Jumped dh's car, now my battery light is on.

[DeniseNJ]

dh's car wouldn't start so I tried to jump start it with my car.  I connected everything correctly, but his car still wouldn't start.  I took my car out later and it was fine.  This was all on Saturday.  This morning, Monday, dh took my car and says my battery light went on while he was driving to work.  What did he do to my car?

Or is it possible to drain your own battery when jumping another car?  But I drove it later that day and it was fine.

Also, I connected everything in the right order with both cars off, then turned my car on for 20 min, then while my car was still running, he tried to start his and it wouldn't start so I turned off my car and disconnected everything.  Are you supposed to leave your car running while the dead car tries to start?  I couldn't find this anywhere. Do you turn off your car and try to start the dead car, or turn off your car and disconnect everything and then try to start dead car?  I left my car on and connected when he tried to start his. 

Waiting to see if he can start my car to get home and if the light is still on.  Also, still not sure what's wrong with his car.

[Car Jack]

Are you supposed to leave your car running while the dead car tries to start?

No.  It's possible that the load of your own car plus his dead battery charge plus the starting current can overload the diodes inside your alternator and blow them.  I see this done all the time and most of the time, it won't cause a problem, but sometimes it does.  You likely need an alternator now.

[DeniseNJ]

Ugh.  That's just great.  All the instructions I read were very specific about what order to connect things and to start with both cars off.  and they all said what order to disconnect and to leave the dead car running if it starts etc., but nothing said when to turn off the good car.  Scotty Kilmer did say don't have the cars both running. And one other site seemed clear that you leave the good car on too but didn't mention when to turn it off, whether before or after removing the wires.  the half dozen other sites I checked didn't specify when to turn off the donor car.

So now I may be shopping for a new alternator.  Fun.

[yachi]

I've always seen the written instructions to connect the negative to a ground in the car with the dead battery (unpainted metal location).  But I've never had success this way, only by connecting + to + and - to -.

[sailinlight]

Are you supposed to leave your car running while the dead car tries to start?

No.  It's possible that the load of your own car plus his dead battery charge plus the starting current can overload the diodes inside your alternator and blow them.  I see this done all the time and most of the time, it won't cause a problem, but sometimes it does.  You likely need an alternator now.

I had no idea, I always assumed that you should leave your alternator running so you don't risk draining the 'good' battery while it's connected. Thanks

[Posthumane]

I disagree about leaving the car running. The "good" car should be left running so as not to drain the battery and provide a higher voltage for starting the dead car. Have boosted many vehicles this way, my own and when I was an army mechanic, as have all my colleagues as this is how we were taught.

The diodes in the rectifier are rated to much higher currents than the alternator can produce even at max rpm, and at idle it won't come close to putting out it's rated current. If the diodes burn out during boosting then they were probably going to burn out soon anyway and it's just the straw that broke the camel's back.

[Syonyk]

So... pull the code and see what it's whining about?  If the MIL is lit, there will be at least one stored code you can pull, or a local parts store can pull or... seriously, just go ask the car what it thinks is wrong instead of speculating.  It could be totally and completely unrelated.

If you own a modern car, you should own an OBD-II reader.  And a voltmeter.  They make life so much easier it's not even funny.  I don't have to guess that the idle issues are a sticky idle air control valve, the computer is shouting, "Hey, my idle air control valve isn't responding right!" if you go listen properly.  Or a weird roughness under load, hey, misfire on cylinder 3.  Yup, plugs are ancient.  Etc.

The little jump start lithium packs are nice enough, I hear, though I've not found any that can do 6V as well, and about half the time I need to jump start something, it's a 6V system.

[yachi]

The little jump start lithium packs are nice enough, I hear, though I've not found any that can do 6V as well, and about half the time I need to jump start something, it's a 6V system.

That's neat.  I didn't realize those were around.  You can get them that have a flashlight and put out usb power for cellphone charging.  The only portable jump start packs I knew of are the old heavy suckers with a lead-acid battery in them.  In college we could borrow them from campus police and lug them back to our cars clear across campus.

[Caoineag]

The little jump start lithium packs are nice enough, I hear, though I've not found any that can do 6V as well, and about half the time I need to jump start something, it's a 6V system.

That's neat.  I didn't realize those were around.  You can get them that have a flashlight and put out usb power for cellphone charging.  The only portable jump start packs I knew of are the old heavy suckers with a lead-acid battery in them.  In college we could borrow them from campus police and lug them back to our cars clear across campus.

The lithium jump starters are worth it. We have used them a couple of times to help others and when we got some as gifts for other people, they came in handy for them too. They work instantly which is nice. No waiting around required.

[JLee]

Are you supposed to leave your car running while the dead car tries to start?

No.  It's possible that the load of your own car plus his dead battery charge plus the starting current can overload the diodes inside your alternator and blow them.  I see this done all the time and most of the time, it won't cause a problem, but sometimes it does.  You likely need an alternator now.

I have always jumped with the engine running - a random "how to jumpstart" article agrees: https://www.pepboys.com/auto-care/maintenance/jump-starting-your-car

[Syonyk]

The lithium jump starters are worth it. We have used them a couple of times to help others and when we got some as gifts for other people, they came in handy for them too. They work instantly which is nice. No waiting around required.

