PLEASE HELP ADVISE Battery Dead but weird as I jumped it and it started but then...

[ZOA NOM]

Current is not regulated, voltage is. This is electronic theory, not physics or chemistry.

Ohm's law is absolute.

[the]

Interesting debate! A dead battery thread didn't interest me, but when it got to FOUR pages, I had to check it out.

But it is interesting. Here's my newbie outsider-to-the-thread view:

I think BOTH sides are right!

On the side that it may cause damage/is bad for your battery, that does have some appeal. The charging system does seem designed to maintain the battery, not charge a dead one. Some off the pages from manual(s) seem to support that view. I note that this view does not contend that it WILL cause your alternator to fail. Just that it is bad for it, and may cause it to fail, maybe immediately, maybe just by shortening its life a bit.

The other side says "But people do it all the time, and the alternator doesn't fail, and may not fail for another 10+ years." Which is also true.

But I think both sides can be reconciled thusly:

1. Recharging a "dead" battery with the alternator puts more stress on the alternator than driving around with a fully charged battery. The regulator is going to ensure that, as it signals the alternator to keep working to get the voltage up to normal range. No disputes there?

2. This extra work is "bad" for the battery. More work = more heat and more wear, by definition. Agreed?

3. The level of "badness" is going to vary from situation to situation. A new car with a robustly designed charging system is not likely to immediately overwork and blow out the alternator. By definition, the extra work isn't *good* for the alternator, though, and will shorten its life.. However, it may only shorten it from 2000 hours to 1995 hours, i.e., not significant. You may own the car another decade, and the alternator may stay functional. That has to be true, because many have done it, over and over. Yes?

An old car with a worn alternator on its last legs - the extra load and heat may be enough to kill it right away. OK?

See, that wasn't so bad!

[Gearbox]

Alternator amp ratings indicate max suggested output of amps that that particular alternator was designed to put out. Correct me if I'm wrong, but your discussion focused on the steady voltage output limited by the regulator suggesting that the alternator produces the same power all the time whether it was used or not. While this is true for voltage, it is not with amps. With the car running, you may pull 10 -20 amps, turn on the lights it it may go to 30 amps, turn on the air conditioning, and it goes to 40 amps and so on. Amps produced, or drawn, from the alternator is limited to use only, so your 90 amp alternator does not produce 90 amps all the time, rather only what is needed. Also, alternators are not designed to run at max for long periods of time. In the normal course of driving, a 90 amp alternator will use about 60% of max output if that. If you hit the horn, play your radio, hit the cigerrette lighter, plug in your GPS and radar detector, turn on the headlights, hit the brake light, open and close all the windows, turn on the wipers, etc all at the same time, you may approach the max, but it should be for a short limited time. Also by pulling amps, as I mentioned before, creates heat which again is the number one enemy in electronic failures. Think of it this way, if you put that 60 amp alternator from your 1968 Bug in a later Vanagon that requires a 90 amp alternator with power everything and air, how long do you think that alternator is going to last? They both produce 12 volts and it will run, but normal amp draw from the Vanagon would fry the 60 amp alternator in no time as it draws the full 60 amps or more in normal running. The way the system works is that the battery acts as an electrical shock absorber for the alternator. As the amps are drawn from the battery by the accessories, the battery draws more amps from the alternator. So if the battery is sulfated which in turn means there is a short, it will draw a tremendious amount of amps as any short would. Approach or exceed the max rating for a period of time, the alternator will get very hot and fry. That's why Porsche warns against this in its factory manual. That's the science.

[ZOA NOM]

I couldn't have said it better myself.

[FinallyGotOne]

It started this morning immediately, I drove it to work and so far so good. We'll see tomorrow how it goes.

[rick-l]

Originally posted by Iwantone
It started this morning immediately, I drove it to work and so far so good. We'll see tomorrow how it goes.

Do you think the problem was your ground cable connection? That could be pretty dangerous in a car with modern electronics, tends to let the magic smoke out of the protection devices. If it was a shorted cell that charging shocked open it will probably return shortly


Now back to the alternator discussion. If you guys can quote Ohm's law, i ought to be able to name drop also and introduce Faradays's law.

Can anyone fill in or guess at the missing elements in the model below? I would guess that the flux linkage times the rotational velocity would be at least 100 volts. The 160 mohms is two windings measured from my Carrera alternator (btw looking at the layout it seems to be 6 phase).

Is this a valid model?

[FinallyGotOne]

fortunately my 79 must not be modern electronics/

I have now driven back and forth across town. parked for an hour. Got back in, drove 10 miles. Parked for 15 minutes. got in droveto work. All seems fine now.

