http://www.babcox.com/editorial/cm/cm80122.htm
Tester
I was all excited to see an “informative” link. Unfortunately, this one is not. There is no mention of alternator death or even 12v systems. This link refers to a 6v system. Although voltage is not important, 6v systems went out in the late 50’s or early 60’s. The documentation included with new or even reman alternators (mostly mid 80’s GM) have a warning about pairing with a battery having less than 11.5v thus causing or increasing the chance of alternator death. This is most likely some lawyer-speak to cover their butts. I personally have not witnessed this sort of failure nor do I believe that a battery that is essentially 1 volt low would cause a diode failure. When jumping a vehicle with a dead battery (no dome light, no horn, no nuthin) you have to wait for the donor battery to achieve 8v-10v before you can crank the engine. I would have to vote Myth, although well accepted.
This thread reminded me to go check my motorcycle battery (almost dead again), so I just went to the local McParts place and bought a 1.5A tender charger. Hopefully, the battery will recover, I’m tired of having to buy $150 batteries every-time I get lazy and let it go dead over the winter. The alternator on my bike takes forever to recharge the battery if it gets low.
I think I have had jump starts perhaps a 10 times in my life. Never a damaged alternator. In fact I never lost an alternator.
I jumped my old POS jeep (with a portable battery pack) last week after my wife took it to work in the snow and left the lights on all day, the battery was very dead and seemed to recharge itself without any problems after running a while. Realistically, I’m not going to have it towed to my garage and recharge the battery.
You can feel the alternator getting hot when you are charging a dead battery. It is most likely to quit when you put it to its greatest stress. We all do it.
I took a look at a number of random Alternaters specs - Most are rated for more than 100 Amps - I didn’t find any rated lass than 100 Amps but I didn’t do a comprehensive search.
No dead battery in the world is going to draw anything near 100 Amps - probably not even 50 Amps - unless it was shorted.
Bottom line - unless the alternater is already bad or about to fail - there is no way you can kill it by using it to charge a dead battery.
Kudos to Opera House for nailing it in the first posting!
Good point, even a dead battery won’t draw enough to overload the alternator, although the initial current may be higher than usual. My diesel glow plugs draw about 50A (until they time out in a couple of minutes), which is probably more than any dead battery could draw.
You can usually get away with charging a completely dead battery with your alternator. I have done it many times. If you gotta do it, keep the engine rpm high, this causes the alternator’s cooling fan to force maximum cooling air through the alternator and at high rpm, the excitation current (field current) is low. The lower magnetic field strength at high rpm also means lower hysteresis and eddy current heating of the stator iron. Also, don’t run any accessories that aren’t essential while the alternator is charging the battery. Headlights, heater fan, AC, etc, all off. That way the alternator only has to charge the battery, not power your accessories and charge the battery.
If it isn’t urgent to drive the car somewhere right now, put a battery charger on the battery.
Your alternator will probably survive it just like guns all survived shooting a proof load at the factory, however, you would not want to make a habit of shooting proof loads.
“Your alternator will probably survive it just like guns all survived shooting a proof load at the factory, however, you would not want to make a habit of shooting proof loads.”
OK, I suppose that means something to folks who know something about firearms, but not to me.
A proof load is a round intentionally loaded to produce extra high chamber pressure to test the strength of a gun. For liability reasons, ammo makers will only sell them to gun manufacturers. They are usually colored blue to identify them.
Now you know.
How hot? there is nothing in an alternator that could get damaged below 200?F, perhaps higher. That is very hot. You couldn’t keep your hand on it. That’s a touch and hisss type temperature.
Interesting, I had never heard of that.
What I are tryna say is that the heat is an indication of stress (work). You are right about getting hot enough so as not to be able to keep a hand on it. Usually from jump starting while cranking and not too bad when your alternator is charging its own battery. We all find the risk of jump starting and letting the alternator work is an acceptable risk. We all would let our dead battery get charged by its own alternator. If an alternator is going to quit, it will be likely to do it when it is called upon to do its hardest work. If you have a battery charger handy, you would want to use it. If you don’t have one and you want to drive your car then you would let the alternator work. We should give machinery a break whenever possible, because those alternators are expensive these days and some are a lot harder to replace. Life always has risks. I agree that the original question has low risk.
Yes, a bad battery can kill an alternator. If the alt. is being forced to produce at near full power this means a lot of heat. The alternator is going to be weakened by this heat; either the soldered joints, insulation, etc. It may quit then and there or weeks/months later but the alternator is damaged goods.
Do the math on wattage, which is nothing more than heat disappation and see what you get.
Figure a 12.6 volt normal battery putting out 10 amps or so and compare that to a weak 10 or 11 volt battery with the alternator producing 80 amps. Big difference.
And Vogelfish, I think Willey knows perfectly well what he’s talking about as does Tester. Dead or defective is irrelevant; the alternator is being punished by a low voltage battery. The only question is the degree and at what point will the alternator give up its life.
Try plugging half a dozen electric space heaters into the same damned wall socket and see what happens. Eventually after enough breaker resetting the house wiring will let go and the house goes up in flames. There is no difference between this and an alt.
