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I know you should be using a chArger and not jump leads as this can affect alternator etc if donor car.

But still want to understand.

If I add the jump cables directly to isolated battery terminals I.e the battery is not connected to the car battery cables will it recharge like this and how long would it take? If no why?

One might be thinking about this if they have a dead battery(which is rechargeable), no charger and a faulty starter too so charging by attaching to cables will not charge the battery.

Thanks

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Yes, you can absolutely do this, with or without the battery being in the car.

If you think about it, many cables aren't capable of providing enough juice from one car to another. When you attach them, you cannot attempt to start the receiving car right away ... you have to let the donor car run while attached to the receiving car. When you're doing this, you're actually charging the battery. Once the dead battery has enough juice from the donor car (has been recharged a certain amount), then you try it, the partially recharged battery as well as the amount of juice through the cables gives you enough to start the vehicle. If you left a battery in this mode for a while, it would be completely recharged.

Here's the thing though. There's a couple of problems with doing this. The problem is it is HIGHLY inefficient. You'd be doing a lot better by putting the dead battery on a charger and letting it charge easily. Secondly, it really isn't good for the donor car to completely recharge the battery using this method. While you could possibly do this, the problem lies in the fact the donor car has to be left idling for a long time, which really isn't good for the car nor is it good for the environment.

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  • Above idle gets you most alternator output 1500 to 2000rpm - consider the pulley drive ratio and alternators start developing full output around 3000rpm (alternator revs)...
    – Solar Mike
    Sep 20, 2020 at 19:20
  • Any idea roughly how long might it take? And why do you say a donor car idling or revolution by without moving is a bad thing? Hope u can amswer. Thanks.
    – James Wilson
    Sep 20, 2020 at 19:23
  • @JamesWilson - There's no telling. It all depends on what "dead" means. Is the battery completely drained? It's going to take longer. How much power can the donor vehicle supply? Less=more time; more=less time. Voltage/amperage output is usually less at lower RPMs. Idle is less power and more time; revving the engine to 3k rpm is more juice and less time.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 20, 2020 at 19:32
  • @PAULSTER2 I guess what I’m asking is might it recharge in 20 minutes or is t more likely to take hours or even days? You did mention it was inefficient.
    – James Wilson
    Sep 21, 2020 at 16:50
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    @JamesWilson - Might point is, it depends on the depletion of the battery. More than likely, the battery will take several hours to completely recharge ... but that depends on all those factors I mentioned. The reason I said it was inefficient is because if you consider how much energy it takes to recharge a battery using a car versus using a 2A recharger, the answer is pretty obvious why the car would lose the battle every time. Also, consider in the UK idling your car for more than a minute is actually illegal, isn't it? Something else to consider. A $20 tender seems like a good option here.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 21, 2020 at 17:58

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