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Let's say I'm unable to start my car. I get a jump from another car, and my car is now running and charging the battery. The advice I get next is "leave it running for 45 minutes so it can charge up the battery."

Which is better, to leave the car parked and idling, or to take it out on the road and rack up more RPMs while it charges? Will one result in a faster or better charge than the other?

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  • This isn't your question but idling means combustion doesn't combust as well so your engine will suffer. Alternator is cheaper than engine so....
    – findwindow
    Jun 28 at 18:11

2 Answers 2

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Of the two, running it at higher RPMs will result in a faster/better charge for the battery.

That said, It should be of note, the alternator really isn't made to be used this way, yet people do it all the time anyways. The alternator is more or less a device which replaces the energy lost to starting the vehicle and the small amounts lost to maintain systems in the vehicle which need a constant power source. Can you use it to recharge? Absolutely. It's just not doing your alternator any favors (or your battery for that matter). Using a battery charger on its lowest setting (usually 2A for most chargers) until fully charged is the by far better way to recharge a battery.

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  • In fact, I've been trying to charge it much of the day with a DieHard 2/10/50 battery charger and starter. After several hours on 10A there was no change in the needle; I've switched it to 2A based on your recommendation, but will have to wait overnight to see if it works better that way... My apparent inability to charge a battery using a battery charger is what led me to ask about proper post-jump actions.
    – gowenfawr
    Jun 28 at 21:53
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    @gowenfawr - If it isn't taking a charge, I'd suggest the problem is you need to replace the battery. Jun 28 at 22:06
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    @Paulster2 You're not wrong... this particular battery has had a tragic if short life.
    – gowenfawr
    Jun 29 at 1:16
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    @gowenfawr point is, if a battery charger doesn't bring it up after several hours of 10A, then you shouldn't expect the alternator to do better (even if it does have, say, a 100A max output). The battery will probably be dead again shortly after you turn off the engine, regardless of whether you drive it around after jumping.
    – hobbs
    Jun 29 at 18:18
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    It wasn't charged this morning, so AAA came out, suggested that the charger should connect red-red terminal and black-black terminal instead of black to engine. That said, he thinks my charger isn't working. Then he hooked a portable battery to mine and it jumped easily, even restarting on its own after a few minutes runtime. None of which explains why the battery was dead in the first place. So now I'm: alternator charging it early today and later after work, trying to diagnose the battery charger with a multimeter, and waiting for Amazon to deliver a jump battery to sit in the trunk.
    – gowenfawr
    Jun 29 at 18:31
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Something of concern: the engine may stall, and if there isn't enough charge in the battery to re-start the engine, you could be screwed in the middle of the traffic.

I'd say some period of idling would be good. Not necessarily 45 minutes, but let it idle at least 15 minutes. By 15 minutes, even if idling (so charging rate is lower), there should be enough charge to start a warm engine and the engine should be warm enough to be easily restartable.

This is especially important if the car is a manual transmission car and you either have a history of stalling engines when accelerating from a stoplight, or if you for some reason are unfamiliar with the clutch feel of the car (for example due to it being a very recent purchase).

Of course you may avoid this problem by giving excessive gas before operating the clutch. It'll burn some clutch, but quite well avoids the possibility of stalling the engine.

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  • +1, but if it's a manual with which you're familiar, stalling should be very rare, so we're talking about rare cases. I have actually had to deal with something similar - not a dead battery but a failed starter in a work car that I'd borrowed. We bump started it then I had to get it to the garage, the other side of the city centre, in heavy traffic.
    – Chris H
    Jun 29 at 8:31
  • @ChrisH it's very easy to stall a manual with a dead battery, brake lights can pull enough juice that the spark plugs fail to ignite. I had an '84 S10 with a dead battery, popped the clutch to get it going, but the minute I touched the brakes the engine would stop, I was in a small parking lot on a hill so I couldn't just let it roll. Of course this is without a jump to give the battery a little juice.
    – rtaft
    Jun 30 at 14:33
  • @rtaft if the battery isn't completely wrecked, 5 minutes at idle will give you enough charge for a few stops. But you must have had a pretty bad battery and probably a small alternator as well, because I've never needed that from bump starting a flat battery. Of course to get to idling stationary, you may need to use the handbrake for the first stop - assuming you have a sane design with a cable-actuated handbrake and not some sort of stupid electronic arrangement. I've only ever owned manuals, and some of them have been pretty old and low-spec, so I've had a few bad batteries
    – Chris H
    Jun 30 at 14:38

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