be will require less input power but 1000 Watts of output power is still the same amount of energy whether it is a Class AB, or Class B topology.

nope, you are 100% wrong
your cars charging system is 0 watts. Watts are a measure of work. The amplifiers draw volts and amps. And they are DC not AC. 
Depending on the topology of the power supply in an amplifier they will draw so many amps depending on the supplied power voltage VS/ how many volts the power rail is being maintained at VS/ the input of the RCA.
A regulated power supply will maintain a standard voltage along the power rail thus reducing the total amperage draw . 
An unregulated power supply will continue to draw more voltage and amperage the further you turn up the volume. This becomes particularly troublesome when you are pushing a speaker to it's limit because as a speaker reaches XMAX the resistance across the coil often approaches zero again multiplying the amount of power the amp is sucking out of the car. 
Basically batteries don't put out watts. That is why if you look at a batteries rating it is in Volts and Amps. 
Amplifiers don't take in watts. They take in volts and amps. That's why you attach the battery to the power in.
Amplifiers don't really put out watts either. It is a measure of the amount of work that the amps can make the speakers do. 
By adding another fully charged battery to your car you add the total number of amperage that that battery can put out into the charging system. The less amperage the amplifiers draw from the alternator the less you will notice a drain on the rest of your audio system. 


_Modified by ssssly at 11:57 AM 5-6-2006_


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## Non_Affiliated (Feb 12, 2002)

*Re: Re: (ssssly)*


_Quote, originally posted by *ssssly* »_
nope, you are 100% wrong
your cars charging system is 0 watts. Watts are a measure of work. The amplifiers draw volts and amps. And they are DC not AC. 
Depending on the topology of the power supply in an amplifier they will draw so many amps depending on the supplied power voltage VS/ how many volts the power rail is being maintained at VS/ the input of the RCA.
A regulated power supply will maintain a standard voltage along the power rail thus reducing the total amperage draw . 
An unregulated power supply will continue to draw more voltage and amperage the further you turn up the volume. This becomes particularly troublesome when you are pushing a speaker to it's limit because as a speaker reaches XMAX the resistance across the coil often approaches zero again multiplying the amount of power the amp is sucking out of the car. 
Basically batteries don't put out watts. That is why if you look at a batteries rating it is in Volts and Amps. 
Amplifiers don't take in watts. They take in volts and amps. That's why you attach the battery to the power in.
Amplifiers don't really put out watts either. It is a measure of the amount of work that the amps can make the speakers do. 
By adding another fully charged battery to your car you add the total number of amperage that that battery can put out into the charging system. The less amperage the amplifiers draw from the alternator the less you will notice a drain on the rest of your audio system. 

I beg to differ, and there is just so many thing wrong with that explainantion I don't know where to begin. I am not sure of your back ground in electronics, but I'll leave at this. You have your idea of how things work, and I have mine. Good day to you sir.


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## ssssly (May 4, 2005)

*Re: Re: (Non_Affiliated)*

My background is 11 years of wave form and electrical engineering. In other words I design and build amplifiers. 
You don't know how amplifiers work.


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## ssssly (May 4, 2005)

*Re: Re: (ssssly)*

To answer the question of the thread if you install another battery you can run around 2000 watts as long as the amps have regulated power supplies. Without an additional battery around 800.


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## afinley (Dec 29, 2004)

*Re: Re: (ssssly)*

im gonna have to say you need a bigger alternator before you need two batteries. the battery starts your car, the alternator runs it. two batteries would give you more playback time with the car off....
and a 150A draw off a 90A alternator isnt gonna be pretty no matter how regulated your power supply is.


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## Non_Affiliated (Feb 12, 2002)

