I left my lights on, and my vehicle's battery died. . . .

(Well, not recently. Just hypothetically.)

I get my vehicle jump-started, which will presumably keep the battery charged assuming the alternator is in good shape.

The question: How long will it be before my battery is fully charged, and I can reliably restart? Instantly? Minutes? I’ve always driven around for a few minutes just in case, but I wonder if that’s necessary

Depends on how dead the battery is. Best bet, if feasible, is to leave it on a charger overnight. I’ve always heard it’s hard on an alternator to recharge a badly discharged battery.

What he said. Alternators are really just designed to “top off” the battery, which in turn is not designed to be very deeply discharged. Enough deep discharge cycles will kill a car battery permanently (more or less), even after as few as three or four cycles. It’s definitely a good idea to have a charger; if you must drive home after a dead battery jump start, get it on the charger as soon as you can.

Despite the ideal advice given above, I have always driven around for 30 minutes or so (with the AC off if you can) and it has never seemed worse for the wear after that. However, that is usually for the kind of discharge where the starter will turn a couple of times and then not have enough juice to go further. I don’t think that is a deep discharge but it is probably the most common type.

When I thought this had happened to me, the tow truck driver who got my car started said that I should drive for about 30 minutes or more.

The problem turned out to be that my battery was old and wouldn’t hold a charge, though, not that it just happened to be run down.

Only IF you keep the engine running.

Per others approximately 30 minutes.
Due to gas prices I would use a small charger that will bring it up to full charge overnight or 24 hours at the most.

I know it’s not cold in S.C., but my mechaic once told me that a dead battery in freezing temps can cause the electrolyte to freeze. When that happens the plates can warp and short out (notcible by the sides of the batteries bulging) which will pretty much kill the battery.

OK, I am going to take the other side of this discussion.
A modern alternator is designed to keep a fully charged fully charged, or to top up an almost fully charged battery. It is not designed to fully charge a dead battery.
It may appear that after you have driven the car for 30 minutes that the battery is “charged”. It most likely isn’t. It is probably charged to say 50-60% which is plenty to crank the car. The problem is if the battery never gets fully recharged, you will over time lose that capacity. When the winter sets in and the battery has to work hard, it will not be long for this world.
My advice? Run the battery down, use a battery charger.

Well a modern alternator is designed to charge the battery, provide the electricity needed to run the car, engage the a/c clutch, vent fan, rear defroster grid, wipers, accessories placed in the cigerette lighter, headlights, ect.

Now if you drive with those things off you should have quite a bit of ‘extra’ capacity to recharge the battery.

A common misconception. Yes the alternator is designed to run all of those things. The car does not run off the battery, it runs off the alternator. But once you have supplied those needs, it is designed to keep a fully charged battery fully charged, or charge an almost fully charged battery. An alternator is not designed to recharge a dead battery. The voltage regulator is designed to do exactly what I am telling you. It does not run wide open all the time.

Actually, if your battery is in good shape to begin with, you might not even need the jump start. The cells chemically make electricity on their own. Turn everything off, and wait 15 or 20 minutes, and try it again. Chances are, you’ll have enough to crank the engine over and start it. I’ve done it many times.

And if course if you have a manual transmission car and you’re facing downhill, you can turn the ignition on, put it 3rd or 4th gear, let it drift until you get a little speed, and pop the clutch to get it started. Just be sure to push in the clutch when it starts so you don’t stall it.

Did this on my motorcycle recently when left the key turned on the headlight drained the battery.

AskNott, control-z neither one of those actions will fully recharge a battery.

No offense Rick but you seem to be dancing around the subject a bit here then just stating out of the blue a ‘so called fact’ that “An alternator is not designed to recharge a dead battery” in a way that you have concluded that by your statements, which you have not. Sorry I’m not buying it, a car can be used to recharge a battery fully (though it may take longer then 30 minutes).

Well if you had demonstrated this to a class as many times as I have you would probably state it as a fact also. :smiley:
We seem to do variations on this theme every few months. Here is one from about 3 months ago where this came up. See posts 26, 27, 28, 35 and 37.
Or perhpas you will take Robert Bosch’s opinion on the matter.

