Jumpstarting...can it kill alternator?


#1

I have heard that jumpstarting a car with a bad alternator can kill my alternator…is this true and why ?


#2

I do not think so. The power for the jump start mainly come out of your battery not from the alternator.


#3

This came from the experience of many drivers who jump started their cars and then let them run to have the alternator charge up the battery. Modern alternators are not very good at this and many have a warning on the box that says: “This alternator is not a battery charger”.

The best way to charge up the battery is to use a charger in the first place, or, after running your errand, shut thecar off and hook up a regular charger. I carry a spare battery booster in the winter, just in case.


#4

maybe I didn’t clarify my question… will jumpstarting another car ,that has a bad alternator , damage the alternator im MY car?


#5

No, as long as you do it correctly. Otherwise, all bets are off.


#6

As long as you use a battery cable prophylactic, you should be safe.

Seriously, no it shouldn’t. Nothing on your car should take any damage unless you hook up the jumper cables INcorrectly. (I stand corrected.)


#7

I think Whitey meant “incorrectly”…


#8

did some other research on the subject,… here is what I found so far…

Well, yes and no.

If your car is jumping the dead car, here’s what happens. Let’s say you put the cables on correctly. The instant you finish the connection, your alternator senses the new load and wants to put out a full charge. But if you’re doing the jump with your car just idling, your alternator isn’t turning fast enough to put out all it can. So, it heats up. And yes, you can end up burning out YOUR alternator by jumping someone else’s car.

Even if you get in your car and rev the engine, you’re still asking your alternator to become a battery charger–something is isn’t designed to do for any length of time.

This is a case where being a good friend can end up costing you hundreds of dollars.

You could try leaving your engine off and just jumping with your battery alone. Leave it attached to the dead battery for about 5 minutes. That shouldn’t drain your battery all the way and should give the other battery enough of a boost to get it to start. Then you can start your car and drive it. The key word is drive it–to get the revs up and keep it cool.

Best way to jump a car is with a booster pack.

Rick

I’m a retired ASE Master/L-1 Technician. I still keep current with the latest automotive technology.


#9

I’m still not fully convinced, but I will say that I have had a couple of situations where I had an alternator start to go out a few days after giving a jumpstart. I think it does have something to do with the fact that the alternator is usually never subjected to the current draw from engaging the starter. I think it’s probably only an issue with marginal alternators that were on their way out anyways.

Like I said, I’m still not 100% convinced, but now I’ll only charge someone else’s battery with my engine running or give them a jump with my engine off.


#10

The broken link is how does the alternator “wanting” to put out max output cause heat? I can see an alternator actualy putting out max output cause heat but just “wanting” too, leaves me scratching my head.

As a tech I expect you to identify exactly what part of the alternator is “burned up” vague statements are to be expected from the general public but you should know better.People read what you write and you are held to a higher standard than the layman.


#11

Interesting. I’m pretty sure that happened to me, years ago. I jumped someone’s car with my '71 Datsun 510 and a couple of weeks later, the charge light comes on and I have to get a rebuilt alternator because the diodes had been fried. I always suspected that the jump did the damage, and I never used that car to jump start anyone else again.


#12

I must be way off in my understanding of the alternator, but I thought it’s job was to keep the battery charged, while the battery supplied power to the car. I understand the operating principles of the alternator, and how it produces power, but am I wrong about it’s “job”?


#13

The instant you finish the connection, your alternator senses the new load and wants to put out a full charge. But if you’re doing the jump with your car just idling, your alternator isn’t turning fast enough to put out all it can. So, it heats up. And yes, you can end up burning out YOUR alternator by jumping someone else’s car.

I can see this happen if the voltage regulator is malfunctioning, but I have used my little Civic to jump start large pick-up trucks. When my Civic alternator senses the new load, it doesn’t overload itself. It simply takes more time to charge the big battery. Sometimes it takes five or ten minutes before the truck can start, and sometimes it will only start if I hold the throttle on the Civic, but after 187,000 miles, 11 years, and several jump starts of larger vehicles, I am still using my original alternator.

In order for your theory to hold, something on the running vehicle (maybe the voltage regulator, maybe the alternator itself), must be malfunctioning or on its last leg. If this theory held true, my alternator would die every time my battery dies. It doesn’t.

I think cases like this where someone suspects a jump start damaged their alternator happen because the alternator was on it’s last leg anyway, and doing the jump start pushed it over the edge a little sooner than would have happened otherwise.

Oldschool, I don’t think stachi is an ASE master tech. I think stachi is quoting a response from another forum.


#14

It keeps the battery charged and supplies operating power to the car’s electrical system when the engine is running.


#15

correct Tardis, the cars electrical system runs off of the power supplied by the alternator, not the battery…the batteryis ONLY used to crank the engine over,…not the other way around, and by the time the car with the bad alternator dies , the battery has been supplying ALL of the power needed by the cars electrical system and has been drained almost completely. If you read the instructions when replacing an alternator , you will see that the warranty for the alternator will be voided if the battery is not replaced as part of the alternator replacement for this very reason.


#16

If the battery is “only used to crank the engine over” then why cant I disconnect the battery, and continue operating the engine, after it’s started?


#17

Removing the negative battery cable is the old school way of testing an alternator, if the engine died , the alternator was bad…now , we use a voltmeter to see if the alternator is puting out more than 14 volts…less than that and the alternator is suspect. I wouldn’t even try removing the battery cables while the engine is running on newer ECM controlled cars,…unless you like buying new ECM’s…thats where the voltage regulator is on most cars now


#18

Learn somthin’ new every day. Thanks.


#19

If you read the instructions when replacing an alternator , you
will see that the warranty for the alternator will be voided
if the battery is not replaced as part of the alternator
replacement for this very reason.

But we can’t deduce the reason from that. Is the reason because the battery is drained or is it because the battery might be internally shorted? Both conditions affect the alternator differently.


#20

You’re welcome Cooper ,I learned a lot today as well