*Re: Re: (ssssly)*

nope, you are 100% wrong
your cars charging system is 0 watts. Watts are a measure of work. The amplifiers draw volts and amps. And they are DC not AC. 
*If a car is running, then how can it be zero watts. Considering that the alternator is set at a higher potential (ideally 14.4V), than the battery 12.6V. This is to allow charging of the battery when it has been discharged and to run the electrical accessories on the car. Now if a alternator can generate 120A. There is a potential energy transfer of 1680 watts. Now I realize that most common applications the alt will only deliverwhat is required by the electrical system. But I am trying to figure out where an amp is only DC, seeing that the output is AC. You could try to sit here and tell me that AC is just a varying DC Voltage. Which in a certain sense may be kind of true. But Then I wouldn't have spent time going through classes broken into DC , and AC wave form analysis.*
Depending on the topology of the power supply in an amplifier they will draw so many amps depending on the supplied power voltage VS/ how many volts the power rail is being maintained at VS/ the input of the RCA.
*The power supply will deliver current up to a certain point, exceed the current demand of the power supply what happens.*
A regulated power supply will maintain a standard voltage along the power rail thus reducing the total amperage draw . 
*Right up until exceed the current regulating capability of the power supply*
An unregulated power supply will continue to draw more voltage and amperage the further you turn up the volume. This becomes particularly troublesome when you are pushing a speaker to it's limit because as a speaker reaches XMAX the resistance across the coil often approaches zero again multiplying the amount of power the amp is sucking out of the car. 
*Nope a speaker at either x-max will not be zero. Explain to me where you have seen a superconductor speaker. Actually you'll see the DC resistance of the coil.*
Basically batteries don't put out watts. That is why if you look at a batteries rating it is in Volts and Amps.
*Your right, Which is done to allow users to know aht the limitations are of the power source. Cause there are all types of batteries. But you wouldn't put a 24V battery into your car. That is how we specify the battery perameters.*
Amplifiers don't take in watts. They take in volts and amps. That's why you attach the battery to the power in.
* Yeah, and what is Voltage x amps again? Watts! Yup but if you have a know amout of power output, and a known amplifier efficency. You can then figure out how much power is needed for the amplifier. Which then it can be calculated how much current is needed. *
Amplifiers don't really put out watts either. It is a measure of the amount of work that the amps can make the speakers do. 
By adding another fully charged battery to your car you add the total number of amperage that that battery can put out into the charging system. The less amperage the amplifiers draw from the alternator the less you will notice a drain on the rest of your audio system. 
*????? If you draw current from your electrical system, the alternator is going to generate more current to, for lack of better term, replace what is being drawn off of the electrical system. Extra batteries allow for more key off playing time.*


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## Non_Affiliated (Feb 12, 2002)

*Re: Re: (ssssly)*


_Quote, originally posted by *ssssly* »_My background is 11 years of wave form and electrical engineering. In other words I design and build amplifiers. 
You don't know how amplifiers work. 

Just wondering where did you get your degree at age 17 and join the work force right after that?
Cause if your 28, eleven years in the field would put you starting at? 17?
Who do you build amps for?
Just a back ground, an do not have a BSEE, But I have a ATA in electronics. I decided to give school a break. I sucked at english, Dyslexia, but excelled in electronics. Math I am much better at now then what I was back in college.
I went to work for Fluke Electronics, as a calibration tech. From there I moved up from a Cal Tech to the Rework Diagnostic Tools division. Funny cause it was a division of one. I serviced many of the distributors returns, and refurbs. Up-grades for marketing and sales force, keeping there equipement up to snuff and working for trade shows. I was primary support for Cert. of calibration program. Which I worked very closely with the engineers at Fluke US and Fluke Europe in Eidenhoven.
For resons I won't get into I left Fluke and currently work for the federal government (for reason I can't go indepthon) working at Regional Calibration Lab and repair center. I could go ahead and say I built amps too but that would be lame cause they were just stupid labs in class making a full class A amp, Class B, And Class AB back in '94.


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## ssssly (May 4, 2005)

*Re: Re: (Non_Affiliated)*

I'm 30, had my degree when I was 19. You need to go study your electronics a little more.


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## ssssly (May 4, 2005)

*Re: Re: (ssssly)*

First off a Watt is not a measure of electricity. It is a measure of work. If there is not a physical device doing work there are no Watts. Charging a battery and powering appliances requires no work. The charging system has the potential to do work but since it doesn't there are no Watts. 
Second everything in a car is DC. The alternator does nothing but charge the battery which then powers everything. All batteries are DC. Everything in a car runs off of DC. 
AC is used for power distribution only. It is used solely because it is much more efficient to transport electricity over long distances with much lower losses. Everything in your house has a AC to DC converter built into the power supply. Because nothing can actually use AC to operate. The power supply turns the AC into DC which can then be used to make it work.
If you exceed the voltage or current capabilities of an amp it will fry. The filter cap will blow and the amp will turn off. I don't see your point.
A large part of a speaker resistance is if it is moving or not. As a speaker moves it constantly changes it's resistance in respect to how far it is moving and how fast it is vibrating. Please look at a resistance graph for a speaker.
Who the hell said to try putting a 24v battery into a car. Yes an additional battery will give you ore "key off" playing time but it also reduces the strain on the electrical system. When the total demand of the electrical system are less than the storage capacities of the batteries they charge. When it is less they drain. The longer it takes to drain the batteries the longer it takes for the other systems in the car to become deprived of the power they are asking for. Without additional storage capacity a larger alternator is a waste.


