Carl Fausett

I would really like to talk to you about your alternator options. I'm sorry, I have no email from you. My email address is in my signature for every post - please just drop me a note and I will be sure to respond. I am hard to get ahold of by phone as I am often in the shop and not at my desk. Email is better.

Simon928

Nuts. Well this thread just became really relevant to me today. I needed a new crank pulley for my '90 AT and I just got a used one from an 85. I thought they were the same but from what Jim quoted from the tech bulletin it looks like the 85 will have a smaller pulley than the damaged one that I am replacing on my '90. Does that mean that if I install the '85 pulley it won't charge my car at idle, or will it do so at the same level as 87-89 S4s? Can I make it better by putting a larger pulley on the alternator or should I just not bother and wait until I get the right sized pulley? I intend to do so eventually regardless but I was just hoping to be able to get by for the next few weeks with what I have.

Speedtoys

"if I install the '85 pulley it won't charge my car at idle"

Is idle your use case?

ROG100

No emails or messages here either.

Simon928

...

Simon928

You're right, I suppose that charge at idle isn't necessarily the most important scenario. I guess what I'm really asking is if I use the 85 pulley on my 90 will the alternator adequately charge the battery (like at 87-89 levels for example which use the same alternator / pulley combo that I am proposing) or are there other reasons which would make installing this pulley a complete waste of time?

Alan

Carl - a few thoughts here. Having a fan at the back & at the front with central exhaust is a better idea for sure - more so if back fan cooling the regulator & rectifiers diodes sucks in cool air - so a rear shrouded feed from somewhere other than the ~header area would be better - like in the stock case. The stock idea of blowing forward through the alternator with a single front fan is a actually a bit odd to me - there is so much flow from the radiator area backwards...

However most of the heating of the alternator is actually internal and proportional to the power being generated - more power made = more internal heating.

Most power is dissipated by the rectifier diode drop power (~proportional to output current), by self heating of the stator windings (~proportional to total power), by the regulator power (highest under idle conditions) and by the self heating of the rotor winding (highest under idle conditions), there is also a small mechanical dissipation in the bearings (~proportional to RPM). The first two are the biggies.

Generating more power means generating more heat - no two ways around it. Fan cooling at idle is quite low, while power demand is generally ~unchanged and regulator and rotor dissipation spikes - so temperatures will always go up quickly at idle. You are right that modern designs have slightly better thermal behavior, but not by very much. A lower de-rating of the temperature curve in the regulator helps much more, this however is a trade off. The stock case is over aggressive for sure - but go too far the other way and you are trading long term reliability for more power now.

A sulfated battery is not good for the car as you have lost much of the charge - however to the alternator is actually a smaller load. The primary reason to recommend to replace it is not to avoid alternator damage or poor output - it is to avoid alternator returns ("it didn't fix my battery charging problem"). It is not generally a bad idea to change both I agree, but it's clear some alternators are actually replaced only because the battery is bad - perhaps because of poor battery testing at auto stores (and a reluctance to make too many under warranty replacements).

Repudiating another oft repeated fallacy - the alternator knows NOTHING about the battery, it certainly does not know how well charged it is.

It cannot distinguish electrons going to the light bulbs, ECUs, AC clutch, or radiator fans from electrons going to the battery. It doesn't even know if there is net current going TO or FROM the battery at any given time - or really even if the battery is actually connected. It tries simply to maintain its temperature de-rated voltage set point regardless of load - or if it can't - to generate as much power as it can under low RPM conditions - the nature of the load is basically irrelevant in this.

There is simply no mechanism in how it is connected or how it works for it to know anything about the battery. Today there are more advanced alternators with more complex connections that can mange generation and charging better - but you can't connect them to a 928.

Alan

the flyin' scotsman

As well as thoughts on alternators and their pulleys give much consideration to the front wiring harness. I posted results of a new harness installed on a S4 with no pulley or alternator change with significant difference to electrical performance.

great product quality and performance by Sean

Alan

Indeed apparent power supply problems can actually be in the battery, alternator, power side connections (any) or ground connections (any) and also even in the measurements tools & methods! On older cars like ours it is most likely to be several of these working in concert - so lots to look at & investigate.

Best to consider and investigate them all - including your own assumptions/memory/abilities!

Alan.

Taguid

I have noticed a dramatic difference with GB's alternator and Sean's engine harness. My windshield wipers and windows operate much faster than before.Along with 13.8 volts at startup in the dash.

Carl Fausett

I politely disagree. While I do agree that alternator does not know much about the battery, it does know resistance to charge when it feels it. Not sure if this is resistance or impedance to flow, Dr. Bob will correct me on that. But the result is the same. a sulfated or partially sulfated battery will resist a charge, and make the alternator look bad when it is actually not.