Okay, I be cornfused. My itty bitty battery charger can fully charge my big honkin WarWagon battery at 14.21 volts and not over 10 amps cause that is all it will put out and my alternator rated an Xity X amps continuous and operating at 14.75 volts and using 25 amps total load which is 75 amps below it rated continuous output of 100 amps? It should not be used to charge the battery if it is discharged to the point that it won’t turn the starter and I have to get a jump? I should not continue my trip but go buy a battery charger and remove my battery and take it into a motel room for 24 hours while I watch “Oprah™” and “Jeapord™”?

Might be the best but I need a little more incentive to do this as so far, with having been driving and killing batteries since 1959, I have never harmed an alternator.

Now, I don’t do a specific gravity test on all my batteries before and after all recharges and do not pull them apart to check condition nor do I do the same on my alternators. Heck, back in the bad old days when alternators were only rated at 20 - 30 amps and were made out of junk materials, I and many folks I know would run cables so an extra battery could be charged for the old farm equipment that the old ‘generators’ had died in.

So, I really need to know why my 100 Amp alternator running a max load of 40 amps is not strong enough nor cooled enough to charge a battery that is not ever taking over 30 amps and that is for less than 20 minutes for as the charge builds, the 14.75 or so volts will only push so much current through.

Splain me more please…

*:: Yeah, I’m being a bit of a smart ass because I know Rick knows his stuff and others here do also, but as an A&P mechanic and an electrical instructor for Spartan School back in 69 and having done the things I have with auto and aircraft electrical systems, I want a bit more in depth answer.

If those who say a totally dead battery should not, or that just any battery that does not have enough to turn the engine should not be and can not be fully charged on a modern car with the higher capacity alternators of today, I want more.

A total dead battery will accept way too many amps if they are available but if they are not due to either regulation or the itty bitty AC powered battery charger won’t put out over 10 anyway. So are we talking a total dead battery that will suck 150 amps if it is available ( skip the part where the battery will self-destruct for that many amps ) and that much load for even 10 minutes will harm the normal
Alternator or … are we talking the normal headlight left on battery rundown where the starter won’t quite turn?

We talking that most folks are just doing short runs and never run the 12 - 24 hours necessary to fully bring back a battery? Or is it a flat ( The alternator will be ruined. )?

Come on those who know, I really want to believe you but I need more better splainin…

Thanks:: *

GusNSpot, ditto.

Once I had an alternator go bad and draw a 3-amp discharge current through it when the engine was turned off (measured with a clamp meter, confirmed with a multimeter) Before I discovered this, I went through a whole lot of cycles of bump-starting every morning (I lived on a hill) and building up a charge on the drive to work. Slowly discharging all day, sometimes the battery would have enough to get me started for the trip home and sometimes I needed a jump-start.

Initially I thought this was a battery problem, replaced it with a nice, fully charged new one, and found it dead in the morning. When I actually found out what was happening, I drove for a week simply disconnecting the battery when the car was parked. The bad alternator gave a recharging current of ~ 50 A with the engine idling, which surprised me but might reflect the state of deep discharge the battery was in.

Eventually I replaced the alternator, and the 3-amp discharge current went away. The new alternator gave a recharge current of ~ 30 A at idle.

That same, abused battery is still in my car a couple of years later, working fine, having been repeatedly completely discharged and only recharged by alternator. I’m sure I knocked some life off it, but I’m not convinced I did my new alternator any harm.

As I understand it, the voltage regulator’s job is to hold the alternator output at 14-and-a-bit volts regardless of whether the engine is idling or revving high. I see no reason why this regulated alternator output shouldn’t be coupled directly to the battery, with a diode to prevent back-discharge and perhaps some high-current protection. What am I missing?

Rick thanks for those links and that latest info. I do agree with that, and your statement that An alternator is not designed to recharge a dead battery, and see hopw if you try to use one as such and don’t drive long enough (hours at a time), you could get into a condition where you are never fully charging the battery.

But it still seems like if you are driving for that long that you could use a altenator to recharge the battery, i.e. if you have a dead battery at a hotel, and have a 8 hour drive the next day.

One further Q about this, if you have a dead battery, just jumpstarted the car and hook up a (12V to 120V) inverter to a (120V to 12V) battery charger to your discharged battery in yoru running car, will that charge the battery faster then just using the altenator by itself?

I once went through a cycle of 3 alternators and 3 batteries in a week with a situation like this. The new battery got run down a couple of times over the course fo the next day due to the bad alternator and then wouldn’t hold a charge at all. I replaced the alternator and the (now) bad battery ruined the alternator. I spent the next 5 days replacing battery and then alternator over and over until I replaced both at once and never had the problem again.