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## Non_Affiliated (Feb 12, 2002)

*Re: Re: (ssssly)*

I'm sorry but can you tell me how a single Transitor amplifier (Simple class A operation, Voltage divider biased) stage works? Seeing that I don't know electronics.


_Modified by Non_Affiliated at 1:01 PM 5-7-2006_


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## afinley (Dec 29, 2004)

*Re: Re: (Non_Affiliated)*


_Quote, originally posted by *Non_Affiliated* »_I'm sorry but can you tell me how a single Transitor amplifier (Simple class A operation, Voltage divider biased) stage works? Seeing that I don't know electronics.

_Modified by Non_Affiliated at 1:01 PM 5-7-2006_

Magic. All amplifiers are powered by magic!


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## Non_Affiliated (Feb 12, 2002)

*Re: Re: (afinley)*


_Quote, originally posted by *afinley* »_
Magic. All amplifiers are powered by magic!

Well Actually P.F.M.


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## ssssly (May 4, 2005)

*Re: Re: (Non_Affiliated)*

Sure what part of it is confusing you exactly?


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## Non_Affiliated (Feb 12, 2002)

*Re: Re: (ssssly)*


_Quote, originally posted by *ssssly* »_Sure what part of it is confusing you exactly?


_Quote, originally posted by *Non_Affiliated* »_I'm sorry but can you tell me how a single Transitor amplifier (Simple class A operation, Voltage divider biased) stage works? Seeing that I don't know electronics.

What part of the question confuses you?


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## ssssly (May 4, 2005)

*Re: Re: (Non_Affiliated)*

If you have a question about something I will be more than happy to answer it. 
If you are being insolent because you were caught trying to explain something that you didn't have the requisite knowledge to explain and fundamentally misunderstood please don't continue to waste anyone else's time.


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## pookz (Jul 5, 2005)

*Re: Re: (ssssly)*


_Quote, originally posted by *ssssly* »_If you have a question about something I will be more than happy to answer it. 
If you are being insolent because you were caught trying to explain something that you didn't have the requisite knowledge to explain and fundamentally misunderstood please don't continue to waste anyone else's time.

uhoh... I think this is gonna get ugly... Someone's trying to avoid the question... Kudos to anyone that knows this answer, cuz I have no clue...


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## afinley (Dec 29, 2004)

*Re: Re: (pookz)*

i already said! it. is. magic!


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## jaybong (Jun 11, 2005)

magic, damn, i want a refund for my MECP tuition.
you guys are too damned smart for your own good.
but i like how the OP got scared away- too funny.
i love the 'tex, theres lotsa fights, but no stabbings.


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## Non_Affiliated (Feb 12, 2002)

*Re:*


_Quote, originally posted by *ssssly* »_If you have a question about something I will be more than happy to answer it. 
If you are being insolent because you were caught trying to explain something that you didn't have the requisite knowledge to explain and fundamentally misunderstood please don't continue to waste anyone else's time.

Considering my rebuttal argument, you seem to go of course a little. Then in an attempt to really fire me off, then again maybe it's your tact.
Which then you fire off,

_Quote, originally posted by *ssssly* »_To answer the question of the thread if you install another battery you can run around 2000 watts as long as the amps have regulated power supplies. Without an additional battery around 800.

When that wasn’t even the question to the thread. I believe the original thread was

_Quote, originally posted by *NP* »_I have a 98 VR6 with a 120 amp alt (if I remember correctly). How far can I go on amp size/power before I outgrow my alternator. To me, that means how much amp witout the lights dimming while driving at night?


_Quote, originally posted by *ssssly* »_Basically batteries don't put out watts. That is why if you look at a batteries rating it is in Volts and Amps.
* And then....*
your cars charging system is 0 watts. Watts are a measure of work. The amplifiers draw volts and amps. 


Watts are Voltage Times Current (which there is no doubt that you know that). But it seems your issue is more rather semantics, you say there is no watts in a Charging system. Well that is true if there is no current draw. The minute the car turns over there is a current draw. Applied voltage x Current demand and viola WATTS can be calculated. 
And in all actuality the definition of watts is; the unit of electrical power equal to 1 ampere under a pressure of 1 volt. Equal to 1/746 horsepower.
Then you crack off with, 

_Quote, originally posted by *ssssly* »_Amplifiers don't take in watts. They take in volts and amps. That's why you attach the battery to the power in.

Uh volts and amps, wait what does that equal = power! You said it your self, and what is power? The time rate at which work is done or Power is transferred. Well in SI power is expressed in Watts, or Joules per second, and Horsepower is the unit of power in the British system of measurement.
Then to this:

_Quote, originally posted by *ssssly* »_By adding another fully charged battery to your car you add the total number of amperage that that battery can put out into the charging system. The less amperage the amplifiers draw from the alternator the less you will notice a drain on the rest of your audio system.
 
