Interesting whizzing match. One thing that hasn’t been overtly mentioned is that the alternator must also supply the demands of the rest of the vehicle in addition to charging the battery. Most people, being quite ignorant of such matters, would more than likely turn on the headlights, heater fan, stereo etc and drive off fat, dumb and happy after their car was jumped back to life. I don’t know about you but I see that most cars today are designed to run on the normal load with a bit of overhead for the unusual. Alternators look more like walnuts today than the melons of days past. Overkill is a thing of the past. The alternator is designed to supply the electrical needs of the car’s electronics and replenish the service battery from starting the engine.
If you add up ALL the loads and figure 100% duty cycle on a device that wasn’t really intended to provide that kind of service, well you’re treading on thin ice. That’s why it’s not recommended.
Hey OK4450, remember star88 the guy who had some industrial A/C knowledge and was trying to apply it to automotive A/C? Vogelfish sounds like the same type of guy, a know-it-all engineer who is trying to apply his so called experience to automotive electrical.
How about a substantive reply? All I’ve herd here is speculation and personal attack.
Hey willey - here is a challange for your class. Take a good battery and run it dead with a couple of headlights. Then use it as a test load on your bench set up - see if the alternator survives. See how many times you can repeate that experiment before it fails.
Also measure the charging current when you start and at 1 hour intervals. And if you can - put a thermocouple on the alternater body and measure the temp at 1 hour intervals.
Why is it so hard for you to understand alternators fail. They can fail charging batteries that are dead but not defective. They will fail charging defective batteries.
What do you think a bench tester simulates? The bench tester has a battery in it. Every alternator we run on it generates heat, imagine the amount of heat there is when it’s under the hood of a car. I have removed alternators from cars that are so hot they could burn your hands.
You assume the alternator can’t fail charging a dead but not defective battery because the charging amps will decrease as the battery charge level increases. Maybe in a perfect world. The enviroment under the hood of a car is far from perfect.
What about the belt that is too tight or too loose? A misaligned alternator. Couple these factors while trying to charge a battery. The heat generated during charging with any of these conditions can be tremendous.
You might be good at building circuit boards but when it comes to automotive applications you don’t know what you’re talking about.
I’m not saying they don’t fail - I’m saying charging a battery with one will not cause it to fail. If there are other things wrong - like the things you list (or others), then the added stress of charging may result in failure.
Do the experiment and demonstrate it with data and I’ll believe you. Until then I think we have to leave it as speculation since neither of us has provided any real data. I’ll put a case of beer on it not failing in the test I described.
willey, i remember star very well. maybe vogelfish is star reincarated. has the same traits…
i’ll be off this forum for day or2
went through major shoulder surgery this morning and they got me in an immobilizer. The only thing moving is my left arm a even that is very shaky
that’s the reason for the crummy writing.
Only have to put up wiyh it for a month and a half. yippee
in the meantime vogelfish, have you had any reall world xpereince with an alt. tester? and I dont mean 10 dollar vom from wal mart
The biggest urban myth is that an alternator is capable of putting out a maximum charge at low rpm. They don’t. This myth dates back to the early days of alternators, before they were introduced, cars had a single phase, permanent magnet, split comutator generator. These did not provide enough current at idle to match the engines need, so it would discharge the battery.
The three phase, electromagnet alternator with a diode rectifier pack does provide enough current at idle to meet the engines normal demand so it will charge a battery at idle. However, contrary to popular belief, it can discharge a battery if the cart is operated at very low speeds with a high load, i.e. fan on high, lights on etc. One of the factors determining the alternators output is the speed at which it turns. The faster it turns, the more current it is capable of producing. Other factors include the size of the alternator and the field voltage that is controlled by the regulator.
Can a dead battery cause an alternator to go bad? It is possible, but not automatic. Even a defective battery won’t necessarily burn out an alternator. Usually the damage caused by a defective or dead battery is damaged diodes due to high sustained current. If an otherwise good battery is drained to the point of being “dead”, it will accept a charge quick enough so that there isn’t a sustained high current.
