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Myth or fact: Does jumping a totally dead battery kill your alternator?

Battery is dead.
You jump start the car.
You let it run for an hour.

Does this kill the alternator?
How exactly does the alternator work harder?
It’s spinning at the same rate.

Alternators have a maximum rated output, say 90 amps. That’s more than adequate to supply power to all the appliances on the car and recharge a battery over a course of an hour. But that rated output comes with something called duty cycle. That talks about how long the rated output can be maintained. And I would say the duty cycle is about 30%.

Look at it this way. You can run up a flight of stairs in 5 seconds. You can walk up a flight in 10. How long can you manage that? 3 floors? 15? 40?

Asking an alternator to recharge a completely dead battery is like asking you to walk up 50 flights of stairs without taking a break.

I would guess it might make a difference in why the battery died and how “dead” it is as a battery may be down just enough not to turn the starter motor. An internal short isn’t a good thing as opposed to leaving the lights on too long.

An alternator isn’t a battery charger. It’s designed to maintain the charge on a fully charged battery.

Asking an alternator to recharge a fully discharged battery causes the rectifiers to become extremely hot. And this is what fries the alternator.

Heres’ the warning that comes with each remanufactured alternator I install.


“An alternator isn’t a battery charger.”

I agree. I just don’t think you can jump a completely dead battery though.

I love that bit about how if your battery is over 30 months old, you should have it professionally tested or install a high quality new battery

The high quality part seems a bit vague to me

What I consider high quality may be cheap, overpriced, or overrated for somebody else

Its all about current. More current, more heat. The weak link in the alternator is the diode rectifier bridge. The diodes are the first to go when the duty cycle mentioned by asemaster mentioned.

But the alternators output is determined by the load. The load is the amount of current the electrical system is asking for and it can be very high with a dead battery to charge as well as high powered accessories. But another factor is RPM. The faster the alternator is spinning, the more current it is capable of generating.

At low RPM, the alternator may not generate enough current to damage the diodes. Even though the electrical system is asking the alternator to “run up 50 flights of stairs”, at idle the alternator will only walk fast, it won’t run. So if you charge a dead battery at idle, you are not likely to damage the alternator. If you rev the engine to get it to charge faster and you rev it too high for too long, then you can burn up the diodes.

If you have a battery with a shorted cell, then the battery puts a big load on the engine. If you are driving at sustained highway speeds, you can end up needing a new alternator along with a new battery.

A dyeing battery puts a lot of strain on an alternator, then trying to charge a completely dead one could be enough to kill it off. Such as trying to swim 5 miles after running up 50 flights of stairs. A younger alternator might be able to handle this, but as they get older…not so much. But you can probably save your expensive alternator with a relatively cheep battery charger, or maybe just a new battery that is a lot less painless than a new alternator.

How does a dead battery demand more output from the alternator?
The alternator does not just provide a fixed power output?
The battery “asks” for more power, and the alt. suppplies it?

LOL. All I could think about when I saw the thread title was that picture that Tester posts once in a while - and there it was.

The alternator is stupid.

It looks at the reference voltage from the battery. If it see’s little or no reference voltage it goes to full output to try to bring full charge back to the battery. The alternator is only supposed to recharge the battery back to full charge after the battery has been discharged from using the starter.

The alternator will continue to try to recharge the discharged battery until the rectifiers heat up to a point where they no longer convert the AC voltage to DC voltage. And AC voltage won’t recharge a DC battery.

Rectifiers are what allows DC voltage out of the alternator. Diodes only allow that voltage to flow in one direction.


The battery “asks” for more power, and the alt. suppplies it?
Yup…at least on a car.

A dynamo spits out a defined current at a given RPM, the voltage is in theory unbounded. A system is needed to prevent the alternator from just running amok and frying everything.