I will give you this:

_Quote, originally posted by *ssssly* »_First off a Watt is not a measure of electricity. It is a measure of work. If there is not a physical device doing work there are no Watts. 

I guess that this statement about 120a x 14V = 1680 watts. Maybe I didn't clarify that with, that is the maximum potential power output that the alt can transfer.

_Quote, originally posted by *ssssly* »_Charging a battery and powering appliances requires no work. The charging system has the potential to do work but since it doesn't there are no Watts.

Well yes they do the reason things again is rated in Voltage is a specification more or less. Cause if we rated stuff in watts you wouldn’t know if the device works on AC or DC, or what the operating voltage was that was my point in the earlier post.
Oh and this comment:

_Quote, originally posted by *ssssly* »_I'm 30, had my degree when I was 19. You need to go study your electronics a little more. 

Hmmmmm, you graduated from where? The fact that I put my real world experience out there and all you can say is this, lends me to question that. Your 30? Hmmm why does your profile say 28? Sorry I just made my argument by what I see as inconsistencies in your argument.
Oh well, I have said my piece, and will just let sleeping dogs lie. Good day to you sir.



_Modified by Non_Affiliated at 7:10 PM 5-8-2006_


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## afinley (Dec 29, 2004)

*Re: Re: (Non_Affiliated)*

in my best charles bronson voice, "'dis, aint, ova'"


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## Furley (Feb 28, 2005)

*Re: Re: (ssssly)*

I'm sorry, what???

_Quote, originally posted by *ssssly* »_First off a Watt is not a measure of electricity. It is a measure of work. If there is not a physical device doing work there are no Watts. Charging a battery and powering appliances requires no work. The charging system has the potential to do work but since it doesn't there are no Watts.



_Quote, originally posted by *ssssly* »_Second everything in a car is DC. The alternator does nothing but charge the battery which then powers everything. All batteries are DC. Everything in a car runs off of DC.

The only reason a battery is in a car is to start the car, nothing more nothing less, once the car is started the alternator provides ALL, wait for it, POWER to run the car, charge the battery and POWER all the electrical components of the car. Adding additional batteries INCREASES the load on the alternator.

Let me go over that again, the battery is in the vehicle to start the car only, to start the car only. did I mention it's in the car to start the car only?
if you start the car, and remove the battery, guess what, the car still runs, how is that possible, because the battery is only there to start the car.


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## rastaone (Apr 15, 2005)

i was going to watch some carlos mencia but this is just as funny. reminds me of the mk4 forum lol. 
i think non_affiliated wins this round


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## jaybong (Jun 11, 2005)

girls will be girls


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## afinley (Dec 29, 2004)

*Re: (rastaone)*

yeah this is why i avoid the mk4 forums...
how often does this kind of thing happen? i got into something similar a few pages ago myself...


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## rastaone (Apr 15, 2005)

these sorta things occur maybe in 1 out of every 4 threads in the mark 4 forum hahah


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## jaybong (Jun 11, 2005)

bump for a good catfight


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## pwnt by pat (Oct 21, 2005)

*Re:*

Let me just throw in my input here.
When the car is running, the alt will be supplying as much power as it can supply. Any extra draw will be supplied from the battery. When the draw is reduced to below the alts maximum draw, the alt will then continue to supply the _current_ draw and charge the battery until full. The alt *only* supplies the amount of power needed.
120x14.4 (idea)=1728w
With the car running, it's probably going to consume 40-60a, however this is only when all electronics are on, stereo, wipers, windows, fans, etc. Everything. 
Like was said before, you will never have a constant load on your alt unless you blast tones, so it's safe to assume you can use the alternators full potential
Also, amplifiers are not 100% efficient. Class a are ~50% efficient, class a/b are ~50-60% depending on design and class D are ~70-80%, again depending on design.
With a class D amplifier (most amps are class d now-a-days. a/b are mostly "specialty" lines anymore), that means that with 1728w to spare, you can power 1200w with a 70% efficient amp.
Something to remember is that when an amp is sent in to clipping, it can put out over twice it's rated power. Current draw is increased accordingly.

Finally, batteries store power. They don't create power. If you draw power from the battery, the alternator has to charge it back up. Adding more batteries only only, *ONLY* help if:
a)You have a huge current draw and don't have or can't run the proper sized cable to support it, in which case a battery close to the amp will help
b)You want to play your system with the car off for a longer period of time.


