I can do a test, but i think there should be no problem... arent the stock alternators rated at 80amps?
lets say its only making 50amps, that's a net loss of 10amps. with a 70AH battery, that's only using up 5amp hours of capacity if you have a net loss of 10amps for 30min. hardly a situation where the the battery would loose much voltage. plus , for this to happen, MOST IMPORTANTLY, you would see a slight discharge in the system , which i can almost guarantee you will NOT see in traffic and 60amps draw. idling, 60amps, should still see a charging voltage of 13.5amps, only a .5 amps (14volts normal idle charge voltage) under a no load , engine running situation.


the only way you will have an issue in traffic , in the heat for 30mins, is if the voltage goes under 12.6 volts on the battery and i dont think that is possible with a good alternator. the ONLY issue here is that the alternator wants to produce that voltage, so at the slower speed, there is more force and wear on the brushes... the alternator is under more load and there is more heat on the brushes and commutator to keep the charging power up at the slower RPM.

 

Thanks for all the suggestions. I put in a new battery, since I did not have a receipt for the old one. It seems fine, and I don't get the low voltages I got before. We shall see in hot weather.
Thanks,
Dave

 

Great read Dave. I have a very similar setup to you on one of my 86 ROW 5.0l cars but not intercooled. No room for a rear fan of significant size but I did fit 10" one to a home made shroud on the lh side of the radiator. I replaced the front fan with a Zirgo ZFU16S which they claim is 3630CFM and 10 amp draw. I measured mine when I got it and it did only draw 10amps in 'free flow' but if I put it up against something, eg radiator it jumped to 12amps. Still not too bad. I run synthetic waterless coolant, so I don't really have to worry about boiling and it gets hot here, we have long summers and ambient temps over 110F are common. My temp gauge rarely goes over the 2/3 mark and has never had the red light come on.
I have had low idle voltage though. I converted both my '86 5.0l cars and my '81S [now my B.I.Ls car] to stock GTS 130amp alternators but this didn't do much to 2 of them. I notice that one '86 5.0l cars, I bought after an engine fire, which required me to completely rewire the front from the fire wall, is fine with just the GTS alternator. I did cheat and run an extra very heavy cable from the starter straight to the CE board [RHD car] which reduces the stock route from about 4 metres to about 400mm. I did run the new stock one as well. This car runs an 18" Lincoln variable speed fan with an 'AutoCool' 85amp PWM controller and it run great engine temps.
I've just replaced the '86 5.0l SC cars GTS alternator with a Ford Courier [Motorcraft 3G I think] which is a straight bolt on swap, belt and all. Now with everything turned on, both fans, SC belt fan, lights [all LED] aircon on, it idles at 12.9v. Lift the revs to 800 and it goes to 14.22v. I measure it with a calibrated Fluke 88VA multimeter.
Interesting note here, both my '86 cars are Aussie delivered and only a few chassis numbers apart, one is supercharged, with all the fuss that takes, the other has Porken's S300 chips and an '87 S4 regulator and it's quicker, especially from low speed.

 

Mark - wow, you need to stop drinking and try again later

Amps ≠ Volts
There are no commutators in alternators
At low RPM's the brush friction heating & wear is always lower
Heat has more effect on the windings & regulator than the brushes
Increased idle field current affects the brushes & slip rings

Lets also remember (earlier posts) that the alternator has no idea what the battery is doing - or if it even exists. It never has any idea if it is charging a battery, let alone by how much. It can't ever know if the battery is fully charged, or fully depleted. It is not a smart device by any definition of the term - it is kept in the dark, in a frequently very hot place and tries to do one thing - output about 13.8v. It is specifically designed to derate the 13.8v as temperature rises - to protect itself and the battery (it implements this a bit too enthusiastically IMO).

The alternator is affected primarily by its total load. The battery charging current (or supply current if the alternator voltage drops below ~12.6v) simply modulates that apparent load - and that is the only way the alternator responds to what the battery is doing, very indirectly. In fact in all these cases the alternator voltage is the key variable driving the outcome.

