Kill alternator by charging a dead battery?

Can you really kill your alternator by charging a dead battery? I’ve charged the (dead) battery and not killed the alternator several times over the years, and I’m sure most of us have also. In fact this is what AAA seems to assume when they do a jump start.

Is this an urban myth or is there any solid evidence to back it up.

Only if it is about ready to fail anyway. It does put a very high load on the alternator and will make it quite hot. It can take several days for the battery to fully charge under normal driving. The extra load can take out an old belt due to slippage.

It’s true. You can damage an alternator by charging a dead battery with it. You have to remember, alternators are intended to maintain a charge on a good battery. If this weren’t true, the people who make battery chargers would be out of business.


So it’s a low probability — most times there will be no problem.

I tend to think that. Remember if not, AAA would have to carry around a truck load full of alternators when they went out on calls to jump start cars.

Of course, most times you get a jump start it is during cold days, which will help.

Also I forgot to add, if you’ve ever purchased a remanufactured alternator, one of the things they instruct that must be done is, either install a new battery, or make sure the original battery is fully charged. Failing to do either will null and void the warranty on the alternator.


If alternaters were that fragile - there would be a tremendous run on alternators! Imagine everyone whom left their lights on and ran their battery down had to now buy a new alternator… I think Opera House has the correct answer - only if it’s already about to fail anyway.

I believe one big variable is the reason the battery was dead. If the battery is shot it’s a risk, but of the battery is good and the lights were left on, it should recharge without incident to the alternator. I suspect that while the initial load on the alternator of any drained battery is high, a good battery will charge and the load to the alternator will go rapidly enough that there should be no damage.

I have on evidence to back this up. Just a hunch. I’m bein’ honest here.

You’re wrong. An alternator that continues to run at full charge, trying to charge a defective battery, will fail. It can overheat and take out bearings, diodes and regulators.

Get your facts straight before weighing in on something you know nothing about.

Alternators can and will fail if they continue to run at full charge. If it is trying to charge a battery that is defective and won’t take a charge it will fail.

You’re talking about trying to charge a defective battery, not a dead battery - there is a difference. Get your terminology streight before you accuse others of having their facts wrong. The fact is that every time your battery is run down you do not also kill your alternater.

There is a difference between a dead battery and a defective battery. At least, there can be. A dead battery might just be low on charge and an alternator should be able to charge it back up with no damage, but a defective battery with internal shorts that forces the alternator to operate at high load constantly certainly could cause it to fail. The flip side is that a defective alternator can fry a battery, also. So replacing a battery and having the new one go bad soon afterward might mean the alternator was bad to begin with. A depressingly large number of people don’t test the charging system after replacing a battery.

There are also differences in alternators. Some can source larger currents than others and would be less likely to be harmed by charging a dead battery. Also, alternators generally are providing current to all electrical loads when the engine is running, the battery only provides power when the engine is either not running or idling too low for the alternator to keep up.

So, there are a lot of “ifs”, “ands”, and “maybes” to the answer.

Can recharging a dead battery kill an alternator? Urban myth… but it is so pervasive that it may well prevail into the 22nd century. Even most automotive “experts” insist it is so.

It is not difficult to design an automotive charging system that will protect the alternator/generator from high loads. The electrical engineers figured out how to make voltage/current regulators back 1n the late 1800s – even before the automobile was invented!

I doubt that Adam and Jamie will ever test this myth. There is nothing to explode. Until someone of stature busts this myth once and for all, everyone here may deal with jump starting a dead car according to his own personal beliefs.

I don’t believe I’ve ever damaged an alternator by charging a dead battery, and I’ve done it plenty of times. I’m sure it does increase the load on the alternator but I don’t know if it’s any worse than operating all the lights and accessories in the car. Personally, I wouldn’t worry about it.

thanks. That’s about what I suspected. I kept seeing posts here that said if you had a dead battery and charged it you automatically had a dead alternator.

As a long time engineer, I found that difficult to believe that anyone would design an alternator system with that obvious flaw. I’ve designed lots of power supplies, and not one that would die driving a short circuit.

Plus, as I said, AAA would have to carry around a truck load of new alternators.

Would you rather spend money on a battery (what? $60-75?), a battery charger (maybe $75?), or replace an alternator at what? $130 on up? And I mean really “on up”. Recharge a battery before your alternator gets fried. Oh, not to mention all of those skinned knuckles for changing out the alternator. Charge the battery or put in a new one.

trying to keep the answer as simple as the question:

there are several reasons why the battery may die. if the battery dies, the alternator will try to work harder to keep it up.

that is the problem with killing the alternators, its not the charging (per se`); it is the damaged system, being incompletely troubleshot (as it were) which leads to the killed alternator.

then the guy who went to get a jump, and complains about the alternator dying, then blaming the alternator as the culprit, when it was an underlying problem which caused the whole issue.

Charging the battery first is certainly preferable, but if you have dead battery someplace jumping it is the only practical alternative. I have done it more than a few times without any alternator damage. I also find it hard to believe that an alternator design would not withstand charging a completely dead (but not shorted) battery occasionally.

An alternator costs more like $300 for many cars, but I would rather risk paying for an alternator (small risk, IMHO) than sit someplace with a dead battery.

  1. It is NOT an urban myth. I killed my alternator by trying to get by on a 5 year old battery. Doh! Listen to the good folks on this board with dirty fingernails and grease stained jeans and not the armchair theorists. I do and it?s served me well.

  2. If your battery is 4-5 years old, don?t be penny-wise pound-foolish. Buy a new battery for $50-$80. Replacing an alternator will cost you a LOT more than that. Then you will be introduced to the murky world of remanufactured alternators. My advice to the board, DO NOT BUY AUTOZONE rebuilds. Stick with OEM rebuilds. My AutoZone ?lifetime warranty? rebuilt died after 2,200 miles. I don?t like replacing an alternator every other month?

  3. If you have a newer car, you might get by without realizing the long-term damage you?re doing to your alternator until it?s much too late. A good alternator will be able to tolerate the abuse for a while before it finally dies, possibly in the middle of the night, in the boonies, in a snowstorm?.

P.S. I would like to know what the board?s experience has been with rebuilt Alternators, both chain store and OEM.

ive bought three alternators in my (as yet) life. 1 autozone, 1 napa, and one from a florida auto parts store i can’t recall.

i have had no problems with any of them. nor the starters, two; waterpumps, three; brake calipers LOTS; and what else i have gotten and forgotten.

the one thing i have noticed is that autozone rotors have NO life. they warp, rust and die quickly. the “made in china” stamp probably has everything to do with it, but i do notice Napa has USA rotors. and i have to do brakes only every 30K to 40K now.

so, i think the rebuilt issue is probably moot from my point of view, since none of the stores do their own rebuilding.

there was a thread about this not too long ago, and one of the guys named the factory out west somewhere that does like 75% of the countries rebuilds and cores.

“P.S. I would like to know what the board?s experience has been with rebuilt Alternators, both chain store and OEM.”

My recent experience with McParts store rebuilds is that they are junk. I had my original (I think) 20 year old OEM alternator fail a few years ago on a weekend. Against my better judgement I had a part store rebuild installed by the local place (my real shop wasn’t opened). About one week and 1500 miles later it failed about 1000 miles from home, I had to drive home without an alternator (diesel). They replaced it with a similar unit, that one lasted another 1000 miles. I found a real shop in chicago and had an OEM unit (rebuilt bosch) installed, that one actually lasted.