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## afinley (Dec 29, 2004)

*Re: Re: (pwnt by pat)*

a. most sub amps are class d. most multi-channel amps are class a/b
b. yeah i thought batteries stored power, and alternators generated power. i looked it up on howstuffworks so i could jump in on the flame war, but HSW called me a liar. batteries generate power though a chemical reaction. damn.


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## 2.ohh (Jun 21, 2002)

*Re: Re: (pwnt by pat)*


_Quote, originally posted by *pwnt by pat* »_
Also, amplifiers are not 100% efficient. Class a are ~50% efficient, class a/b are ~50-60% depending on design and class D are ~70-80%, again depending on design.



i wish i had a class A amp that was 50% efficient







the maximum efficiency they can achieve is 25%, and that is reaching for the stars. think of a class A amp as always at full output, whether it be sending the power to the speaker or converting it to heat. a true class A amp will be rediculously hot when at idle, but will cool down when it is at full audio output.
class D amps are getting over 90% efficient. they theoretically can get to 100% efficiency, but that would require zero on-impedance switches with infinitely fast switching time.


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## pwnt by pat (Oct 21, 2005)

What class a amps are that low in efficiency? 
The lowest class D I have seen is 75, highest I've personally seen was very low 80s. I used 75 as a safe number.


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## 2.ohh (Jun 21, 2002)

*Re: (pwnt by pat)*


_Quote, originally posted by *pwnt by pat* »_What class a amps are that low in efficiency? 
The lowest class D I have seen is 75, highest I've personally seen was very low 80s. I used 75 as a safe number.


class A operation is where both devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. the key ingredient of class A operation is that both devices are always on. there is no condition where one or the other is turned off. because of this, class A amplifiers are single-ended designs with only one type polarity output devices. class A is the most inefficient of all power amplifier designs, averaging only around 20%. because of this, class A amplifiers are large, heavy and run very hot. all this is due to the amplifier constantly operating at full power. the positive effect of all this is that class A designs are inherently the most linear, with the least amount of distortion.

a better question is not what class A amps are that low in efficiency, but what 12 volt class A amps are actually produced







you are probably mistaking amps that say class A, such as the older soundstream reference class A amps from about a decade ago, with true class A amps. the SS reference amps, for example, use class A output topology, but are not true class A amps. it uses a switching power supply, which is not in a true class A amp.

another aspect of class A amps is that they are LOW in power. typically less than 50 watts of total output. the largest one i can recall was made by monolithic and was only 100 watts. and from what i heard, it does not operate in deep class A1 mode, but rather A2, which is inbetween class A and class A/B.


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## pwnt by pat (Oct 21, 2005)

*Re: (2.ohh)*


_Quote »_a better question is not what class A amps are that low in efficiency, but what 12 volt class A amps are actually produced you are probably mistaking amps that say class A, such as the older soundstream reference class A amps from about a decade ago, with true class A amps. the SS reference amps, for example, use class A output topology, but are not true class A amps. it uses a switching power supply, which is not in a true class A amp.

Exactly what I was making reference to. I wasn't awaire that the old chrome references weren't true class A.


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## afinley (Dec 29, 2004)

*Re: (pwnt by pat)*

yeah SS Human Reign amps are billed as pure class A, but they just cant make that much power with 12v in class A.


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## 2.ohh (Jun 21, 2002)

*Re: (pwnt by pat)*


_Quote, originally posted by *pwnt by pat* »_
Exactly what I was making reference to. I wasn't awaire that the old chrome references weren't true class A.


there are a ton of amps marketed as class A amps, but there are less than a handful that actually are true class A amps. their design just doesn't make sense for automotive applications, unless your heater is broken and you live at the north pole







the amps marketed as class A amps but are not true class A have efficiencies in the class A/B range.


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## ssssly (May 4, 2005)

*Re: (2.ohh)*

Everyone who knows what a watt is raise their hand








I am not arguing a matter of semantics I am simply explaining the extent to which you have misstated things. A Watt is a measure of work. You can predict the amount of work a given electrical device is capable of doing by multiplying it's available voltage by it's available current. However if you don't have said device hooked up to something that does actual work there is no work being done hence no Watts. The key in the definition is "pressure". 
You can calculate the potential energy of anything, but unless it is actual doing something it's potential energy is just that, potential. As long as the electrical system is not doing physical work there are no Watts.
The amount of wattage that the alternator is capable of producing is only relevant if you were to hook the alternator directly to the speakers, The relevant measurement is what amperage the alternator can supply at a given voltage vs the amperage draw.
Nothing but amplifiers are rated in Watts because almost nothing else is hooked up to something that does work. Speakers do actual work so it is useful, to a point, to understand the amount of physical work the amplifier can make the speaker do. 
The fundamental problem with your explanations as I previously stated is that you understand the equations but do not understand how to apply them. 
Class A amps are notoriously inefficient and you can use them to heat a small apartment. My class A mono-block tube amps put out enough heat that they raise the temperature of my listening room 10-15 degrees on average when I have them on. I have never measured them but in theory they should be about 8% efficient as designed.
My class Ds are 32% on paper, again I have never measured them.


