Yeah. It dove down to 4, but you could hear the car just shut down as it dropped. Again, I had cables connected to my Lexus 300 sedan and it was running (lexus) just peachy. As long as the cables were connected to my battery the 911 ran fine, (a bit high idle tho) and then literally as soon as I took off the positive off the lexus Jay put the meter on the battery and it dropped over the next 15 seconds i would say it went all the way down to 4, car shut off and there was not any juice even to run the dome light. NOTHING. so I was hoping the battery was bad, but it has been on the charger now for 5 hours or so and the charger shows at 6 amps still. I'll see where it is in the morning.

If it is still at 6, then I think it is not chargeing and I can go get another Bosche Battery. So if the battery is bad, when the car is running and you have a meter connected to the battery that wont tell you what the alternator is spitting back will it? I also actually after jumpingthe car did not see the alternator light on except for the initial flash I think. Well hopefully more info will be found out tomorrow.

According to the Porsche manual, it's not a good idea to disconnect the battery while the car is running, actually they warn against it for fear of damaging the alternator. Any good battery would have kept the car running for at least 20-30 minutes or longer if the alternator is bad. What you have is a sulfated battery that's shorting out and is like disconnecting the battery each time it shorts and runs down. I still say get a new battery before you do damage to the car.

As for the argument about recharging a dead battery, I found the following passage off an automotive site:

"Alternators are designed to keep a battery charged and power the accessories in the vehicle. They are not designed to recharge a dead battery so you must test the battery before you attempt to diagnose the rest of the charging system. Alternators seem like they are working 100% of the time but in actuality they are turning on/off at regular intervals. Attemping to recharge a dead battery will overwork the alternator and may burn it out."

Not taking sides, but it was the first one I found. The site is at http://www.ratwell.com/technical/ChargingSystem.html Hope this helps.

+1 for a new battery, and I would have your alternator checked. It may have sustained damage to one of the windings, rendering it 2/3 effective, which may not be noticeable without a full load test. I would also be concerned about the DME, since you may have been running it with a low voltage for some time. The DME is robust, but it is TTL circuitry, which can be damaged by out-of-tolerance voltage.

An automotive site? Like this one?

Are you quoting Zonas?

No specs like max power, continuous power, temperature derateing etc???

Or you could save yourself the labor of removing the alternator and just put a battery in and see if it works.

You could assume the Porsche designers could add up electrical loads and sized the alternator correctly (including battery charge load).

You could also assume the Bosch/Marchall/Paris-Rhone (whoever) designed the alternator so it wouldn't burn up under rated load.

His 79 probably doesn't have a DME. If it did I wouldn't worry too much about power input in the range of 0-14 volts. On the other hand no battery or an open battery with large spikes coming from the alternator get out your soldering iron.

Didn't want to get sucked into this one, but is the factory manual ok?


http://forums.pelicanparts.com/uploa...1165907332.jpg

Factory manual for what?

Damaged I'll buy, I don't think you could run the alternator into a short.

Define discharged, Battery voltage 12 volts? 12.5 volts? How much difference does it make to the alternator?

Looking from the alternator into the battery do you model the battery as a resistance in series with a voltage source?

Below a certain set point is the field coil full on?

There are so many things that can consume our lives with worry and concern...or be utterly irrelevant. I remember once hearing Click and Clack say that if you back a car out of a parking spot and then put it in gear (or drive) while still rolling backward and let the clutch out (or gas it), you're causing unnecessary wear. You should first apply the brakes, come to a complete stop and _then_ go forward.

Am I concerned? Only if I'm backing up at 60 or so.

I am going to contact Delco or Bosch, under the guise of a journalist doing research, and ask them about this tempest in a teapot. I'll post what I learn, either way.

bad battery is a relative term???? come on. put a battery on a battery tester, which puts a load on it, if voltage and current cant be maintained, its bad, not relatively bad, unless u want to use it power your remote. ive even seen LOW batteries test good, they just needed a charge. a batt that can be recharged is just a low batt...not bad.

Sorry for the digression, but....

Actually, this very thing is known to cause failures in VW 020 & 02a transmissions. Evidence as seen in my very own transmission:

Reverse idler:
http://forums.pelicanparts.com/uploa...1165934683.jpg

Back to our regular scheduled programming...

s

OK, Here is a brief update. I went out this morning and the charger was at 2. So it seemed to have taken the charge. Next up is when I get home we will test with the meter again after the car has sat for 8 hours or so.,

What do you all think I will find?

WHERE is the voltage regulator? I have a few items on my driver side fender wall. Is it there? thanks!