Alan

Carl - You should trust me here - it knows nothing at all - exactly nothing about the battery.

It CANNOT know because based on its connections there is no way for it to know any of the things you attribute to it.

A sulfated battery won't work very well for the car and will make the regulation of the alternator slightly worse (so likely the voltage will have more ripple), but the alternator won't actually really know this and won't really look bad on any typical measurement - except that the battery won't really charge any better than before (because its a battery problem).

I don't make this stuff up, and I'm not trying to bash you - I'm trying to get you to realize how things actually work. Sometimes you have to listen, understand and let go of what you think you know...

We've been through this before - most people have no idea how alternators work and it is a much more confusing topic than anyone thinks. I like to keep the story ~straight. Alternators in 928s are all dumb - they know nothing except: their RPM, their load, their temperature and their voltage/temperature set point curve. They do what they can. There are better performing alternators - sure - but because of how they are connected they all must still be considered 'dumb' - they can only do so much.

Most of the time even the best alternators are outputting a very non-optimal voltage to the battery for re-charging. But lead acid batteries are pretty forgiving, pretty robust and have a decent enough capacity - so if you meet just good enough - it doesn't really even matter that much - that is what we rely on.

So just don't undercharge it too badly, don't overcook it too badly and it will all work out in the end.

Alan

dr bob

Perhaps the biggest issue with tired batteries is that they are way less charge-efficient. It takes more amp-hours of charge per amp-hour of capacity recovery needed compared with a new battery. The alternator has to work harder (more amps) to maintain the target battery terminal voltage and for a longer time, as a percentage of the charge current is wasted into heat at the battery due to the salt bridges forming between the bottoms of the plates as the battery ages. The extra amps and extra time mean the alternator is working harder for a longer time, and the current spent there isn't available to run accessories.

The thermal limits on the alternator are defined based on the insulation used on the stator and rotor windings, usually some kind of varnish. Heat dissipation is obviously important, as you can stand more generated heat if you can pump more out via airflow. The voltage regulator has a protection curve based on temperature, the same curve that drops the excitation when it gets hot. As Alan stated above, the alternator has no clue what's causing the heat, no clue what's using the power it generates, etc. It only knows a maximum terminal voltage and the temperature correction curve for its own protection.


More "efficient" alternators...

The major heat generators include winding heat due to current passing through. The voltage drop and heating from current flow through the diode rectifiers is very predictable. Silicon diodes typically see about 0.67 Volts drop. There are better diodes with less voltage drop but they get expensive when you are buying a dozen of them for the high-current capacity needed. Making an alternator more "efficient" has a few options beyond the diode heating. Perhaps the biggest improvements have come from the way that the windings are shaped, and how tight the windings can be placed with better insulation. Using a winding of square-section wire instead of round takes care of both of these handily. Next-next gen alternators will have the square wire in the windings, wound in a direction that is directly at odds with the excitation field, and uses high-power FET's instead of diodes for switching. Going to a many-more-pole stator winding will help too. Lots of little things add up, but in reality not fast enough to justify the manufacturing costs just yet. Except for savings in the copper, there's no easy justification for picking up those last few percent of total efficiency. Yet.

Alan

There is some truth here - it would certainly take more time to charge a sulfated battery, however generally I will say I don't think this stresses an alternator more.

The actual cold battery charging current will be lower due to battery's higher internal resistance. As the engine and alternator warm up normally the voltage de-rating will mean that the charge current will then go down before much recharging has actually happened.

The battery will then be charging at a lower rate and will likely never get fully satisfied because it missed the benefit of the high bulk charging phase when the alternator was ~cold. It will continue to take current for longer than a good battery but it just won't be that much current.

The alternator is well able to handle a good % of its rated current indefinitely at high RPM for accessories - and the sulfated battery just isn't really that big of a load @~13-13.5v - certainly not compared to the cooling fans, blower motor, rear defogger, headlights etc.

If the alternator voltage droops (say at idle) - the battery charge current will go down very much first - before any accessories lose much power - by about 12.6v charge current will be zero.

To me this is no issue at all for a healthy alternator, and wouldn't make it look bad in any obvious way - but it would still soon be obvious you'd need to change the battery.

Alan

SeanR

^^ these guys know what they say.

Customers been fighting a slow start, charging issue and swears the battery is good, RT Optima battery that's many years old. Same issue I had for years using 3 different Red Top Optimas. Waiting to find out the results if he puts in a standard new battery. It solved issues on my car.