If the battery is defective and the rpm’s are kept low, you probably won’t hurt the alternator because the sustained current won’t be that high. But if you increase the rpms to the point where the sustained current exceeds the current rating of the diodes, they will burn out. Many stock alternators are only rated at 46 amps max sustained.
If you are jump starting another vehicle and your engine is running at high rpm to “provide enough juice”, under certain circumstances, you could draw enough instantaneous current to burn out the diodes immediately. This is rare, but it can and has happened. If I give someone a jump, I leave my engine off and use small gauge cables to protect my vehicle. It takes longer, about 5 minutes to transfer enough charge to the other battery, but its safer.
The reason so many rebuilt alternators go bad is the rebuilders, for time and monetary expediency, only replace parts that are completely worn out. Of the chain store alternators I have found NAPA to be the best. They also have a store in almost every little town in America and are good about honoring their warrantee. The most reliable option is to find a rebuild shop and ask them if they do a complete rebuild, that is to say replace all wear parts - bushings, bearings, brushes, diodes… May cost more, but it’s worth it!
cappy208 is absolutely correct about brake rotors. The American rotors are a MUCH better metal and it shows. If you have a chinese and an American rotor side-by-side, look at them, feel them, taste them and scratch them with your jack-knife. The differences will be apparent.
Thanks for this post…as I was reading through I was wondering if it followed that continuing to use an older battery would put a similar overload on an alternator as trying to jump-start a dead battery…When I bought a new car on moving to AZ, I was told no battery would last more than 2 years out there. 5 years later mine has never so much as hesitated, even on cold Ohio mornings. But I think now I will add getting a new battery to my list of weekend errands…Thanks again!
craig, (pardon grammar/punctuation; my shoulder has been fileted from 1 side to the other and i’m poking at the keyboard with a pencil eraser) but here is my point about glow plugs. vw 4 cyl. diesels
the gl. pl. draw is 9 amps per plug for a total of 36 amps. the vircuit uses a 50 a. fuse so theoretically no problem.
however there is a near 80 amp surge when the plugs are first energized but the fuse will not blow on that surge. the fuse is weakened and may blow within months or even years dep. on how many cold start cycles it goes through
your benz should be no different. ck the cold surge and youll see wf i mean
Back to gas engines and a partial list of energy consumers other than a defective battery. ecm, fuel pump (6-7 amps right there)heated 02s, injectors, trans solenoids, dash panel operation, any one of dozns of eng. control sensors, various relays, etc etc and the biggest one of all - the starter motor.
A defective battery and jumper cables means the alt. will be punished because the starter id going to easily draw 300-400 amps on the surge with a steady 125 amp draw during normal cranking; assuing the eng. is warm and has no faults draggging it fown even futher
hope you can read this garbage; i feel ike one of those birds perched on the side of a glass bobbing for watere
I understand your point, the dead battery will result in a high initial (in-rush) current that will take it’s toll on the alternator over time. I agree, but I don’t think charging a dead battery occasionally is likely to cause an immediate alternator failure, unless it’s already degraded. I also agree that charging a dead battery with the alternator should be avoided if possible.
Just for info, the cold resistance on each of my GPs is about 0.6 ohms and they see about 11V at the GP. That gives an initial current surge of about 90 amps, but it drops off very quickly as the GPs start to heat up. The GP relay fuse is rated for 80 amps. Starting that car with a weak battery is a challenge.
You are talking about the alternater on the vehicle being used to supply the jump. I doubt the alternater on the car being jumped is generating much current during starting.
Regarding overstressing the fuse. A 50 Amp fuse can handle >50 amps without weakening if the duration and magnitude of overcurrent is sufficiently “under the curve”. To determine that you need to look at the data sheet for the fuse type and figure out how long the overcurrent is lasting and how much overcurrent you have. I would assume the designers have taken this into account when selecting the fuse for the glow plug circuit. However - there are some simple things a circuit designer can do to eliminate the surge altogether - a thermister can be used to “soften” the inrush current in this type of circuit. For circuits with shorter duration inrush - an inductor can be used to get the same result. I don’t know how the automotive industry typically handles this, but I know that turning on your glow plugs under normal operating conditions should not weaken your fuse.