I am really just assuming that the problem was a non-grounded battery. It must have come loose the other day. I honestly was driving my car harder than ever. flying around turns etc... having fun! Must have loosened from an already not fully tightened state. Just think I discharged the battery too low.

[rick-l]

Come on...

Nobody knows the design specifics of alternators???

Something else we can debate is the flux in the above equation (capitol phi) is controlled by the ampere turns in the field winding. This is controlled by the regulator. Is this an on/off signal or is it proportional control? I know in the mechanical (relay) type regulators it was on/off.

[ZOA NOM]

Originally posted by rick-l
Can anyone fill in or guess at the missing elements in the model below?

Both diodes are reverse biased, so no current will flow. Unless your AC generator sine wave negative peak drops below ground. You do have the negative terminal of the battery run to ground, right?

[rick-l]

Originally posted by ZOANAS
Both diodes are reverse biased, so no current will flow.

During 1/2 the cycle. The diodes are only in there to show power loss.

This is supposed to represent the 2 windings out of the 6 that are conducting over any 60 degree period. For any particular pair you could write 0 < omega*t < pi/3 (I think 6 phase AC comes out of the coils, 6 legs)

You can put a ground reference wherever you want.

[livi]

I seem to have read in a 911 buyer´s guide, that one 'standard' problem with the 80´s 911´s is that they are eating alternators for breakfast. That they usually did not last for more than 40k miles.

Don´t know how true that is, but maybe it is caused by a lot of people doing like me - recharging the battery by driving.

[ZOA NOM]

Originally posted by rick-l
During 1/2 the cycle. The diodes are only in there to show power loss.

This is supposed to represent the 2 windings out of the 6 that are conducting over any 60 degree period. For any particular pair you could write 0 < omega*t < pi/3 (I think 6 phase AC comes out of the coils, 6 legs)

You can put a ground reference wherever you want.

Huh? How do diodes show power loss? And an alternator has three windings, called stators that provide the AC to the regulator. Where are coming up with 6 windings, and ...

doh! you got me. Good one

[rick-l]

Originally posted by ZOANAS
How do diodes show power loss?

Vf of a power diode like that is about 1.2 volts. P=VI When you put 92 amps through them they are dissipating 110 Watts each.

Originally posted by ZOANAS
And an alternator has three windings, ...provide the AC to the regulator. Where are coming up with 6 windings,

I took mine apart and looked at it. It sure looks like it has 6 windings. The workshop manual shows two Y connected winding sets also. The way the wires are distributed in the shell it looks like they are spaced 60 degrees apart.

Originally posted by ZOANAS
And an alternator has three windings, called stators

A stator is defined as the part of a motor, generator or alternator that does not rotate. Mine only has one of those

[randywebb]

rotors...


stator = STATionary...

rotor = ROTatin'

[rusnak]

I think Gearbox got it right. I only want to add what Iwantone alluded to, which is that the output is related to RPM. I'd guess that the original design was for more sustained high speed driving, and that we are probably draining our batteries faster in the US, what with 20+ year of urban growth, clogged freeways, US speed limits, etc = slower driving = less alternator output.

[rick-l]

Originally posted by rusnak
I think Gearbox got it right. I only want to add what Iwantone alluded to, which is that the output is related to RPM.

That would be the omega in the equation above. Looking at the pulley size it looks like the alternator/fan turns 2X as fast as the engine.

Looking at the alternator, it looks (inside and out) to be a pretty rugged machine.

[FinallyGotOne]

Today, here I am at work, no troubles at all. So I think, happy to say, no need for a new alternator ...

[T77911S]

gearbox is right on the money. an alt only puts out the amount of current that is asked of it. much like the transformer outside your house. the more lights u turn on, the more current u use and the more u pay the electric man. the volts are regulated. look at a batt. charger. the volts stay the same but for a low batt. the current is high, but as it reachs full charge, the current goes down. i would think a batt charger has a current sensing circuit to regulate the amount of current through the batt were an alt does does not have this. so, with a low batt, the alt may be putting out a lot of current, maybe max, until the batt gets charged. imagine driving your car for 10 miles in 2nd gear at 5000 rpm, wont blow it up today, but it wouldnt want to do it.

in the diagram above, both diodes cunduct on the pos. cycle and are off on the neg. cycle. put a capacitor in there and it will smooth out the ripples. most diodes have a .7v drop across them.

i am a radar tech for the FAA. i havent studied motors/generators in a long long time. at idle, the alt may put out 12v (13v), but the current will be low. u need to get the rpm's up for the alt to reach its full potential of its rated current output.