Don't know what model 911 you have, but on many midyears cars, it's an integral part of the alternator and is on the back of it (i.e. toward the front of the car) inside the engine shroud. It can be changed out, but it ain't easily accessible...

What I believe is that the battery has taken a surface charge, soon as you put load on it again, it will short and it once again discharge. Instead of beating this dead horse, why haven't you just swap batteries with your Lexus and see what happens? It'll be the fastest way of proving whether the battery is bad or not.

1979 911 SC.

well I did not want to swap batteries in case I had some issue shorting out my battery. I thought bes to charge it fully withthe battery charger and go from there. When i get home today we'll see whats up. The battery appeared to have taken a full charge now. So we'll be able to start it and let it run and so forth without any jumpers. Once I do start it, if I check at the battery what the meter is saying can that tel me if the alt is bad? Like what would a brand new battery show on the mater if the car was running, but the alternator was bad.? would the battery just show like 11 or 11.5 volts? or would it show a continuos drain? f the alt is good but the regulator is bad and teh battery is good what would i see at the meter on the battery? and lastly if the alt and regulator are both fine but the battery is bad somehow, what would the meter show at idle?

Your car may perform normally for awhile with an alternator that has a bad winding. My advice is to have the charging system checked by a professional, in spite of all the brilliant advice you've had here.

If he wanted professional help why would he be asking questions here?

Is it worth taking it out and testing it?

If you have a short in the car, or the alternator is not charging, the battery will just discharge. Either put the Lexus battery in the Porsche, or the Porsche battery in the Lexus and see what happens. Voltage should be 12-13 volts with the car off and 13-14 volts with the car running. By putting the same battery back into the car will just lead to another discharged battery with no conclusions gained and perhaps damaging the car. Your choice. Good luck Allan

Iwantone,

Your voltage regulator is on the back of the alternator. It is removable, and should be changed if you're charging like over 14-ish volts.

An interesting experiment: try warming up the engine, then idle with all accessories off. You should get 13-14 volts or so (it will jump around). Then turn on all electrical devices that you would on a worse case (AC or heater, radio, headlights, whatever) and re-check. You will be getting 12 volts or less. A fully charged batter is 13 volts. You're draining your battery just by running the motor.

Alrighty, I think it is time to get a Bentley Manual. Check with our gracious host, or any Borders or Barnes and Noble can special order it for you.

If you are charging much less than 13 volts in normal conditions, then you either have a weak alternator, or loose belt, or something else is not connected. I'd recommend cleaning and checking the battery clamps, and fuses while you're at it. I think you correctly diagonosed the loose ground cable.

As for the rest: www.howstuffworks.com

Okay, I called ACDelco, spoke to an engineer there, and he said it's nonsense that re-charging a dead battery will hurt the alternator. "The voltage is regulated," he basically said. I told him about this thread and he said that it's simply not true that you can hurt an alternator by using it instead of an external charger to bring the battery back up to its normal state.

Good enough for me.

yeah, the "engineers" at Kragen, Pep Boys, and Napa are quite helpful. They can even recommend what brand of chewing tobacco is best too.

On the other hand, there is a lot of info out there that indicates it is bad to charge a "bad" battery with your alternator. Again, do whatever you want with your car, it's your wallet:
http://www.ericpetersautos.com/forum/index.php?PHPSESSID=7359221fd739a1da1d9f28606095d8 79&topic=324.msg5435

if the battery was near the motor you could easily check the alternator by connecting the positive terminal off the alternator and the negative from the battery. If it reads above 13.5v you are fine.

Oh for god sake rusnak, I called their offices, in Detroit. I didn't call a store. I'm a writer with a reasonable reputation. I do research. Been dooing it as a writer for national magazines since--odds are--you were in grade school.

I know who Eric Peters is. Which is why I'm not even going to bother to read that link.

I have just registered with the site so I can post on this thread. I have literally been killing myself laughing at the posts on here. This has been superb entertainment, thanks to everyone.

Oh man, what a wonderful place to start your Pelican Parts career! If you're nice, I'll tell you how to find the Motormeister thread, and if you're _really_ nice, I'll introduce you to the K&N filter symphony.

Wow Steve, you called AC Delco? Must have had some doubts. :)


Did the "engineer" talk to you about duty cycle? Did he tell you what damage might be caused if you run your alternator at 100% duty cycle for an extended period? Ask him what the affect is on the alternator of the heat dissipated through its windings when it is forced to revive a dead battery, and run at full duty cycle, and provide power for the rest of the electrical system.