They’re both overloaded and eventually both house and alterantor will go up in smoke.
Forgot something. Before adding an additional comment I would like to know if any of you have experience with load testing equipment of the type used for alternator testing.
Especially Vogelfish in regards to the 100 amp/50 amp comment. There’s a reason why I ask. 
Good point, I too would like to know what kind of experience he has with alternators. I guess the hundreds of alternators I’ve replaced over the years were not really defective.
Right now in my class we are studying charging systems. We are disasembling/reassmbling alternators and then bench testing them. We have actually fried alternator components such as diode trios’, stators, and regulators simply by load testing them. These are not 20 year old components that are “about” to fail, these are new components.
“Bottom line - unless the alternater is already bad or about to fail - there is no way you can kill it by using it to charge a dead battery”. Quote from vogelfish. Please tell me how do you tell a alternator is about to fail? If it is about to fail how did it get to that point?
The bottom line is if a battery is dead the alternator can recharge it without any problems. If a battery is defective and won’t take a charge you can and will overheat it and cause it to fail.
With crickets chirping quietly in the background, thought I would bump this back up to the top in an attempt to find out if anyone has any hands-on experience with alternator testers.
Craig’s comment about glow plug amperage leads me to think of an electrical analogy that could be applied to an alternator but I’ll save that for a bit.
Yes, a bad battery can kill an alternator. If the alt. is being forced to produce at near full power this means a lot of heat. The alternator is going to be weakened by this heat; either the soldered joints, insulation, etc. It may quit then and there or weeks/months later but the alternator is damaged goods.
A BAD battery that draws high current continuously can ? One that is dead but otherwise good is another story.
Do the math on wattage, which is nothing more than heat disappation and see what you get.
Figure a 12.6 volt normal battery putting out 10 amps or so and compare that to a weak 10 or 11 volt battery with the alternator producing 80 amps. Big difference.
126 W vs 800 W (that?s equivalent to 8 100 watt light bulbs)
And Vogelfish, I think Willey knows perfectly well what he’s talking about as does Tester. Dead or defective is irrelevant; the alternator is being punished by a low voltage battery. The only question is the degree and at what point will the alternator give up its life. [b]I make no judgement on what these guys know.
Fact is ? a defective battery that won?t take a charge draws high current continuously ? a dead battery that is charging up will draw less and less current until it reached full charge at which the current become steady at some value ? maybe around 4 amps or less.[/b]
Try plugging half a dozen electric space heaters into the same damned wall socket and see what happens. Eventually after enough breaker resetting the house wiring will let go and the house goes up in flames.
There is no difference between this and an alt.
They’re both overloaded and eventually both house and alterantor will go up in smoke.
This is just a crock.
Forgot something. Before adding an additional comment I would like to know if any of you have experience with load testing equipment of the type used for alternator testing.
Especially Vogelfish in regards to the 100 amp/50 amp comment. There’s a reason why I ask.
I don?t load test vehicle alternators. I am an electrical and electronic circuit designer and have extensive experience load testing power supplies, generators, and other electronic equipment. In addition, I have extensive experience with HALT and HASS testing, Hi-Pot testing and much more. As far a load testing ? there is much more than meets the eye to doing this and there are many different types of loading you can do. In the case of alternators, you need to also consider the method by which the power is generated and delivered. It is not continuous DC. As you know the output is a ?diode OR? of the coils and requires a capacitive load to operate correctly.
Good point, I too would like to know what kind of experience he has with alternators. I guess the hundreds of alternators I’ve replaced over the years were not really defective.
I can?t speak to your skill as an automotive troubleshooter.
Right now in my class we are studying charging systems. We are disasembling/reassmbling alternators and then bench testing them. We have actually fried alternator components such as diode trios’, stators, and regulators simply by load testing them. These are not 20 year old components that are “about” to fail, these are new components.
I?m sure this is true ? and I?m sure the testing you did that destroyed your alternator is not analogous to charging up a dead battery.
“Bottom line - unless the alternater is already bad or about to fail - there is no way you can kill it by using it to charge a dead battery”. Quote from vogelfish. Please tell me how do you tell a alternator is about to fail? If it is about to fail how did it get to that point?
There is no way to know if it?s about to fail ? there are many ways it can get to that point ? charging a dead battery is not one of them.
The bottom line is if a battery is dead the alternator can recharge it without any problems. If a battery is defective and won’t take a charge you can and will overheat it and cause it to fail.
This is what I?ve been saying ? exactly what I?ve been saying ? don?t know what all your fuss is about.
“Craig’s comment about glow plug amperage leads me to think of an electrical analogy that could be applied to an alternator but I’ll save that for a bit.”
The question (that I don’t know the answer to) is how much internal resistance a fully discharged automotive battery would have. That will limit how much current current it can initially draw from the alternator. In the case of my glow plugs, they will draw about 60 amps when they are cold (their resistance increases as they heat up after the first few seconds). My alternator is only rated for 65 amps, so I have to assume it is operating at almost full capacity whenever my engine starts (the glow plugs time out in about 3 minutes).
What analogy are you thinking of?