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## 2.ohh (Jun 21, 2002)

*Re: (ssssly)*


_Quote, originally posted by *ssssly* »_
Class A amps are notoriously inefficient and you can use them to heat a small apartment. My class A mono-block tube amps put out enough heat that they raise the temperature of my listening room 10-15 degrees on average when I have them on. I have never measured them but in theory they should be about 8% efficient as designed.
My class Ds are 32% on paper, again I have never measured them.



which class A and class D amps you rockin? or are they homemade? most tube amp guys at least upgrade some of the components







i'd guess you at least upgraded an off the shelf amp, if not built your own.


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## Furley (Feb 28, 2005)

*Re: (ssssly)*


_Quote, originally posted by *ssssly* »_Everyone who knows what a watt is raise their hand








My class Ds are 32% on paper, again I have never measured them.

*Raises Hand*
oh, and you have the most ineffecient class D amp on the planet then, because the latest generation products are approaching 90% effecient.
And the origional question was not "how many watts can my 120A alternator make?" it was "how many watts can my 120a alt handle?"
there is a big difference in questions there and you answer should be worded accordingly.


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## pwnt by pat (Oct 21, 2005)

I think it has been assumed that the qustion was the rephrased one, at least that's what I understood it as.

_Quote »_However if you don't have said device hooked up to something that does actual work there is no work being done hence no Watts. The key in the definition is "pressure".

What IS happening then when the accesory belt turns the alternator pully, spinning the magnet inside, creating a electron pressure difference? Last time I checked, the engine does the work of spinning the pully. The efficiency of the alt, it's winding, etc, determines what kind of power it can produce. (power as refering to amount of amperage at a given voltage)

SSSS, this is what you're getting at I think. It'd be nice if you actually tried to describe what you were talking about as opposed to reiterating yourself with no examples.
Alts aren't rated in Watts because they produce amps at a given voltage, just like a battery. You dont' buy a 3000w battery. You buy a battery based off of it's supportive amperage. You buy alts because they create amperage. Last time I checked, forcing electroncs to move is by definition work. 

And like Furley said 

_Quote »_And the origional question was not "how many watts can my 120A alternator make?" it was "how many watts can my 120a alt handle?"

edited out: violence



_Modified by pwnt by pat at 10:51 AM 5-13-2006_


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## ssssly (May 4, 2005)

*Re: (pwnt by pat)*

The engine does work by rotating the alternator. The combustion of the engine creates torque to spin the alternator which then creates electricity. 
The alternator itself does no physical work. It creates electricity and stores it in the battery. That is not considered work. Technically moving the electricity through the wires from the alternator to the battery is work, since the wires dissipate some of the electricity as heat it can be considered work. But since that is not the intended goal when calculating things it is not considered.
The confusion is often what is considered work. Powering electrical devices is not considered work. In order for something to be considered work there must be physical actual work completed. IE something moving such as a speaker or electric engine, an actuator etc.
Without actual work, regardless of how much voltage and amperage is present, no Watts can be measured. You can calculate the total potential Wattage but there is no actual expenditure of Wattage. It is like calculating the potential energy of something. 
When you are holding a wrench it has the potential to produce ~200lbs of torque. But until you actually put the wrench on something that supplies resistance and pull on it there is no torque. Same concept.


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## ssssly (May 4, 2005)

*Re: (Furley)*


_Quote, originally posted by *Furley* »_*Raises Hand*
oh, and you have the most inefficient class D amp on the planet then, because the latest generation products are approaching 90% efficient.
And the original question was not "how many watts can my 120A alternator make?" it was "how many watts can my 120a alt handle?"
there is a big difference in questions there and you answer should be worded accordingly.

yeah, I already answered that question I was responding to something else. The whole thing started because I pointed out that it is irrelevant how many Watts the amps are. The only thing that is relevant is what the amperage draw of the amps is. This information should be available in the front of the amps manual with warning message saying don't kill yourself when you hook it up. 
Add up the total amperage draw of your system, if it is more than 15-20% of your alternators output you are likely going to run into problems. Try adding another battery. If you still have problems get your alternator rebuilt so that your system draw is less than 15% of total output.
Yes my amps are not exactly the most efficient in the world. I didn't design them to be. They are designed to sound good not conserve energy. Increasing the efficiency would have added additional components into the signal path that id didn't want there. Wasn't worth the trade off since I have a dedicated main for the audio room.


