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Hi,

I've spent some time searching for answer and I'm clueless :dunno:
Right now I have 90A Bosch alternator. I want to install more powerful unit as I have ~1000W audio system. I am completely lost as I don't know if any other alternator will fit. If it helps, here's last part of my VIN - BN52735.

Please help!
 

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Not sure how powerful it is, but I put an alternator from an E39 M5 on my car. I have a 528i w/M52TU engine and it bolted right on.
 

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So you say I could risk buying 120A/140A from diesel or from 523i?
This is an interesting question - which I don't think has been asked recently - and - which I'm not myself sure of the correct answer.

Of course, it would only matter if your current alternator is maxing out, so, I would think the entire question is moot unless you have measured your maximum output current at maximum load to be maxed out at the 90 amps that your alternator is capable of outputting.

If you haven't proven that you've maxed out the existing alternator, then you really won't get anything whatsoever out of increasing the current output capability of the replacement alternator setup.

As for your question itself ... normally, greater-than-needed amperage capability of a power supply doesn't matter as the system will only draw what it needs. My 2002 525i, for example, uses the 120 amp Bosch air-cooled alternator. Of course, you'd have to see if the fitment works for your vehicle.

Other than fitment, the next problem, I would think, is "calibration". For example, the alternator spins at a certain speed (relative to the engine RPM), and, is basically a coil of wire spinning inside a magnetic field.

The strength of that magnetic field is what "moderates" the amount of current that is output. For example, if the battery sense circuit shows the battery to need charging, the field current will be increased so that the alternator will output more current.

Likewise, if the battery is full, and the load on the vehicle is low, then the sense circuit will lessen the current to the field, thereby lessening the current output by the always-spinning alternator.

So, I'd say it could work IF the calibration of that field-to-output ratio would work with the newer alternator.

This is all guesswork - but it would be interesting to see the following:
a) You must measure your current output at maximum load
b) There would be no point in proceeding unless that maximum current exceeds 90 Amps <== IMPORTANT!
c) Check the fitment of the replacement 120 amp alternator <=== A side-by-side physical comparison would be easy
d) Learn how the field current is calibrated to the alternator <=== I have no idea how to do this!
 

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Discussion Starter #5
Thank you for extensive input.
Honestly I don't think there's need to check if 90A is sufficient. I have enough audio hardware to make this alternator sweat. On some moderately powerful bass drum kicks there is a noticeable light dimming. My engine even started to stall when I played some mighty sub-bass so I had to turn it down to a normal spl level.

I didn't know about calibration. I know that there are universal alternators for mobile audio purposes so I thought, hey why not to upgrade my old alternator. Guess I'm stuck now. If YOU don't know, then who?
 

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The car's electrics will only draw what it needs, therefore a larger capacity alternator will only be less stressed because it will be taxed less relative to its maximum power draw and will last longer. Personally I've never head of calibrating an alternator. I know some BMW V8s have water cooled alternators, so as long as you don't get a water cooled one you should be good to go.
 

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Bmwfans shows that 120A alternator should be compatible with your car. So go to bmwfans and note the part number and which car uses the same altenator. In any case I don't think a bigger alternator will solve your issue. Alternators can't keep up with amps because the power needed fluctuates too fast. Capacitors or AGM battery would be able to handle fluctuating load a lot better. There are AGM batteries designed for audio use. If your setup really has output of 1000w, even with amps that 80% effiency you'll need over 1.2kW. That would mean that even 120A wouldn't be enough. Even 200A alternator wouldn't eliminate the dimming light effect when the bass hits hard enough, because it just can't adjust the output fast enough.
 

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Personally I've never head of calibrating an alternator.
I didn't have better words for it. What I meant was that the field current is X amps and that makes the alternator put out Y amps. If X & Y are the same for all alternators, then they're all compatible; but I was just guessing that maybe they're not.

Bmwfans shows that 120A alternator should be compatible with your car.
This is good to know!

Alternators can't keep up with amps because the power needed fluctuates too fast. Capacitors or AGM battery would be able to handle fluctuating load a lot better.
This is a very good point!

The question is how quickly does a change in load affect the field current, which then has to affect the alternator output.

It makes a lot of sense that this would be too slow.

But, then, I must wonder if the OP really is drawing so much current that the battery can't handle it? How can that be? The battery can handle the 80 amps (or so) that it takes to start the car, and it can handle that multiple times, and for 30 seconds at a time (or whatever it takes to start the car).

If his stereo is consuming MORE than that kind of wattage (80 amps x 12 volts is ~ 1,000 watts), then, yes, it seems a higher CCA would be needed.

But isn't the CCA of our BMW batteries pretty high already?
 

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The car's electrics will only draw what it needs, therefore a larger capacity alternator will only be less stressed because it will be taxed less relative to its maximum power draw and will last longer..
This almost certainly is true - but it doesn't change the point that, if the current draw isn't at least 90 amps, then a larger alternator won't help.

I won't disagree that a larger alternator may last longer though - but that wasn't the question.

However - I think the argument already put forth is probably true, in that the battery is really the limiting factor, and not the alternator (whose field current sensing circuit may be too slow anyway).
 

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I bought a used 97 528i and it came with a 140 watt alternator. Not sure that was the original equipment but would assume so. I am guessing it would fit your car as well.
 

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This is a very good point!

The question is how quickly does a change in load affect the field current, which then has to affect the alternator output.

It makes a lot of sense that this would be too slow.

But, then, I must wonder if the OP really is drawing so much current that the battery can't handle it? How can that be? The battery can handle the 80 amps (or so) that it takes to start the car, and it can handle that multiple times, and for 30 seconds at a time (or whatever it takes to start the car).

If his stereo is consuming MORE than that kind of wattage (80 amps x 12 volts is ~ 1,000 watts), then, yes, it seems a higher CCA would be needed.

But isn't the CCA of our BMW batteries pretty high already?
During cranking voltage can drop to 9V or even below. You could see this with a voltmeter, but the drop is really fast just the time the starter turns. That isin't an issue because as soon as the engine starts the alternator gets the voltage up to proper levels. When you got audio system drawing huge sudden loads the voltage will drop during driving, which can cause the lights to dim. When the voltage drops enough it will cause even more issues. Car electronics are designed to work at centain voltage range.

We need to remember that CCA values are for new batteries. OP could have an old weak battery that only has lets say 300 CCA, it would be still enough to start the car in above freezing weather.

OP could be drawing that much power, if he has 1000W RMS system. Flickering lights indicate loads that are higher than the current system can handle. As I said before OP needs to get something to even out the load fluctuations, because the alternator can't do it. You can see this even at your home if you got power tools. Some angle grinders draw 2.5kW+, when you use them you can sometimes see lights flickering, if the household wiring is thin.

I know many have installed secondary AGM battery like Optima yellow top to even out the fluctiations. Large capacitors can also be used. 2F capacitor should be plenty. If OP is going full blast all the time he would need an alternator that has 160A or higher output.
 

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My 528 has the 140 AMP alternator, so it will fit right in, and you could put in a second battery in the trunk with that 140 AMP !
 

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I bought a 50 farad capacitor off my old boss. It came off one of his show cars. Im running a 3000w sub system in my trunk now. I was unsure why my lights were still dimming a lot. Then I realized I had to charge the cap first so I left my music off for a few trips and the cap charged like having a second battery. Now the dim went down significantly. 99 540iT. Id start by getting a capacitor. 50 farads is way too much. Depending what your running between 2 and 10 should be plenty. Youll see a difference. My system is way too much for the car but its just a hobby of mine. I like base.
 
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