Can a low battery cause damage to an alternator?

[crw]

So, if we use the same logic, then a poor battery could effect gas mileage? That is, the alternator is continuing to work harder than normal, causing more drag on the engine?

For some time I've been thinking that the battery in my Jetta is original. (2001) I'm over 99K miles now, and that would make it very old... but I think it's possible. The starter seems very smooth, and I don't think the starter in this car takes as much power to start as, say, my old Chevy. This would tend to keep the battery in operation longer than normal.

Every time I throw the charger on the battery, it says something around 85%, but quickly charges up to 100%.

 

[brianl703]

A battery with a shorted cell could cause an alternator to be damaged. It happened to me. As long as you started the car every day it was OK but as soon as you let it sit for a couple of days it would need to be jump-started. Apparently the last owner just kept jump-starting it instead of replacing the battery...

A battery with a shorted cell will show excessive water loss in all the cells but the shorted one, which will use very little water at all.

Hey, another reason why it's good to have a battery with removable vent caps...

 

[George7941]

Originally Posted By: meep
It's the diodes that I suspect are the most sensitive.

As for the coils, yep, there's three of them, but remember, they pulse, so while yes each might be pushing 75 amps, it's not constant... on-off-on-off in a sine wave. The *average* by time may actually be closer to ~33 per example above.

Typical diodes lose ~0.7 volts. That loss becomes heat. Ideally they'd be adequately cooled, but they aren't. They stink bad when they let go.

Some off-grid folks tie an alternator to a small engine for days "when the sun don't shine." There's a neat writeup somewhere on homepower.com . They offer a [free] regulator design that limits current output first, voltage second, to charge properly and avoid burnout. I have a similar project at home. The alternator by itself gets surprisingly warm in the open air at 15 amps. The coils definently get warm.

RPM also plays a factor. Higher rpms means less field current needed, lower load through brushes.

Mike.






I have to disagree on a couple of points.

When we refer to a 100Amp alternator, the 100 Amps is the RMS value of the current output, the peak current is higher by a factor of 1/.707. So we have already taken into account the varying sine wave nature of the current and the heating effect of the current is exactly as if it was a steady 100amp DC current.

At high rpms, the field current will be low, but as long as we are talking about heat generation, the contribution from the field current is low. It is the stator current thru the stator windings and the diodes that generates most of the heat.

 

[George7941]

Originally Posted By: crw
So, if we use the same logic, then a poor battery could effect gas mileage? That is, the alternator is continuing to work harder than normal, causing more drag on the engine?

For some time I've been thinking that the battery in my Jetta is original. (2001) I'm over 99K miles now, and that would make it very old... but I think it's possible. The starter seems very smooth, and I don't think the starter in this car takes as much power to start as, say, my old Chevy. This would tend to keep the battery in operation longer than normal.

Every time I throw the charger on the battery, it says something around 85%, but quickly charges up to 100%.



A poor battery will affect mileage to a very very minor degree. Whatever extra energy the poor battery consumes will show up as heat from the battery, and there is only so much heat that a battery wil put out before it explodes. Compared to the heat dissipated thru the radiator, the energy consumed by the battery is negligble.

 

[glennc]

This has turned out to be a very interesting thread.

A few days ago I bought a gadget at Checker that they call a battery monitor - basically a digital voltmeter that plugs into the cigarette lighter socket. On my car that is a convenient spot to leave it plugged in all the time. Since most cars these days don't have voltmeters, it is very useful to have and I've learned a few things by keeping an eye on it. Also, since it is digital, very small changes are apparent - for example, the exact change in voltage when the turn signal flashes, or when powering up a window or turning the fan one notch higher.

A good deal at $20.

FYI.

 

[PT1]

I usually replace all batteries at 36-42 months of service. WHo wants to deal with the dead one because they always go bad at the most inconvienient time.

 

[deeter16317]

Originally Posted By: George7941

A poor battery will affect mileage to a very very minor degree.



Actually a low voltage situation can cause a loss of power, mileage, and all sorts of "other" symptoms attributed to the ignition system...it causes the ignition system to react poorly. I had a ramcharger with a corroded connection between the battery and alternator...high speed miss, low power, and low mileage were symptoms...the difference once the connection was fixed was very, very noticeable.

And in some internally regulated alternators (I'm specifically thinking of newer GM alternators), they will literally burn themselves up if the battery is either low on charge or not connected to the alternator. I had a friend burn a new replacement alternator up (as well as the original) in his Park Avenue because his battery was bad and showing under 10VDC.

 

[George7941]

Deeter. Both your points are very valid.

As to your first point, a low system voltage can cause all kinds of problems. However, it is the voltage that is causing the problems, not the poor battery. It is the low voltage that is causing the poor mileage.

As to your second point about alternators burning up, THANK YOU1 I finally have someone backing up what I have been saying all along on this thread. There has been a surprising amount of resistance from Bitogers to the notion that operating modern alternators at full capacity will eventually burn them up.

Forty years ago, a sixty amp alternator was a powerful alternator and they were rougly the same size as a current 150amp alternator. They both use copper windings and, because of space limitations, use the same gauge wire. Copper is no more conductive now than it was forty years ago, hence the problem with operating a high amp alternator at full capacity for long periods of time.

Also, a valid point was made about the heat generated from the diodes. Here is a calculation of the amount of heat generated. We will use a 100amp 3-phase alternator as an example. Each individual phase has 58 amps flowing thru 1.4V (positive and negative diodes), generating 58x1.4 = 81W, leading to a total of 243W of heat that the diode heat sink has to dissipate. And I am neglecting the heat generated by the field diodes.