NB
Your fans on full almost certainly consume > than 10A each & if you have the HVAC blower on full it consumes >20A also, then you have to run the car...

Alternators at hot idle typically generate <<1/2 full rated load.

Alan

 

Drinking? whats in your glass?

1. Are we going to debate that commutator don't exist on an alternator but brushes do? ha ha ha... put down the pipe! news flash, if you have a brush, it rides on a commutator. (or I guess more properly called, "slip ring" ) (edit: yes, a modern alternator, uses the slip ring to transfer current, but doesn't play a part in the switching of polarity, as the solid state devices do that now...... but for this discussion semantics )

2. Amps don't equal volts, but the more the voltage drop, the greater the load on the alternator, and the greater the amperage it produces.

3. brush friction is relatively constant, based on spring pressure, but as the load increases, there is more current flowing across the brushes, which creates more heat.

4. yes, the alternator does know if there is a battery or not. (or the condition of the battery) the lower the voltage, the more current flows into it, and through the system.

5. yes, the fans are about 15-20amps each... just measured them and there are two of them on an S4

in a test I just did, as you put a 15 to 20amp load on a healthy battery (albeit only 14amp hour in my case), the voltage drops about .3volts. with both fans on , the system voltage is about 13.3 volts. put on the headlights and fog lights, and it drops to about 13volts. this is hot or cold in a test yesterday. So, in traffic, if the alternator is good, it will continue to charge the battery at differential of potential of 12.5 for the battery and 13volts. depending on the internal resistance the battery, current will flow through it and be charging it. if the battery is half discharged, there will be more load on the alternator and more current will flow through the system to charge the battery.

again, if you put a full load on the battery, it will drop to a voltage near 9volts during start up. that's a 300amp load . you find that kind of load when its running, and you will see the same voltage drop.

so, back from the "hot" observation, if you have a 10volt reading , you have a bad alternator. it will take a lot of load to get the voltage to dip below 12volts, during any running conditions. and yes, over a hour that might be 10amps over the rating of the alternator, and that will kill a battery in a matter of about 5 hours in traffic.. one hour, not a issue. I ran a race at the track, with NO alternator, just using a standard battery. 10ams to keep the engine running , was no problem for a 30min race.

 

there is the answer then....

 

yep, as you should see, that kind of load is no issue for a working alternator. hot or not. when you went to 800rpm, that entire load was still showing 14.2volts. so, it might have an issue at the slow RPM, but AC on should kick up the RPM to 800rpm so you might want to address that.


again, the voltage will drop in the system based on current load, with or without an alternator attached. however that drop on a stronger alternator will be a lot less.... stock seems to be about .5volts for each 30amps of draw.

 

You may think it's semantics but hardly - no alternator ever had a commutator, they all have slip rings. They look similar but the function is not the same, (DC dynamos have commutators - but they are not alternators).

No you are simply completely wrong! - when the voltage goes down the current goes down - ALWAYS! (I=V/R). When the voltage goes down the power goes down too - with square law Vs. voltage reduction so the load goes down (quickly) - not up.

The wear and heating are of course higher at higher RPM, and lower at idle. Yes friction itself it likely quite constant due to the brush spring pressure. The higher rotor current just challenges the electrical connection between the slip ring & the brush - until there are problems transferring the current - it likely has a quite small heating effect. It isn't the load increasing (that is usually flat to reducing) that causes problems at idle - it's the magnetic field reduction due to lower RPM's that causes the rotor current from the regulator to have to increase dramatically.

Quite nonsense... you aren't thinking.

Answer this: how does it actually know anything? NOT your BS answer above - a real answer, one that makes sense - How does the battery look any different to any other load? Say between a battery or a cooling fan - if you are an alternator - how can you tell the difference?

I'll answer for you: from the perspective of our alternators all loads look exactly the same. SO IT HAS NO IDEA IF A BATTERY EXISTS - LET ALONE IT'S CHARGE STATE. Dumb alternators like ours can never know... there is simply no mechanism for them to know. Think about it - HOW? They have an output B+ and an initiation line D+ that's it - current is current- all the amps look exactly the same...