OK was talking about the initial current that will be drawn by the dead battery immediately after the car starts, I agree that the cranking current will be supplied by the running car (which may be a problem for some cars).
You are correct about the GP circuit, the “slow blow” fuse is not challenged by the initial high current to the GPs, and the GP resistance rapidly increases as it heats up (in effect, the GP is acting like a Thermistor).
this is becoming like which came first, the chicken or the egg? question.
the OP never said the battery was defective, he just said dead. this makes a whole lot of difference. to help other readers figure out how this discussion helps them: how can you tell if the battery WILL take a charge? if the battery won’t take a charge will the alternator shoot the bed? how can one check the alternator? how long should one try to “jump” a battery? is the “jump donor car” at risk to burn up their alternator giving a car with a defective battery a jump?
i am not saying for one minute tha the alt. output is being used to power the starter, i’m saying that the second the engine is being cranked over that a load will be applied to the alt. on the non-running vehicle because the fields will be energized by the car providing the jump.
consider the following. i use a sun vat 40 tester and when testing an alt. heavy load is applied only for a second at the risk of baking the alt. it’s easy to roast an alt. even with a tester; which mimics a dead or junk batt.
thought of this one also. motorcycle related but electrics are the same.
A 1975 harley of mine went thru 3 alts. in 1 year. they woud work fine for x months and fry on the open road.
the removed alts. all tested bad but looked like new.
Being curious i soaked the stators in carb solvent until everything was removed. Windings looked perfect ubtil i unraveled all of the wire from the stator frame.
it was found that the separate regulating winding was toast in all 3 of them.
at this point the aftermarket volt reg. was cut apart and sure enough, the reg. was the problem.
now, what does this have to do with batteries and alts?
the alt is a 4 pin unit as is the reg. it was found that the am reg. was missing a power lead to the reg. windings, was made like this, and could not be noticed visually short of cutting it open.
remember my comment about the alt having to sense the batt condition in determining output?
in all 3 instances here you have the alt thinking there is a junk or dead batt in the circuit so it tries to compensate of course, cannot do this, and eventually steadt elevated rpms fries the alt.
Some of the stators worked fine for 3-4 months before committing hari-kari.
I guess I misunderstood you when you said, “A defective battery and jumper cables means the alt. will be punished because the starter id going to easily draw 300-400 amps on the surge with a steady 125 amp draw during normal cranking;”
WILL SOMEONE ACTUALLY MEASURE THE AMPERAGE flowing from the donor alternator / battery into the donee circuit during donee starter actuation?!!!
WILL SOMEONE ACTUALLY MEASURE THE AMPERAGE flowing from the donee alternator/battery during donee starter actuation!!!
After a while, the anecdotes, though true, show the weakness of anecdotes. Such statements as, "The amperage is probably too high for the alternator, leaves the question, “How much is the actual amperage draw, and what is the duration?” Did ANYBODY answer this?
Charging the battery won’t kill a normal alternator. The proof is easy. A headlight bulb on low beam draws about 4.5 amps. 9 amps for the pair. Double that when you’re running fog lights. 18 amps total. Add 2 amps for the tail lights, and you’re routinely drawing 20 amps at night. Continuously. And your alternator handles that without failing, doesn’t it? So, assuming you start running with a dead battery during the day, sending 20 watts to charge the battery (which would be some very fast charging, indeed) would not be an unusual load for your alternator.
Like all urban myths, this starts from a seed of truth. If you have a dying alternator, your battery will take up the slack, likely without you noticing. Until the battery finally runs out of juice and you get a jump start which leaves a little extra charge in the battery. When that is gone, you notice the alternator can’t keep up, and you blame the failure on trying to charge a dead battery.
Maybe somewhere there is someone who runs a high-power stereo with their high beams and fog lamps and continuously fiddles with the electric windows immediately after getting a jump start for a dead battery. Then maybe you’ll overwork the alternator. But in any real scenario, it won’t happen, but do refer the previous paragraph if you think it happened to you.