The parts house that had originally diagnosed the battery as being bad covered everything under warranty (except for 1 battery and 1 alternator) because they felt that they should’ve seen the alternator was bad to begin with. This was on a 1982 Chrysler.

After that, I learned to test the alternator with a multimeter.

Later, I had a similar problem with a 2001 Mustang and the dealership was handling the whole thing under warranty. I had already determined that the alternator was bad. When I brought the car in, I told them they needed to replace the alternator, and 2 days later, I got the car back and got stranded the next day. I made the dealership come get the car with a tow truck (even though I could’ve gotten it started by pushing it). Sure enough, the alternator was bad, as I’d already determined and the new battery they’d put in was ruined. I still wonder if they ever did a voltage check the first time to see what was wrong or if they just chunked a new battery in there to get it out of the way.

The moral of the story is to test the battery and the alternator thoroughly to determine which one is bad or you’re going to end up with a big headache. And if you’re bringing it to a mechanic/dealer, make sure they have someone competant who can make the correct diagnosis.

First off guys there is dead and there is D-E-A-D. Pretty much anytime the battery won’t start the car it is considered dead. Now it could be that the battery needs 75% state of charge to start the car, and it is at 70%. Car won’t start, customer says battery is dead. You know what, he is correct. Or it could be a temp related problem like old Chevy pickups with big engines. When parked after driving the solenoid got so hot from heat soak, that a voltage drop was created across the solenoid that was so great, it cased either a no crank, or a very slow crank condition. Customer says battery is dead, in this case he would be incorrect. What I am trying to get across here is that there are different conditions that need to be considered. (Cue the Monty Python dead battery sketch)

Also you need to look at just how the battery got dead. Left the lights on, or did you leave the car parked for several months. Both batteries could be equally dead, but they will take recharging in a totally different way.

Gus it doesn’t have to do with the alternator it has to do with the design of the regulator and the battery itself. Before we go any deeper, let me tell you a story
Back in 1991 a customer bought a car from the dealer in Santa Monica California. If you recall 1991 car sales were in the toilet. Cars sat on the lot for months and months. Any way this guy buys a car that has been sitting for OG knows how long. The salesman has to jump start it. The salesman tells him that it will charge up just fine on the trip home. The customer lives in Apple Valley, which is maybe 120 miles from Santa Monica. So he had a good 2-3 hour drive home. When he gets home he shuts off the car and goes to get his wife to show her the new car. It would not start. All he got was click, click out of the starter.
It got towed to the nearest dealer, which was me. All I could do at that point was replace the battery, which I did. I then tested the alternator every which way, and it was fine. So I shipped the car. The customer never came back with a battery problem. So I fixed the car. The kicker to this story is, I then put the “dead” battery on a slow charge for a day or so (2-3 amps). After it was fully charged I tested it. It was fine. Not wanting a good battery go to waste, I put it in my car. It lasted for 7 years after that. Not to bad for a dead battery. Now why didn’t his original battery charge up on the 3 hour drive home? Because the alternator was not designed to charge a dead battery.
A battery charger works somewhat different than an alternator. When attached to a battery the charger will put out 1.2V above the battery voltage. If the battery is dead enough this means that the charger may only put out 2-3Volts to start. At this output the amperage the charger is supplying is also very small, maybe an amp or so. As the battery starts to take a charge, the voltage and the battery’s ability to take current increase. Therefore the voltage and amperage output from charger will increase. Gus if your battery charger is similar to this one when you first hook it to a dead battery the amp gauge will barely deflect. As the charge progresses the gauge will gradually deflect to max, and then as the battery finishes it charge the charge will taper back off to zero. The voltage all the while will be increasing. The 14.21 Volts would be the voltage at or near the end of the charge. As I said at the beginning the voltage will be much lower than that.
The alternator on the other hand has to put out 13.5-14.5 Volts all the time. However if the battery is really and truly dead, it is not going to want to take a charge, and the alternator can’t / won’t provide it.
Another problem is charging batteries creates heat. The higher the charge rate, the more heat is created. Heat is what kills batteries. Alternator designers limit the charge current to limit the heat, to limit the damage to the battery.

jasonh300 There is an excellent chance that with the proper charging technique those batteries that were “ruined” could have been brought back to life.