Smarter alternators could know charge state by monitoring (uniquely/separately) the battery current (or measuring a voltage across known resistance in the path), or could know a battery exists by monitoring their own output and seeing they have no real ballast (output noise). But our alternators don't do any of this.

OK so finally we can agree 10A was way too low. So its more like 30-40 Amps for a pair (or more) for the cooling fans and another 20A for the blower so 50A-60A plus whatever else you need for everything else.

You are mixing Batteries and Alternators. You were loading an Alternator - not a battery.

The battery was irrelevant here - it stays irrelevant until the voltage hits 12.6-12.7v. - until that point all the current comes from the alternator. After that point it would be coming from both - so the results would be indeterminate for either stand alone anyway... In this you are quite correct
When you are running the battery shouldn't be doing much of anything except charging.. if it is supplying current then the voltage already has to be be below ~12.6v Then significant currents - even in the 50-100A range cannot be supplied from the battery for more than a matter of minutes without significant voltage drop and battery depletion (and possibly damage - a battery heats up fast like this - if it was already hot - extended very high current draw may cause it to explode).I agree - but I never talked about a 10V case so I assume you are talking to Dave.

I consider any time the alternator fails to supply all the required current to run the vehicle (and do whatever residual charging is needed on a healthy battery) then this is a system that doesn't work well enough for me. This means anything less that 12.6-12.7v is very inadequate - really allowing for any charging - it needs to stay above ~13v.

I know you like to be right in all things Mark - but you are just way off on your fundamental understanding of how all this works.

Alan

 

Thanks Alan,
I've heard that you should never disconnect the battery while the engine and alternator are running. If the alternator does not know the battery is there, why is disconnection bad. Is that a wives' tale?
Dave

 

The alternator needs a load on it to keep peak voltages in check. Particularly, the diodes typically have a PIV in the 16V range. Remember that the alternator is a three-phase AC 'generator', so peak-to-peak voltages can get quickly out of hand; the regulator doesn't respond to individual phase voltages, instead effectively to a moving-average combined voltage aftr each phase's output has passed through all the diodes. Turns out the battery smooths out the variations, making it possible for the single regulator to manage three separate phases.

Then, especially on the 928, the delicate electronics are sensitive to excessive voltage. Caps in the fan controller, for instance, are rated 16V, as are the ones in the LH controller. I suspect the EZx modules have similarly-rated capacitors inside. Unrestrained by the hopefully-low internal resistance of the battery, peak voltages applied to those components could easily fail the modules. Good News, sort of, is that most auto alternators reach a magnetic saturation point at about 15 volts RMS. Doesn't mean peaks are that low.


I think we sometimes forget that alternator current ratings are at voltages lower than what one might think ideal. The 120 Amp alternators we buy only deliver that when terminal voltage is abot 12V. We'd love that current capability curve to extend the full current capacity up to 13.8 - 14V, but it seldom does.

 

Thanks for the explanation, Dr. Bob.
Dave

 

Alan, Its not a matter of wanting to be right, its producing actual information. I you want to debate a commutator (which I conceded is not equal to a slip ring, although they are sometimes used interchangeably) sure you will win... again, its semantics for our argument.

the point here is that you say I am "completely" wrong. Really?? voltage goes down, current goes down.... now think about what you are saying vs what I said! you are having a hard time following this, because you think the voltage is lowering and the current is rising. you are looking at it backward. the current vs voltage curve is KNOWN by all but you it seems. as you put more current load on the battery. (meaning, you have a lower resistance in the circuit), the voltage of the battery will go down..... current goes up, voltage goes down. its because the "load" is the resistance. Im very familiar with E=IR.

so, think about it, you put more load on the battery or alternator, and the voltage goes down. this is how you can actually measure the current load too. I know if my voltage goes from 13.9 to 12.6, I have a big current load on the batter. (yes, bigger than if there was no load, which means a higher resistance, if that's how you want to look at it)

So, please tell me... how I have a misunderstanding. From your response, it seems you are not really understanding what is going on. clearly!!