OPTION 1: A really wimpy alternator. Example–vintage motorcycles, esp those without electric start.
OPTION 2: Minimize iron. You can “saturate” the ferromagnetic material somehow as a cap. This is how bicycle “bottle genetators” (really alts) don’t explode the headlight on fast downhills.
OPTION 3: dump the excess to ground. That is how permanent magnet alternators do it. Not terribly efficient, but effective.
OPTION 4: Vary the electromagnetic current. That is what a voltage regulator does (how? dunno) to a car alternator…vary the strength of the electromagnet.

I’ve jumped engines with totally dead batteries, and it’s never cost me an alternator, but it only works when both batteries are about the same size or the dead battery is the smaller of the two. I’ve done it with my family’s van and an old 1969 Dodge Dart (which probably had a generator, not an alternator). Coincidentally, on my previous cars, the 1985 Buick Shyhawk and the 1984 Mercury Marquis, I had alternators go bad (for what I presume are unrelated reasons), but my 1998 Civic still has the original alternator, and I’ve jump started quite a few engines with dead batteries - on other small engines.

The thing to remember is that when I’m jump starting something with a larger battery, my car’s electrical system isn’t going to be enough to start the larger engine unless the larger battery is capable of holding a charge. If the battery is totally dead, it won’t matter how long we sit there and wait for the dead battery to charge while my car idles (or even if I give it some gas). However, if the larger battery can hold a charge, but is just currently depleted, my little Civic can charge the battery enough to start the larger engine. We may need to sit there for 10 or 20 minutes waiting for the larger battery to charge, but it will be the larger battery eventually starting the engine, not the alternator on my little car.

I’m confused. So you’re saying if your OWN battery is dead and you get a jump to start your car, then YOUR alternator will possibly be destoryed while recharging your own battery?

How is one supposed to recharge a drained battery if this is the case? How would you take said battery to the store to get it recharged if you can’t even drive your car! I guess you need another one? I used to have a parasitic drain on my battery…finally just took out the fuses to the radio/clock. But it’s been dead many times and I’ve jumped it and recharged it many times.

I hope my alternator is OK.

The posters are correct; a dead battery or bad battery can kill an alternator. There are some variables involved and it’s a flip of the coin as to whether or not the alternator will die.

Somewhat busy right now but will post an example later today in which 3 alternators were fried because of an issue which did not involve a dead battery but a situation in which the alternator thought the battery was dead.

How is one supposed to recharge a drained battery if this is the case?

You buy a dedicated trickle charger and recharge the battery in the car before returning it to service.

But it’s been dead many times and I’ve jumped it and recharged it many times.

This is kind of like my hamburger analogy I use at work:

Eating hamburgers once in a while isn’t likely to kill you.

If you eat them every day, that practice can catch up with you.

Some people can eat hamburgers every day for their entire life and not suffer any noticeable, detrimental effect.

However, if you eat a 100 of them in one day, it might kill you right there depending on your condition going in…

Well then I guess the important thing is to look at the % probability it’d kill the alt, times the purchase price of a rebuilt alt. Then consider how frequently you run your battery down and consider if it’s worth bench charging or not.

You might want to add some factors to consider in your risk analysis-

How long do you need to drive around to completely re-charge that depleted battery? If you don’t completely re-charge it, you’ll shorten the life of the battery too.

Labor to exchange the alternator can be more than the cost to buy the part.

What if it fails at some bad location/time?
Will you need a tow?

What is the cost of a trickle charger vs all of the other factors?

Somewhat moot, seeing that the most likely cause of a depleted battery (IME) is leaving the lights on, likely away from home, and hoofing it 5 mi to the parts store is out of the question. If I can bump-start my truck with a weak battery, I’m not going to call a friend to come and get me in an hour’s time…I’ll bump it and go, and damn the torpedos/alt wear!

The only benefit I can see is…if you already own a trickle charger and aren’t in any hurry AND are at home, throw a discharged battery on the charger a few hours’ ahead of scheduled departure. NOT worth it to me when the alt is on with two bolts and two pigtails, and easily accessible.

@kenberthiaume, you can reduce the chance of alternator damage by letting the engine idle for 15 minutes or so after a jump; don’t drive or rev it up.
This limits the current the alternator puts out while the battery is very low, as @keith explained.