Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Jump-starting with booster car running?

Like any good truck owner, I feel that I should always carry jumper cables, because I never know when I might run across a damsel in distress or come face to face with my own stupidity in the form of leaving the headlights on. However, like any good jumper cable owner, I feel that I should know how to use them properly without blowing my car and myself up, or worse, making a fool of myself.

In every jump in which I have seen or participated, the booster vehicle was always left running for the duration of the affair. This seems to make logical sense. However, all the jump-starting instructions I have read, including the one on your site, have recommended that the cables be attached with the booster engine off and starting it post-hookup.

I would be afraid that the with the extra demand of the dead battery, the booster battery wouldn?t have enough power left to start the booster engine, and you would end up with two half-dead batteries.

So essentially, I have two questions: why is it better or safer to turn off the booster engine before hooking up the jumper cables? And secondly, have you ever heard of a case where the booster vehicle was unable to be restarted (i.e. if the dead battery was very low)

This has become a point of family contention, so any help would be greatly appreciated. Thanks!


Guess that is what I get for not reading directions, can you put a link so I can see the error of my ways? Always had running car running to provide makeup charge for charge lost in jumping and seems logical to me too! I am not too entrenched in my ways but will need to look this one over.

Here is Car Talk’s guide:

Your vehicle’s owner’s manual probably has a similar section too (the manuals for my Nissan Xterra and Ford Excursion both do)

Thanks for the quick reply :slight_smile:

There are some who claim that jumping with the engine running can lead to alternator failure very soon thereafter. I’m personally agnostic to it-- I can’t really understand why it would happen, but there seems to be some anecdotal evidence for it.

I personally don’t jump with the donor car running unless I really have to and I won’t do it at all with my Honda, which has an expensive and difficult to replace alternator.

Very good questions about jumpstarting; I’d be very curious to see the responses.

I’ve jumpstarted about a dozen or so cars over the past 20 years. I typically always refresh my memory and read the manual before doing so. I’ve noticed that the directions seem to always vary slightly between manufacturers but with a couple primary differences. Some manufacturers recommend connecting the neutral cable to a grounded surface on the dead car, and others recommend connecting it to the neutral terminal on the dead battery.

The same goes in order of connections, some manufacturers recommend connecting the positive cable to the good car first and others recommend going backwards and connecting to the dead battery first.

On my Jeep:

  1. Connect cable to positive on booster car;
  2. Connect other end to positive on dead battery;
  3. Connect cable on neutral of booster battery;
  4. Connect other end to neutral battery terminal on dead battery;

I think whether or not the boosting vehicle should be running depends on the size. For example, you can jump start a motorcycle with a car, but most people recommend the car be shut off for the whole process. However, if I use my Honda Civic to jump start your truck, there is no way my little battery could do the job with the engine off. My vehicle would have to be running, and it might take 10-20 minutes for my little alternator to recharge your dead battery to the point where it can start your truck. As a result, if you use your truck to help a damsel in distress who left the headlights of her Dodge Neon on all night, it is probably safe to shut down your truck before you hook up the cables and leave it off until the cables have been disconnected. On the other hand, if you are jump-starting another full-sized truck, SUV or minivan, you might want to have your truck running before you even connect the cables.

Dude, there’s no role for your alternator at all in this scenario, other than keeping your own battery charged. Your alternator has no role in charging the battery of the vehicle with the dead battery.

Why would I leave my car’s engine off while charging a motorcycle? Nothing you said here makes sense except for the last sentence.

I beg to differ. My point is that a vehicle with a large battery usually has the capacity to jump start a vehicle with a smaller battery without running. However, when you try to jump start a vehicle with a large battery using a vehicle with a smaller battery, the larger one usually won’t start right away after you hook up the cables. You have to leave them connected with the good vehicle running for a while before the larger dead vehicle will start. That is just the way it is. I can use my Civic to jump start my mother’s Sienna, but if my Civic isn’t running, it isn’t up to the job. If the Civic is running, and is given time to build up a charge, the Civic can be used to jump start the Sienna. In this case, the alternator in the small vehicle is being used to charge the dead battery in the larger vehicle until it has enough power to start. If the alternator had no role in this exchange, the larger dead vehicle would never start at all, no matter how long both vehicles were left connected with the small one running.

No, you’re [ZombieWoof] wrong. If the donor car is running, then it’s alternator will be supplying current to the dead car. Perhaps electrical is not your thing.

More likely than not they tell you not to have the “booster” car running for safety’s sake. i.e. fingers or clothes being caught in belts, etc. I’ve always left the car running and never had any problems.

Tardis and Whitey:

Notice I was talking about “charging” the “battery” not “starting” the “car”. Once you disconnect the jumper cables after “starting” the “car” the “alternator” in the “car” with the bad “battery” will “charge” that “car”'s “battery”. The “alternator” of the jumping “car” is not needed to “charge” the “battery” in the jumped “car”.

Then you are talking about some other scenario than the one that Whitey described. He clearly describes hooking up the cables and running the donor car with cables connected for thirty minutes before starting and then disconnecting the bad car.

Actually, I often disagree with him, but he’s right in this case. There are several cars (usually V8s) that won’t start with just jumper cables and no battery. The cables simply won’t carry enough current (or more accurately, they have too much voltage drop at the current required). Just because you haven’t experienced it, doesn’t mean it doesn’t happen.

For me I like the fact that with the donor car running the difference in voltage potential in the two cars is greater so this mkes it possible for more current to flow from the donor to the car being jumped.

Secondary, the load placed on the donor cars battery can be attenuated if the donor car is running.

Third, nothing is going to work (OK 99% sure nothing is going to work) when the dead cars battery is totaly flat (like under 6V standing voltage)donor running or not, and this is the time the donors cars electrical system is pushed to the max and it is not good to push the donor car to the max, this is when those anecdocal stories get their legs.

I had this happen to me after jumping a neighbor’s car. Next day the alternator was dead. I’ve jumped many cars before and since then though. So I think it just weeds out the weak ones.