This really is a ridiculous thread, and I can't believe there is any debate at all about this issue. There is no mystery to the design and purpose of an alternator. That's apparent to anyone who has used a battery charger.

I'll take my car to get fixed by an engineer/writer next time. Either that or I'll plug my battery into the Battery Charger (n. "Bah ter ee - Char ger", not run it on the alternator and just avoid that repair bill altogether.

I don't understand how there is not a 100% duty cycle to the alternator. I have had alternator problems and when I have diagnosed it using my voltmeter I have yet to see it not sit at 13.5v or better with a good voltage regulator. I have tried to frankenstein them and there isn't much to them. Based on physics you create can electricity when a magnet spins within a winding of copper wire. Doesn't the alternator work that way? I never recommend trying to charge a dead battery with the alternator because you don't really know how much you need to drive to get it fully charged. A trickle charger can be done over a day. If the voltage regulator is fine you I am pretty sure you won't do damage to the alternator charging a dead battery.

The link above made me laugh. How fast do you need to go to get good charge? The voltage regulator controls it. If the voltage regulator is good it doesn't matter how fast your alternator is spinning it will reach a maximum of around 13.5-14.5V. You may get a .1-.25V change with increase of rpms over idle.

edit: never mind.

You forgot eRAM and cool collars.....

So I have results:
Got home, started immediately. Meter showed at 1st 13 volts at battery. let it idle it eventually dipped to 12.3 or so with just idle. Then hit lights, radio, A/C, heater fogs and it dipped to about 12.03 and stayed there for a bit, then revved up the engine to around 4500 RPM's for a split second and the volts jumped to 13.5. as car dropped back to idle, volts stayed at 13ish. and then hit all accessories on again, and it maintained about 12.7 volts. So I think my synopsis is this:

car was running around town just perfectly. somehow cable got loose at negative side. went back and forth to work a few days must have just been maintaining from the starter each time, but with loose cable, alternator was not chargingback battery. Then left a light on (glove box) over 2 days must have drained battery completey. With just jump starting, was not sufficient to get battery charged to keep alernator running.

therefore, alternator does not appear to work correctly with a completely drained battery.

OK Guys comments please...

does the extra current draw wear out the brushes faster?

Told you so. :rolleyes:

I don't think it is a matter of the alternator not charging the dead battery. Charging is a matter of time vs. voltage output to the battery. If your battery is around 10v how long will it take you to completely charge the battery? I don't know but I am pretty sure 10-20min drive will not. A battery will have some drop over time it. The minimum volts a battery needs to start the car is about 12.5V at most. So if you get your car charged to say 12.5v when it was pretty dead, you may be able to start it after running a short errand but then if left overnight/longer period of time it may drop just too low for the next time around. That is why I recommend a trickle charger on a dead battery.

As for wearing out the voltage regulator drawing more current I think current is steady regardless. So I don't expect the brushes to wear out any faster but that is just my personal opinion and I have been known to be wrong more than once for sure.

Steve - that answer does seem surprising to me. Was the AC/Delco engineer aware that you were talking about an "old-timey" alternator/Vreg in a Porsche?

Voltage has to be constant, amps are not. The more demand for power, the more amps are drawn from the alternator causing the magnetic field to intensify. More amps causes more heat. Diodes don't like heat and are the common cause for alternator failures.

E=I*R, therefore, current must rise if voltage rises (actually, the difference in potential between the battery and the alternator) and resistance is constant.

P=I*I*R, therefore, if current rises while resistance is constant, power will rise, causing more heat to be dissipated in the alternator.


The greater the difference in voltage between the alternator and the battery, the greater the power that will be dissipated in the alternator, especially over time, as in an attempt to revive a dead battery.

The big problem is that you will most likely only damage one winding of the alternator, which may go unnoticed, depending on your electrical needs. Then it will fail on you at a bad time.

Again, I strongly suggest a charging system check-up at a mechanic. They'll tell you everything about the health of the system, and identify any potential problems.

I'm not saying DIY is a bad thing, believe me, I DIY plenty of my own work, but this is a situation where you could do serious damage to your car's electrical system that could cost plenty if it damages your DME.

Aren't amps regulated on an alternator? I seem to recall on my VWs I could have either a 55amp or 90 amp depending on the accessories on my car. So is it safe to say you can only draw so many amps as the rating on the alternator? Although you can pump, say 10amps, to your battery that doesn't mean you will charge your battery 10x faster than pumping 1amp in. It depends on the energy required to reverse the chemical reaction in your battery. I still say it is a matter of time vs. voltage.

-note: these are only my opinion based on fair knowledge of physics, chemistry and experience.