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## pwnt by pat (Oct 21, 2005)

Adding a second battery does NOT help as it's not part of the charging system. The alternator will STILL have to charge the second battery. *ALL* a second battery is good for is powering the system longer with the engine off or providing a shorter run between bat and amp.
A fairly accurate way of estimating current draw is adding up the fuses 3x60a fuses will equate to roughly a 180 max current draw at 14.4v, which would be 2592w.
One of my rubicon 501s had two 30a fuses. approximately 864w of power drawn at full blast. 500w w/0 clipping leaves a 57.87% class a/b amp








Oh, and you can use a HELL of a lot more powerful amp than one that will draw only 15-20% of the alternators potential output. You figure a 120a alt will leave you with an amp that could draw 18a. at 14.4v that's 250w BEFORE inefficiences....
Any extra power that the alternator can not support is drawn from the battery, which the alt charges after the power draw from the amp fades down.


_Modified by pwnt by pat at 12:50 AM 5-14-2006_


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## ssssly (May 4, 2005)

*Re: (pwnt by pat)*

I suppose you think caps are great too.
Powering the system longer is exactly what you are trying to do. You are trying to provide power to the stereo system long enough for the batteries to be charged by the alternator. Your alternator charges your battery, your battery powers everything in the car. Your amps are not hooked up to your alternator.
There is a specific reason that your alternator puts out a certain amount of amps. Because that is how much your car needs to work. Exceeding that by more than 20% is a very bad idea as it can cause major problems over long periods of time. Your ignition system requires a large amount of energy to properly work. 
Adding up fuses is a very ineffective way of determining how much power your amps draw. They are designed to blow at the rated amperage so obviously that is not how much power the amps are using.
Your car does not work at 14.4 v.
Nothing you said is correct.


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## Non_Affiliated (Feb 12, 2002)

*Re: (ssssly)*


_Quote, originally posted by *ssssly* »_I suppose you think caps are great too.
*I personally don't care for caps. the 1 Frad eletrolytics may somewhat buffer about a 16 mS of a bass note. and those 5, 10, 15, 50 F hybrid caps are just crap. Too high of ESL, and/or ESR. But that is not what the dispute is here. Carsound.com Forum is much better to bring that subject on







*
Powering the system longer is exactly what you are trying to do.
You are trying to provide power to the stereo system long enough for the batteries to be charged by the alternator. 
*Adding batteries just increases the load on the alternator, especially if they are different ratings*
Your alternator charges your battery,
*Agreed*
your battery powers everything in the car.
*Right! When starting the car, and when the motor is not running.*
Your amps are not hooked up to your alternator.
*your right*
There is a specific reason that your alternator puts out a certain amount of amps. Because that is how much your car needs to work.
*Hey wait, I thought the battery powers the car, not the alt. You just said that in the last paragraph. Now your just talking in circles. To exact the Battery in conjunction with the alternator provide power to the cars electrical system*
Exceeding that by more than 20% is a very bad idea as it can cause major problems over long periods of time.
*What exactly are you trying to say here? Are you saying that the alternator is is powering the car, and the current draw is at 100% cpacity output when the engine is running?*
Your ignition system requires a large amount of energy to properly work.
Adding up fuses is a very ineffective way of determining how much power your amps draw. They are designed to blow at the rated amperage so obviously that is not how much power the amps are using.
Your car does not work at 14.4 v.
*Yup, usally due to IR drops in the system you'll see 14 to 13.8V in properly working electrical system when the car is running*
Nothing you said is correct.
*Nobody in this forum is correct except you, Correct?







*




_Modified by Non_Affiliated at 12:07 AM 5-14-2006_


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## 2.ohh (Jun 21, 2002)

*Re: (pwnt by pat)*


_Quote, originally posted by *pwnt by pat* »_*ALL* a second battery is good for is powering the system longer with the engine off or providing a shorter run between bat and amp.