 

[glennc]

George, your posts in this thread have been very interesting and I've learned a good bit by reading them. It had not really occurred to me how much heat an alternator generates nor what the challenges are in shedding it. My '89 BMW has a 90 amp alternator and my '94 has a 140 amp alternator. The latter car has special ducting under the hood to provide airflow to the alternator for cooling. 140 amps is nominally 1680 watts or well over two full horsepower.

Anyway, it has been enlightening reading.

 

[Donald]

A very knowledgeable starter/alternator rebuider said to never let your alternator charge a battery after you left your lights on, etc, and the battery died. It might burn out the alternator. Better to charge the battery with an AC battery charger.

 

[severach]

Another reason to not let your alternator charge a dead battery is because even though most alternators can stand it, the battery requires several hours of charge before it can start the engine. You wouldn't start a several hour trip with a dead battery so the AC charger is the only practical choice.

Allowing a battery to go dead destroys the bulk of its useful life. A second discharge is pretty much the end of the battery. You can charge it up and use it in warm weather but it will fail when the cold weather hits. You must keep the voltage above a minimum level at all times for a battery to have a long life. Fortunately computer controlled interior lights have removed most of the reasons that batteries go dead.

If you discharge a battery in the winter you must bring it inside and let it thaw out before charging.

 

[Lazy JW]

Originally Posted By: meep
...

Some off-grid folks tie an alternator to a small engine for days "when the sun don't shine." There's a neat writeup somewhere on homepower.com . They offer a [free] regulator design that limits current output first, voltage second, to charge properly and avoid burnout....


Gotta love those old Volkswagens; never would burn out the alternator (they had generators). Most generator systems used regulators that limited current; I never could figure out why alternators lack that protection.

I have an old Leece-Neville alternator that is rated at 70 amps continuous, it has REALLY BIG diodes. The whole unit is much bigger than a modern 100 amp alternator.
Joe

 

[meep]

"When we refer to a 100Amp alternator, the 100 Amps is the RMS value of the current output, the peak current is higher by a factor of 1/.707. So we have already taken into account the varying sine wave nature of the current and the heating effect of the current is exactly as if it was a steady 100amp DC current."

....Hmph. didn't know that.

"At high rpms, the field current will be low, but as long as we are talking about heat generation, the contribution from the field current is low. It is the stator current thru the stator windings and the diodes that generates most of the heat."

....Agreed. I was thinking in terms of brush degradation from excessive load. I should have been more clear.

Diodes in the FC circuit? That I haven't seen, with the exception of a reverse-biased diode across the Fet/transistor/switching device to prevent burnout from back-pulse. Where are they used?

"I never could figure out why alternators lack that protection."

....What I've seen is pretty rudimentry - that is that they are allowed to produce lower V as the load increases, by a small amount. Yes, the old relay-type regulators actually passed output current through a field-coil cutout relay - if max amperage was passed, the relay would oscillate rapidly, momentarily breaking the circuit to the field coil, reducing output. My old '57 chevy had that type of regulation on its 27A genset.

Mikey!

 

[George7941]

"Diodes in the FC circuit? That I haven't seen, with the exception of a reverse-biased diode across the Fet/transistor/switching device to prevent burnout from back-pulse. Where are they used?"

The field current is generated from the stator AC output by a separate diode pack and then applied to the rotor windings after being regulated by the voltage regulator. These diodes are smaller than the main diodes since the field current is max 7amp.

 

[deeter16317]

Originally Posted By: Lazy JW


Gotta love those old Volkswagens; never would burn out the alternator (they had generators). Most generator systems used regulators that limited current; I never could figure out why alternators lack that protection.

Joe



The reason they most likely quite using generators is due to size, a comparable 100A generator would be the size of some engines used in these modern cars.

I have a M37 with a 78A (?, CRS right now) generator, its about 8"D by 16"L...a monster.

I still wish they had mechanical regulators...you could adjust those, repair them, and they didn't mean the generator had to be replaced to replace the regulator...

 

[George7941]

"I still wish they had mechanical regulators...you could adjust those, repair them, and they didn't mean the generator had to be replaced to replace the regulator...


I don't. The mechanical ones wore out and had to be replaced every so often. They were far less reliable than a solid state regulator. Comparable to Points Ignition vs Electronic Ignition.

It is industry trends that did away with the adjustability and the location of the regulator, not some inherent characteristic of mechanical regulators vs elctronic. Some electronic regulators are adjustable and it is just as easy for the manufacturer to externally mount an electronic regulator as it is a mechanical one.

 

[meep]

"The field current is generated from the stator AC output by a separate diode pack and then applied to the rotor windings after being regulated by the voltage regulator. These diodes are smaller than the main diodes since the field current is max 7amp."

...Makes sense. The chrysler-spec alts I've been playing with use external regs, so there are just 2 terminals for FC. I like it b/c there's a bit more flexibility there for roll-your-own regulators. Thanks!

Mikey

 

[Rabbler]

Chrysler doesn't really use any regulator. The engine controller modulates the field strength directly with either high or low side drivers. You could install an external regulator if the PCM failed for a chap fix though.

 

[George7941]

It is not just Chrysler, a lot of cars have alternators regulated by the PCM now. This way, the voltage can be more tightly controlled, with the regulated voltage varied to suit driving conditions.

Meep's reference was to older Chrysler alternators that had external voltage regulators.