Have you ever seen a voltage vs current plot for a battery? Guess what, power goes up too!!! I^2 R = power. volts x amps = Power too!!

as you have a greater currentl load, the battery and alternator fight to keep the voltage as high as possible, but they fall as current flows through the fans for example. 40amps for both of them. voltage goes down to 13.3 volts. as I saw.... you see, voltage went down, current went up, power went up, but the think you are not thinking about, is the R went down due to the "load" being present in the circuit.
So, because you are challenging this, Im wondering how you can profess to know as much as you think you know. seriously.

you can easily put a load on a battery, like starting the car..... 300amps from a battery is a lot...it is very capable of doing this and guess what, from the AH of the battery, you can calculate when it will die. it wont explode, and is well within its capability. right?? 300cca or higher numbers for max potential are known values. so, with my 14amp hour battery, 300 amps will bring the voltage down to about 9volts and will produce that much current for about 3mins.. (actually a little less due to the voltage decay as the capacity of the battery is exhausted..... (still more battery curves..... voltage vs capacity)

 

No it's quite true - it is a very bad idea. Partly exactly because the alternator has no idea if the battery is there or not - so it cannot do anything to compensate.

When there is no battery there is nothing to support loads between the alternator phases so you see a lot of ripple on the top of the DC. The regulator interprets this as a varying load and starts to swing the field current around to compensate, but no sooner than it does the load appears to change again. This totally confuses the regulator and at certain speeds the response time of the regulator vs the frequency of the ripple cause this to get extremely bad and voltage regulation goes to hell. The output voltage may rise and ripple voltage may increase significantly and at these time any significant load dump (like turning off the cooling fans) might cause quite major spikes on the output (B+ supply) due to the alternator stator inductance and the lack of a high current receptor (the battery acts a lot like a big capacitor here). It is possible to see many 10's of volts spike (even up to 100v) when operating without a battery. High voltage spikes kill delicate ECU's and the like... so a very bad idea. The alternator usually has avalanche diodes built in to protect against this - but they only eliminate the highest voltage spikes and may not survive long under these conditions.

Alan

 

Thanks Alan.
Let me ask if cooling the alternator provides any real advantage. Will it put out more current, maintain voltage, or have a longer service life? It seems like all three should be true, but I don't know.
Dave

 

Mark

This discussion is about what happens to a heat soaked alternator at hot idle, it is NOT about what happens to a battery or an alternator when you switch loads on and off. You seem to want to argue that... it would be a topic for a different thread.

The case in point here is a 928 that has been cruising at high speed in a hot ambient environment in some fixed 'configuration' and then comes to a stop in traffic and sits idling in the same fixed 'configuration' (nothing new is being switched on or off).

Let's say the 'configuration' is: Engine is running, AC on, cooling fans full on, blower on full, brakes lights are on, head unit is playing music through an amplifier at a volume sufficient to be heard over the blower noise. For me this is a pretty normal summer configuration. On my car this is about 85A at ~13.8v cold (including battery charging current).

So the voltage droops as the RPM's hit idle: the voltage goes down AND the current goes down and and the power generated goes down by the square of the voltage reduction. My point was exactly this: V/I =R and R is not changing at any significant rate* so I reduces directly with V.

You seem intent on varying R - there is no good reason for this in this case... It's primarily about magnetic flux changing due to lower rotational speed and the alternators eventual inability to generate enough power - so voltage drops, current drops and power drops by a larger % until it comes into a range it can support.

You can evaluate the effects of load on batteries all you want - but it's largely irrelevant here. I consider that by the time the alternator output has reached 12.6v and the battery comes into play the system is already outside the range I want to consider acceptable operation, and it's time for a redesign.

Does the stock system go outside this range - yes on cars other than yours it does - so to me it IS a failure.

* One could argue about the battery load marginally changing over time due to charging - but it's not really material here.

So Battery and Hot Alternator - I was talking about the Hot Alternator... As long as it is working normally the battery really doesn't matter much to this question.

Alan