And if you have a car that can't jump start other cars (our Volt is in the "You will probably burn up the power converter if you try..." category), one would be nice to have.  I had someone's car die around our hill while he was moving rocks (left the lights on), and it turns out, it's a royal pain for me to jump start 12V stuff around the hill... truck had a trailer attached, car won't jump start stuff, and the stuff that was easily moved wasn't 12V. :/

I've seen claims that some of those packs will cold start a diesel in the winter, and I've been tempted to try it.  The actual energy required to run the glow plugs and crank the engine isn't that much, but the power requirements are substantial (I think my truck cranks well north of 500A, glow plugs alone are 160A...).

[lthenderson]

The little jump start lithium packs are nice enough, I hear, though I've not found any that can do 6V as well, and about half the time I need to jump start something, it's a 6V system.

That's neat.  I didn't realize those were around.  You can get them that have a flashlight and put out usb power for cellphone charging.  The only portable jump start packs I knew of are the old heavy suckers with a lead-acid battery in them.  In college we could borrow them from campus police and lug them back to our cars clear across campus.

The lithium jump starters are worth it. We have used them a couple of times to help others and when we got some as gifts for other people, they came in handy for them too. They work instantly which is nice. No waiting around required.

If you have kids though, be forewarned that they can also use those lithium battery packs to recharge their electronic devices so when you go to use it, there may not be enough charge left anymore. Ask me how I learned that! I now have a standing rule that they must notify me if they use it so I can recharge it when we get back home.

[sonofsven]

dh's car wouldn't start so I tried to jump start it with my car.  I connected everything correctly, but his car still wouldn't start.  I took my car out later and it was fine.  This was all on Saturday.  This morning, Monday, dh took my car and says my battery light went on while he was driving to work.  What did he do to my car?

Or is it possible to drain your own battery when jumping another car?  But I drove it later that day and it was fine.

Also, I connected everything in the right order with both cars off, then turned my car on for 20 min, then while my car was still running, he tried to start his and it wouldn't start so I turned off my car and disconnected everything.  Are you supposed to leave your car running while the dead car tries to start?  I couldn't find this anywhere. Do you turn off your car and try to start the dead car, or turn off your car and disconnect everything and then try to start dead car?  I left my car on and connected when he tried to start his. 

Waiting to see if he can start my car to get home and if the light is still on.  Also, still not sure what's wrong with his car.

How old is your battery? It could be a coincidence, I've had that happen where something has been working and you fix something that wasn't working and then what had been working quits too.
It makes troubleshooting difficult.

[DeniseNJ]

Mechanic confirms, need new alternator.

Do not leave your car running when you jump another car.  Let it run for a while to charge the battery then TURN IT OFF and try to start the other car.
https://www.youtube.com/watch?v=xNOjEuyRRO0
Listen to Scotty.  Even if it's rare, when it happens to you and you need a new alternator you will be as sad a I am.

[DeniseNJ]

Oh, I didn't mention what happened last night.  Battery light stayed on. When I dropped my kid off at his class, my power steering started to go, then the lights were going off, the AC got weak, and the car started jerking.  I turned it off, rest a few minutes and it started right up, but within minutes it was the same.  Battery had power to start but alternator wouldn't keep it charged.  So I got it towed and mechanic confirmed what I already knew, I ruined my alternator.

[Syonyk]

How bad is it to swap out?  Reasons I hate transverse mount engines: Alternators are more of a pain to replace than a longitudinal mount.

Though if you get a new alternator, have them test it before you leave with it - they're occasionally bad out of the box, which makes troubleshooting radically more frustrating, because you know it's got a reman alternator on it, so that can't be the problem... etc.

[Car Jack]

A way you can leave the good car running is if you make the connections and just wait.  Leave it there for a good 10 minutes.  This is the same thing you would do with a jumper pack and what's going on is that you're charging the bad battery.  Just before attempting to start the dead battery car, shut off the good one.

I learned this hard lesson twice when I was maybe 20.  First was getting a jump from someone else and they were revving the crap out of their van.  I got my car started and their alternator immediately died.

The other time, I jumped someone else, leaving my car running.  I brought my dead alternator to an auto electric shop and they confirmed that I blew up the diodes.  Unfortunately, the diode replacement pack would have cost as much as a new alternator.

A fact for tow trucks jumping cars, they are running while jumping, but they have a separate, protected battery pack with a 24V battery to do the jumping. 

[DeniseNJ]

OK, Well now I've seen everything!  The mechanic said he doesn't think my wrecked alternator was due to jumping dh's car.  He said nothing was blown out and, while the stress didn't help, it did not cause the alternator to die.  It was a coincidence?!

And then as we talked and I was telling him what happened, I realized or remembered that the jump start didn't work--dh tried to start his car a few times while mine was running but it never turned over.  We had to have his towed bc we couldn't get it started.  So the alternators weren't running at the same time bc we never got his car running.

How could my alternator die the next day I tried to jump start another car and it not be related?  Am I just lucky?  should I buy a lottery ticket?

[Artem_F]

to answer your question, one needs to know what exactly died in the alternator. I guess, the brushes are worn, so they do not contact the rotor anymore. If this is the case, the repair should not cost more than few $, and it has nothing to do with jump starting, you were just lucky. If not, the second plausible explanation after burnt rectifier is a faulty contact on the way from the alternator to the system, and this can be affected by jump starting (I mean, the contact was already bad and high current just finished it).