i need to add a little to this. the alt does still need to charge a second battery, but the benefits of a second battery or even a larger battery bank outweighs the small added strain to keep them maintained. imagine you have two identical batteries. identical in size, age, and performance in every last detail. (not really feasible that they would be 100% identical - but go with me on this one) now let's say that the system demands current sufficient to start dropping the voltage to below 12 volts on a single battery. as opposed to the pair of batteries which would have greater current capacity and would not require being discharged as deeply as the single 12 volt battery. perhaps the single batter might drop to 11.5 volts. (from 12.5 for the sake of discussion) well, that pair of batteries will handle the same current draw, but will be more stable and not have the voltage swing as far downward due to their increased capacity. they may only discharge to 12 volts in that given scenario. not to mention that paralleling batteries is like paralleling caps, you decrease the ESR when they are paralleled. they will deliver current more efficiently and not be as deeply discharged as a single battery would. the batteries are also not a load on the alt when they are discharging and they also recharge quicker, since they represent less of a load than a single heavily discharged battery.
here's a good analogy:
Take one glass and set it outside in the rain and let it fill up. Now pour 50% of the contents out of the glass and measure how long it takes to fill back up. Now set two of them out there. Pour 10% out of each one and watch them fill back up. (batteries do not discharge in a linear fashion, 50% from one does not equal 25% from two) With the glasses setting side by side and the rain falling at a constant rate (which is similar to an overtaxed alternator) the two glasses with 10% discharge will refill in a fraction of the time.


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## pwnt by pat (Oct 21, 2005)

Yes, the car doesn't run at 14.4v. However, when dealing with theoriticals it's always better to overestimate the maximum draw. I'd rather have an electrical system prepaired for 2500w and actually see 2000w vs the other way around.
With a good battery, voltage drop should be minimal. Add to this large enough sized wire and a direct line from both positive and negative post on the battery and you should be all set.
Unless you are running massive power, or trying to draw more than the alt and battery can sustain at one time, you should see very little benefit from the second battery under "normal" circumstances.' Perhaps I should have added relocating the battery would probably be more beneficial (and less costly) than adding a second batt on a DD setup.
I'll agree that for extremely intense current draws, IE spl burps, they will benefit but that isn't what I was talking about.
As for caps, I'm a firm believer that with an electrical system up to snuff the built in capacitor bank on your amplifier is all you will need.









_Quote »_Adding batteries just increases the load on the alternator, especially if they are different ratings
Your alternator charges your battery

Exactly, that's what I said. The more your underwhelmed alternator to charge.

_Quote »_Your amps are not hooked up to your alternator.

If you follow the wire from your alternator you'll see it meets up at the battery + terminal. Now you have a run from the + terminal to the amp. The alt's - terminal is it's mount to the engine block, which is tied in to the chassie (and negative batt terminal). Where's your amps ground? Oh yeah, the chassie. Take out the battery and you have a circuit between the alt and amp, correct?
[quoteYour ignition system requires a large amount of energy to properly work.
Adding up fuses is a very ineffective way of determining how much power your amps draw. They are designed to blow at the rated amperage so obviously that is not how much power the amps are using.
Your car does not work at 14.4 v.[/quote]
Like I said, I'd rather overestimate than understimate.


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## 2.ohh (Jun 21, 2002)

*Re: (pwnt by pat)*


_Quote, originally posted by *pwnt by pat* »_
I'll agree that for extremely intense current draws, IE spl burps, they will benefit but that isn't what I was talking about.



i wasn't talking about SPL vehicles. the bank of batteries are there in an SPL setup because unless you have a dozen or so alts, you won't be able to keep up with the current demand of tens of thousands of watts. batteries, espeically a high voltage bank, can keep up with the current demands of a system like that. but once again, i wasn't talking about that. i'm talking daily driving with over a 1k watt system. some might think that is a lot, but power is cheap these days, and more and more kids are buying several thousand watt amps. heck, my sub amp is only 1,200 watts and is conservatively set at around 900 rms for daily driving. i've also got 840 watts dedicated to my active front sound stage. with only upgrading the main battery, i was getting a pretty healthy voltage drop. after i replaced the engine compartment battery and added an identical battery in the back, the voltage did not drop nearly as much. and the two batteries i installed were the same make and model as the original one i had, but i wanted two brand new batteries that were manufacturered at nearly the same time for my dual battery setup. i used to think that dual battery setups were just for engine off listening, until i properly implemented them. when the system is on low volume, there are no noticable differences and there is no noticable sign of increased current draw on the alt (there is, but i don't drive around with a meter on the power wire). but when i crank the system, the benefits are very clear in more stable voltage.


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## Noobyatbass (Jun 26, 2021)

ssssly said:


> *Re: Re: (Non_Affiliated)*
> 
> how many watts your amps are has little to do with the size alternator you will need. Has more to do with the topology which governs how many Amps the Amplifier will draw. If the Amp has a regulated power supply then it will cause little to no problem with your charging system. High powered amplifiers with unregulated power supplies will suck juice out of your car right quick.
> If you are worried or having problems with dimming just install another battery.


Can you please tell me if I install a 800 watts amp will it be okay on stock alternator?


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