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.